[Robert Latest]

["Followup-To:" header set to sci.electronics.design.]
William Sommerwerck wrote:

I'm not sure that's right. My memory (which could be faulty) is that this
can be shown mathematically.

I'll ask around (I know a few people in high places) and see if I can get a
reference.

If you want real information, don't ask people in high places. Ask techs.

robert

[Robert Latest]

If you put in a FS 1 KHz tone, you get out a nearly FS 1KHz tone, DC that is
46 dB down, and 2 KHz that is 46 dB down. Run the output back through again,
and you get a nearly FS 1 KHz tone, DC that is still about 46 dB down, a 2
KHz tone that is about 46 dB down, and a 3 KHz tone that is about 92 dB
down.

Wouldn't you get 4 kHz (2nd harmonic of 2kHz) rather than 3 kHz?

robert

[Paul Stamler]

Dammit, I've got that paper around here *someplace*.

The paper I'm referring to is by an English author, I think not Reg
Williamson and I think not Self, showing the generation of higher harmonics
on the application of moderate amounts of feedback in a simple FET circuit
which produces only low-order harmonics without feedback. As the feedback is
increased the high harmonics get smaller; they're at their worst in
low-feedback circuits. The measurements were real, not simulations.

Meanwhile, as I looked for that $%^$# article, I found this:

www.ucop.edu/research/micro/98_99/98_074.pdf

It's a theoretical discussion of the generation of higher-order IM products
in feedback amps. The theory is supplemented by simulations, but
unfortunately not by real-world measurements, and the authors note that
their models are oversimplified. Still interesting reading as a possible
stimulus to further work. In their model FETs behave worse than BJTs,
tubes -- sometimes -- behave a bit better than FETs.

Meanwhile, can anyone help my blocked memory? Who the hell wrote that paper?

Peace,
Paul

[William Sommerwerck]

It does, because a stage which is audibly blameless
by itself may turn into a sonic disaster when it appears
a few hundred times in the signal path.

** Huh ??
A few HUNDRED times ???????
The colossal fool must be on LSD.

Mr. Dorsey is being only slightly hyperbolic.

Mixing boards use huge numbers of op amps. If you bounced a signal from one
track to another, it wouldn't be difficult to pass the signal through 50 to
100 gain stages.

[William Sommerwerck]

I do wish you'd glom onto a Crown K1. You really need to
hear this amplifier -- and run it through some blind tests.

What's the K1 like then ?

Something like The Second Coming?

No, more like something out of Revelations.

If there were a Fifth Horseman, "Grundge", that would be the K1. It's so
bad-sounding, you can hear what's wrong with it without directly comparing
it with anything else.

[Arny Krueger]

"Eeyore" wrote in message
...


"Scott Dorsey wrote

It does, because a stage which is audibly blameless by itself may turn
into a sonic disaster when it appears a few hundred times in the signal
path.

The EQ section alone on a Neve V series (and derivatives) has 18 op-amp
stages.

Can't find a schematic for that one, but I'm looking at the schematic of a
Neve 83022EQ which seems to be representative.

http://www.danalexanderaudio.com/nev...49/83022EQ.jpg

There are a ton of op amps, but they aren't all cascaded on the signal path.
For example, 16 op amps are in 4 state-variable filters each composed of 4
stages, plus a helper amplifier.

In actual use, the full bandwidth and amplitude of the output signal of the
equalizer rarely if ever flows through all 16 op amps.

The state variable filters are typically used as hi pass, lo pass, shelving,
peaking or nulling filters, so only a fraction of the audio band is affected
by each. When each parametric section's boost/cut control is centered as it
often is, very little of the output signal passes through them.

There are 5 op amps with gain either -1 or +1, cascaded across the top of
the schematic. They are always in the signal path of the eq. They each pass
the entire audio band. However, it looks like it may be possible for the
whole eq to be bypassed.

My analog parametric eqs include individual bypass switches for each
section, and a bypass the whole eq. I can see maybe 20 ops amps actually
interposed full-band and full-signal in a record/play signal path, but 100
seems like a reach.

I've done experiements where we built up a string of 20 unity and 10 dB
stages, using fairly primitive op amps like TL074s. No reliable detection
in level-matched, bias-controlled tests, using very clean sources, very
clean monitors, and a variety of listeners who were either audio engineers
and/or audiophiles, and thought they would hear a difference.

[Arny Krueger]

"Robert Latest" wrote in message
...

