Distortion products [Archive]

Patrick Turner

April 2nd 11, 10:55 AM

On Apr 2, 7:23*am, Newbie > wrote:
> It seems well known that negative feedback can reduce lower order harmonics, but
> increase the higher order ones.
>
> Can anyone direct me to references that show how this happens with mathematical
> analysis? *Perhaps a paper in the journal of the AES, or elsewhere; on the web,
> perhaps?

During the last 40 years there have been at least two in depth
articles in Wireless World which later became Electronics World. I
read all the WW and EW magazines from 1917 to about 1997 and you'll
probably find the articles you want to read if you go where there is
an archived set of the magazines like I did. In about 1993 I spent
days reading in several university library archives to see what had
been said about anything to do with audio engineering. Unfortunately I
don't have copies of all I read, but the phenomena of increasing
distortion spectra after applying NFB is well known, and remains
rivetted into my brain cells.

The conditions required to "make matters worse" with NFB are about as
follows :-

The open loop THD distortion of the amp approaches 10% at 2dB below
clipping.

The frequency of an input test signal is say 1kHz.

The open loop bandwidth of the amp is reducing at 6dB/octave above say
1kHz, worst case.

The open loop phase shift is high at F above the input signal, but not
so high
to allow NFB to become positive FB and causing oscillations.

A low amount of global NFB is connected, say 10dB.

Suppose the THD open loop spectra contains 2H, 3H in declining %
amounts in that order.

With 10dB NFB applied, and at 2dB below clipping where there is still
enough forward gain to drive the signal
without clipping, there will be a reduction of 2H, but not as much as
one might predict using the formula THD = THD without FB / ( 1 + [A x
ß] )
The 2H which appears at the amp output is fed back and intermodulation
products of H and 3H are generated because of the nonlinearity of the
open loop gain. These add to the mix already there depending on phase
and and sure the 2H is reduced, and I don't know what happens to H,
but the 3H is only marginally reduced. The 3H when run around the loop
generates IMD of 2H and 4H, and these also feed the mix and you get
4H and 6H and so on. These harmonics are called second order harmonic
products.
Now if the OL phase shift is large and OLG declines after 1kHz then
the effective amount of NFB also declines
and there are lousy conditions present for reducing THD or any other
damn artifacts by NFB.

Now if the amount of global NFB is increased a lot, to say 20dB, then
the effect of "fixing up the mess" with NFB increases and low numbered
H are reduced as theory predicts but second order products become more
numerous in number and they might resolve down to very complex
composition especially when many signal frequencies are present in
music.
To get around this problem in SS amps there is usually an enormous
total amount of NFB applied. An OP stage might have the output mosfets
connected in source follower mode which could be a 20dB application of
local NFB and depending on load value. The OP stage might then make
only 2% THD at a dB below 100W and the driver stage may make 5%. The
total THD will then be approx 5.1%. There may be 52dB of global NFB so
we see 0.0127% with GNFB. The OL bandwidth of most SS amps is very
high at 100Hz, but above this F we may see the OLG decline at 6dB/
octave or 20dB/decade so that by 10kHz applied GNFB is only +12dB.
There is 90 degrees of OL phase lag determined by the capacitor
between VAS collector output and VAS base input. Phase shift of 90d is
easily corrected while the shift does not exceed 90d, so that the
amplifier with GNFB "measures pretty well."

If the amp is tested with a 20kHz sine wave 2H is 40kHz, and all the
NFB in the world does very little to reduce it because the A reduction
is so slight because OLG has become so low.

If you examine the characteristics of opamp gain, you'll find out
about. Not a huge amount of info is given by makers of audio amps with
regard to OLG curves and serious analysis because its such a sad
boring story which can only be understood by engineers.

In a tubed amp, There may also be 5% THD at 2dB blow clipping, but
usually OLG is up to 10kHz before gain tailoring zobel networks reduce
gain and reduce the initial F2 pole down a decade and insert a higher
F3 pole so that stability with moderate GNFB of say 20dB may be
applied.
The OL THD of 5% is reduced to 0.5% typically.

But where only 10dB of GNFB is applied, there will be second order
products which are unresolved to negligible levels and they may be all
too audible, bearing in mind that above 100Hz, tones of the same
amplitude seem to sound louder as F rises.
A 10W SET amp using poor output tube load matching and using 10dB GNFB
and which is being forced to use all of its headroom may sound worse
than with NFB disconnected.

But where the SET is used sensibly with Horn loaded speakers needing
only 1/20 of the power, the THD with GNFB is much reduced as *simple*
math and theory may predict because initial OLG THD level is less than
perhaps 0.8% at 0.5Watts. I find 12dB GNFB is about enough for SET
amps so that typically there is 1.3% THD at 10W and 0.2%at 0.5W, and
perhaps we may agree that the IMD produced in an amp with
predominantly even numbered H products produces a less agonizing sound
than the PP amp with odd number H and the same % level at the same
PO.

In 10W class A and AB PP amps the OL THD might be 1% if triodes are
used and at 0.5W OLG THD = 0.2%. The 20dB of GNFB will then reduce the
tube amp THD to 0.1% at 10W
and 0.02% at 0.5W. The Wiliamson amp is a classic example.

There have been other attempts to reduce THD to vanishingly low levels
by Professor Ed Cherry with is Nested FB idea, and by the makers of
Halcro amps whose schematic does not seem to have entered the public
domain where it achieves 0.0001% THD at 20kHz, slightly better than
you can do I would bet.

In a nutshell, SS amps need all the NFB thay might muster, but tube
amps get way with much less.

Where OL BW is wide and THD is below 1% at clipping, 10dB NFB is OK,
but you'll see artifacts at low levels which were not present on the
OLG analysis.

After reading the papers I have seen I try to make my amps with OLG
giving low THD, low phase shift, wide BW at full power and leave very
little work for the GNFB to achieve. Sound is fine with NFB.
Even if I knew all the math, I'd not make better tube amplifiers
because how good they are depends on my understanding of concepts and
simple methods to reduce OLG problems. The amount of GNFB applicable
around a tube amp is limited by the phase shifts caused by stray C and
L in RC couplings and Miller C and in the OPT. All the math in the
world won't make tube amps very much more linear than they have been
for the last 50 years. In fact, many tube amps are LESS linear than 50
years ago when many makers tried to use 26dB or even 30dB of GNFB so
they could quote sales propelling figures. These days output tubes are
run with class B amp loading, only 15dB GNFB, and with speakers which
are much less sensitive than in 1960. THD and IMD are rarely ever
mentioned. In many ways tube amp design has flowed backwards, and the
average THD and IMD being listened to has increased +15dB, and
difficulty of servicing increased +10dB, while the need for servicing
occurs +7dB more often.

I raise my hat to all makers charging such high prices and with much
more THD/IMD. They are giving me a better chance to sell my amps.

And one other thing. Triodes have internal NFB built into them. The
NFB is most effective when Ia change is zero, permitting a single
triode connected 6550 to produce 100Vrms at only 1% THD.
Professor child wrote about this in one of Terman's hard cover old
books written in 1937.

Much of the understanding of my father's generation was not widely
understood 60 years ago, and the written article have been lost, and
not uploaded, and now there is the buffoonery of the short attention
span ****ter and Farcebook generation where they agree that history
may be safely ignored.

Patrick Turner.

It's that Guy again...

April 2nd 11, 12:17 PM

On Sat, 2 Apr 2011 02:55:54 -0700 (PDT), Patrick Turner
> wrote:

>Much of the understanding of my father's generation was not widely
>understood 60 years ago, and the written article have been lost, and
>not uploaded, and now there is the buffoonery of the short attention
>span ****ter and Farcebook generation where they agree that history
>may be safely ignored.

where I live there are/were a few old used book stores that had
all those old tube books, with math my (then) calc was blind to.

SOMEONE local here bought all the candy, then dumped them.

Too often, a 'estate' just dumps what is left because the people
running it don't understand/know/care what the stuff is worth.

Years ago, a estate dumped about 500 old Boyds teddy bears
and my Mom, a long time collector, bought them for chump change.

Value now is about 400%+ of pure profit.

I bought as many books I could, got a pile of them, yet there ARE
sources of said books in .pdf. Yeah, I know, have to have it in ya
hands, I agree, but otherwise, I couldn't find them at all.

Check Usenet even, so very often for free.

>Patrick Turner.

Great post, as usual. Thanx...

JJTj

COMING SOON

FRITZMANIA 2011

www.fritztronics.com

Andre Jute[_2_]

April 2nd 11, 01:16 PM

On Apr 1, 9:23*pm, Newbie > wrote:
> It seems well known that negative feedback can reduce lower order harmonics, but
> increase the higher order ones.
>
> Can anyone direct me to references that show how this happens with mathematical
> analysis? *Perhaps a paper in the journal of the AES, or elsewhere; on the web,
> perhaps?

Here's an article to help you distinguish between added-on negative
feedback and natural negative feedback.
http://www.audio-talk.co.uk/fiultra/KISS%20104%20by%20Andre%20Jute.htm

This article explains why added-on negative feedback is a bad thing in
tube audio if you have really good speakers; it focuses on how the
higher harmonic artifacts are added.
http://www.audio-talk.co.uk/fiultra/KISS%20123%20by%20Andre%20Jute.htm

More articles on Harmonic Distortion on the KISS Amps site, available
through those pages.

Andre Jute
Visit Jute on Amps at
http://www.audio-talk.co.uk/fiultra/
"wonderfully well written and reasoned information for the tube audio
constructor"
John Broskie TubeCAD & GlassWare
"an unbelievably comprehensive web site containing vital gems of
wisdom"
Stuart Perry Hi-Fi News & Record Review

mike s

April 2nd 11, 01:35 PM

John L Stewart

April 2nd 11, 03:20 PM

It seems well known that negative feedback can reduce lower order harmonics, but
increase the higher order ones.

Can anyone direct me to references that show how this happens with mathematical
analysis? Perhaps a paper in the journal of the AES, or elsewhere; on the web,
perhaps?

---------------------------------------------------------------------

For an authoritative coverage of the subject, see the text 'Electronic & Radio Engineering', Frederick E. Terman, 4th Edition. All covered well in Chapter 6, in particular around p204 then again in Chapter 10, p 326. Lots of power series stuff.

For those not familiar with Terman, he was the head of EE at Stanford U. Among his students were both Hewlett & Packard. Also the Varian Brothers, inventors of the Klystron.

Later Schockley, co-inventor of the transistor showed up at Stanford. Many spinoffs followed. With heavy weights like these it is no wonder the SFO Bay area became known as Silicon Valley.

You can find most of Terman's textbooks at-

http://www.alibris.com/booksearch?mtype=B&author=terman+frederick+emmons

And many others. Terman & his work are often found & referenced in RDH4.

I have several of Terman's books. They are a great help in understanding the vacuum tube era.

A few years ago I built two simple RC amplifiers for making some comparisons of distortion. Both used the same twin triode, it was a 12AU7 I think. One circuit was connected as a mu-follower while the other was a feedback pair. The feedback pair was adjusted so that its gain was the same as the mu-follower circuit.

Both showed low distortion on a THD meter, the HP334A. But when a spectrum analyzer was connected to the feedback pair the new higher order distortion components showed up. These are the distortions that are said to cause discomfort.

Cheers to all, John

Don Pearce[_3_]

April 2nd 11, 08:46 PM

On Sat, 02 Apr 2011 14:38:18 -0500, flipper > wrote:

>On Sat, 2 Apr 2011 02:55:54 -0700 (PDT), Patrick Turner
> wrote:
>
>>On Apr 2, 7:23*am, Newbie > wrote:
>>> It seems well known that negative feedback can reduce lower order harmonics, but
>>> increase the higher order ones.
>>>
>>> Can anyone direct me to references that show how this happens with mathematical
>>> analysis? *Perhaps a paper in the journal of the AES, or elsewhere; on the web,
>>> perhaps?
>>
>>
>>During the last 40 years there have been at least two in depth
>>articles in Wireless World which later became Electronics World. I
>>read all the WW and EW magazines from 1917 to about 1997 and you'll
>>probably find the articles you want to read if you go where there is
>>an archived set of the magazines like I did. In about 1993 I spent
>>days reading in several university library archives to see what had
>>been said about anything to do with audio engineering. Unfortunately I
>>don't have copies of all I read, but the phenomena of increasing
>>distortion spectra after applying NFB is well known, and remains
>>rivetted into my brain cells.
>>
>>The conditions required to "make matters worse" with NFB are about as
>>follows :-
>
>The only condition required to increase higher order harmonics is NFB.
>
Negative feedback can never make harmonics worse. Feedback can make
harmonics worse when the phase shifts far enough to make it positive.

d

Don Pearce[_3_]

April 2nd 11, 09:33 PM

On Sat, 02 Apr 2011 15:21:55 -0500, flipper > wrote:

>On Sat, 02 Apr 2011 19:46:54 GMT, (Don Pearce) wrote:
>
>>On Sat, 02 Apr 2011 14:38:18 -0500, flipper > wrote:
>>
>>>On Sat, 2 Apr 2011 02:55:54 -0700 (PDT), Patrick Turner
> wrote:
>>>
>>>>On Apr 2, 7:23*am, Newbie > wrote:
>>>>> It seems well known that negative feedback can reduce lower order harmonics, but
>>>>> increase the higher order ones.
>>>>>
>>>>> Can anyone direct me to references that show how this happens with mathematical
>>>>> analysis? *Perhaps a paper in the journal of the AES, or elsewhere; on the web,
>>>>> perhaps?
>>>>
>>>>
>>>>During the last 40 years there have been at least two in depth
>>>>articles in Wireless World which later became Electronics World. I
>>>>read all the WW and EW magazines from 1917 to about 1997 and you'll
>>>>probably find the articles you want to read if you go where there is
>>>>an archived set of the magazines like I did. In about 1993 I spent
>>>>days reading in several university library archives to see what had
>>>>been said about anything to do with audio engineering. Unfortunately I
>>>>don't have copies of all I read, but the phenomena of increasing
>>>>distortion spectra after applying NFB is well known, and remains
>>>>rivetted into my brain cells.
>>>>
>>>>The conditions required to "make matters worse" with NFB are about as
>>>>follows :-
>>>
>>>The only condition required to increase higher order harmonics is NFB.
>>>
>>Negative feedback can never make harmonics worse.
>
>I didn't say it made "harmonics worse." I said it increases *higher
>order* harmonics.
>

Exactly - in terms of harmonics, increasing means making worse.

