Feedback in audio esp wrt op-amps.

[Mr.T]

Pull the cover on an SSL 4000 and look inside. Or just look at the
schematic.

You keep saying that as if it proves something all by itself ???

Well, what comes out sure sounds a whole lot different than what went in,
so I would suspect a measurement would indicate that too.

Ah, what you "suspect" may be wrong too. That's why I prefer to actually
measure these things.

Bob Pease has a wonderful classroom demo in which he shows a 1 KC square
wave through a fairly clean op-amp stage, then through a hundred, and
then through a huge board with a thousand op-amps on it. A small error
gets exaggerated substantially.

And many people have put a kHz square wave into a SSL console without any
problem, or they would simply use a Behringer instead!

MrT.

 

[Scott Dorsey]

You keep saying that as if it proves something all by itself ???

Well, you take your finger from the input... then you move it stage by
stage until you get to the tape send... then you move it stage by stage
from the tape return to the 2-buss... and count how many op amps you
pass through. Hint: it's more than fifty.

Ah, what you "suspect" may be wrong too. That's why I prefer to actually
measure these things.

It would be interesting to measure, but thank God I don't have one sitting
around here.

And many people have put a kHz square wave into a SSL console without any
problem, or they would simply use a Behringer instead!

I have heard plenty of stuff come out of SSL consoles that sure sounded like
square waves, but that's an operator issue.

Still, the 1KC square wave test is a hell of a good qualitative measurement.
You get to see any stability issues and any frequency response variations
very quickly on the scope.
--scott

 

[D from BC]

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

The idea that you can 'get away' with sloppy circuitry for replay because the
source was in some way 'impaired' is totally false.

I don't think anybody proposed "sloppy" circuitry for replay. The point is
that studio audio gear is just solid, reliable, conventional good audio
stuff (none of that high-end low-oxygen power cord crap). Plenty of opamps,
plenty of NFB, plenty of digital processing, plenty of all the things that
high-enders loathe.

Since the recording studio already did 90% of the work of completely
destroying the audio signal beyond repair, it doesn't matter how much your
home audio gear adds to that.

Sometimes when I hear the golden earers talk I'm surprised that I can make
out any music at all when listening with my Cantons fed from an old Sony amp
through particularly oxygen-rich cables.

robert

Designing audio playback gear that has PPM distortion levels, and
noise so low it's dominated by the source material and room
background, is now so easy it's not worth discussing. Just grab some
National appnotes and opamp datasheets. Because it's so easy to make
measurable noise and distortion vanishingly small, the audiophools
have had to move on to debating the unmeasurable, in long threads with
no content.

John

The inner ear probably has distortion and there's probably auditory
brain distortion.
Since technology has superceded some biological functions, it maybe
that audiophools should turn to improving inner ear and auditory brain
distortion.

A brief history of hearing...

Cavemen listening for predators....

"Was that the sound of a wolf on moss?"
"Nah..sounds more like tiger on grass?"
(Possible the first audiophools.)

To spoken language...

"Hello...hello...can you hear me now?...How about now?...Wait let me
step out in the open..How about now?"

To mp3's downloaded off the internet....

"I'm a black gangsta...yeah man...black gangsta...motherfkr...yo
man...Boom Boom ka boom...Got me an Uzi cause I smoozee...Cause I
boozy.. yeah ... I'm a black gangsta."

Is there really a need for perfect audio... I don't think so..
D from BC

 

[Mr.T]

It would be interesting to measure, but thank God I don't have one sitting
around here.

Still, the 1KC square wave test is a hell of a good qualitative measurement.
You get to see any stability issues and any frequency response variations
very quickly on the scope.

So when you've actually done it, get back to us.

MrT.

 

[Eeyore]

Unless they have to play with tempered instruments.

Then it's the instruments with the equal temperament that are excluded from such
performances.

Graham

 

[Eeyore]

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

Original groups replaced

Designing audio playback gear that has PPM distortion levels, and
noise so low it's dominated by the source material and room
background, is now so easy it's not worth discussing.

Simply not true. But seemingly believed by some people who should know better and
who apparently foolishly think audio is entirely trivial these days.

Power amplifiers in particular still regularly have THD levels as high as 1000 ppm
(and more sometimes). And that's normally measured at full power which is a
relatively easy figure to obtain. THD @ real listening levels of around the 100mW to
1W mark may be considerably higher.

Just grab some National appnotes and opamp datasheets. Because it's so easy to
make
measurable noise and distortion vanishingly small,

What's the ppm THD of a National IC power amplifier ? Certainly not in the single
digits ppm !

the audiophools have had to move on to debating the unmeasurable, in long threads
with
no content.

More uninformed ignorance.

