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[email protected] ethanw@ethanwiner.com is offline
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Default Slew rate and slew rate limiting

I wrote an addendum for my Audio Expert book about slew rate limiting, and thought peeps here might find it useful:

http://www.ethanwiner.com/slew_rate.htm
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Don Pearce[_3_] Don Pearce[_3_] is offline
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Default Slew rate and slew rate limiting

On Mon, 11 Feb 2013 11:26:37 -0800 (PST), wrote:

I wrote an addendum for my Audio Expert book about slew rate limiting, and thought peeps here might find it useful:

http://www.ethanwiner.com/slew_rate.htm

Good explanation, but another picture may make it even clearer. If you
show a plot of current rather than voltage for a slew rate limited
signal, it is the familiar flat-topped, clipped sine shape. This kind
of drives home the idea of limiting better than a voltage wave with
sloping sides.

d
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polymod polymod is offline
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Default Slew rate and slew rate limiting


wrote in message
...
I wrote an addendum for my Audio Expert book about slew rate limiting, and
thought peeps here might find it useful:

http://www.ethanwiner.com/slew_rate.htm


Thank you Ethan!
(btw, excellent article on "mastered For iTunes" in the February edition of
Recording)

Poly


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Mark Mark is offline
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Default Slew rate and slew rate limiting



I wrote an addendum for my Audio Expert book about slew rate limiting, and
thought peeps here might find it useful:




hmmmm.... it would be very interesting if someone could make a demo
of what various amounts of slew rate limiting sounds like...


Mark



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Les Cargill[_4_] Les Cargill[_4_] is offline
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Default Slew rate and slew rate limiting

Mark wrote:


I wrote an addendum for my Audio Expert book about slew rate limiting, and
thought peeps here might find it useful:




hmmmm.... it would be very interesting if someone could make a demo
of what various amounts of slew rate limiting sounds like...


Mark




Download libsndfile and write some 'C' code.



--
Les Cargill


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Arny Krueger[_5_] Arny Krueger[_5_] is offline
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Default Slew rate and slew rate limiting


"Mark" wrote in message
...


I wrote an addendum for my Audio Expert book about slew rate limiting,
and
thought peeps here might find it useful:




hmmmm.... it would be very interesting if someone could make a demo
of what various amounts of slew rate limiting sounds like...


You can find a circuit of a real-time adjustable slew rate limiter he

http://home.provide.net/~djcarlst/abx_slew.htm

This is part of a device we used to demonstrate ABX testing:

http://home.provide.net/~djcarlst/abx_coh.htm

Bottom line - slew rate has to be rediculously low to be audible with
real-world music. The exagerrated reports about the audibility of slew rate
limiting from the late 1970s were due reliance on sighted evaluations which
we now know to be invalid for determining the audibility of subtle
differences.





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Scott Dorsey Scott Dorsey is offline
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Default Slew rate and slew rate limiting

Mark wrote:

hmmmm.... it would be very interesting if someone could make a demo
of what various amounts of slew rate limiting sounds like...


It sounds like the Seventies.

It's interesting to note that slew-limiting (also going under the name
transient intermodulation distortion) isn't really a serious problem with
modern electronics any more. In great part that's the result of work by
Marshall Leach and Matti Otala back in the seventies when it was recognized
as a massive, massive problem.

When it sounds fine at low levels but it starts to get mushy as you turn
the gain up, slew limiting is apt to be the problem.
--scott

Oh, it _is_ still a huge problem with transducers, but what isn't?


--
"C'est un Nagra. C'est suisse, et tres, tres precis."
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Scott Dorsey Scott Dorsey is offline
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Default Slew rate and slew rate limiting

Arny Krueger wrote:

Bottom line - slew rate has to be rediculously low to be audible with
real-world music. The exagerrated reports about the audibility of slew rate
limiting from the late 1970s were due reliance on sighted evaluations which
we now know to be invalid for determining the audibility of subtle
differences.


Strange, I thought they were due to power amplifiers from major manufacturers
with slew rates only around 0.1V/microsecond. (At least two of these vendors
were also noted for amplifiers that burst into flame, as well, so clearly
stability concerns were not addressed well.)