>THD decreases because the reduction in low order harmonics is greater
>than the increase in high order harmonics.
>
>> Feedback can make
>>harmonics worse when the phase shifts far enough to make it positive.
>
>That will certainly screw things up but it has nothing to do with the
>basic principle of NFB always increasing higher order harmonics.
>
>Read Baxandall's paper.
>
I've read it. I suggest you read it again.

d

Phil Allison[_3_]

April 2nd 11, 11:15 PM

"flipper the ****wit "
Patrick Turner
>
>>
>>The conditions required to "make matters worse" with NFB are about as
>>follows :-
>
> The only condition required to increase higher order harmonics is NFB.

** That is absolute BULL**** !!

..... Phil

Phil Allison[_3_]

April 2nd 11, 11:18 PM

"flipper the ****wit "

>
> That will certainly screw things up but it has nothing to do with the
> basic principle of NFB always increasing higher order harmonics.
>
> Read Baxandall's paper.

** He says nothing of the sort.

As usual, you have taken the comment OUT OF CONTEXT .

..... Phil

Phil Allison[_3_]

April 2nd 11, 11:27 PM

"flipper is reading challenged "

>
> Take it from 'the horses mouth' or, at least, one of the well known
> thoroughbreds
>
> http://www.linearaudio.nl/Documents/Baxandall%20power%20amp%20design.pdf
>
> I believe it's in issue 5 about 80% of the way down.

** What is ??

One of the oldest CHEATS in the world is to claim that proof of one's
point is hidden in some massive pile of data somewhere.

When it ain't.

..... Phil

Phil Allison[_3_]

April 2nd 11, 11:35 PM

"Don Pearce"
>
>>Read Baxandall's paper.
>>
> I've read it. I suggest you read it again.

** No matter how many times someone like " flipper " reads that paper -
he will continue to misinterpret it.

Cos he ignores the context and always takes the literal meaning of words to
be the case.

Peter B writes well, but at a level that is intended for engineers and the
like to read.

Not hobbyists and fools like "flipper".

..... Phil

Phil Allison[_3_]

April 3rd 11, 02:54 AM

"flipper"

> "Conclusions

** The "conclusions" are about what ?

A little circuit with a single j-fet (a 2N5456) operated at high level and
hence much waveform asymmetry.

That is all his " conclusions " apply to.

..... Phil

Phil Allison[_3_]

April 3rd 11, 02:57 AM

"flipper is reading challenged "
>
>>
>>> Take it from 'the horses mouth' or, at least, one of the well known
>>> thoroughbreds
>>>
>>> http://www.linearaudio.nl/Documents/Baxandall%20power%20amp%20design.pdf
>>>
>>> I believe it's in issue 5 about 80% of the way down.
>>
>>** What is ??
>
>>One of the oldest CHEATS in the world is to claim that proof of one's
>>point is hidden in some massive pile of data somewhere.
>
> I gave the issue number and rough physical location,

** Not nearly enough.

You have to show HOW the material proves YOUR point.

..... Phil

Phil Allison[_3_]

April 3rd 11, 03:11 AM

>"flipper"
>>
>>> "Conclusions
>>
>>
>>** The "conclusions" are about what ?
>
> What was asked for.

** Oh dear - we are not getting any actual answers.

>>A little circuit with a single j-fet (a 2N5456) operated at high level
>>and
>>hence much waveform asymmetry.
>
>>That is all his " conclusions " apply to.
>

** ( snip insane drivel)

Game over.

..... Phil

Phil Allison[_3_]

April 3rd 11, 03:14 AM

"flipper is reading challenged "
>>
>>>>One of the oldest CHEATS in the world is to claim that proof of one's
>>>>point is hidden in some massive pile of data somewhere.
>>>
>>> I gave the issue number and rough physical location,
>>
>>
>>** Not nearly enough.
>>
>> You have to show HOW the material proves YOUR point.
>
> I already did.

** Peter B. had a point and made it well - back in 1978.

Shame it is nothing like the same one "flipper" posted here.

..... Phil

Patrick Turner

April 3rd 11, 03:29 AM

On Apr 2, 9:17*pm, It's that Guy again... >
wrote:
> On Sat, 2 Apr 2011 02:55:54 -0700 (PDT), Patrick Turner
>
> > wrote:
> >Much of the understanding of my father's generation was not widely
> >understood 60 years ago, and the written article have been lost, and
> >not uploaded, and now there is the buffoonery of the short attention
> >span ****ter and Farcebook generation where they agree that history
> >may be safely ignored.
>
> where I live there are/were a few old used book stores that had
> all those old tube books, with math my (then) calc was blind to.
>
> SOMEONE local here bought all the candy, then dumped them.

That's exactly what has happened here. Over the last 15 years since
the Internet became mainstream, book sales have plummeted and book
shops continue to close. Most second hand book stores have also
closed. About all of book traders listed all their books, found out
what was worth keeping, and I guess the cream went to the rich cats
via Internet sales. Where there is charity book fair you won't find
any hard cover stuff on electronics written before 1960.

What you may find is the vast mountains of books that if read, may
reduce your level of education.
>
> Too often, a 'estate' just dumps what is left because the people
> running it don't understand/know/care what the stuff is worth.

The worth of much of a person's collected junk over a lifetime may be
less than the cost of dumping it. Most older ppl end up with their
little hill of junk, and almost nobody under 40 wants any of it. My
mother of 94 has a house fulla crap. Lots of stuff I don't want. When
she is Asked Upstairs, I will have to dispose of it all after going
through it all and I doubt I'll want anything because I already have a
house fulla junk. But she has some big furniture items which might be
worth a bit.I have no room for them. I am not emotionally attatched
any more. But it is possible she might outlive me and my sister, and I
won't have to deal with anything. She used to buy books, half read
them, and I asked her to give me a few but she couldn't manage that.
OK, she's 94, and it does not matter what she can't do, but all those
books will end up going through charity fairs on their way to the tip.
There are guys I know with a wall-full of shelves with CDs and LPs,
and maybe there are 3,000 recordings, all cost about $20, and that's
$60,000 total expense. Their sound gear may have cost them $40,000.
When they Drop Off The Perch this $100,000 worth of stuff is unlikely
to fetch more than $1,000, so its a -40dB decline in value. Some would
say I'm worth $500,000, but after cremation and as a small can of
rather dodgy fertilizer I will be worth 50c, which is loss in value of
-120dB. My other relatives will buy a car from the loot I leave and
forget me easily because they don't ever remember me now. The money
will just slosh around the system.
>
> Years ago, a estate dumped about 500 old Boyds teddy bears
> and my Mom, a long time collector, bought them for chump change.
>
> Value now is about 400%+ of pure profit.
>
> I bought as many books I could, got a pile of them, yet there ARE
> sources of said books in .pdf. *Yeah, I know, have to have it in ya
> hands, I agree, but otherwise, I couldn't find them at all.

I have not looked around for books online, but there might be
something which could anseer your questions. I have a couple of
shelves of books I bought between 1993 and 2001, but nothing about THD
change when you begin adding NFB. Without those books I'd be really
dumb. Some might say I still am dumb, but no matter how much anyone
thinks he knows, someone around the corner will know more. But for
sure the man who says he knows everything knows nothing. The secret to
my survival is to know enough about what I am doing. Once you get to a
sufficient knowledge level if you increase knowledge by +20dB, your
productivity may not even rise +1dB, and maybe you get distracted by
wonderment, and you end up doing -3dB less, losing money while
fascinated by some damn thing or other....

Patrick Turner.

>
> Check Usenet even, so very often for free.
>
> >Patrick Turner.
>
> Great post, as usual. *Thanx...
>
> JJTj
>
> * * * * * COMING SOON
>
> * * * * FRITZMANIA 2011
>
> * * *www.fritztronics.com

Patrick Turner

April 3rd 11, 03:35 AM

On Apr 2, 10:35*pm, mike s > wrote:
> On Apr 1, 9:23*pm, Newbie > wrote:
>
> > It seems well known that negative feedback can reduce lower order harmonics, but
> > increase the higher order ones.
>
> > Can anyone direct me to references that show how this happens with mathematical
> > analysis? *Perhaps a paper in the journal of the AES, or elsewhere; on the web,
> > perhaps?
>
> In most things it pays to start at the beginning and proceed as far as
> you are interested or able -
>
> http://www.ece.ucdavis.edu/courses/notes/old/S09/EEC110B/feedback.pdf
>
> BTW, it is also "well known" that the heavier an object is, the faster
> it falls. *Of course, it's also untrue.

Ah, and so it is with common sense.

But a 250 Watt class A Krell amplifier will definately reach the
ground faster than a pillow when both are chucked out of a 10th story
penthouse apartment after having an argument with She Who Must Be
Given Attention.

They say a man can do a lot.

But a woman, wow, she can do a whole lot more

DAMAGE.

Patrick Turner.

Phil Allison[_3_]

April 3rd 11, 03:38 AM

"flipper is a retard "

** Oh dear - we are not getting any actual answers.

Game over.

..... Phil

Phil Allison[_3_]

April 3rd 11, 03:39 AM

"flipper is a lying ratbag"

** Peter B. had a point and made it well - back in 1978.

Shame it is nothing like the same one "flipper" posted here.

.... Phil

Phil Allison[_3_]

April 3rd 11, 03:56 AM

"flipper is a lying ratbag"

> Says the cripple who knows not 'what'.

** Says a vile, anonymous, psycho cretin who puts tube whatsits in wooden
boxes.

The sooner someone puts him in one - the better.

.... Phil

Patrick Turner

April 3rd 11, 04:08 AM

On Apr 3, 5:34*am, flipper > wrote:
> On Sat, 2 Apr 2011 05:35:07 -0700 (PDT), mike s
>
>
>
>
>
> > wrote:
> >On Apr 1, 9:23*pm, Newbie > wrote:
> >> It seems well known that negative feedback can reduce lower order harmonics, but
> >> increase the higher order ones.
>
> >> Can anyone direct me to references that show how this happens with mathematical
> >> analysis? *Perhaps a paper in the journal of the AES, or elsewhere; on the web,
> >> perhaps?
>
> >In most things it pays to start at the beginning and proceed as far as
> >you are interested or able -
>
> >http://www.ece.ucdavis.edu/courses/notes/old/S09/EEC110B/feedback.pdf
>
> >BTW, it is also "well known" that the heavier an object is, the faster
> >it falls. *Of course, it's also untrue.
>
> And, apparently, some people don't believe in gravity at all.
>
> NFB does, indeed, increase higher order harmonics, depending on the
> order of the harmonic and how much NFB is applied.
>
> Take it from 'the horses mouth' or, at least, one of the well known
> thoroughbreds
>
> http://www.linearaudio.nl/Documents/Baxandall%20power%20amp%20design.pdf

This adress took me to a 3.3M .pdf file which has an enormous wealth
of useful information.

There is a 'page 56' about 80% down the scroll....

And there is the article I once read from Wireless World, December,
1978. It has a graph of harmonic product levels for the amount of NFB
applied.

>
> I believe it's in issue 5 about 80% of the way down.
>
> Note that as the order of harmonic increases so does the amount of NFB
> required to reduce it back to just the non FB level. However, if it's
> low enough to begin with (and, fortunately, harmonics are usually
> created in ever decreasing magnitude, at least in Class A) then even
> the 'increase' is inaudible, not to mention that above some order N
> the harmonics lie outside the frequency range of human hearing. Or, a
> Cyrano Jones opined when negotiating the price of Tribbles: Twice
> nothing is still nothing.-

Indeed.

Patrick Turner.

Don Pearce[_3_]

April 3rd 11, 08:09 AM

On Sat, 02 Apr 2011 20:28:59 -0500, flipper > wrote:

>On Sat, 02 Apr 2011 20:33:33 GMT, (Don Pearce) wrote:
>
>>On Sat, 02 Apr 2011 15:21:55 -0500, flipper > wrote:
>>
>>>On Sat, 02 Apr 2011 19:46:54 GMT, (Don Pearce) wrote:
>>>
>>>>On Sat, 02 Apr 2011 14:38:18 -0500, flipper > wrote:
>>>>
>>>>>On Sat, 2 Apr 2011 02:55:54 -0700 (PDT), Patrick Turner
> wrote:
>>>>>
>>>>>>On Apr 2, 7:23*am, Newbie > wrote:
>>>>>>> It seems well known that negative feedback can reduce lower order harmonics, but
>>>>>>> increase the higher order ones.
>>>>>>>
>>>>>>> Can anyone direct me to references that show how this happens with mathematical
>>>>>>> analysis? *Perhaps a paper in the journal of the AES, or elsewhere; on the web,
>>>>>>> perhaps?
>>>>>>
>>>>>>
>>>>>>During the last 40 years there have been at least two in depth
>>>>>>articles in Wireless World which later became Electronics World. I
>>>>>>read all the WW and EW magazines from 1917 to about 1997 and you'll
>>>>>>probably find the articles you want to read if you go where there is
>>>>>>an archived set of the magazines like I did. In about 1993 I spent
>>>>>>days reading in several university library archives to see what had
>>>>>>been said about anything to do with audio engineering. Unfortunately I
>>>>>>don't have copies of all I read, but the phenomena of increasing
>>>>>>distortion spectra after applying NFB is well known, and remains
>>>>>>rivetted into my brain cells.
>>>>>>
>>>>>>The conditions required to "make matters worse" with NFB are about as
>>>>>>follows :-
>>>>>
>>>>>The only condition required to increase higher order harmonics is NFB.
>>>>>
>>>>Negative feedback can never make harmonics worse.
>>>
>>>I didn't say it made "harmonics worse." I said it increases *higher
>>>order* harmonics.
>>>
>>
>>Exactly - in terms of harmonics, increasing means making worse.
>
>No, not 'exactly' because you don't distinguish between *which*
>"harmonics."
>
>
>>>THD decreases because the reduction in low order harmonics is greater
>>>than the increase in high order harmonics.
>>>
>>>> Feedback can make
>>>>harmonics worse when the phase shifts far enough to make it positive.
>>>
>>>That will certainly screw things up but it has nothing to do with the
>>>basic principle of NFB always increasing higher order harmonics.
>>>
>>>Read Baxandall's paper.
>>>
>>I've read it. I suggest you read it again.
>
>I've read it. I suggest you read it again.
>
>I quote
>
>"Conclusions
>.
>.
>.
>A small amount of negative feedback (e.g. 6dB) in a single ended
>stage, though reducing the second-harmonic distortion, and also the
>total (unweighted) distortion, by about 6dB, will increase the
>higher-order distortion and the quality of reproduction may well
>become worse as judged subjectively.
>.
>.
>.
>The magnitude of harmonics of extremely high order will be increased
>by the application of negative feedback, no matter what practical
>amount of negative feedback is employed, but this is of no consequence
>if, when thus increased, they are, say, 120dB below the fundamental.
>.
>.
>."
>
>He also explains the mechanism producing the 'new' distortion as the
>modulator/mixer action of the non-linear amplifier, which we know is
>non-linear or else we would not be wrapping NFB around the thing
>trying to 'improve' it's linearity, acting on the fed back
>harmonic(s).
>
>To illustrate he uses the 'simple' case of an amplifier with a 'pure'
>square law transfer function so that it, sans NFB, produces 'only' the
>2'd harmonic.
>
>Again I quote
>
>"Thus while the amplifier without negative feedback gives nothing but
>second-harmonic distortion on a single sine-wave input, as soon as a
>little feedback is applied, a third harmonic output appears. This is
>not the end of the story, however, for this third harmonic, like the
>second harmonic, gets fed via the B network into the input circuit,
>where sum and difference signals are again generated. This time the
>sum products are at f+3f, which gives a fourth order harmonic, and at
>2f+3f, which gives a fifth harmonic. Clearly there is theoretically no
>end to this process - every new harmonic considered, when fed back,
>gives rise to harmonics of yet higher order."
>
>The mechanism has nothing to do with slew rate limiting or lousy
>bandwidth, and he explicitly explains his analysis presumes the
>amplifier is flawless in that regard. It's inherent to NFB because the
>non-linear amplifier is a mixer.