If you put in a FS 1 KHz tone, you get out a nearly FS 1KHz tone, DC that
is
46 dB down, and 2 KHz that is 46 dB down. Run the output back through
again,
and you get a nearly FS 1 KHz tone, DC that is still about 46 dB down, a
2
KHz tone that is about 46 dB down, and a 3 KHz tone that is about 92 dB
down.

Wouldn't you get 4 kHz (2nd harmonic of 2kHz) rather than 3 kHz?

You get both third and fourth. The 4th is another 46dB or so down, or about
138 dB down from the fundamental. I felt safe ignoring it. ;-)

I think that the third harmonic is actually due to the modulation of the DC
term from the first time through. The fourth harmonic is the second harmonic
of the second harmonic, of course.

[Arny Krueger]

"MooseFET" wrote in message
oups.com...

I tried my harmonic of the harmonic argument again. Sometimes it
works sometimes not.

It all comes out if you do the math, which involves a few simple trig
identities.

It also comes out if you simulate it in Matlab or Audition/CEP. I did my
simulation in CEP using Edit, Mix, Paste and appropriate choice of the mix
and modulate options.

The same basic technique can be used to create music with controlled amounts
of various orders of added nonlinear distortion. Here is worked-out
example:

http://www.pcabx.com/technical/nonlinear/

[Arny Krueger]

"Paul Stamler" wrote in message
...
Dammit, I've got that paper around here *someplace*.

The paper I'm referring to is by an English author, I think not Reg
Williamson and I think not Self, showing the generation of higher
harmonics
on the application of moderate amounts of feedback in a simple FET circuit
which produces only low-order harmonics without feedback. As the feedback
is
increased the high harmonics get smaller; they're at their worst in
low-feedback circuits. The measurements were real, not simulations.

Meanwhile, as I looked for that $%^$# article, I found this:

www.ucop.edu/research/micro/98_99/98_074.pdf

It's a theoretical discussion of the generation of higher-order IM
products
in feedback amps. The theory is supplemented by simulations, but
unfortunately not by real-world measurements, and the authors note that
their models are oversimplified.

Really? I see an article about sample-and-holds, and the like.

[Eeyore]

Arny Krueger wrote:

"Eeyore" wrote
"Scott Dorsey wrote

It does, because a stage which is audibly blameless by itself may turn
into a sonic disaster when it appears a few hundred times in the signal
path.

The EQ section alone on a Neve V series (and derivatives) has 18 op-amp
stages.

Can't find a schematic for that one, but I'm looking at the schematic of a
Neve 83022EQ which seems to be representative.

http://www.danalexanderaudio.com/nev...49/83022EQ.jpg

Yes at a casual glance it looks much the same.


There are a ton of op amps, but they aren't all cascaded on the signal path.
For example, 16 op amps are in 4 state-variable filters each composed of 4
stages, plus a helper amplifier.

In actual use, the full bandwidth and amplitude of the output signal of the
equalizer rarely if ever flows through all 16 op amps.

Depending on the cut and boost, the signal may be affected by all of them.

Graham

[Eeyore]

Arny Krueger wrote:

"Robert Latest" wrote in message

If you put in a FS 1 KHz tone, you get out a nearly FS 1KHz tone, DC that
is 46 dB down, and 2 KHz that is 46 dB down. Run the output back through
again, and you get a nearly FS 1 KHz tone, DC that is still about 46 dB
down, a
2 KHz tone that is about 46 dB down, and a 3 KHz tone that is about 92 dB
down.

Wouldn't you get 4 kHz (2nd harmonic of 2kHz) rather than 3 kHz?

You get both third and fourth. The 4th is another 46dB or so down, or about
138 dB down from the fundamental. I felt safe ignoring it. ;-)

I think that the third harmonic is actually due to the modulation of the DC
term from the first time through. The fourth harmonic is the second harmonic
of the second harmonic, of course.

Where does this DC term come from ?

Graham

[john jardine]

"Paul Stamler" wrote in message
...
Dammit, I've got that paper around here *someplace*.

Meanwhile, can anyone help my blocked memory? Who the hell wrote that
paper?

Peace,
Paul

Baxendall? in Wireless world magazine about 35 years ago.
Seem to remember the example was a diff amp pair. Article hinged on power
series expansions.

[Arny Krueger]

"Eeyore" wrote in message
...