After folks learned to stop using TV horizontal sweep transistors for audio
output stages, and working engineers learned that not all amplifier distortion
issues can be solved by adding more feedback, things got much better.
--scott


--
"C'est un Nagra. C'est suisse, et tres, tres precis."
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Don Pearce[_3_] Don Pearce[_3_] is offline
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Default Slew rate and slew rate limiting

On 12 Feb 2013 15:55:35 -0500, (Scott Dorsey) wrote:

Arny Krueger wrote:

Bottom line - slew rate has to be rediculously low to be audible with
real-world music. The exagerrated reports about the audibility of slew rate
limiting from the late 1970s were due reliance on sighted evaluations which
we now know to be invalid for determining the audibility of subtle
differences.


Strange, I thought they were due to power amplifiers from major manufacturers
with slew rates only around 0.1V/microsecond. (At least two of these vendors
were also noted for amplifiers that burst into flame, as well, so clearly
stability concerns were not addressed well.)

After folks learned to stop using TV horizontal sweep transistors for audio
output stages, and working engineers learned that not all amplifier distortion
issues can be solved by adding more feedback, things got much better.
--scott


Slew rate limiting doesn't happen in the power stage. It is due to the
input stage's inability to source and sink enough current to charge
and discharge the dominant pole cap around the voltage amplifier
stage. It current-limits on large HF signals. You can generally fix it
by increasing the emitter current in the long tail pair and decreasing
the dominant pole cap.

d
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Scott Dorsey Scott Dorsey is offline
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Default Slew rate and slew rate limiting

Don Pearce wrote:
On 12 Feb 2013 15:55:35 -0500, (Scott Dorsey) wrote:
Arny Krueger wrote:

Bottom line - slew rate has to be rediculously low to be audible with
real-world music. The exagerrated reports about the audibility of slew rate
limiting from the late 1970s were due reliance on sighted evaluations which
we now know to be invalid for determining the audibility of subtle
differences.


Strange, I thought they were due to power amplifiers from major manufacturers
with slew rates only around 0.1V/microsecond. (At least two of these vendors
were also noted for amplifiers that burst into flame, as well, so clearly
stability concerns were not addressed well.)

After folks learned to stop using TV horizontal sweep transistors for audio
output stages, and working engineers learned that not all amplifier distortion
issues can be solved by adding more feedback, things got much better.


Slew rate limiting doesn't happen in the power stage. It is due to the
input stage's inability to source and sink enough current to charge
and discharge the dominant pole cap around the voltage amplifier
stage. It current-limits on large HF signals. You can generally fix it
by increasing the emitter current in the long tail pair and decreasing
the dominant pole cap.


This is true. Sorry for my not being clear and piling a number of different
amplifier problems of the seventies together in one category.
--scott

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


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Don Pearce[_3_] Don Pearce[_3_] is offline
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Default Slew rate and slew rate limiting

On 12 Feb 2013 16:21:36 -0500, (Scott Dorsey) wrote:

Don Pearce wrote:
On 12 Feb 2013 15:55:35 -0500,
(Scott Dorsey) wrote:
Arny Krueger wrote:

Bottom line - slew rate has to be rediculously low to be audible with
real-world music. The exagerrated reports about the audibility of slew rate
limiting from the late 1970s were due reliance on sighted evaluations which
we now know to be invalid for determining the audibility of subtle
differences.

Strange, I thought they were due to power amplifiers from major manufacturers
with slew rates only around 0.1V/microsecond. (At least two of these vendors
were also noted for amplifiers that burst into flame, as well, so clearly
stability concerns were not addressed well.)

After folks learned to stop using TV horizontal sweep transistors for audio
output stages, and working engineers learned that not all amplifier distortion
issues can be solved by adding more feedback, things got much better.


Slew rate limiting doesn't happen in the power stage. It is due to the
input stage's inability to source and sink enough current to charge
and discharge the dominant pole cap around the voltage amplifier
stage. It current-limits on large HF signals. You can generally fix it
by increasing the emitter current in the long tail pair and decreasing
the dominant pole cap.