Let me put this as few words as the subject will allow. Global NFB is
capable - through intermod - of generating tiny amounts of higher
harmonics where none existed. And I do mean tiny. These higher
harmonics are, in practice always swamped massively by naturally
occurring higher harmonics. These higher harmonics respond normally to
the application of NFB and are reduced proportionately to the ratio of
open loop to closed loop gain.

So no, NFB does not cause higher harmonics to increase.

d

Don Pearce[_3_]

April 3rd 11, 09:39 AM

On Sun, 03 Apr 2011 03:32:50 -0500, flipper > wrote:

>>Let me put this as few words as the subject will allow. Global NFB is
>>capable - through intermod - of generating tiny amounts of higher
>>harmonics where none existed. And I do mean tiny. These higher
>>harmonics are, in practice always swamped massively by naturally
>>occurring higher harmonics. These higher harmonics respond normally to
>>the application of NFB and are reduced proportionately to the ratio of
>>open loop to closed loop gain.
>>
>>So no, NFB does not cause higher harmonics to increase.
>>
>>d
>
>I'll be even more brief. Your 'intuition' is incorrect.
>
>Read Baxandall's paper.

Nicely argued. I clearly stand corrected.

d

Don Pearce[_3_]

April 3rd 11, 11:14 AM

On Sun, 03 Apr 2011 04:46:27 -0500, flipper > wrote:

>On Sun, 03 Apr 2011 08:39:55 GMT, (Don Pearce) wrote:
>
>>On Sun, 03 Apr 2011 03:32:50 -0500, flipper > wrote:
>>
>>>>Let me put this as few words as the subject will allow. Global NFB is
>>>>capable - through intermod - of generating tiny amounts of higher
>>>>harmonics where none existed. And I do mean tiny. These higher
>>>>harmonics are, in practice always swamped massively by naturally
>>>>occurring higher harmonics. These higher harmonics respond normally to
>>>>the application of NFB and are reduced proportionately to the ratio of
>>>>open loop to closed loop gain.
>>>>
>>>>So no, NFB does not cause higher harmonics to increase.
>>>>
>>>>d
>>>
>>>I'll be even more brief. Your 'intuition' is incorrect.
>>>
>>>Read Baxandall's paper.
>>
>>Nicely argued. I clearly stand corrected.
>
>I already tried 'argument', including quoting his exact words, but you
>are immune to a rational discussion, and I see no reason to re type
>the entire paper when the original is just sitting there for you to
>read, but if you can't handle the prose he also provided a pretty
>picture of it.
>
>And it won't 'go away' just because you chose to ignore what it says.
>
>>d

I've already told you, I know what it says. Copying and pasting a
piece of text does not prove that you have understood it - and you
clearly haven't.

d

mike s

April 3rd 11, 12:04 PM

On Apr 2, 1:16*pm, Andre Jute > wrote:
> On Apr 1, 9:23*pm, Newbie > wrote:
>
> > It seems well known that negative feedback can reduce lower order harmonics, but
> > increase the higher order ones.
>
> > Can anyone direct me to references that show how this happens with mathematical
> > analysis? *Perhaps a paper in the journal of the AES, or elsewhere; on the web,
> > perhaps?
>
> Here's an article to help you distinguish between added-on negative
> feedback and natural negative feedback.http://www.audio-talk.co.uk/fiultra/KISS%20104%20by%20Andre%20Jute.htm
>
> This article explains why added-on negative feedback is a bad thing in
> tube audio if you have really good speakers; it focuses on how the
> higher harmonic artifacts are added.http://www.audio-talk.co.uk/fiultra/KISS%20123%20by%20Andre%20Jute.htm
>
> More articles on Harmonic Distortion on the KISS Amps site, available
> through those pages.
>

This in intriguing in that what Jute has re-invented or re-created is
pretty much exactly the type of amplifier that Harold Black was faced
with in the 1920s when he invented negative feedback. So it would be
interesting to hear from someone with such an amplifier as to what
happens if negative feedback is employed.

Certainly well designed and built valve amplifiers can perform
superbly as domestic audio systems without negative feedback. Though
building two with exactly matched characteristics for stereo can be a
(not unrewarding) challenge. Of course before the signal reaches the
domestic system it's been through many other higher performance
amplifiers, all of which will employ NFB.

My own system has no global NFB in the output stage, but it uses push
pull triodes, driven by another pair of triodes, so it's hardly worth
it. And I like the "traditional" sound.
http://mike.wepoco.com/Home/retro-geekery/valve-amplifiers/Building-a-Quality-Amplifier

> Andre Jute
> Visit Jute on Amps at
> *http://www.audio-talk.co.uk/fiultra/
> "wonderfully well written and reasoned information for the tube audio
> constructor"
> John Broskie TubeCAD & GlassWare
> "an unbelievably comprehensive web site containing vital gems of
> wisdom"
> Stuart Perry Hi-Fi News & Record Review

Andre Jute[_2_]

April 4th 11, 05:56 PM

On Apr 3, 12:04*pm, mike s > wrote:
> On Apr 2, 1:16*pm, Andre Jute > wrote:
>
>
>
>
>
> > On Apr 1, 9:23*pm, Newbie > wrote:
>
> > > It seems well known that negative feedback can reduce lower order harmonics, but
> > > increase the higher order ones.
>
> > > Can anyone direct me to references that show how this happens with mathematical
> > > analysis? *Perhaps a paper in the journal of the AES, or elsewhere; on the web,
> > > perhaps?
>
> > Here's an article to help you distinguish between added-on negative
> > feedback and natural negative feedback.http://www.audio-talk.co.uk/fiultra/KISS%20104%20by%20Andre%20Jute.htm
>
> > This article explains why added-on negative feedback is a bad thing in
> > tube audio if you have really good speakers; it focuses on how the
> > higher harmonic artifacts are added.http://www.audio-talk.co.uk/fiultra/KISS%20123%20by%20Andre%20Jute.htm
>
> > More articles on Harmonic Distortion on the KISS Amps site, available
> > through those pages.
>
> This in intriguing in that what Jute has re-invented or re-created is
> pretty much exactly the type of amplifier that Harold Black was faced
> with in the 1920s when he invented negative feedback. *So it would be
> interesting to hear from someone with such an amplifier as to what
> happens if negative feedback is employed.

You should be careful, Mike. My T39, ZNFB SE 300B, would have baffled
Black and a whole bunch of his students till well after WW2. They
would have thought me mad. Check the WE recommended operating
conditions. Now check what I do. I call it my HIGH concept. High
voltage, high current, high impedance. I even run huge driver current
to absolutely murder Miller impedance (http://www.audio-talk.co.uk/
fiultra/KISS%20125%20by%20Andre%20Jute.htm "Using Slew Rate to make
Miller work for you"). The only thing I don't run high is the fils,
which I starve about 10% further to tilt the distortion artifacts.
This entire scheme is to make a very, very silent amp.

At http://www.audio-talk.co.uk/fiultra/KISS%20114%20by%20Andre%20Jute.htm
a standard cookbook amp by a couple of morons dumb enough to challenge
me is compared to one of my amps. Of course I leave him for dead in
total distortion but note that 0.03% 3rd harmonic on my amp against
0.83% 3rd harmonic on the other fellow's wretched noisemaker. Ouch!
That is what all my work is about, tailoring the distortion spectrum
to put the 3rd and higher harmonics so far under any likelihood of
perception that for practical purposes it is non-existent. I doubt
Black would have understood or, if he understood, approved.

> Certainly well designed and built valve amplifiers can perform
> superbly as domestic audio systems without negative feedback. *Though
> building two with exactly matched characteristics for stereo can be a
> (not unrewarding) challenge. *Of course before the signal reaches the
> domestic system it's been through many other higher performance
> amplifiers, all of which will employ NFB.

We don't mention that last bit here since the time an audiophile got
into the tower with a rifle and started picking us off for being
"heretics".

> My own system has no global NFB in the output stage, but it uses push
> pull triodes, driven by another pair of triodes, so it's hardly worth
> it. *And I like the "traditional" sound.http://mike.wepoco.com/Home/retro-geekery/valve-amplifiers/Building-a...

Yes, one of my favourite amps, my Type 113 Triple Threat EL34 Class A
PP EL34 (not on my site as it is commercially licensed), is normally
run triode-tied, in which format it is impossible even for
professional classical musicians to distinguish from an SE 300B; on
fact, they normal prefer the faux triode PP amp.

Interesting set of schematics you have there. When I first came into
tube amp design over twenty years ago, I touched base there as well.

Andre Jute
Visit Jute on Amps at
http://www.audio-talk.co.uk/fiultra/
"wonderfully well written and reasoned information for the tube audio
constructor"
John Broskie TubeCAD & GlassWare
"an unbelievably comprehensive web site containing vital gems of
wisdom"
Stuart Perry Hi-Fi News & Record Review

Big Bad Bob

April 4th 11, 07:24 PM

On 04/02/11 02:55, Patrick Turner so witilly quipped:
> On Apr 2, 7:23 am, > wrote:
>> It seems well known that negative feedback can reduce lower order harmonics, but
>> increase the higher order ones.
>>
>> Can anyone direct me to references that show how this happens with mathematical
>> analysis? Perhaps a paper in the journal of the AES, or elsewhere; on the web,
>> perhaps?
>
>
> The conditions required to "make matters worse" with NFB are about as
> follows :-

/me adding summaries to main points and clipping details to condense
what is otherwise a very detailed and technical discussion of why this
happens.

> The open loop THD distortion of the amp approaches 10% at 2dB below
> clipping.

putting a large number of high order odd harmonics into the signal

> The frequency of an input test signal is say 1kHz.

so plenty of freq response is left for higher order harmonics

> The open loop bandwidth of the amp is reducing at 6dB/octave above say
> 1kHz, worst case.

so negative FB has less of an effect at higher frequencies

> The open loop phase shift is high at F above the input signal, but not
> so high to allow NFB to become positive FB and causing oscillations.

so that you get an effect of having a bit of 'positive FB' for very high
frequencies, emphasizing the higher order harmonics

> A low amount of global NFB is connected, say 10dB.

minimizing the positive benefit

> To get around this problem in SS amps there is usually an enormous
> total amount of NFB applied.

yes - typified by an 'op-amp' design which have high open loop gain and
high non-linearity to go with it, with output transistors typically
biased very close to class B. Large amounts of NFB needed. FET input
helps a lot to avoid mismatching forward transconductance and causing
oscillation (don't ask me to do the math, it makes my brain hurt).

> In a tubed amp, There may also be 5% THD at 2dB below clipping

partially correctable by NOT using cheap designs/components.

> But where only 10dB of GNFB is applied, there will be second order
> products which are unresolved to negligible levels and they may be all
> too audible, bearing in mind that above 100Hz, tones of the same
> amplitude seem to sound louder as F rises.

'louder as F rises' - yeah, sort of. At high volumes all tones 'sound
about the same' when you compare 'apparent tone volume' to frequency
with equal power. But at lower volumes this is true due to the ear's
tendency to 'amplify' midrange tones. At higher frequencies this effect
reverses, almost like an old POTS telephone. This was actually part of
the design of old-style phones, where the 300-3khz range needed to be as
clear as possible, and everything else could be filtered out. Human
ears usually aren't that bad, but it's the same basic idea.

> There have been other attempts to reduce THD to vanishingly low levels
> by Professor Ed Cherry with is Nested FB idea, and by the makers of
> Halcro amps whose schematic does not seem to have entered the public
> domain where it achieves 0.0001% THD at 20kHz, slightly better than
> you can do I would bet.

FYI It's unlikely you can NORMALLY hear anything lower than 0.1% for
THD, though I believe the perception threshold is somewhat lower for IM
distortion. This would also depend a lot on the nature of the
distortion, which is why a lot of people prefer tube audio. 'Nuff.

> In a nutshell, SS amps need all the NFB thay might muster, but tube
> amps get way with much less.

yes

> After reading the papers I have seen I try to make my amps with OLG
> giving low THD, low phase shift, wide BW at full power and leave very
> little work for the GNFB to achieve. Sound is fine with NFB.

works for me. the simplest solutions are usually the best.

> All the math in the world won't make tube amps very much more linear
> than they have been for the last 50 years.

not surprised.

> In fact, many tube amps are LESS linear than 50 years ago when many
> makers tried to use 26dB or even 30dB of GNFB so they could quote
> sales propelling figures.

I'd do that much feedback for better quality and then throw in an extra
triode preamp to compensate for lower power stage gain. An extra 12AX7
isn't that expensive.

> These days output tubes are run with class B amp loading, only 15dB
> GNFB, and with speakers which are much less sensitive than in 1960.

'less sensitive' speakers? I'll have to disagree with you on THIS one.
Modern speakers DO use heavier cones with higher gauss magnets, and
you'll need multi-way crossovers to make the best use of them. But I'll
take a modern JBL-based 3 or 4-way system over a 1960-design system any day.

> In many ways tube amp design has flowed backwards

hmm... maybe. Then again, maybe not. My expertise is mostly in the
area of guitar/keyboard/PA amplifiers, and I think many of these are
still using the same basic designs they were using in the 1950's, and
that's due to a demand for "that sound". You can find the schematic for
a Fender Twin on-line. I'm not a fan of how Fender handles certain
aspects of their design (the P-P phase spitter being one of them) but in
general it's still about like it was 'back then'.