Arny Krueger wrote:

"Eeyore" wrote
"Scott Dorsey wrote

It does, because a stage which is audibly blameless by itself may
turn
into a sonic disaster when it appears a few hundred times in the
signal
path.

The EQ section alone on a Neve V series (and derivatives) has 18 op-amp
stages.

Can't find a schematic for that one, but I'm looking at the schematic of
a
Neve 83022EQ which seems to be representative.

http://www.danalexanderaudio.com/nev...49/83022EQ.jpg

Yes at a casual glance it looks much the same.

Pretty typical for a 4-section parametric eq, plus/minus some details.

There are a ton of op amps, but they aren't all cascaded on the signal
path.
For example, 16 op amps are in 4 state-variable filters each composed of
4
stages, plus a helper amplifier.

In actual use, the full bandwidth and amplitude of the output signal of
the
equalizer rarely if ever flows through all 16 op amps.

Depending on the cut and boost, the signal may be affected by all of them.

No doubt, but it is not the same as every ounce of signal going through all
of them cascaded, no matter what.

And, the channel strips are not usually cascaded, either. This one nets out
to being like 5-6 stages cascaded full time, more if you use EFX.

[Scott Dorsey]

Mr.T MrT@home wrote:
"MooseFET" wrote in message
oups.com...
It isn't hard to end up with that many. 1 per band per channel plus a
few will get you to 20 without working at it. To get above 100, you
are talking about a serious amount of more signal processing.

100 op amps on parallel channels is a far different situation than 100 *ALL
in series* with the signal.
Of course in the real world the situation is somewhere in between those
extremes.

Pop the cover on an SSL 4000 some time...
--scott

--
"C'est un Nagra. C'est suisse, et tres, tres precis."

[Arny Krueger]

"Eeyore" wrote in message
...


Arny Krueger wrote:

"Robert Latest" wrote in message

If you put in a FS 1 KHz tone, you get out a nearly FS 1KHz tone, DC
that
is 46 dB down, and 2 KHz that is 46 dB down. Run the output back
through
again, and you get a nearly FS 1 KHz tone, DC that is still about 46
dB
down, a
2 KHz tone that is about 46 dB down, and a 3 KHz tone that is about 92
dB
down.

Wouldn't you get 4 kHz (2nd harmonic of 2kHz) rather than 3 kHz?

You get both third and fourth. The 4th is another 46dB or so down, or
about
138 dB down from the fundamental. I felt safe ignoring it. ;-)

I think that the third harmonic is actually due to the modulation of the
DC
term from the first time through. The fourth harmonic is the second
harmonic
of the second harmonic, of course.

Where does this DC term come from ?

A DC term is a natural consequence of a second order nonlinearity. Comes out
of the trig identity for X squared:

Sine squared(x) = 1/2 - 1/2 Cos (2x) = (1 - Cos (2x) ) /2

http://en.wikipedia.org/wiki/Trigono...ction_formulae

Please see "Power-reduction formulae" for second and third orders. As I
recall the CRC tables have them for several orders beyond 3. Or, you can
derive them from the formulae for orders 2 and 3.

[MooseFET]

On Aug 20, 10:51 pm, Les Cargill wrote:
MooseFET wrote:
On Aug 20, 6:13 pm, Mark wrote:
On Aug 20, 6:06 pm, Eeyore
wrote:

Mark wrote:
Eeyore wrote:
What is the case AIUI is that NFB can create 'new' (higher) harmonics that don't
exist with the open-loop situation. It's down to the maths of how feedback works.
And I am saying NFB CANNOT create new higher harmonics.
And it seems you are incorrect (at least when the amplifier having the feedback applied
has some non-linearity).
Graham
take something like crossover distortiuon for example...

No, I don't want crossover distortion.

How about thinking about a distortion that only adds, lets say the 2nd
harmonic to a sine wave. Think about what happens when that is
enclosed in a feedback loop. You take some of that second harmonic
from the output and feed it back into the input. The nonlinear
circuit takes the 2nd harmonic of the 2nd harmonic giving the forth
and sends that out the output. That forth comes back around and
around and around. A nonlinear cicrcuit that only made 2nd a harmonic
is now resulting in an infinite chain of frequencies.

in an open loop amp, crossover dist. creates lots of harmonics.

add neg feedback and they are all reduced. The high order ones are
not reduced AS MUCH as the low order ones, but they are certainly not
increased (assumming a proper design not on the verge of instability
and assuming the feedback componets themselves are linear, resistors
are usually linear for our purposes).