This is true. Sorry for my not being clear and piling a number of different
amplifier problems of the seventies together in one category.
--scott


And there ere plenty of problems. It wasn't until we had the serious
circuit analyses courtesy of folks like Doug Self that all of the
problems found solutions.

d
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Default Slew rate and slew rate limiting



And there ere plenty of problems. It wasn't until we had the serious
circuit analyses courtesy of folks like Doug Self that all of the
problems found solutions.

d


my point for bringing up a demo of what it sounds like is because slew
rate issues are a good example of something that is now easily
MEASURED at levels well below where it impacts the sound....

but there was a time when slew was not clearly identified as an issue
to be measured.....

to make progress (as was made with slew) the "golden ears" and the
"propeller head engineers" (thats me) have to work together to solve
these problems...

if you think you hear something, the name of the game is to identify
something measurable associated with what you (think you) hear so that
the engineers can go fix it.

Mark


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Trevor Trevor is offline
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Default Slew rate and slew rate limiting


"Mark" wrote in message
...
to make progress (as was made with slew) the "golden ears" and the
"propeller head engineers" (thats me) have to work together to solve
these problems...

if you think you hear something, the name of the game is to identify
something measurable associated with what you (think you) hear so that
the engineers can go fix it.


Nah the "golden ears" will never believe you can ever measure what they
*think* they can hear, and for the rest of us amplifier tech has already
surpassed the human auditory system capabilities (unlike speaker tech).
Which is not to say a bad amp can't still be made (and are) by those who try
hard enough :-(

Trevor.


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Default Slew rate and slew rate limiting


"Scott Dorsey" wrote in message
...
Arny Krueger wrote:

Bottom line - slew rate has to be rediculously low to be audible with
real-world music. The exagerrated reports about the audibility of slew
rate
limiting from the late 1970s were due reliance on sighted evaluations
which
we now know to be invalid for determining the audibility of subtle
differences.


Strange, I thought they were due to power amplifiers from major
manufacturers
with slew rates only around 0.1V/microsecond.


I don't think that there ever were any serious audio amplifiers any where
near that bad, at least not by the early 1970s.

My calculations show that it takes about 3.55 v/microsecond slew rate to
produce a low distoriton sine wave at 20 KHz and 20 volts rms (equivalent to
50 watts into 8 ohms).

Just about any even halfways competent amp in those days could do that.


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PStamler PStamler is offline
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Default Slew rate and slew rate limiting

On Tuesday, February 12, 2013 9:19:39 PM UTC-6, Arny Krueger wrote:

My calculations show that it takes about 3.55 v/microsecond slew rate to

produce a low distoriton sine wave at 20 KHz and 20 volts rms (equivalent to

50 watts into 8 ohms).



Just about any even halfways competent amp in those days could do that.


That would imply that an amplifier putting out 7.75V rms (+20dBu) would need a slew rate of only 1.38 V/us to produce a low-distortion 20kHz sine wave..

Samuel Groner's measurements of opamps suggest otherwise.

Arny, I think you're assuming that if an amplifier isn't actually slewing, it's okay -- in other words, that slewing is an all-or-nothing phenomenon. Groner's tests suggest, instead, that well before the point of overt slewing, the misbehavior of the voltage amplifier stage is producing real and measurable distortion.

And by the way -- in the 70s we saw a lot of consoles with LM301 opamps, which had abysmal slew rates under usual operating conditions. And those consoles sounded like crap, too.

Peace,
Paul


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Scott Dorsey Scott Dorsey is offline
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Default Slew rate and slew rate limiting

Mark wrote:

to make progress (as was made with slew) the "golden ears" and the
"propeller head engineers" (thats me) have to work together to solve
these problems...

if you think you hear something, the name of the game is to identify
something measurable associated with what you (think you) hear so that
the engineers can go fix it.


Amen!
--scott

--
"C'est un Nagra. C'est suisse, et tres, tres precis."
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Scott Dorsey Scott Dorsey is offline
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Default Slew rate and slew rate limiting

Arny Krueger wrote:
My calculations show that it takes about 3.55 v/microsecond slew rate to
produce a low distoriton sine wave at 20 KHz and 20 volts rms (equivalent to
50 watts into 8 ohms).

Just about any even halfways competent amp in those days could do that.


There were lots of not-halfways competent amps out there at the time.
Lots of people out there developed a great distaste for transistors,
believing that transistors all sounded bad, when it fact it was just
the Dynaco ST120/ Phase Linear / SWTPC amplifier that sounded bad.