> I raise my hat to all makers charging such high prices and with much
> more THD/IMD. They are giving me a better chance to sell my amps.

I can understand your position.

> And one other thing. Triodes have internal NFB built into them.

mmmm, ok that's an interesting way to put it.

> Much of the understanding of my father's generation was not widely
> understood 60 years ago, and the written article have been lost, and
> not uploaded, and now there is the buffoonery of the short attention
> span ****ter and Farcebook generation where they agree that history
> may be safely ignored.

Nice 'hacker names', '****ter' and 'Farcebook'. I'll have to remember
them. FYI the oldest popular electronics I've found online is THIS one:

http://www.swtpc.com/mholley/PopularElectronics/Apr1955/PE_Apr1955.htm

Big Bad Bob

April 4th 11, 08:46 PM

On 04/02/11 15:15, Phil Allison so witilly quipped:
> "flipper the ****wit"
> Patrick Turner
>>
>>>
>>> The conditions required to "make matters worse" with NFB are about as
>>> follows :-
>>
>> The only condition required to increase higher order harmonics is NFB.
>
>
> ** That is absolute BULL**** !!

uh, did you REALLY read it?

Big Bad Bob

April 4th 11, 08:52 PM

On 04/02/11 13:21, flipper so witilly quipped:
> On Sat, 02 Apr 2011 19:46:54 GMT, (Don Pearce) wrote:
>
>> On Sat, 02 Apr 2011 14:38:18 -0500, > wrote:
>>
>>> On Sat, 2 Apr 2011 02:55:54 -0700 (PDT), Patrick Turner
>>> > wrote:
>>>
>>>> On Apr 2, 7:23 am, > wrote:
>>>>> It seems well known that negative feedback can reduce lower order harmonics, but
>>>>> increase the higher order ones.
>>>>>
>>>>> Can anyone direct me to references that show how this happens with mathematical
>>>>> analysis? Perhaps a paper in the journal of the AES, or elsewhere; on the web,
>>>>> perhaps?
>>>>
>>>>
>>>> During the last 40 years there have been at least two in depth
>>>> articles in Wireless World which later became Electronics World. I
>>>> read all the WW and EW magazines from 1917 to about 1997 and you'll
>>>> probably find the articles you want to read if you go where there is
>>>> an archived set of the magazines like I did. In about 1993 I spent
>>>> days reading in several university library archives to see what had
>>>> been said about anything to do with audio engineering. Unfortunately I
>>>> don't have copies of all I read, but the phenomena of increasing
>>>> distortion spectra after applying NFB is well known, and remains
>>>> rivetted into my brain cells.
>>>>
>>>> The conditions required to "make matters worse" with NFB are about as
>>>> follows :-
>>>
>>> The only condition required to increase higher order harmonics is NFB.
>>>
>> Negative feedback can never make harmonics worse.
>
> I didn't say it made "harmonics worse." I said it increases *higher
> order* harmonics.
>
> THD decreases because the reduction in low order harmonics is greater
> than the increase in high order harmonics.

I can believe that high order harmonics would increase RELATIVE TO THD
but not in actual power/amplitude as a component of the output. What
you describe sounds like negative feedback CAUSING high harmonic
distortion, under ALL conditions. If you consider an ideal unity gain
amplifier with infinite open loop gain, this could not be the case.
Alternately, in the case of a wide bandwidth high open loop gain
amplifier with low distortion, and large negative feedback, higher
distortion for high harmonics (over open loop) is also highly unlikely.
Other cases, as described by Patrick, make sense (and these are driven
by specific design inadequacies, phase shift at high freq's and low open
loop gain at high freq's being the biggest contributing factors).

If you can uniquely identify the cause in the 'ideal' and 'high gain
high bandwidth low distortion plus large NFB' case, please let me know.
Thanks. Otherwise I'll have to proceed on the basis that overall
distortion (including higher harmonics) goes DOWN with sufficient
negative feedback.

Don Pearce[_3_]

April 4th 11, 08:56 PM

On Mon, 04 Apr 2011 11:24:07 -0700, Big Bad Bob
> wrote:

>On 04/02/11 02:55, Patrick Turner so witilly quipped:

You might at least spell wittily correctly - please?

d

Big Bad Bob

April 4th 11, 08:57 PM

On 04/04/11 12:56, Don Pearce so witilly quipped:
> On Mon, 04 Apr 2011 11:24:07 -0700, Big Bad Bob
> > wrote:
>
>> On 04/02/11 02:55, Patrick Turner so witilly quipped:
>
> You might at least spell wittily correctly - please?
>

it's that stupid computer of mine. it can't spell, add, type, or answer
my questions without lying to me. stupid computer.

Big Bad Bob

April 4th 11, 09:19 PM

On 04/02/11 05:16, Andre Jute so witilly quipped:
> This article explains why added-on negative feedback is a bad thing in
> tube audio if you have really good speakers; it focuses on how the
> higher harmonic artifacts are added.
> http://www.audio-talk.co.uk/fiultra/KISS%20123%20by%20Andre%20Jute.htm

don't believe EVERYTHING you read on the intarweb

About half of what this guy says is pure opinion. I see no math to back
it up. basic 'servo theory' should easily disprove it.

Big Bad Bob

April 4th 11, 09:41 PM

On 04/04/11 09:56, Andre Jute so witilly quipped:
> At http://www.audio-talk.co.uk/fiultra/KISS%20114%20by%20Andre%20Jute.htm
> a standard cookbook amp by a couple of morons dumb enough to challenge
> me is compared to one of my amps. Of course I leave him for dead in
> total distortion but note that 0.03% 3rd harmonic on my amp against
> 0.83% 3rd harmonic on the other fellow's wretched noisemaker. Ouch!
> That is what all my work is about, tailoring the distortion spectrum
> to put the 3rd and higher harmonics so far under any likelihood of
> perception that for practical purposes it is non-existent. I doubt
> Black would have understood or, if he understood, approved.

how's your frequency response? No resonance or phase shift issues I
take it? Keep in mind that NFB is used for a LOT of things, reduction
of distortion being ONE of those. Also keep in mind that tube
characteristics will change over time, and if you want consistent
performance for many years of frequent or continuous operation, you'll
consider NFB as being one of those things you'll put into your existing
amplifiers to deal with it. And I think you'll also find that your
overall THD will be even LOWER.

By the way - don't forget that a triode tube has a form of 'negative
feedback' built into it (grid position with respect to plate and cathode
being one of the factors) that limits ideal gain to the 'mu' factor of
the triode. Also a self-bias circuit without a bypass cap in the
cathode will add NFB, limiting gain to Rp/Rk [I prefer stage gain that's
significantly below the mu factor, like a gain of 20 with a mu of 100]
and also improving linearity at the same time. So you add a few more
pre-amp stages, make sure your input stages use low noise components,
add plenty of "global" NFB to give you a nice flat frequency response
and low distortion, and the end result should outperform yours without
too much trouble, especially at the edge conditions. That's what the
really high end units are doing, last I checked.

FYI - pump a square wave through your amp (under load), at 50Hz and
10khz, and if it doesn't look 'square' on an o-scope, it's because you
don't have any NFB. Phase distortion can be bad, too.

Just remember two words: servo theory - With NFB you're amplifying the
_DIFFERENCE_ signal between NFB and input by the OL amplification
factor, and _NOT_ just 'adding distortion' to the signal. Like an
op-amp. Yeah. You can limit the effects of additional 'added
distortion' by putting various kinds of negative feedback into the
individual stages, and that would be NECESSARY to prevent the conditions
you describe ("adding" distortion by distorting the distortion that's
part of the feedback). And if you use AB2 bias on P-P output WITH
screen taps (which are a form of NFB), your low signal level distortion
from the output stage should be VERY low.

Anyway, these are all 'the basics' of amp design.

mike s

April 4th 11, 10:49 PM

On Apr 4, 5:56*pm, Andre Jute > wrote:
> On Apr 3, 12:04*pm, mike s > wrote:
>
...
>
> > This in intriguing in that what Jute has re-invented or re-created is
> > pretty much exactly the type of amplifier that Harold Black was faced
> > with in the 1920s when he invented negative feedback. *So it would be
> > interesting to hear from someone with such an amplifier as to what
> > happens if negative feedback is employed.
>
> You should be careful, Mike. My T39, ZNFB SE 300B, would have baffled
> Black and a whole bunch of his students till well after WW2. They
> would have thought me mad. Check the WE recommended operating
> conditions. Now check what I do. I call it my HIGH concept. High
> voltage, high current, high impedance. I even run huge driver current
> to absolutely murder Miller impedance (http://www.audio-talk.co.uk/
> fiultra/KISS%20125%20by%20Andre%20Jute.htm "Using Slew Rate to make
> Miller work for you"). The only thing I don't run high is the fils,
> which I starve about 10% further to tilt the distortion artifacts.
> This entire scheme is to make a very, very silent amp.

I did notice that some of the component values were rather "off", but
was too polite to comment on it.
I expect Black, as an engineer, would have thought us all mad by the
old engineering standard of -
"An engineer does for a shilling what a fool does for a pound".

Starving filaments is interesting. It's another of those things I
think about from time to time, but there are always other more
pressing things to do. What made me wonder about it is that my preamp
uses EF37A (Mullard version of the 6J7) which I still have quite a
lot of despite selling many on Ebay. EF37A pentode was also sold as
ME1400 electrometer pentode. As ME1400 it was rated with a heater
voltage of 4.5V rather than the usual 6.3V but even as a triode the
gain is formidable so NFB would seem necessary :-)

>
> Athttp://www.audio-talk.co.uk/fiultra/KISS%20114%20by%20Andre%20Jute.htm
> a standard cookbook amp by a couple of morons dumb enough to challenge
> me is compared to one of my amps. Of course I leave him for dead in
> total distortion but note that 0.03% 3rd harmonic on my amp against
> 0.83% 3rd harmonic on the other fellow's wretched noisemaker. Ouch!
> That is what all my work is about, tailoring the distortion spectrum
> to put the 3rd and higher harmonics so far under any likelihood of
> perception that for practical purposes it is non-existent. I doubt
> Black would have understood or, if he understood, approved.
>
> > Certainly well designed and built valve amplifiers can perform
> > superbly as domestic audio systems without negative feedback. *Though
> > building two with exactly matched characteristics for stereo can be a
> > (not unrewarding) challenge. *Of course before the signal reaches the
> > domestic system it's been through many other higher performance
> > amplifiers, all of which will employ NFB.
>
> We don't mention that last bit here since the time an audiophile got
> into the tower with a rifle and started picking us off for being
> "heretics".
>
> > My own system has no global NFB in the output stage, but it uses push
> > pull triodes, driven by another pair of triodes, so it's hardly worth
> > it. *And I like the "traditional" sound.http://mike.wepoco.com/Home/retro-geekery/valve-amplifiers/Building-a...
>
> Yes, one of my favourite amps, my Type 113 Triple Threat EL34 Class A
> PP EL34 (not on my site as it is commercially licensed), is normally
> run triode-tied, in which format it is impossible even for
> professional classical musicians to distinguish from an SE 300B; on
> fact, they normal prefer the faux triode PP amp.
>

Williamson states that triode strapped KT66 was almost exactly the
same as PX25 and since he worked for the manufacturer I expect he was
right. Whether my KT44s will perform as well in the long term I don't
know, but they're cheap enough to replace at present and I can always
switch to 807s if I have a problem.

> Interesting set of schematics you have there. When I first came into
> tube amp design over twenty years ago, I touched base there as well.
>

I've acquired most copies of Wireless World from the birth of this
amplifier in 1934 to the Williamson and on into the 1950s. They're an
enjoyable read on winter evenings and from time to time I'll scan
interesting bits and add them to my website.

> Andre Jute
> Visit Jute on Amps at
> *http://www.audio-talk.co.uk/fiultra/
> "wonderfully well written and reasoned information for the tube audio
> constructor"
> John Broskie TubeCAD & GlassWare
> "an unbelievably comprehensive web site containing vital gems of
> wisdom"
> Stuart Perry Hi-Fi News & Record Review

Andre Jute[_2_]

April 4th 11, 11:39 PM

On Apr 4, 9:19*pm, Big Bad Bob <BigBadBob-at-mrp3-
> wrote:
> On 04/02/11 05:16, Andre Jute so witilly quipped:
>
> > This article explains why added-on negative feedback is a bad thing in
> > tube audio if you have really good speakers; it focuses on how the
> > higher harmonic artifacts are added.
> >http://www.audio-talk.co.uk/fiultra/KISS%20123%20by%20Andre%20Jute.htm
>
> don't believe EVERYTHING you read on the intarweb
>
> About half of what this guy says is pure opinion. *I see no math to back
> it up. *basic 'servo theory' should easily disprove it.

That's a humorous article, Bubba. The math is elsewhere on the site.
But by all means bring on your math. We had a clown here called Henry
Pasternack who was last seen running away with his halls in one hand
and his MSEE from Stanford in the other. I sincerely hope you're
better prepared than he was.

Phil Allison[_3_]

April 5th 11, 12:04 AM

"Big Bad Bob"
> On 04/02/11 15:15, Phil Allison so witilly quipped:
>> "flipper the ****wit"
>> Patrick Turner
>>>
>>>>
>>>> The conditions required to "make matters worse" with NFB are about as
>>>> follows :-
>>>
>>> The only condition required to increase higher order harmonics is NFB.
>>
>>
>> ** That is absolute BULL**** !!
>
> uh, did you REALLY read it?

** The comment as presented is absolutely false.

..... Phil

Andre Jute[_2_]

April 5th 11, 12:07 AM

On Apr 4, 9:41*pm, Big Bad Bob <BigBadBob-at-mrp3-
> wrote:
> On 04/04/11 09:56, Andre Jute so witilly quipped:
>
> > Athttp://www.audio-talk.co.uk/fiultra/KISS%20114%20by%20Andre%20Jute.htm
> > a standard cookbook amp by a couple of morons dumb enough to challenge
> > me is compared to one of my amps. Of course I leave him for dead in
> > total distortion but note that 0.03% 3rd harmonic on my amp against
> > 0.83% 3rd harmonic on the other fellow's wretched noisemaker. Ouch!
> > That is what all my work is about, tailoring the distortion spectrum
> > to put the 3rd and higher harmonics so far under any likelihood of
> > perception that for practical purposes it is non-existent. I doubt
> > Black would have understood or, if he understood, approved.
>
> how's your frequency response?