This is not correct. You have to have an extraordinarily large phase
margin to not have a boost in the harmonic near the gain crossover.

If G is the forward gain from the point where the distortion is made
to the output and H is the rest feedback the math looks like:

G /(1 + GH)

Here's the very ugly bit:

The distortion is often created in the output section making the G
part unity or nearly so. A stable servo loop can have a phase margin
of 30 degrees.

1/(1 + 1 * 1@(180-30)) = 1/(1 - 0.866 + j0.5)

= 1/(0.134 + j0.5)

Take ABS()

ABS(1/(0.134 + j0.5)) = 1/sqrt(0.134^2 + 0.5^2) = 1.93

Even though this amplifier is very stable, the feedback loop doubles
the amplitude of the harmonic near the gain crossover.

So for audio, put the gain crossover way out of band. Right?

That tends to happen if you have a high amount of feedback at the
normal audio frequencies. You want to put the gain crossover high and
use a large amount of feedback so it works out nicely.

[MooseFET]

On Aug 20, 11:08 pm, D from BC wrote:
[.....]
Cool...
Maybe call it a distortion loop. :P

+---------------------------------------+
| |
sine--summation-------nonlinear transfer (inverting)-+
|
Not completely containing a signal to cancel out the
nonlinear transfer. So some 2nd harmonic gets to pass through the
nonlinear transfer again to make...the 4th....and so and so on..
(IIRC that would be the harmonic generation sequence for a 2nd order
nonlinear transfer.)

Take 2 tone and then there's the intermodulation products.
What a painful thing to think about...

Now add some noise and follow it around. I'm sure your head will
explode. You will discover that the signal modulates the noise and
intermixes with it. The peak in the noise near the gain cross over
gets mixed down with the harmonics of the signal that also land
there. If you make many very accurate frequency measurements on the
signal after the signal has been through such a process, you will find
that there is an increased low frequency modulation of the signal.


Significant magnitudes???

If it can be measured it can be called significant. Someone will
care.


Cheerleader in electronics...
"2,4,6,8 what distortion do I hate."

D from BC

[Mark]

On Aug 21, 9:37 am, MooseFET wrote:
On Aug 20, 11:08 pm, D from BC wrote:
[.....]



Cool...
Maybe call it a distortion loop. :P

+---------------------------------------+
| |
sine--summation-------nonlinear transfer (inverting)-+
|
Not completely containing a signal to cancel out the
nonlinear transfer. So some 2nd harmonic gets to pass through the
nonlinear transfer again to make...the 4th....and so and so on..
(IIRC that would be the harmonic generation sequence for a 2nd order
nonlinear transfer.)

Take 2 tone and then there's the intermodulation products.
What a painful thing to think about...

Now add some noise and follow it around. I'm sure your head will
explode. You will discover that the signal modulates the noise and
intermixes with it. The peak in the noise near the gain cross over
gets mixed down with the harmonics of the signal that also land
there. If you make many very accurate frequency measurements on the
signal after the signal has been through such a process, you will find
that there is an increased low frequency modulation of the signal.

Significant magnitudes???

If it can be measured it can be called significant. Someone will
care.





Cheerleader in electronics...
"2,4,6,8 what distortion do I hate."

D from BC- Hide quoted text -

- Show quoted text -- Hide quoted text -

- Show quoted text -

Is this the article?

http://stereophile.com/news/10065/

Someone mentioned a perfect second order ONLY device that open loop
produces ONLY 2x. When you put neg feedback around it you could get
the "harmonic of the harmonic" i.e. 4th harmonic which wasn't there
before. OK maybe in this special case. But this is a theoretical
math excersize then, any practical device that has a second order non-
linearity will also have high order terms and the neg feedback will
reduce those.

If you start with a hypothetical perfect 2nd order device, I MIGHT be
ready to concede that neg feedback might produce some small level of
4th order that wasn't there before. Someone needs to simulate this
case.

This may be an interesting mental exersize, but it has very little
connection to actual practice. In practice using any REAL amplifer,
neg feedback REDUCES all the harmonics. (another exception someone
mentioned would be those harmonics near the gain crossover frequency
if the neg feedback causes the gain to peak a few dB then the harmonic
could also be increased a few dB. Again in paractice, this is well
above 20 kHz. If there is any large amount of peaking, then the
system is only marginally stable.

Neg feedback is your friend.