If you ever get a chance to measure slew rate on those SWTPCs or on some
of the bigger Phase Linears, you'll be horrified.

I think you forget just how bad things used to be, and in terms of
amplifier sound quality things went _down_ in the seventies as we
made the transition from tubes to solid state while trying to design
circuits with the same capacitively-coupled techniques.

We're still dealing with the fallout today. There is a lot of misinformation
floating around in the audiophile world about "solid state sound" which was
in fact nearly true at one time but is effectively bunkum today.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."
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Scott Dorsey Scott Dorsey is offline
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Default Slew rate and slew rate limiting

PStamler wrote:

And by the way -- in the 70s we saw a lot of consoles with LM301 opamps, wh=
ich had abysmal slew rates under usual operating conditions. And those cons=
oles sounded like crap, too.=20


Although... surprisingly enough Studer made some consoles using LM301s that
actually sounded very good. I boggled when I discovered those things were
full of 301s. Then I looked at the schematics and noticed that almost all
of them were working at unity gain, or maybe a gain of five or ten at most...
--scott

--
"C'est un Nagra. C'est suisse, et tres, tres precis."
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Default Slew rate and slew rate limiting

On 2/12/2013 10:19 PM, Arny Krueger wrote:

I don't think that there ever were any serious audio amplifiers any where
near that bad, at least not by the early 1970s.


That was the golden age of golen ears. Their high end amplifiers were
indeed pretty good, but they were the ones who pointed out the problem
in the "popular" amplifiers of the day. The San Francisco Lunatic Audio
Fringe was instrumental in defining the problem.



--
For a good time, call http://mikeriversaudio.wordpress.com
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William Sommerwerck William Sommerwerck is offline
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Default Slew rate and slew rate limiting

We're still dealing with the fallout today. There is a lot of misinformation
floating around in the audiophile world about "solid state sound" which
was in fact nearly true at one time but is effectively bunkum today.


Perhaps the biggest piece of misinformation is the belief that because modern
solid-state amplifiers are good, solid-state amplifiers have always been good.

The following is an blatant plug for a friend's products...

I find it interesting that both Stereophile and Abso!ute Sound both recommend
the Parasound A21 (which I own) as one of the best amplifiers available,
despite its modest price (for an amp that can dump 300 W/ch into an 8-ohm
load).



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hank alrich hank alrich is offline
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Default Slew rate and slew rate limiting

Scott Dorsey wrote:

PStamler wrote:

And by the way -- in the 70s we saw a lot of consoles with LM301 opamps, wh=
ich had abysmal slew rates under usual operating conditions. And those cons=
oles sounded like crap, too.=20


Although... surprisingly enough Studer made some consoles using LM301s that
actually sounded very good. I boggled when I discovered those things were
full of 301s. Then I looked at the schematics and noticed that almost all
of them were working at unity gain, or maybe a gain of five or ten at most...
--scott


The studio board we built at onion audio used LM301AN's, the military
version, which could run at a higher supply voltage, for buffer amps. We
ran them just under their max. v. spec. The board sounded great.

--
shut up and play your guitar * http://hankalrich.com/
http://hankandshaidrimusic.com/
http://www.youtube.com/walkinaymusic
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Don Pearce[_3_] Don Pearce[_3_] is offline
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Default Slew rate and slew rate limiting

On Tue, 12 Feb 2013 22:16:14 -0800 (PST), PStamler
wrote:

On Tuesday, February 12, 2013 9:19:39 PM UTC-6, Arny Krueger wrote:

My calculations show that it takes about 3.55 v/microsecond slew rate to

produce a low distoriton sine wave at 20 KHz and 20 volts rms (equivalent to

50 watts into 8 ohms).



Just about any even halfways competent amp in those days could do that.


That would imply that an amplifier putting out 7.75V rms (+20dBu) would need a slew rate of only 1.38 V/us to produce a low-distortion 20kHz sine wave.

Samuel Groner's measurements of opamps suggest otherwise.

Arny, I think you're assuming that if an amplifier isn't actually slewing, it's okay -- in other words, that slewing is an all-or-nothing phenomenon. Groner's tests suggest, instead, that well before the point of overt slewing, the misbehavior of the voltage amplifier stage is producing real and measurable distortion.