Limited by the iron both as a selfstanding amp and as a booster for my
75W SE/PSE kilovolt transmitter tube amp.

>*No resonance or phase shift issues I
> take it? *

Why should there be. You're making very large assumptions here, Bubba,
when you should be studying the circuit instead. I don't build amps
with your assumptions.

>Keep in mind that NFB is used for a LOT of things, reduction
> of distortion being ONE of those. *

Uh-huh. So what? I'm not interested in those other things. I'm not
even interested in reduction of distortion by NFB. I don't need it.

>Also keep in mind that tube
> characteristics will change over time, and if you want consistent
> performance for many years of frequent or continuous operation, you'll
> consider NFB as being one of those things you'll put into your existing
> amplifiers to deal with it. *

Why?

>And I think you'll also find that your
> overall THD will be even LOWER.

I'm sure that's important to you. But, d'you see, Bubba, my amp starts
out on the blank sheet with the statement, "I don't care **** about
THD because I know mine will be lower than yours. What I care about is
lowering the proportion of 3rd and higher harmonics. And I know how to
do that too without NFB."

Now, explain to me again why I need you and your NFB, Bubba. We get a
guy like you running in here every few years, knowing better than I
do, throwing out wild statements like yours, rote-learned at some
third-rate polytechnic. Without fail, they leave with their tails
between their legs.

> By the way - don't forget that a triode tube has a form of 'negative
> feedback' built into it (grid position with respect to plate and cathode
> being one of the factors) that limits ideal gain to the 'mu' factor of
> the triode. *

My netsite has only said so for about 20 years. The article you
condemn actually says so and refers the reader to more detail on my
netsite. You should pay attention, Bubba.

>Also a self-bias circuit without a bypass cap in the
> cathode will add NFB, limiting gain to Rp/Rk [I prefer stage gain that's
> significantly below the mu factor, like a gain of 20 with a mu of 100]
> and also improving linearity at the same time. *

We all have our preferences. I wonder why it is only mine that the
clowns come to sneer at.

> So you add a few more
> pre-amp stages, make sure your input stages

You should learn to read, Bubba. The concept for my amp is KISS. It
stands for Keep it Simple, Stupid, and it is a mnemonic for people
like you. I don't need more stages because I want only 3.8W out of an
8W tube. You really should calculate up the circuit before you start
pronouncing on it, Bubba.

> use low noise components,
> add plenty of "global" NFB to give you a nice flat frequency response
> and low distortion, and the end result should outperform yours without
> too much trouble,

Well then, build it and show us, and then we'll get some musicians in
and play your amp and mine behind a curtain and discover which they
think sounds more natural.

>especially at the edge conditions. *That's what the
> really high end units are doing, last I checked.

Who's interested in emulating what everyone else is doing? You're
projecting your dreams unto me. You clearly don't know what my
equipment consists of. I can afford any amp I want. I build my own
because what I want isn't out there.

> FYI - pump a square wave through your amp (under load), at 50Hz and
> 10khz, and if it doesn't look 'square' on an o-scope, it's because you
> don't have any NFB. *Phase distortion can be bad, too.

I'm not in the kindergarten class at wherever you teach, Bubba.

> Just remember two words: *servo theory - With NFB you're amplifying the
> _DIFFERENCE_ signal between NFB and input by the OL amplification
> factor, and _NOT_ just 'adding distortion' to the signal. *

I'm still not in the kindie class, Bubba.

>Like an
> op-amp. *Yeah. *

Perhaps you should go away and go live with that op-amp before you
suffer an accident to your bluster here on RAT.

> You can limit the effects of additional 'added
> distortion' by putting various kinds of negative feedback into the
> individual stages,

Gee. first lots of Global Negative FeedBack, now lots of little
monkeys to cling to the back of the big monkey on the back of my
sound. Thanks, but no thanks, I already have a chimpanzee for a pet, I
don't want these ugly NFB monkeys.

>and that would be NECESSARY to prevent the conditions
> you describe ("adding" distortion by distorting the distortion that's
> part of the feedback). *

Good golly, the monkeys on the backs of other monkeys are
*compulsory*. Now I've heard everything. Bring back Dumb and Dumber,
Arny and Poopie, all is forgiven, there's an even bigger idiot on RAT.

>And if you use AB2 bias on P-P output WITH
> screen taps (which are a form of NFB), your low signal level distortion
> from the output stage should be VERY low.

Oh dear. KISS means Keep It Simple, Stupid. The concept is for an SE
amp. The concept for every amp in my collection is Class A1 sound. Why
don't you take your Class AB2 and stick it where it hurts, and stick
your NFB by screen taps the same place.

> Anyway, these are all 'the basics' of amp design.

What an offensive jerk you are, Bubba. WTF makes you think I need you
to tell me "the basics"?

If you're smart, you'll apologize and shut up. If you're not, I'll
destroy your little self-confidence for life. I'm bored with fools
coming here and reading me lectures from the depth of their ignorance.

Andre Jute
Visit Jute on Amps at
http://www.audio-talk.co.uk/fiultra/
"wonderfully well written and reasoned information for the tube audio
constructor"
John Broskie TubeCAD & GlassWare
"an unbelievably comprehensive web site containing vital gems of
wisdom"
Stuart Perry Hi-Fi News & Record Review

Andre Jute[_2_]

April 5th 11, 12:27 AM

On Apr 4, 10:49*pm, mike s > wrote:
> On Apr 4, 5:56*pm, Andre Jute > wrote:

> > The only thing I don't run high is the fils,
> > which I starve about 10% further to tilt the distortion artifacts.
> > This entire scheme is to make a very, very silent amp.
>
> Starving filaments is interesting. *It's another of those things I
> think about from time to time, but there are always other more
> pressing things to do. *What made me wonder about it is that my preamp
> uses EF37A *(Mullard version of the 6J7) which I still have quite a
> lot of despite selling many on Ebay. *EF37A pentode was also sold as
> ME1400 electrometer pentode. As ME1400 it was rated with a heater
> voltage of 4.5V rather than the usual 6.3V but even as a triode the
> gain is formidable so NFB would seem necessary :-)

If you haven't yet, you should find the site of Steve Bench and read
him. He's the main man on starved fils, and quite a bit besides.

> I expect Black, as an engineer, would have thought us all mad by the
> old engineering standard of -
> "An engineer does for a shilling what a fool does for a pound".

Anyone worried about the obscene waste of building a tube amp to
listen to music while people starve in Africa is too sensitive to be
an audiophile.

Andre Jute
Visit Andre's books
http://coolmainpress.com/andrejute.html

Paul G.

April 5th 11, 02:10 AM

On Sat, 02 Apr 2011 14:19:58 -0500, flipper > wrote:

>On 1 Apr 2011 15:23:04 -0500, Newbie > wrote:
>
>>It seems well known that negative feedback can reduce lower order harmonics, but
>>increase the higher order ones.
>
>The oft cited source of that observation is Baxandall's experiments
>with a single stage FET amplifier and what he observed is that higher
>order distortion is increased for 'small' values of feedback ('worst'
>being about 6dB) but turn downward when more feedback is applied, with
>it taking 'more' NFB the higher the harmonic order.
>
>Note that in figure 7 he provides both observed and calculated
>distortion under the given assumptions.
>
>Conceptually he attributes this to the fed back harmonic(s)
>intermodulating with the source and summing with the original
>harmonic(s). I.E. in a simplified case a 'pure' input produces an
>output with fundamental plus the 2'd (and more) harmonic so the
>'feedback' now contains the 2'd (and more) harmonic which
>intermodulates with the 'pure' input to produce the 3'd (and more)
>harmonic.
>
>>Can anyone direct me to references that show how this happens with mathematical
>>analysis? Perhaps a paper in the journal of the AES, or elsewhere; on the web,
>>perhaps?
>
>Take it from 'the horses mouth' or, at least, one of the well known
>thoroughbreds
>
>http://www.linearaudio.nl/Documents/Baxandall%20power%20amp%20design.pdf
>
>I believe it's in issue 5 about 80% of the way down.

The assumption of generated odd harmonics in a "closed loop"
amplifier that should only generate even harmonics (when open loop) is
entirely reasonable. The math makes sense. The thing that puts the
whole excercise into meaningful proportion is "alpha" (from the paper)
which is roughly the level of 2nd harmonic distortion when open loop.
On open loop the output contains the 2nd harmonic, about 20 db
(alpha) below fundamental. On closed loop, the 2nd harmonic is reduced
further by feedback as you'd expect.
On closed loop the 3rd harmonic pops up, and is approximately alpha
(20db) below 2nd harmonic.
On the next modulation product (4th) is reduced by alpha again.
This process continues until bandwidth limitations come in.
Each harmonic is reduced approximately by "alpha", looking at the
ratios of harmonic amplitudes in Table 1 of "Amplifier Design -5".
The curves in figure 7 of "Amplifier Design -5" are based on alpha
of .1 (10% distortion of 2nd harmonic in open loop parabolic
amplifier). Similiar values are obtained with a simple single FET
amplifier. In "Amplifier Design -6" figures are obtained for a
junction transistor voltage input. The consecutive harmonic levels are
not spaced as far apart (depends on amount of drive).
In both cases modulation products rapidly decrease in amplitude
with increasing order.

I set up a LTSpice simulation for the JFET circuit that Baxandall
used, and followed his protocol. With no feedback, 3rd or greater
harmonics were less than 64db below fundamental. With small amounts of
feedback (5 db gain reduction, same 3 v output), they jumped up by
10-13 db, ie, -51db instead of -64db. That's still well below 2nd
harmonic (-28 db with feedback). Applying more feedback showed the
same behaviour as in the paper (each harmonic was about 17-20 db below
the previous one). The 2nd harmonics followed the classic reduction by
feedback (1/(1+BA), and the higher harmonics were reduced by "alpha"
for each one.

With a push-pull circuit, the reduction of the 2nd harmonic will
mean that the above effects are greatly reduced. "alpha" could 40-60
db down for each harmonic.

It's worth quoting the Baxandall paper's last paragraph.....quote:
"It can thus be concluded that providing plenty of feedback is assumed
right in the beginning, the more awkward parts of the theory outlined
in this article, though academically interesting, do not need to be
taken into account for design purposes"

If you can understand that quotation, then the whole issue is a
"tempest in a teapot". It's more a curiosity of how did the 2nd
harmonic distortion give rise to much smaller amounts of high order
distortion products when feedback is applied. It also gives you the
math and a table to calculate the harmonics in a closed loop
transistor amplifier.

To conclude from Baxnadall's paper:
1- feedback doesn't create distortion where there wasn't already some.
2- new harmonics are created from existing 2nd harmonic
3- the new harmonics are much smaller than each preceding harmonic
4- they are smaller by roughly the 2nd harmonic distortion figure
5- the effect is more pronounced in bipolar junction transistors than
FETs
6- reducing 2nd harmonic (ie, push-pull) will nearly eliminate the
effect

Paul G.

Phil Allison[_3_]

April 5th 11, 02:47 AM

"Paul G."
>
>
> To conclude from Baxnadall's paper:
> 1- feedback doesn't create distortion where there wasn't already some.
> 2- new harmonics are created from existing 2nd harmonic
> 3- the new harmonics are much smaller than each preceding harmonic
> 4- they are smaller by roughly the 2nd harmonic distortion figure
> 5- the effect is more pronounced in bipolar junction transistors than
> FETs
> 6- reducing 2nd harmonic (ie, push-pull) will nearly eliminate the
> effect

** Another way of summing up the article is to conclude that NFB should be
applied around an *amplifier stage* free of gross amplitude non-linearity
and with sufficient excess gain to obtain the result needed.

A square law mixer is simply not an amplifier.

Which is stating the bleeding obvious - really.

This article, from my friend Rod, covers the same territory and is worth a
read:

http://sound.westhost.com/articles/distortion+fb.htm

..... Phil

Phil Allison[_3_]

April 5th 11, 03:40 AM

Big Bad Bob

April 5th 11, 08:44 AM

On 04/04/11 15:38, flipper so wittily quipped:
>> What
>> you describe sounds like negative feedback CAUSING high harmonic
>> distortion, under ALL conditions.
>
> It does, although, if you have 'enough' NFB then the increase in
> (much) higher order distortion components, which are generally small
> to begin with, is inconsequential.
>

> The saving grace is that higher order harmonics manifest in ever
> decreasing magnitude and eventually you reach the Cyrano Jones price
> of Tribbles condition that "twice nothing is still nothing."
>
> For those unfamiliar with that Star Trek episode, The Trouble with
> Tribbles, the original offer was 1 credit and when the bartender
> doubled it to 2 credits Cyrano made his well known quip.
>
> Of course, Cyrano was not being 'mathematically correct', since 1 is
> not "nothing" and neither is 2, but we understand his meaning: the
> original offer was so low that doubling it was of no consequence (to
> him anyway).
>
> It's the same with NFB as long as there's 'enough' of it. OMG, you
> doubled the -150dB n'th harmonic!!! Yes, well, twice nothing is still
> nothing.
>
> It is an 'increase' but we don't care about it.

ok, then that makes more sense. To suggest a theoretical increase
verses a measurable one. I can accept that.

>> Other cases, as described by Patrick, make sense (and these are driven
>> by specific design inadequacies, phase shift at high freq's and low open
>> loop gain at high freq's being the biggest contributing factors).
>
> Of course they do but Baxandall's paper specifically excludes those
> conditions because he was investigating NFB caused distortion and not
> 'lousy amplifier' caused distortion.

if someone tries to match measurements to his theory, he will probably
succeed, especially if he ignores the more practical aspects of science
in the process. In this case, apply NFB to a crappy amplifier, and
measure the "increase". Patrick's "crappy amplifier" description makes
a good case for that.

Incidentally it's a standard engineering practice to assume 5 time
constants as "discharging" a capacitor, even though a capacitor never
truly 'discharges'. It's really at 1/e^5 of the original potential but
there ya go. I suppose measuring those 'added' harmonics are the same
way. Maybe the theory says they're there but you won't be able to
measure any effect from them. With sufficient NFB and proper amp design
they'll be virtually undetectable.

Big Bad Bob

April 5th 11, 08:58 AM

On 04/04/11 16:07, Andre Jute so wittily quipped:
> What an offensive jerk you are, Bubba. WTF makes you think I need you
> to tell me "the basics"?

there seemed to be a general misunderstanding of 'the basics' coming
from you, but NOW it seems you must already have achieved some level of
superiority over the rest of us. Well, I suppose not all of us can be
the next greatest genius in electronics engineering.

May your level of success directly reflect exactly what your skill
levels are. That's either a blessing or a curse, depending on how you
look at it.

as for me, I don't care which. Both extremes can be equally entertaining.