Mark



Mark

[Mark]

If you start with a hypothetical perfect 2nd order device, I MIGHT be
ready to concede that neg feedback might produce some small level of
4th order that wasn't there before. Someone needs to simulate this
case.



OK I ran the sim...yes you are correct adding neg feedback to a
perfect 2nd order device creates higher order harmonics 3rd 4th etc
that were not there before.

Some PSPICE code for those that want to play....

Neg Feedback Amp does neg feedback create high order distortion

..TRAN 1uS 10ms

*transient analysis sine wave
Vin 1 0 Sin(0 1 1KHz)

Eamp 2 0 poly(1) (1,2) 0 100 -10 ;with 100% neg feedback
*Eamp 2 0 poly(1) (1,0) 0 100 -10 ;with NO neg feedback

Rloadin 1 0 600
Rloadout 2 0 600

..probe

..end

Small amounts of feedback created the most distortion. As I increased
the closed loop gain, as expceted all the distortion levels were
reduced.

In most any real amplifier, there will be high order non-linearities
in the device and adding neg feedback will reduce them. (with the
exceptions near the crossover frequency noted in the previous post)

Thank you for the interesting observation.

Mark

[D from BC]

On Tue, 21 Aug 2007 07:43:10 -0700, Mark wrote:

On Aug 21, 9:37 am, MooseFET wrote:
On Aug 20, 11:08 pm, D from BC wrote:
[.....]



Cool...
Maybe call it a distortion loop. :P

+---------------------------------------+
| |
sine--summation-------nonlinear transfer (inverting)-+
|
Not completely containing a signal to cancel out the
nonlinear transfer. So some 2nd harmonic gets to pass through the
nonlinear transfer again to make...the 4th....and so and so on..
(IIRC that would be the harmonic generation sequence for a 2nd order
nonlinear transfer.)

Take 2 tone and then there's the intermodulation products.
What a painful thing to think about...

Now add some noise and follow it around. I'm sure your head will
explode. You will discover that the signal modulates the noise and
intermixes with it. The peak in the noise near the gain cross over
gets mixed down with the harmonics of the signal that also land
there. If you make many very accurate frequency measurements on the
signal after the signal has been through such a process, you will find
that there is an increased low frequency modulation of the signal.

Significant magnitudes???

If it can be measured it can be called significant. Someone will
care.





Cheerleader in electronics...
"2,4,6,8 what distortion do I hate."

D from BC- Hide quoted text -

- Show quoted text -- Hide quoted text -

- Show quoted text -

Is this the article?

http://stereophile.com/news/10065/

Someone mentioned a perfect second order ONLY device that open loop
produces ONLY 2x. When you put neg feedback around it you could get
the "harmonic of the harmonic" i.e. 4th harmonic which wasn't there
before. OK maybe in this special case. But this is a theoretical
math excersize then, any practical device that has a second order non-
linearity will also have high order terms and the neg feedback will
reduce those.

If you start with a hypothetical perfect 2nd order device, I MIGHT be
ready to concede that neg feedback might produce some small level of
4th order that wasn't there before. Someone needs to simulate this
case.

This may be an interesting mental exersize, but it has very little
connection to actual practice. In practice using any REAL amplifer,
neg feedback REDUCES all the harmonics. (another exception someone
mentioned would be those harmonics near the gain crossover frequency
if the neg feedback causes the gain to peak a few dB then the harmonic
could also be increased a few dB. Again in paractice, this is well
above 20 kHz. If there is any large amount of peaking, then the
system is only marginally stable.

Neg feedback is your friend.

Mark



Mark

Feedback does the job but like with most things in electronics...you
don't get something for nothing.
Usually something else gets fk'd when there's a large benefit.
So that's why there's some feedback bashing.
Trust nothing..
D from BC

[Scott Dorsey]

Mark wrote:

Neg feedback is your friend.

It absolutely is. However, in the 1970s it was regarded as a cure-all that
could fix all ills, and it's not. The resulting sonic issues were severe,
and the current backlash you see in the community against the use of feedback
is mostly a reaction to that. This is a shame, since feedback is a useful
tool.
--scott

--
"C'est un Nagra. C'est suisse, et tres, tres precis."

[William Sommerwerck]

Pop the cover on an SSL 4000 some time...

When I worked at Rupert Neve, I learned that SSL stood for "sure sounds
lousy".