And by the way -- in the 70s we saw a lot of consoles with LM301 opamps, which had abysmal slew rates under usual operating conditions. And those consoles sounded like crap, too.

Peace,
Paul


It is certainly fair to say that if it isn't slew-rate limiting, then
slew rate isn't a problem. Slew rate limiting is the same as any kind
of limiting. It's doing it, or it isn't. There's plenty of scope for
other stuff to be wrong of course.

d
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Default Slew rate and slew rate limiting


"PStamler" wrote in message
...
On Tuesday, February 12, 2013 9:19:39 PM UTC-6, Arny Krueger wrote:

My calculations show that it takes about 3.55 v/microsecond slew rate to
produce a low distoriton sine wave at 20 KHz and 20 volts rms (equivalent
to
50 watts into 8 ohms).


Just about any even halfways competent amp in those days could do that.


That would imply that an amplifier putting out 7.75V rms (+20dBu) would
need a slew rate of only 1.38 V/us to produce a low-distortion 20kHz sine
wave.


My spread sheet says 1.381737, so check!

Samuel Groner's measurements of opamps suggest otherwise.


I have his 2009 book right here and I don't see that. Page number?

For lurkers, it is a freebie download that you can probably search for and
find.

Arny, I think you're assuming that if an amplifier isn't actually slewing,
it's okay -- in other words, that slewing is an all-or-nothing phenomenon.
Groner's tests suggest, instead, that well before the point of overt
slewing, the misbehavior of the
voltage amplifier stage is producing real and measurable distortion.


Your conclusion seems to be several miles west of what I actually said. ;-)

At this point we know that "real and measurable distortion" can be
fantastically low compared to what it takes to have flawless SQ.

And by the way -- in the 70s we saw a lot of consoles with LM301 opamps,
which had abysmal slew rates under usual operating conditions. And those
consoles sounded like crap, too.


The performance of a 301 can be improved to about 0.88 v/uSec which gives
you a 5 volt RMS signal at 20 KHz. Not +20 but a pretty useful signal
level, especially if this is a consumer device and reference level is -10.
These days there is a fair amount of pro stuff that only goes up that high.
It's only a few dB shy of +20.

I think that those consoles may have sounded like carp but SR wasn't the
reason why if the designers knew what they were doing.

http://www.ti.com/lit/ds/snosbs0c/snosbs0c.pdf page 7


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Les Cargill[_4_] Les Cargill[_4_] is offline
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Default Slew rate and slew rate limiting

hank alrich wrote:
Scott Dorsey wrote:

PStamler wrote:

And by the way -- in the 70s we saw a lot of consoles with LM301 opamps, wh=
ich had abysmal slew rates under usual operating conditions. And those cons=
oles sounded like crap, too.=20


Although... surprisingly enough Studer made some consoles using LM301s that
actually sounded very good. I boggled when I discovered those things were
full of 301s. Then I looked at the schematics and noticed that almost all
of them were working at unity gain, or maybe a gain of five or ten at most...
--scott


The studio board we built at onion audio used LM301AN's, the military
version, which could run at a higher supply voltage, for buffer amps.



That sounds like the critical issue then - it's an impedance
lower-er, not a gain-add-er-er.


We
ran them just under their max. v. spec. The board sounded great.


Clipping* parts are clipping.

*realizing of course that they not have actually been clipping, but
there was distortion based in abuse of the parts on the basis of
expected Vout....

--
Les Cargill
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Default Slew rate and slew rate limiting

Scott Dorsey wrote:
Arny Krueger wrote:
My calculations show that it takes about 3.55 v/microsecond slew rate to
produce a low distoriton sine wave at 20 KHz and 20 volts rms (equivalent to
50 watts into 8 ohms).

Just about any even halfways competent amp in those days could do that.


There were lots of not-halfways competent amps out there at the time.
Lots of people out there developed a great distaste for transistors,
believing that transistors all sounded bad, when it fact it was just
the Dynaco ST120/ Phase Linear / SWTPC amplifier that sounded bad.

If you ever get a chance to measure slew rate on those SWTPCs or on some
of the bigger Phase Linears, you'll be horrified.