/me passing the bucket of popcorn to the right

Big Bad Bob

April 5th 11, 09:02 AM

On 04/04/11 15:39, Andre Jute so wittily quipped:
> On Apr 4, 9:19 pm, Big Bad Bob<BigBadBob-at-mrp3-
> > wrote:
>> On 04/02/11 05:16, Andre Jute so witilly quipped:
>>
>>> This article explains why added-on negative feedback is a bad thing in
>>> tube audio if you have really good speakers; it focuses on how the
>>> higher harmonic artifacts are added.
>>> http://www.audio-talk.co.uk/fiultra/KISS%20123%20by%20Andre%20Jute.htm
>>
>> don't believe EVERYTHING you read on the intarweb
>>
>> About half of what this guy says is pure opinion. I see no math to back
>> it up. basic 'servo theory' should easily disprove it.
>
> That's a humorous article, Bubba. The math is elsewhere on the site.
> But by all means bring on your math. We had a clown here called Henry
> Pasternack who was last seen running away with his halls in one hand
> and his MSEE from Stanford in the other. I sincerely hope you're
> better prepared than he was.

bubba's your momma. who's your daddy?

Big Bad Bob

April 5th 11, 09:07 AM

On 04/02/11 05:35, mike s so wittily quipped:
> BTW, it is also "well known" that the heavier an object is, the faster
> it falls. Of course, it's also untrue.

I like that one.

Big Bad Bob

April 5th 11, 09:14 AM

On 04/02/11 20:57, flipper so wittily quipped:
>> And there is the article I once read from Wireless World, December,
>> 1978. It has a graph of harmonic product levels for the amount of NFB
>> applied.
>
> People seem to be having a kind of "how dare you insult my
> girlfriend?" reaction but, according to Baxandall, it's just a matter
> of getting 'enough' NFB.

and that's pretty much what I've been seeing/hearing for decades.

you could say the same thing about FM modulation, generating infinite
harmonics in both directions from the carrier at the modulation
frequency. Typically they're filtered out to avoid crossing over into
the next channel or exceeding licensed bandwidth [whichever]. And yet,
if you look on a spectrum analyzer, you won't see the infinite
harmonics. You'll only see the DETECTABLE ones. sure, the others are
"there" according to the theory, but they drop below the sea of chaos
known as 'background noise' and unless you're writing a demodulation
algorithm for cell phones, you can assume it's not there.

twice nothing IS nothing. I learned that in calculus class, but the
star trek reference is more fun.

You could also call this a religious war between mathemeticians and
engineers. Just don't mention accountants.

Big Bad Bob

April 5th 11, 09:18 AM

On 04/04/11 18:10, Paul G. so wittily quipped:
>
> To conclude from Baxnadall's paper:
> 1- feedback doesn't create distortion where there wasn't already some.

that makes the most sense yet.

Big Bad Bob

April 5th 11, 09:43 AM

On 04/04/11 16:27, Andre Jute so wittily quipped:
> Anyone worried about the obscene waste of building a tube amp to
> listen to music while people starve in Africa is too sensitive to be
> an audiophile.

sensitivity is overrated anyway. best to abandon it entirely.

Big Bad Bob

April 5th 11, 09:57 AM

On 04/03/11 00:09, Don Pearce so wittily quipped:
>> A small amount of negative feedback (e.g. 6dB) in a single ended
>> stage, though reducing the second-harmonic distortion, and also the
>> total (unweighted) distortion, by about 6dB, will increase the
>> higher-order distortion and the quality of reproduction may well
>> become worse as judged subjectively.

hmmm... so the test conditions are specific, and not general (as was
originally implied). My argument all along has been that a high gain
amplifier with a large amount of negative feedback would not exhibit
this particular problem due to its operation being closer to that of an
operation amplifier, assuming no phase shift or stability issues.

>> The magnitude of harmonics of extremely high order will be increased
>> by the application of negative feedback, no matter what practical
>> amount of negative feedback is employed, but this is of no consequence
>> if, when thus increased, they are, say, 120dB below the fundamental.

thanks for that part. It is the common sense application of the theory
where NFB isn't generically bad any more. Now it does what I think all
of use have assumed it would do, to bring distortion down to a 'below
detectability' level.

>> He also explains the mechanism producing the 'new' distortion as the
>> modulator/mixer action of the non-linear amplifier, which we know is
>> non-linear or else we would not be wrapping NFB around the thing
>> trying to 'improve' it's linearity, acting on the fed back
>> harmonic(s).

that would be IM distortion, and that's very very bad for the reason
stated here.

> Let me put this as few words as the subject will allow. Global NFB is
> capable - through intermod - of generating tiny amounts of higher
> harmonics where none existed. And I do mean tiny. These higher
> harmonics are, in practice always swamped massively by naturally
> occurring higher harmonics. These higher harmonics respond normally to
> the application of NFB and are reduced proportionately to the ratio of
> open loop to closed loop gain.

agreed.

> So no, NFB does not cause higher harmonics to increase.

also agreed, to the extent that some other 'bad design' factor like
clipping below max power or drawing current on a grid or using a sharp
cutoff pentode instead of triodes in the pre-amp or using a B+ voltage
that isn't significantly higher than expected voltage swing on the
preamp tubes doesn't throw a wrench into the works. Those other things
might make the original premise correct, sorta like what Patrick said
early on.

Big Bad Bob

April 5th 11, 10:12 AM

On 04/04/11 14:49, mike s so wittily quipped:
> Starving filaments is interesting. It's another of those things I
> think about from time to time, but there are always other more
> pressing things to do.

I would think that 'use the right component for the job' might be a
better way to go, rather than forcibly de-rating the device by starving
the filament. It's also more likely to give you unpredictable results,
or require tweaking each unit. Tube characteristics change over time,
after all, though I suppose lower heater voltage might slow that process
down a bit.

Now, something else comes to mind with reduced heater potential, and
that is a cooler cathode, which for a triode might reduce the noise
level. If you can squeeze out enough current to operate properly, the
reduced cathode temp might produce less entropy in the electron cloud.
That might result in lower noise, so for a 1st stage in a preamp it
might be a good idea on that basis. It's worth an experiment. But then
again, 'right component for the job' is probably the better choice.

But reducing heater current on the power output stage? I think I'd shop
for a better tube instead. Maybe one of the high end tube makers would
spec one out specifically for low distortion power amps.

Patrick Turner

April 5th 11, 11:05 AM

On Apr 3, 1:57*pm, flipper > wrote:
> On Sat, 2 Apr 2011 20:08:50 -0700 (PDT), Patrick Turner
>
>
>
>
>
> > wrote:
> >On Apr 3, 5:34 am, flipper > wrote:
> >> On Sat, 2 Apr 2011 05:35:07 -0700 (PDT), mike s
>
> >> > wrote:
> >> >On Apr 1, 9:23 pm, Newbie > wrote:
> >> >> It seems well known that negative feedback can reduce lower order harmonics, but
> >> >> increase the higher order ones.
>
> >> >> Can anyone direct me to references that show how this happens with mathematical
> >> >> analysis? Perhaps a paper in the journal of the AES, or elsewhere; on the web,
> >> >> perhaps?
>
> >> >In most things it pays to start at the beginning and proceed as far as
> >> >you are interested or able -
>
> >> >http://www.ece.ucdavis.edu/courses/notes/old/S09/EEC110B/feedback.pdf
>
> >> >BTW, it is also "well known" that the heavier an object is, the faster
> >> >it falls. Of course, it's also untrue.
>
> >> And, apparently, some people don't believe in gravity at all.
>
> >> NFB does, indeed, increase higher order harmonics, depending on the
> >> order of the harmonic and how much NFB is applied.
>
> >> Take it from 'the horses mouth' or, at least, one of the well known
> >> thoroughbreds
>
> >>http://www.linearaudio.nl/Documents/Baxandall%20power%20amp%20design.pdf
>
> >This adress took me to a 3.3M .pdf file which has an enormous wealth
> >of useful information.
>
> >There is a 'page 56' about 80% down the scroll....
>
> Yep, that's it.
>
>
>
> >And there is the article I once read from Wireless World, December,
> >1978. It has a graph of harmonic product levels for the amount of NFB
> >applied.
>
> People seem to be having a kind of "how dare you insult my
> girlfriend?" reaction but, according to Baxandall, it's just a matter
> of getting 'enough' NFB.

Ah, Once Upon A Time, when I was about 26, I rooted my girlfriend so
often she walked about bow legged and cross eyed in a delerious
romantic swoon from one day to the next.

Neither of us could get enough of each other. NFB didn't get thought
about very much. But after the wedding
it sure was considered, and enough of lots of things were also
considered. After the divorce the gettering within both of us kinda
faded, and we couldn't get enough.......

But between then and now, girls of 62 have become sad old chooks and
won't ****, can't ****, and since I am a fiddle with many a tune still
able to be played if there was a girl who could use a bow, then the
old chooks can't look at me, and think a BJ is a Blue Jumper, and a
bicycle doesn't have enough "class", and makes an old man look
distortedly young, and strange.

I happen to think the business of enough NFB is a fair enough Idea,
providing the amp is fairly linear, quiet, and has sufficient BW and
can be listened to without displeasure before FB is applied.
Adding NFB usually vastly improves bass and creates low output
resistance which ensures the speaker crossover filters all work as
intended and that the response level is flatter than if the Rout was
higher than say 1 ohm.

To me, the NFB enables the tubes to display their grandure with music.
If you want good music, or good sex, you must bring the performer in
from the street where its cold and windy, and have them relax and
focus while comfortable, in the absense of noise and distractions. NFB
kinda takes care of things.

But many of my customers don't mind tube amps without a stitch of or
global NFB. All these amps are triode amps. Myself, I prefer the
shiela without make-up, and wearing only a birthday suit, au-natural.
No need for lingerie. But for music some NFB is OK.

At 63, at my favourite cafe the waitress of 20 rushes over to me with
a smile and brings me edible and drinkable treats which are my humble
idea of what is enough. She outshines the others among the cafes
around the square, and if she was a vacuum tube, she'd be a little
triode that was musical, engaging, warm, smooth, erotic, emotional,
practical, and vivacious and plain damn desirable. When I have
finished and paid I say "I'll catch you later..." Trouble is I am 30
years too late already. Nobody can be young enough. If I was a vacuum
tube I'd probably be big tired old 845, but capable of sweeping a girl
off her feet in her dreams if only I was directly coupled to a good
little triode. A good butler called Mr NFB would make the experience
better if he worked in the background to take care of details which
romance forgets.

Patrick Turner.

Patrick Turner

April 5th 11, 11:24 AM

On Apr 3, 9:04*pm, mike s > wrote:
> On Apr 2, 1:16*pm, Andre Jute > wrote:
>
>
>
>
>
> > On Apr 1, 9:23*pm, Newbie > wrote:
>
> > > It seems well known that negative feedback can reduce lower order harmonics, but
> > > increase the higher order ones.
>
> > > Can anyone direct me to references that show how this happens with mathematical
> > > analysis? *Perhaps a paper in the journal of the AES, or elsewhere; on the web,
> > > perhaps?
>
> > Here's an article to help you distinguish between added-on negative
> > feedback and natural negative feedback.http://www.audio-talk.co.uk/fiultra/KISS%20104%20by%20Andre%20Jute.htm
>
> > This article explains why added-on negative feedback is a bad thing in
> > tube audio if you have really good speakers; it focuses on how the
> > higher harmonic artifacts are added.http://www.audio-talk.co.uk/fiultra/KISS%20123%20by%20Andre%20Jute.htm
>
> > More articles on Harmonic Distortion on the KISS Amps site, available
> > through those pages.
>
> This in intriguing in that what Jute has re-invented or re-created is
> pretty much exactly the type of amplifier that Harold Black was faced
> with in the 1920s when he invented negative feedback. *So it would be
> interesting to hear from someone with such an amplifier as to what
> happens if negative feedback is employed.
>
> Certainly well designed and built valve amplifiers can perform
> superbly as domestic audio systems without negative feedback. *Though
> building two with exactly matched characteristics for stereo can be a
> (not unrewarding) challenge. *Of course before the signal reaches the
> domestic system it's been through many other higher performance
> amplifiers, all of which will employ NFB.
>
> My own system has no global NFB in the output stage, but it uses push
> pull triodes, driven by another pair of triodes, so it's hardly worth
> it. *And I like the "traditional" sound.http://mike.wepoco.com/Home/retro-geekery/valve-amplifiers/Building-a...
>

What exactly was "traditional" sound?

In 1925, reproduced sound was mostly pretty damn awful mainly because
transducers such as microphones and speakers and recording discs were
so woefully limited in their ability to create hi-fi. Triode
amplifiers could always be easily made without global NFB and to be
able to perform flawlessly compared the other crappy gear which was
used.

But now we have had hi-fi discs since vinyl days. The mics and
speakers and record cutters, FM transmiters, and digital technology
etc have quite low distortions, so the amp needs to perform better
than the worst generic crappy tube stuff which was used in 1925. There
were crappy triode amps. The WE movie theatre amps were not so hot
IMHO.

There has always been plenty of damn awful tubed audio electronics
available at a cheap price for those who wouldn't know hi-fi even it
it bit them on the arse.

Patrick Turner.

Ian Bell[_2_]

April 5th 11, 03:25 PM

flipper wrote:
>
> I already tried 'argument', including quoting his exact words, but you
> are immune to a rational discussion, and I see no reason to re type
> the entire paper when the original is just sitting there for you to
> read,

I have not read this paper. Can you provide me with a link to it please?

Cheers

Ian

Paul G.

April 6th 11, 03:29 AM

On Mon, 04 Apr 2011 20:57:58 -0500, flipper > wrote:

>On Mon, 04 Apr 2011 22:10:11 -0300, Paul G. >
>wrote:
......[snip!]