[Paul Stamler]

Yes, the article was written by Peter Baxandall. I haven't been able to find
the article itself online, but Stereophile published a short summary he

http://stereophile.com/news/10065/

I've read Baxandall's article, and it's written with his usual thoroughness.
He explicitly stated that the absolute level of higher harmonics rose when
he introduced feedback, and noted that with 40dB of feedback the level of
5th harmonic was higher, on an absolute basis, than it was with no feedback.
At higher levels of feedback (on the order of 60dB) the levels of high
harmonics began to decrease again.

Thank you for pointers that got me to the right place, or at least its
vicinity!

Peace,
Paul

[William Sommerwerck]

If you start with a hypothetical perfect 2nd order device, I MIGHT be
ready to concede that neg feedback might produce some small level of
4th order that wasn't there before.

That isn't what we're talking about.

This may be an interesting mental exersize, but it has very little
connection to actual practice. In practice using any REAL amplifer,
neg feedback REDUCES all the harmonics.

True. The claim (which I made) was that the spectrum of the harmonics
changes.

[William Sommerwerck]

Small amounts of feedback created the most distortion.
As I increased the closed loop gain, as expected all the
distortion levels were reduced.

Okay, but what about the spectral distribution?

[Bob Myers]

"William Sommerwerck" wrote in message
. ..

To be sure, in the practical case, the open-loop
gain of the amplifier is non-linear, but even then
you can clearly create an amplifier employing
negative feedback which does NOT "create new
harmonics" to an appreciably greater degree than its
open-loop cousin.

How do you know that?

The question was whether or not negative feedback
NECESSARILY resulted in more distortion than operating
a comparable set-up in an "open loop" fashion. While
we have seen various mathematical treatments that show
how additional harmonics/distortions may be generated
via non-linearities, none have shown that the total distortion
is necesseraly greater in the negative-feedback case. On
the contrary, the math regarding the feedback case shows
how such distortion will actually be reduced in total.

Bob M.

[Kevin Aylward[_2_]]

Mark wrote:
On Aug 20, 2:30 pm, Eeyore
wrote:
Mark wrote:
Eeyore wrote:
There was part of a thread a while back about how adding negative
feedback can create higher order harmonic distortion products than
exist open-loop in an amplifier stage.

This premise is NOT correct. Do not believe everything you read on
the Internet.

Feedback done correctly ADDS nothing. Perhaps what you are
thinking about is that feedback is generally more effective at
reducing low order distortion compared to reducing high order
distortion. Feedback (implemented correctly) does not INCREASE
either form of distortion. It reduces them both.

I know it decreases overall THD numbers. I'm not one of those nuts
who's anti-NFB per se.

What is the case AIUI is that NFB can create 'new' (higher)
harmonics that don't exist with the open-loop situation. It's down
to the maths of how feedback works.

Graham

And I am saying NFB CANNOT create new higher harmonics.



Ho humm.... no.

Look, it is this, a 1st order approximation is:

Vo = aVipsin(wt) + b(VipSin(wt) )^2 ++...

This means Vo will have some 2nd after you expand the sin squared term. Now,
that 2nd harmonic of Vo added to the input as in when feedback is applied,
means that the net Vin to the amp is say, (Vp qSin(wt) + kSin(2wt))^2. Now
expand this and you will get a Sin(3wt) term.




--
Kevin Aylward

[Mark]

And I am saying NFB CANNOT create new higher harmonics.

Ho humm.... no.

Look, it is this, a 1st order approximation is:

Vo = aVipsin(wt) + b(VipSin(wt) )^2 ++...


Read the later posts.. I have conceeded the point.

It is an interesting observation but not very relevant to real world
audio amplifier circuits that are not ideal square law devices.

Mark

[Eeyore]

Mark wrote:

If you start with a hypothetical perfect 2nd order device, I MIGHT be
ready to concede that neg feedback might produce some small level of
4th order that wasn't there before. Someone needs to simulate this
case.

OK I ran the sim...yes you are correct adding neg feedback to a
perfect 2nd order device creates higher order harmonics 3rd 4th etc
that were not there before.

Some PSPICE code for those that want to play....

Neg Feedback Amp does neg feedback create high order distortion

.TRAN 1uS 10ms

*transient analysis sine wave
Vin 1 0 Sin(0 1 1KHz)

Eamp 2 0 poly(1) (1,2) 0 100 -10 ;with 100% neg feedback
*Eamp 2 0 poly(1) (1,0) 0 100 -10 ;with NO neg feedback

Rloadin 1 0 600
Rloadout 2 0 600

.probe

.end

Small amounts of feedback created the most distortion. As I increased
the closed loop gain, as expceted all the distortion levels were
reduced.