I think you forget just how bad things used to be, and in terms of
amplifier sound quality things went _down_ in the seventies as we
made the transition from tubes to solid state while trying to design
circuits with the same capacitively-coupled techniques.


So it's the designs that were bad, not the parts? Something I
had forgotten is that the entry of solid state into consumer
electronics was more driven by market-seeking from Fairchild
and folks like NatSemi ( teh Fairchildren ) after the space
thing cooled off.

SFAIK, NASAians are and always have been obsessively
spec-driven, so I am wondering how part quality could
have dropped when the transition happened ( becoming
price driven perhaps?)

IOW, it was bad slew rate because the poor three-legger
was charging a cap? Hoo boy...

We're still dealing with the fallout today. There is a lot of misinformation
floating around in the audiophile world about "solid state sound" which was
in fact nearly true at one time but is effectively bunkum today.
--scott


--
Les Cargill


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Default Slew rate and slew rate limiting

Les Cargill wrote:

hank alrich wrote:
Scott Dorsey wrote:

PStamler wrote: And by the way -- in the 70s
we saw a lot of consoles with LM301 opamps, wh= ich had abysmal slew
rates under usual operating conditions. And those cons= oles sounded
like crap, too.=20

Although... surprisingly enough Studer made some consoles using LM301s
that actually sounded very good. I boggled when I discovered those
things were full of 301s. Then I looked at the schematics and noticed
that almost all of them were working at unity gain, or maybe a gain of
five or ten at most... --scott


The studio board we built at onion audio used LM301AN's, the military
version, which could run at a higher supply voltage, for buffer amps.



That sounds like the critical issue then - it's an impedance
lower-er, not a gain-add-er-er.


We
ran them just under their max. v. spec. The board sounded great.


Clipping* parts are clipping.

*realizing of course that they not have actually been clipping, but
there was distortion based in abuse of the parts on the basis of
expected Vout....


Board was clean and very quiet. It worked, very well. API pres, EQ's,
summing amps, and some of the limiters.

--
shut up and play your guitar * http://hankalrich.com/
http://hankandshaidrimusic.com/
http://www.youtube.com/walkinaymusic
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Scott Dorsey Scott Dorsey is offline
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Les Cargill wrote:

So it's the designs that were bad, not the parts? Something I
had forgotten is that the entry of solid state into consumer
electronics was more driven by market-seeking from Fairchild
and folks like NatSemi ( teh Fairchildren ) after the space
thing cooled off.


There was some of each going on. Also, the transition to solid state
made it possible to have truly enormous amounts of power, and people
went berserk over that. The market became very focussed on high power
operation, often leading to amplifiers with lots of crossover distortion
because the designs were optimized for getting great numbers at high power.

SFAIK, NASAians are and always have been obsessively
spec-driven, so I am wondering how part quality could
have dropped when the transition happened ( becoming
price driven perhaps?)


The transition from tubes to solid state was a very dramatic change in
philosophy. A lot of the simple and reasonable techniques that worked well
in the tube era fell apart in the solid state world.

You build a tube amp, you have an input transformer, a tube stage, a
coupling capacitor, another tube stage, an output transformer. The
impedances and voltages are high, so you can use paper or film caps
for coupling.

In the solid state world to do the same thing the impedances and voltages
are much lower, which drives you to electrolytics for coupling.
A lot of the simple straightforward designs wind up having a lot of
side-effects.

Now, careful engineering managed to work around those side-effects, in
part because transistors are tiny and cheap so it suddenly becomes easy
to just add a constant current source with another transistor rather than
using a resistor to fake a current source (a tube era technique that works well
when you have lots of voltage to throw away, but not so well with a 12V
rail).

It was worse for the audio guys than the RF guys by a long shot, since the
RF guys always keep a tight handle on distortion products (and usually
filter them out if they can) so they at least know where they are. Lots of
audio designers just didn't know what their distortion spectrum was until
the age of FFT analyzers....
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."
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Chuck[_10_] Chuck[_10_] is offline
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Default Slew rate and slew rate limiting

On 13 Feb 2013 07:38:00 -0500, (Scott Dorsey) wrote:

Arny Krueger wrote:
My calculations show that it takes about 3.55 v/microsecond slew rate to
produce a low distoriton sine wave at 20 KHz and 20 volts rms (equivalent to
50 watts into 8 ohms).