>>
>> With a push-pull circuit, the reduction of the 2nd harmonic will
>>mean that the above effects are greatly reduced. "alpha" could 40-60
>>db down for each harmonic.
>
>Modulation effects from the second will certainly be reduced but
>harmonics from the rest, usually dominated by the 3'd, will do the
>same thing.
>
>
>> It's worth quoting the Baxandall paper's last paragraph.....quote:
>>"It can thus be concluded that providing plenty of feedback is assumed
>>right in the beginning, the more awkward parts of the theory outlined
>>in this article, though academically interesting, do not need to be
>>taken into account for design purposes"
>
>Yes, I've explained the 'enough' NFB point till I'm blue in the face.
>
>> If you can understand that quotation, then the whole issue is a
>>"tempest in a teapot"
>
>Not entirely because people in this forum don't always employ 'tons of
>NFB."
>
>In fact, I think in the tube genre there's a following who believes in
>'modest' (global) NFB, which Baxandall's paper suggests might not be
>as good a thing as they imagine. On the other hand it might be because
>they're often thinking about clipping effects and Baxandall wasn't
>addressing that issue.
>
>>. It's more a curiosity of how did the 2nd
>>harmonic distortion give rise to much smaller amounts of high order
>>distortion products when feedback is applied. It also gives you the
>>math and a table to calculate the harmonics in a closed loop
>>transistor amplifier.
>>
>> To conclude from Baxnadall's paper:
>>1- feedback doesn't create distortion where there wasn't already some.
>
>A rather moot point since there's no such thing as a distortionless
>amplifier and if there were you wouldn't using NFB to reduce what
>wasn't there.
>
>>2- new harmonics are created from existing 2nd harmonic
>
>There's nothing 'special' about the 2'd that make it 'uniquely' a
>modulator and new harmonics will be created from any existing
>harmonic. 2'd just happens to be the first and largest produced by a
>single square law device (his FET approximating)
>
>>3- the new harmonics are much smaller than each preceding harmonic
>>4- they are smaller by roughly the 2nd harmonic distortion figure
>
>Depends on which harmonic is doing the modulation.
>
>>5- the effect is more pronounced in bipolar junction transistors than
>>FETs
>>6- reducing 2nd harmonic (ie, push-pull) will nearly eliminate the
>>effect
>
>It'll reduce 2'd harmonic modulation but not the 3'd, which will then
>modulate when fed back. They all will.
>

The issue is now whether with push-pull or symetric distortion (all
odd products ie., 3rd,5th,7th, etc) will have the same problem of
feedback increasing the amplitude of upper harmonics.
I would say "no" because the harmonic that would be fed back would
be the 3rd, and that would be modulated by the fundamental to become
the 4th harmonic. The modulation mechanism is the distortion itself.
Now the 4th harmonic won't get through the amplifier because of the
symmetry - it's push-pull.
To verify this, I made up a circuit in LTSpice in which an ideal
opamp drove a circuit with 2 diodes and a few resistors so the
clamping wan't too extreme. The diodes (part of the shunt to ground)
were in opposite polarity to give symmetry. I measured the distortion
products, then wrapped an opamp around the circuit, and varied the
internal gain of the opamp, and watched tha amplitude of the
harmonics. In all cases I forced the output signal to stay at a
constant level.
The results of the simulation showed no even harmonics under any
condition. The ratios of the harmonics to each other was about 5db
less for each consecutive odd harmonic (3rd..5th..7th..etc) when open
loop. With small amounts of feedback, the ratios varied between 3-10
db. With larger amounts of feedback the ratios varied between 3-7 db,
with most of the variation occuring between 3rd and 5th harmonics.
The absolute values of the harmonics roughly decreased in accordance
with the feedback (loopgain).
Feedback is decreasing the distortion (all harmonics), but there is
variation with respect to each harmonic. For an overall decrease of
20db in all distortion, the individual harmonics will vary plus-minus
3-4 db with respect to each other.
Some of the variation will be from the FFT, I chose a signal
frequency of 1 KHz, and on the time domain simulation I forced a max
timestep of .2us in order to give a reasonable results.
The opamp I used was the "Universal Opamp", and I set GBW to
100MHz. That would cause frequency response issues for open loop gains
greater than 10000, or very large amounts of feedback. The result of
limited frequency response (GBW=100MHz is a VERY fast audio amp) gave
a limit to the distortion reduction (60-70db) for large amounts of
feedback.

The verdict? Feedback still works..... for small amounts of feedback
the ratios of the harmonics vary with respect to each other, but much
less than what the overall reduction from feedback does.

Paul G.

Big Bad Bob

April 6th 11, 11:55 PM

On 04/05/11 03:24, Patrick Turner so wittily quipped:
> On Apr 3, 9:04 pm, mike > wrote:
>> My own system has no global NFB in the output stage, but it uses push
>> pull triodes, driven by another pair of triodes, so it's hardly worth
>> it. And I like the "traditional" sound.http://mike.wepoco.com/Home/retro-geekery/valve-amplifiers/Building-a...
>>
>
> What exactly was "traditional" sound?

that's a very good question. gramophone maybe? "his master's voice"
indeed.

> There has always been plenty of damn awful tubed audio electronics
> available at a cheap price for those who wouldn't know hi-fi even it
> it bit them on the arse.

I refurbed one of those 'damned awful tube audio' units back in the
70's, 'cause it had a reasonably nice wood cabinet. The phono used a
crystal cartridge (and diamond a needle, but it had trouble with 1970's
vinyl), the amplifier had an open loop SE (6BQ5 or similar) with output
transformers that saturated due to current flowing in one direction for
too long. I reversed polarity on them to compensate for the hysteresis
problem, added a bit of negative feedback (not too much since gain
sucked) to improve freq. response a bit, then later changed it out for
an external 10W/channel tube power amp that I found at the local swap
meet and fixed up a bit. It used better components and sounded pretty
good. Oh, yeah, I built matching speakers for it too. I still have
those speaker cabinets, with newer speakers, better [Peavey style] grill
cloth, modern components.

Big Bad Bob

April 6th 11, 11:58 PM

On 04/05/11 11:51, flipper so wittily quipped:
>> You could also call this a religious war between mathemeticians and
>> engineers. Just don't mention accountants.
>
> No, it's an 'argument' created by people who don't read the paper, nor
> what's written about it, and misrepresent what's said... which is a
> neat trick seeing as how they haven't read it.

I actually didn't read it myself, except for the quotes that were posted
here [who has that much time, right?]. But the argument being made
sounded so ridiculous that I rejected it outright based on my own
experience and a few mental calculations.

Also picked up a couple of trolls. that's always fun.

Big Bad Bob

April 7th 11, 01:00 AM

On 04/05/11 19:29, Paul G. so wittily quipped:
> The issue is now whether with push-pull or symetric distortion (all
> odd products ie., 3rd,5th,7th, etc) will have the same problem of
> feedback increasing the amplitude of upper harmonics.
> I would say "no" because the harmonic that would be fed back would
> be the 3rd, and that would be modulated by the fundamental to become
> the 4th harmonic. The modulation mechanism is the distortion itself.
> Now the 4th harmonic won't get through the amplifier because of the
> symmetry - it's push-pull.

well, that's an interesting point, but the even harmonic cancellation is
for ADDED distortion, and if the IM distortion (creating the 4th
harmonic) is exactly the same for both legs, well then yeah. It cancels
out. But the real point is that the amplitude of the 'fed back'
distortion is so small as to be negligible, and inverted with respect to
the 'generated' distortion. The IM components that are of concern are
between the odd harmonics and one another, and the rest of the audio
(usually not a pure tone).

But of course large NFB brings these levels to "below detectable" which
is essentially zero. And that was the question, whether NFB is always
resopnsible for more harmonic distortion at the higher harmonics, or
whether it does what it claims to do, reducing THD and not creating more.

> To verify this, I made up a circuit in LTSpice in which an ideal
> opamp drove a circuit with 2 diodes and a few resistors so the
> clamping wan't too extreme. The diodes (part of the shunt to ground)
> were in opposite polarity to give symmetry. I measured the distortion
> products, then wrapped an opamp around the circuit, and varied the
> internal gain of the opamp, and watched tha amplitude of the
> harmonics. In all cases I forced the output signal to stay at a
> constant level.
> The results of the simulation showed no even harmonics under any
> condition.

that's actually very interesting. Can you inject multiple tones and see
what THAT does?

> Feedback is decreasing the distortion (all harmonics), but there is
> variation with respect to each harmonic. For an overall decrease of
> 20db in all distortion, the individual harmonics will vary plus-minus
> 3-4 db with respect to each other.

real world vs theory maybe? Or just an artifact of the nature of
harmonic distortion, because the harmonics aren't necessarily in phase
with the original tone. In fact they probably aren't.

> Some of the variation will be from the FFT, I chose a signal
> frequency of 1 KHz, and on the time domain simulation I forced a max
> timestep of .2us in order to give a reasonable results.

FFT's are fine, so long as your sample width is a power of 2. BTW I
have a nice multi-thread DFT algorithm that you can use if you want.
Works great on multi-core machines. Each harmonic is calculated as a
separate work unit, sort of like those distributed projects like dnetc
or seti@home. I suppose anyone could code it now that I gave my
'secret' away...

> The verdict? Feedback still works..... for small amounts of feedback
> the ratios of the harmonics vary with respect to each other, but much
> less than what the overall reduction from feedback does.

thanks for experimental confirmation.

Big Bad Bob

April 7th 11, 01:10 AM

On 04/06/11 16:44, flipper so wittily quipped:
>> I actually didn't read it myself, except for the quotes that were posted
>> here [who has that much time, right?]. But the argument being made
>> sounded so ridiculous that I rejected it outright based on my own
>> experience and a few mental calculations.
>
> And what argument is that?

heh, did I leave out too much information?

going back to the beginning, it was being (incorrectly) argued that NFB
always increased higher level harmonic distortion. Has the thread
gotten so 'deep' that the original post has been forgotten? Not
surprising really, what with trolling and self-promotion from a couple
of posters and common sense from the rest.

it's beer-o-clock anyway and I still need to get work done. My boss
(me) is a real slavedriver and keeps yelling at me for wasting time on
USENET instead of working.

Big Bad Bob

April 7th 11, 01:13 AM

On 04/05/11 03:05, Patrick Turner so wittily quipped:
>> People seem to be having a kind of "how dare you insult my
>> girlfriend?" reaction but, according to Baxandall, it's just a matter
>> of getting 'enough' NFB.
>
> Ah, Once Upon A Time, when I was about 26, I rooted my girlfriend so
> often she walked about bow legged and cross eyed in a delerious
> romantic swoon from one day to the next.
>
> Neither of us could get enough of each other. NFB didn't get thought
> about very much. But after the wedding
> it sure was considered, and enough of lots of things were also
> considered. After the divorce the gettering within both of us kinda
> faded, and we couldn't get enough.......
>
> But between then and now, girls of 62 have become sad old chooks and
> won't ****, can't ****, and since I am a fiddle with many a tune still
> able to be played if there was a girl who could use a bow, then the
> old chooks can't look at me, and think a BJ is a Blue Jumper, and a
> bicycle doesn't have enough "class", and makes an old man look
> distortedly young, and strange.
>
> I happen to think the business of enough NFB is a fair enough Idea,
> providing the amp is fairly linear, quiet, and has sufficient BW and
> can be listened to without displeasure before FB is applied.
> Adding NFB usually vastly improves bass and creates low output
> resistance which ensures the speaker crossover filters all work as
> intended and that the response level is flatter than if the Rout was
> higher than say 1 ohm.
>
> To me, the NFB enables the tubes to display their grandure with music.
> If you want good music, or good sex, you must bring the performer in
> from the street where its cold and windy, and have them relax and
> focus while comfortable, in the absense of noise and distractions. NFB
> kinda takes care of things.
>
> But many of my customers don't mind tube amps without a stitch of or
> global NFB. All these amps are triode amps. Myself, I prefer the
> shiela without make-up, and wearing only a birthday suit, au-natural.
> No need for lingerie. But for music some NFB is OK.
>
> At 63, at my favourite cafe the waitress of 20 rushes over to me with
> a smile and brings me edible and drinkable treats which are my humble
> idea of what is enough. She outshines the others among the cafes
> around the square, and if she was a vacuum tube, she'd be a little
> triode that was musical, engaging, warm, smooth, erotic, emotional,
> practical, and vivacious and plain damn desirable. When I have
> finished and paid I say "I'll catch you later..." Trouble is I am 30
> years too late already. Nobody can be young enough. If I was a vacuum
> tube I'd probably be big tired old 845, but capable of sweeping a girl
> off her feet in her dreams if only I was directly coupled to a good
> little triode. A good butler called Mr NFB would make the experience
> better if he worked in the background to take care of details which
> romance forgets.

heh, full quote simply because it was a great read

Big Bad Bob

April 7th 11, 01:30 AM

On 04/05/11 11:23, flipper so wittily quipped:
>>> Let me put this as few words as the subject will allow. Global NFB is
>>> capable - through intermod - of generating tiny amounts of higher
>>> harmonics where none existed. And I do mean tiny. These higher
>>> harmonics are, in practice always swamped massively by naturally
>>> occurring higher harmonics. These higher harmonics respond normally to
>>> the application of NFB and are reduced proportionately to the ratio of
>>> open loop to closed loop gain.
>>
>> agreed.
>
> Except what he wrote is wrong, as the measured data in the paper
> unquestionably shows.

that would be the theory, actually, and mathematically you could prove
something like that happening.

>>> So no, NFB does not cause higher harmonics to increase.
>>
>> also agreed,
>
> Also wrong, as the measured data in the paper unquestionably shows.

should qualify as 'above the detectability point' but I thought that was
implied.

I think you must be a myers-briggs 'SJ' (guess ESTJ) type, where I'm a
'ENTP' type. You appear to see things in terms of details and direct
observations, whereas I look at the general case based on real world
experience and my understanding of the theories behind it all (besides
the details being boring). Problem is I see why you make these
statements and I'm just as likely to agree with you as I am the other
side [from your perspective]. And so I look back at every NFB audio
circuit I've ever made, and what the o-scope traces looked like, and how
they performed, and things of that nature, and intuitively understand
that yes, NFB [when done right] does NOT increase any harmonic
distortion, at least not in a way that's detectable.

In any case, the elephant really isn't like a tree, he's like a snake.
Or not. Heh.

In the real world, ESTJ's pair up nicely with ENTP's as part of an
engineering team because input from each perspective helps make a better
design. ENTPs are mad scientists, while ESTJs are bit fiddlers. An
ENTP will keep an ESTJ from running in an infinite loop to solve an
unnecessary problem [such as refactoring computer code 3 dozen times or
moving components around in circles on a circuit board], wherease an
ESTJ will keep an ENTP from doing something that generally violates the
laws of physics or doesn't coincide with the goals of the department.

Big Bad Bob

April 7th 11, 01:35 AM

On 04/05/11 10:35, flipper so wittily quipped:
>> Incidentally it's a standard engineering practice to assume 5 time
>> constants as "discharging" a capacitor, even though a capacitor never
>> truly 'discharges'. It's really at 1/e^5 of the original potential but
>> there ya go.
>
> Really? "Really" it is? Is that a 'theory' or a crappy capacitor?

a crappy capacitor would discharge itself due to poor insulation. but I
digress.