In most any real amplifier, there will be high order non-linearities
in the device

Why ?

Graham

[Eeyore]

Scott Dorsey wrote:

Mark wrote:

Neg feedback is your friend.

It absolutely is. However, in the 1970s it was regarded as a cure-all that
could fix all ills, and it's not. The resulting sonic issues were severe,
and the current backlash you see in the community against the use of feedback
is mostly a reaction to that. This is a shame, since feedback is a useful
tool.

Yes, as abused in the 70s, vast quantities of NFB were used in attempts to
correct significant non-linearities.

It seems to make a lot more sense to apply NFB in rather more moderate amounts to
a gain stage that's already quite linear.

Graham

[Eeyore]

William Sommerwerck wrote:

Pop the cover on an SSL 4000 some time...

When I worked at Rupert Neve

When and where was this ?

I was at Neve Melbourn myself for 3 years (1985-1988).

Graham

[Eeyore]

William Sommerwerck wrote:

If you start with a hypothetical perfect 2nd order device, I MIGHT be
ready to concede that neg feedback might produce some small level of
4th order that wasn't there before.

That isn't what we're talking about.

This may be an interesting mental exersize, but it has very little
connection to actual practice. In practice using any REAL amplifer,
neg feedback REDUCES all the harmonics.

True. The claim (which I made) was that the spectrum of the harmonics
changes.

And the spectrum is important.

Graham

[Eeyore]

Bob Myers wrote:

"William Sommerwerck" wrote

To be sure, in the practical case, the open-loop
gain of the amplifier is non-linear, but even then
you can clearly create an amplifier employing
negative feedback which does NOT "create new
harmonics" to an appreciably greater degree than its
open-loop cousin.

How do you know that?

The question was whether or not negative feedback
NECESSARILY resulted in more distortion than operating
a comparable set-up in an "open loop" fashion.

That's not what I asked.

I aked if overall NFB can create 'new' harmonics, and it's now clear from
responses here that it can. I'm curious about the effect of local NFB in this
respect too. Does linearising a single gain stage with e.g. emitter degeneration
do the same ?


While we have seen various mathematical treatments that show
how additional harmonics/distortions may be generated
via non-linearities, none have shown that the total distortion
is necesseraly greater in the negative-feedback case.

No, I wouldn't expect it to be *numerically* greater. That would make no sense.
Hiowever the ear responds not only to the quantity of distortion but it's
spectrum, with higher order harmonics sounding more unpleasant.

It's therefore entirely possible to have an amplifier with a numerically smaller
THD figure that actually sounds worse than an amplifier with higher THD.


On the contrary, the math regarding the feedback case shows
how such distortion will actually be reduced in total.

The headline x % THD is not actually especially helpful when being very
critical.

Graham

[Eeyore]

Mark wrote:

And I am saying NFB CANNOT create new higher harmonics.

Ho humm.... no.

Look, it is this, a 1st order approximation is:

Vo = aVipsin(wt) + b(VipSin(wt) )^2 ++...

Read the later posts.. I have conceeded the point.

It is an interesting observation but not very relevant to real world
audio amplifier circuits that are not ideal square law devices.

That's a simple proof of the effect. It's not as if gain stages with other
non-linearities won't be similarly affected.

Graham

[Karl Uppiano]

"Eeyore" wrote in message
...
There was part of a thread a while back about how adding negative feedback
can
create higher order harmonic distortion products than exist open-loop in
an
amplifier stage.

This made me think about the application of op-amps in audio generally.
Negative
feedback is used primarily to linearise the transfer function and is used
in
huge quantites as much as 80dB @ 1 kHz for example.

Since this amount of NFB is not required to provide an accurate gain
setting, it
struck me that it's somewhat counter productive. If instead the open-loop
transfer characteritic was made more linear by degeneration of the
open-loop
gain for example, when NFB is applied, the overall result should be
largely
similar (i.e. no worse) but would presumably also suffer less from the
creation
of these new distortion products .

There is a good analysis of a "blameless" amplifier he
http://www.dself.dsl.pipex.com/ampins/dipa/dipa.htm

Note that DC amplifiers of the type described here are remarkably similar to
the topology used in a typical op-amp.