Just about any even halfways competent amp in those days could do that.


There were lots of not-halfways competent amps out there at the time.
Lots of people out there developed a great distaste for transistors,
believing that transistors all sounded bad, when it fact it was just
the Dynaco ST120/ Phase Linear / SWTPC amplifier that sounded bad.

If you ever get a chance to measure slew rate on those SWTPCs or on some
of the bigger Phase Linears, you'll be horrified.

I think you forget just how bad things used to be, and in terms of
amplifier sound quality things went _down_ in the seventies as we
made the transition from tubes to solid state while trying to design
circuits with the same capacitively-coupled techniques.

We're still dealing with the fallout today. There is a lot of misinformation
floating around in the audiophile world about "solid state sound" which was
in fact nearly true at one time but is effectively bunkum today.
--scott



The SWTPCs were also inherently unstable and went into hf oscillation
even when just driving resistor loads. I used to modify them by
adding caps to the driver stage so they would even be functional.
Does anyone know of a worse designed commercial amps than these? The
Dynaco ST120 had a flammable circut board that was stocked with carbon
resistors. One amp of a customer of ours burst into flames and was
sitting under some curtains. His house burned down. On the bright
side, with amps failing right and left back then, being an audio
technician was far more lucrative occupation than today. Chuck
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Scott Dorsey Scott Dorsey is offline
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Default Slew rate and slew rate limiting

Chuck wrote:
The SWTPCs were also inherently unstable and went into hf oscillation
even when just driving resistor loads. I used to modify them by
adding caps to the driver stage so they would even be functional.
Does anyone know of a worse designed commercial amps than these? The
Dynaco ST120 had a flammable circut board that was stocked with carbon
resistors. One amp of a customer of ours burst into flames and was
sitting under some curtains. His house burned down. On the bright
side, with amps failing right and left back then, being an audio
technician was far more lucrative occupation than today. Chuck


The phase linears were the best, though. I remember going to a concert
at the Little Five Points Pub in Atlanta when the monitor amp spewed
a three-foot jet of flame out the top. The audience cheered!
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."
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gregz gregz is offline
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Default Slew rate and slew rate limiting

Chuck wrote:
On 13 Feb 2013 07:38:00 -0500, (Scott Dorsey) wrote:

Arny Krueger wrote:
My calculations show that it takes about 3.55 v/microsecond slew rate to
produce a low distoriton sine wave at 20 KHz and 20 volts rms (equivalent to
50 watts into 8 ohms).

Just about any even halfways competent amp in those days could do that.


There were lots of not-halfways competent amps out there at the time.
Lots of people out there developed a great distaste for transistors,
believing that transistors all sounded bad, when it fact it was just
the Dynaco ST120/ Phase Linear / SWTPC amplifier that sounded bad.

If you ever get a chance to measure slew rate on those SWTPCs or on some
of the bigger Phase Linears, you'll be horrified.

I think you forget just how bad things used to be, and in terms of
amplifier sound quality things went _down_ in the seventies as we
made the transition from tubes to solid state while trying to design
circuits with the same capacitively-coupled techniques.

We're still dealing with the fallout today. There is a lot of misinformation
floating around in the audiophile world about "solid state sound" which was
in fact nearly true at one time but is effectively bunkum today.
--scott



The SWTPCs were also inherently unstable and went into hf oscillation
even when just driving resistor loads. I used to modify them by
adding caps to the driver stage so they would even be functional.
Does anyone know of a worse designed commercial amps than these? The
Dynaco ST120 had a flammable circut board that was stocked with carbon
resistors. One amp of a customer of ours burst into flames and was
sitting under some curtains. His house burned down. On the bright
side, with amps failing right and left back then, being an audio
technician was far more lucrative occupation than today. Chuck


My tigersaurises, or however you spell them, had those big bias resistors
that ran hot. Eventually had to redo them, and added fan.

With all the paranoia about SRL, one way to deal with it was to put a low
pass on the signal before amplification.

After redoing an overly complex Crown amplifier, I watched as it lit up. I
hope I never go back to fixing that beast.

Greg


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Default Slew rate and slew rate limiting

Worst case signal for slew rate tests is to jangle keys in front of a high-bandwidth microphone. Of course once it hits an a/d converter it gets band-limited to 20k so after that there is no issue.