>> I suppose measuring those 'added' harmonics are the same
>> way. Maybe the theory says they're there but you won't be able to
>> measure any effect from them. With sufficient NFB and proper amp design
>> they'll be virtually undetectable.
>
> If you had read the paper you'd know his whole point is to use
> 'enough' NFB.

details, details. That was covered in a different post anyway. It's
back to theory vs real world vs measureable vs what really matters. I
thought I was posting in English but I guess not. Anyway the day's
getting too long and I need more caffeine. Or alcohol. Or both.

Big Bad Bob

April 7th 11, 01:40 AM

On 04/06/11 17:24, flipper so wittily quipped:
>> going back to the beginning, it was being (incorrectly) argued that NFB
>> always increased higher level harmonic distortion.
>
> It does, in Baxandall's FET experiment with 'how high' the order
> depending on the amount of NFB.

ok - I'll split the difference with ya on this one. 'ALWAYS' vs
'depending upon'. Your statement counteracts the 'ALWAYS', as in "find
one counterexample and 'always' no longer applies." That's what I
implied by the 'always' part. So yeah, the theory would suggest that
you get an increase [based on the experiment] but with sufficient NFB it
drops below the point where you can detect it. Or something like that.
The math would suggest an infinitesimal increase in a higher harmonic
somewhere 'out there' regardless, but the reality would show it to be so
small as to become like the star trek 'twice nothing is still nothing'
example.

I'm out of breath. did I miss anything? No, don't say it (I know
you're thinking it) or we'll end up in a causality loop.

Patrick Turner

April 7th 11, 03:39 AM

On Apr 6, 12:29*pm, Paul G. > wrote:
> On Mon, 04 Apr 2011 20:57:58 -0500, flipper > wrote:
> >On Mon, 04 Apr 2011 22:10:11 -0300, Paul G. >
> >wrote:
>
> .....[snip!]
>
>
>
>
>
>
>
> >> * With a push-pull circuit, the reduction of the 2nd harmonic will
> >>mean that the above effects are greatly reduced. "alpha" could 40-60
> >>db down for each harmonic.
>
> >Modulation effects from the second will certainly be reduced but
> >harmonics from the rest, usually dominated by the 3'd, will do the
> >same thing.
>
> >> * It's worth quoting the Baxandall paper's last paragraph.....quote:
> >>"It can thus be concluded that providing plenty of feedback is assumed
> >>right in the beginning, the more awkward parts of the theory outlined
> >>in this article, though academically interesting, do not need to be
> >>taken into account for design purposes"
>
> >Yes, I've explained the 'enough' NFB point till I'm blue in the face.
>
> >> * If you can understand that quotation, then the whole issue is a
> >>"tempest in a teapot"
>
> >Not entirely because people in this forum don't always employ 'tons of
> >NFB."
>
> >In fact, I think in the tube genre there's a following who believes in
> >'modest' (global) NFB, which Baxandall's paper suggests might not be
> >as good a thing as they imagine. On the other hand it might be because
> >they're often thinking about clipping effects and Baxandall wasn't
> >addressing that issue.
>
> >>. It's more a curiosity of how did the 2nd
> >>harmonic distortion give rise to much smaller amounts of high order
> >>distortion products when feedback is applied. It also gives you the
> >>math and a table to calculate the harmonics in a closed loop
> >>transistor amplifier.
>
> >> *To conclude from Baxnadall's paper:
> >>1- feedback doesn't create distortion where there wasn't already some.
>
> >A rather moot point since there's no such thing as a distortionless
> >amplifier and if there were you wouldn't using NFB to reduce what
> >wasn't there.
>
> >>2- new harmonics are created from existing 2nd harmonic
>
> >There's nothing 'special' about the 2'd that make it 'uniquely' a
> >modulator and new harmonics will be created from any existing
> >harmonic. 2'd just happens to be the first and largest produced by a
> >single square law device (his FET approximating)
>
> >>3- the new harmonics are much smaller than each preceding harmonic
> >>4- they are smaller by roughly the 2nd harmonic distortion figure
>
> >Depends on which harmonic is doing the modulation.
>
> >>5- the effect is more pronounced in bipolar junction transistors than
> >>FETs
> >>6- reducing 2nd harmonic (ie, push-pull) will nearly eliminate the
> >>effect
>
> >It'll reduce 2'd harmonic modulation but not the 3'd, which will then
> >modulate when fed back. They all will.
>
> * *The issue is now whether with push-pull or symetric distortion (all
> odd products ie., 3rd,5th,7th, etc) will have the same problem of
> feedback increasing the amplitude of upper harmonics.
> * *I would say "no" because the harmonic that would be fed back would
> be the 3rd, and that would be modulated by the fundamental to become
> the 4th harmonic. The modulation mechanism is the distortion itself.
> Now the 4th harmonic won't get through the amplifier because of the
> symmetry - it's push-pull.
> * *To verify this, I made up a circuit in LTSpice in which an ideal
> opamp drove a circuit with 2 diodes and a few resistors so the
> clamping wan't too extreme. The diodes (part of the shunt to ground)
> were in opposite polarity to give symmetry. I measured the distortion
> products, then wrapped an opamp around the circuit, and varied the
> internal gain of the opamp, and watched tha amplitude of the
> harmonics. In all cases I forced the output signal to stay at a
> constant level.
> * *The results of the simulation showed no even harmonics under any
> condition. The ratios of the harmonics to each other was about 5db
> less for each consecutive odd harmonic (3rd..5th..7th..etc) when open
> loop. With small amounts of feedback, the ratios varied between 3-10
> db. With larger amounts of feedback the ratios varied between 3-7 db,
> with most of the variation occuring between 3rd and 5th harmonics.
> The absolute values of the harmonics roughly decreased in accordance
> with the feedback (loopgain).
> * *Feedback is decreasing the distortion (all harmonics), but there is
> variation with respect to each harmonic. For an overall decrease of
> 20db in all distortion, the individual harmonics will vary plus-minus
> 3-4 db with respect to each other.
> * *Some of the variation will be from the FFT, I chose a signal
> frequency of 1 KHz, and on the time domain simulation I forced a max
> timestep of .2us in order to give a reasonable results.
> * *The opamp I used was the "Universal Opamp", and I set GBW to
> 100MHz. That would cause frequency response issues for open loop gains
> greater than 10000, or very large amounts of feedback. The result of
> limited frequency response (GBW=100MHz is a VERY fast audio amp) gave
> a limit to the distortion reduction (60-70db) for large amounts of
> feedback.
>
> * The verdict? Feedback still works..... for small amounts of feedback
> the ratios of the harmonics vary *with respect to each other, but much
> less than what the overall reduction from feedback does.
>
> Paul G.- Hide quoted text -
>
> - Show quoted text -

I think It may have been Ian Hinkman or some name like that who re-
visited the NFB causes distortion idea in a 1990s article in
Electronics World. He used a circuit where a resistor plus diode was
used to make a simple circuit produce copious even number H which
would be then converted to additional second ordrr products as NFB was
added, but which then diminished when a larger amount of NFB was used,
typically over 14dB. PP amps like the Williamson or Mullard 520 have
SE input stages so any FB will create IMD even No products to appear
even though the OP stage is PP. But these products are usually at very
low levels because the amp input stages usually produce far less THD
than the OP stage which may only make 2% open loop just under
clipping.

So all the concerns ppl have about NFB creating more mud than it
washes away is largely un-founded, and the measurements end to confirm
all this.

Time and time again, many people find they don't mind if global FB is
either used or not used with a good triode amp.

Patrick Turner.

Paul G.

April 7th 11, 03:57 PM

On Wed, 06 Apr 2011 18:54:24 -0500, flipper > wrote:

.......[snip ! ]........
>
>Interesting observation about PP cancellation.
>
>That won't happen on a multistage amplifier if the input is SE,
>though, because the modulation becomes part of the signal at that
>stage.
>
>I realize that's not what you tested and just mean it can't be
>universally extrapolated to just 'any ole' PP amplifier.
>
>Might be a good argument for using a differential input, though.
>
>> The verdict? Feedback still works.....
>
>That's never been in dispute.
>
>> for small amounts of feedback
>>the ratios of the harmonics vary with respect to each other, but much
>>less than what the overall reduction from feedback does.
>>

Although the front end of a power amp can be single ended, the
signal levels (if designed properly) are quite low compared to the
final section. You'd expect 2nd harmonic distortion to be 1% or less
at that stage. Using 1%, would give the 3rd harmonic at roughly .01%
(-80db), and the 5th at -120db as per the Baxandall paper. The odd
harmonics DO pop up, but they're pretty small.
A bit off topic, if you ARE using a single ended topology
throughout, you can bias the driver stage in such a way to partially
cancel the 2nd harmonics, and thereby reducing some of the odd
harmonic component as a side effect. I built a single ended tube amp
some years ago to see what all the fuss was about. Using the above
"trick" and optimum feedback, it both measured and sounded well.
It had about 15w/ch, under .1% at full power, and way less than .1% at
lower power. Using level matched A/B testing by changing the amps
only, I found it very difficult to hear the difference between it and
a good modern amplifier. I suppose that's heresy.

Paul G.

Big Bad Bob

April 8th 11, 09:27 AM

On 04/07/11 07:57, Paul G. so wittily quipped:
> A bit off topic, if you ARE using a single ended topology
> throughout, you can bias the driver stage in such a way to partially
> cancel the 2nd harmonics, and thereby reducing some of the odd
> harmonic component as a side effect. I built a single ended tube amp
> some years ago to see what all the fuss was about. Using the above
> "trick" and optimum feedback, it both measured and sounded well.
> It had about 15w/ch, under .1% at full power, and way less than .1% at
> lower power. Using level matched A/B testing by changing the amps
> only, I found it very difficult to hear the difference between it and
> a good modern amplifier. I suppose that's heresy.

sounds like a good design to me

I usually bias for 'middle of the spec' operation. Seems to work ok.

Big Bad Bob

April 8th 11, 09:40 AM

On 04/06/11 20:38, flipper so wittily quipped:
>> I think you must be a myers-briggs 'SJ' (guess ESTJ) type, where I'm a
>> 'ENTP' type. You appear to see things in terms of details and direct
>> observations, whereas I look at the general case based on real world
>> experience and my understanding of the theories behind it all (besides
>> the details being boring).
>
> Uh huh. Well, if you aren't 'observing' the real world then what makes
> you think you've got a handle on it?

It's a combination of lots of experience, extremely good insight,
incredibly high intelligence, and the attitude of a hacker.

thanks for confirming my observation. keep in mind that some of us can
see the entire alphabet after only 2 letters. Others have to go through
all of the letters before it becomes clear. I'd rather not be bored by
the detail of the other 24, so I'll stop after 'B' and say "alphabet".
Occasionally there may be a missing letter, and the detail guy will pick
up on that. But it's not interesting enough for me to care about 1
letter missing. The 'alphabet' concept is sufficient.

Big Bad Bob

April 8th 11, 11:21 PM

On 04/08/11 03:18, flipper so wittily quipped:
> On Fri, 08 Apr 2011 01:40:44 -0700, Big Bad Bob
> > wrote:
>
>> On 04/06/11 20:38, flipper so wittily quipped:
>>>> I think you must be a myers-briggs 'SJ' (guess ESTJ) type, where I'm a
>>>> 'ENTP' type. You appear to see things in terms of details and direct
>>>> observations, whereas I look at the general case based on real world
>>>> experience and my understanding of the theories behind it all (besides
>>>> the details being boring).
>>>
>>> Uh huh. Well, if you aren't 'observing' the real world then what makes
>>> you think you've got a handle on it?
>>
>> It's a combination of lots of experience, extremely good insight,
>> incredibly high intelligence, and the attitude of a hacker.
>>
>> thanks for confirming my observation. keep in mind that some of us can
>> see the entire alphabet after only 2 letters. Others have to go through
>> all of the letters before it becomes clear. I'd rather not be bored by
>> the detail of the other 24, so I'll stop after 'B' and say "alphabet".
>> Occasionally there may be a missing letter, and the detail guy will pick
>> up on that. But it's not interesting enough for me to care about 1
>> letter missing. The 'alphabet' concept is sufficient.
>>
>
> I guess that explains why you haven't been seeing what's actually
> said, only 1/13 of the letters get through.

more likely I'm just explaining things inadequately (or leaving out
explanations for things that appear intuitively obvious to me) for
someone that sees things the way you do. So the letters get through,
but I would view it as an explanation of a concept and not a set of details.

Big Bad Bob

April 11th 11, 08:21 AM

On 04/08/11 15:33, flipper so wittily quipped:
> On Fri, 08 Apr 2011 15:21:05 -0700, Big Bad Bob
> > wrote:
>
>> On 04/08/11 03:18, flipper so wittily quipped:
>>> On Fri, 08 Apr 2011 01:40:44 -0700, Big Bad Bob
>>> > wrote:
>>>
>>>> On 04/06/11 20:38, flipper so wittily quipped:
>>>>>> I think you must be a myers-briggs 'SJ' (guess ESTJ) type, where I'm a
>>>>>> 'ENTP' type. You appear to see things in terms of details and direct
>>>>>> observations, whereas I look at the general case based on real world
>>>>>> experience and my understanding of the theories behind it all (besides
>>>>>> the details being boring).
>>>>>
>>>>> Uh huh. Well, if you aren't 'observing' the real world then what makes
>>>>> you think you've got a handle on it?
>>>>
>>>> It's a combination of lots of experience, extremely good insight,
>>>> incredibly high intelligence, and the attitude of a hacker.
>>>>
>>>> thanks for confirming my observation. keep in mind that some of us can
>>>> see the entire alphabet after only 2 letters. Others have to go through
>>>> all of the letters before it becomes clear. I'd rather not be bored by
>>>> the detail of the other 24, so I'll stop after 'B' and say "alphabet".
>>>> Occasionally there may be a missing letter, and the detail guy will pick
>>>> up on that. But it's not interesting enough for me to care about 1
>>>> letter missing. The 'alphabet' concept is sufficient.
>>>>
>>>
>>> I guess that explains why you haven't been seeing what's actually
>>> said, only 1/13 of the letters get through.
>>
>> more likely I'm just explaining things inadequately (or leaving out
>> explanations for things that appear intuitively obvious to me) for
>> someone that sees things the way you do. So the letters get through,
>> but I would view it as an explanation of a concept and not a set of details.
>
> Perhaps, but you'll never know till you read what's said because not
> everything that starts with AB is a rendition of the English alphabet,
> and it only seems that way to you because you don't bother to go past
> the AB.

uh, no. not even close.