This amplifier is optimized for a specific amount of feedback. Op-amps are
typically general-purpose devices that, while usually well-designed, trade
optimal performance for convenience (you program their operational transfer
function with feedback - hence the name "op-amp"). When used not too close
to their design limits, I think high quality op-amps can be quite acceptable
even for high-fidelity applications, but probably not for the "bleeding
edge" audiophile.

On the other hand, some of the esoteric circuits that attract audiophiles
are measurably inferior to an equivalent circuit that uses op-amps, so in
many cases there is more to it than mere audio fidelity.

[Scott Dorsey]

Eeyore wrote:

Yes, as abused in the 70s, vast quantities of NFB were used in attempts to
correct significant non-linearities.

Some of which (like crossover distortion) are not really solvable with
feedback.

It seems to make a lot more sense to apply NFB in rather more moderate amounts to
a gain stage that's already quite linear.

Yes, but then you need a gain stage that has plenty of extra gain, good
margins, and good linearity. That's quite a bit to ask for.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."

[Paul Stamler]

"Eeyore" wrote in message
...

It absolutely is. However, in the 1970s it was regarded as a cure-all
that
could fix all ills, and it's not. The resulting sonic issues were
severe,
and the current backlash you see in the community against the use of
feedback
is mostly a reaction to that. This is a shame, since feedback is a
useful
tool.

Yes, as abused in the 70s, vast quantities of NFB were used in attempts to
correct significant non-linearities.

It seems to make a lot more sense to apply NFB in rather more moderate
amounts to
a gain stage that's already quite linear.

It does, intuitively. But Baxandall's results suggest otherwise; if one
wishes to avoid high-order distortion components, one should either use a
lot of feedback or none at all.

I'd be very interested to see Baxandall's experiment (which I believe was
done using a JFET) repeated on other active devices such as MOSFETS, vacuum
tubes and of course bipolar transistors.

Peace,
Paul

[Eeyore]

Scott Dorsey wrote:

Eeyore wrote:

Yes, as abused in the 70s, vast quantities of NFB were used in attempts to
correct significant non-linearities.

Some of which (like crossover distortion) are not really solvable with
feedback.

It seems to make a lot more sense to apply NFB in rather more moderate amounts to
a gain stage that's already quite linear.

Yes, but then you need a gain stage that has plenty of extra gain, good
margins, and good linearity. That's quite a bit to ask for.

It's not actually *that* difficult as I keep saying.

Going back to the 60s/70s when transistors were actually quite expensive (I recall the
BC109 cost 6s/6d from hobby stores - that's 32p in decimal plus adding the inflation
makes it well over £1 or $2 in today's money) designers used all manner of tricks to
screw the last dB of gain from them, potentially at the cost of linearity.

With a typical general purpose low noise transistor costing around 2 cents in quantity
these days, there is absolutely no need to have to do that any more and linearity
should be the goal.

Graham

[Mr.T]

"Scott Dorsey" wrote in message
...
It isn't hard to end up with that many. 1 per band per channel plus a
few will get you to 20 without working at it. To get above 100, you
are talking about a serious amount of more signal processing.

100 op amps on parallel channels is a far different situation than 100
*ALL
in series* with the signal.
Of course in the real world the situation is somewhere in between those
extremes.

Pop the cover on an SSL 4000 some time...

Yes, but do you understand what I said, or are you just choosing to ignore
the difference between parallel circuits and series circuits?

MrT.

[Arny Krueger]

"Eeyore" wrote in message
...


Mark wrote:

If you start with a hypothetical perfect 2nd order device, I MIGHT be
ready to concede that neg feedback might produce some small level of
4th order that wasn't there before. Someone needs to simulate this
case.

OK I ran the sim...yes you are correct adding neg feedback to a
perfect 2nd order device creates higher order harmonics 3rd 4th etc
that were not there before.

Some PSPICE code for those that want to play....

Neg Feedback Amp does neg feedback create high order distortion

.TRAN 1uS 10ms

*transient analysis sine wave
Vin 1 0 Sin(0 1 1KHz)

Eamp 2 0 poly(1) (1,2) 0 100 -10 ;with 100% neg feedback
*Eamp 2 0 poly(1) (1,0) 0 100 -10 ;with NO neg feedback

Rloadin 1 0 600
Rloadout 2 0 600

.probe

.end

Small amounts of feedback created the most distortion. As I increased
the closed loop gain, as expceted all the distortion levels were
reduced.

In most any real amplifier, there will be high order non-linearities
in the device

Why ?

Electronic devices seem to tend to have exponential characteristics.