Bob

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Trevor Trevor is offline
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Default Slew rate and slew rate limiting


wrote in message
...
Worst case signal for slew rate tests is to jangle keys in front of a
high-bandwidth microphone. Of course once it hits an a/d converter it gets
band-limited to 20k so after that there is no issue.


Not with a 96 or 192k sample rate it doesn't, but you'd have to look hard to
find an amp that can't handle the frequency response of a microphone without
slew problems these days anyway.

Trevor.


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PStamler PStamler is offline
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Default Slew rate and slew rate limiting

On Thursday, February 14, 2013 12:23:37 AM UTC-6, Trevor wrote:
wrote in message

...

Worst case signal for slew rate tests is to jangle keys in front of a


high-bandwidth microphone. Of course once it hits an a/d converter it gets


band-limited to 20k so after that there is no issue.




Not with a 96 or 192k sample rate it doesn't, but you'd have to look hard to

find an amp that can't handle the frequency response of a microphone without

slew problems these days anyway.


Which is a result of people paying attention to the issue back in the 70s and 80s.

Part of the problem back in the way-back was that this was the beginning of the transformerless era; people were champing at the bit to eliminate the transformers at the outputs of mics and the inputs of preamps which colored the low frequencies (and took up space and added weight and cost more money). What they didn't necessarily realize was that the ultrasonic bandlimiting the transformers added helped prevent nasty slewing issues and IM distortion.

By the way, my apologies for an error: the tests that showed high-frequency distortion in opamps that were not into actual slewing came from Walt Jung, not Sam Groner. They're in "Audio IC Op Amp Applications"; unfortunately, my copy is buried at the moment, so I can't get the page numbers.

Oh, and anyone who wants the Groner opamp test results can download them (the file is about 35 megs) from:

http://tinyurl.com/opamptests

Peace,
Paul
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swanny swanny is offline
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Default Slew rate and slew rate limiting

On 14/02/2013 7:54 AM, Scott Dorsey wrote:
Chuck wrote:
The SWTPCs were also inherently unstable and went into hf oscillation
even when just driving resistor loads. I used to modify them by
adding caps to the driver stage so they would even be functional.
Does anyone know of a worse designed commercial amps than these? The
Dynaco ST120 had a flammable circut board that was stocked with carbon
resistors. One amp of a customer of ours burst into flames and was
sitting under some curtains. His house burned down. On the bright
side, with amps failing right and left back then, being an audio
technician was far more lucrative occupation than today. Chuck


The phase linears were the best, though. I remember going to a concert
at the Little Five Points Pub in Atlanta when the monitor amp spewed
a three-foot jet of flame out the top. The audience cheered!
--scott


I remember mixing a gig in the 70's where two of those things shot
flames, and the W bins caught fire. Real pyrotechnics!

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Default Slew rate and slew rate limiting

Agreed that there are plenty of good choices out there with slew-rate to spare. It's a good idea to over-design the slew-rate, because the performance of most op-amps begins to degrade as you approach the hard slew-rate limit.. This occurs because the gain of the input stage shows some soft clipping behavior and therefore the loop gain is dynamically lowered during a high-slope input event that almost but not quite hits the limit. This shows up in the the+n versus frequency plot on the data sheet. I wouldn't call this a major factor and its audibility is questionable, but its common practice in the design world to over-specify slew-rate compared with the simple 2*PI*A*F calculation.

Bob


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Default Slew rate and slew rate limiting

On Thursday, February 14, 2013 5:39:26 AM UTC-6, wrote:
Agreed that there are plenty of good choices out there with slew-rate to spare. It's a good idea to over-design the slew-rate, because the performance of most op-amps begins to degrade as you approach the hard slew-rate limit. This occurs because the gain of the input stage shows some soft clipping behavior and therefore the loop gain is dynamically lowered during a high-slope input event that almost but not quite hits the limit. This shows up in the the+n versus frequency plot on the data sheet. I wouldn't call this a major factor and its audibility is questionable, but its common practice in the design world to over-specify slew-rate compared with the simple 2*PI*A*F calculation.



And with many high-quality opamps available, it has become quite easy to over-spec the slew rate.

Peace,
Paul
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