The difference between the audiophools who argue for such nonsense voodoo as
cryogenically treated and 'broken in' cables and the like is quite different from
this discussion which is based in sound science.

Graham

 

[Arny Krueger]

I disagree with this.

An overtone is the natural resonances of a sound source.

....above the fundamental, although the term "first overtone" is in use.

That is, the natural modes of vibration. I disagree that
the definition has anything to do with integral multiples
of a fundamental.

The set of tones that are "..of greater frequency.." includes both harmonic
and inharmonic tones. Natural resonances of a sound source may be harmonic
or inharmonic, depending on the source.

It just so happens that overtones are
often quite close to harmonics. For example, the 2nd
overtone of a drum is 2.4 times its fundamental (given by
the roots of Jo, the Bessel function).

Probably not a happenstance. Some objects vibrate quite nonlinearly, and
thus produce inharmonic overtones. Percussion instruments seem to be more
prone to this sort of thing.

For example, even guitar "harmonics" are not harmonics.
The string does not vibrate exactly at a length set by
the nut and bridge. The string does not move until it is
a little away from its fulcrums. This is aproximinatly a
fixed length, that depends on the string thichness/mass
density/stiffness. A first order correction to this is to
angle the bridge so that the thinner strings are shorter
than the thicker strings. The net effect is that string
overtones are not integral multiples of a fundamental as
halving the string length, does not half the actual
vibration length.

If you're describing what different strings do, then this is different than
vibrating bodies that product harmonic and inharmonic tones.

 

[Peter Larsen]

Probably not a happenstance. Some objects vibrate quite
nonlinearly, and thus produce inharmonic overtones.
Percussion instruments seem to be more prone to this sort
of thing.

This issue is most interestingly explained in a 1981 +/- one or two
years issue of Scientific American on piano tuning

If you're describing what different strings do, then this is
different than vibrating bodies that product harmonic and
inharmonic tones.

What strings do depend on what their anchor points do. See the
literature reference above.

Kind regards

Peter Larsen

 

[Mogens V.]

Arny Krueger wrote:




This issue is most interestingly explained in a 1981 +/- one or two
years issue of Scientific American on piano tuning




What strings do depend on what their anchor points do. See the
literature reference above.

That's a bit too generalized. It also depends on the rest of the
physical swinging system. Unless counting all beneath strings as anchor,
mounting the same set of strings, tuners, nut, bridge on a solid steel
railway chunk, an acoustic, an electric solidbody and a archtop jazz
guitar yeilds different string movements.
Sorry, no references I can remember. And a bit OT to the discussion.

 

[Rich Grise]

Then it's the instruments with the equal temperament that are excluded from such
performances.

Yeah, ha ha, but temper and temperament are in entirely different
ballparks.

In High School, I broke my collarbone while horsing around. When I got
home from getting it set, my sister asked, "Did you break your well-
tempered clavicle?" ;-)

We all laugued politely.

Thanks,
Rich

 

[Mogens V.]

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 .

Comments ?

Graham

Maybe useful to the discussion on harmonics, YMMV.. it's some graphs of
audibility for different harmonics:

http://members.cox.net/alexhardware/opa.htm

 

[Arny Krueger]

message

Maybe useful to the discussion on harmonics, YMMV.. it's
some graphs of audibility for different harmonics:

http://members.cox.net/alexhardware/opa.htm

Figure one seems to be highly speculative. Where did it come?

 

[Mogens V.]

message



Figure one seems to be highly speculative. Where did it come?

I really can't say.. I stumbled over this from some audiophile forum
while searching for references to peoples experiences with opamps.

The article has a nof links to some modified 'board' seemingly in some
CD/SACD player, the make and model of which I can't make out, which
seems to be the basis for the tests.
Right above Fig 1 this 'board' seemingly used for measurements are
mentioned, indicating they did measure out those opamps - or maybe the
player was measured as a whole.
But you're right, the way it's written, Fig 1 could be based on some
measurements coupled with a mix of theory, experiences and speculations
(or superstition, it you will).

The article has a link to Part 1 somewhere near the top.
Both papers have a nof subjective listening comments, so YMMV..

The reason I put some faith in the papers is that most all comments on
subjective sonic performance seems to fit with what I read elsewhere,
which of cause doesn't nessesarily make it technically valid.

 

[Arny Krueger]

message

I really can't say.. I stumbled over this from some
audiophile forum while searching for references to
peoples experiences with opamps.

The article has a nof links to some modified 'board'
seemingly in some CD/SACD player, the make and model of
which I can't make out, which seems to be the basis for
the tests.

The tests they allude to seem to, er lack rigor.

Right above Fig 1 this 'board' seemingly used for
measurements are mentioned, indicating they did measure
out those opamps - or maybe the player was measured as a
whole.

The analog domain circuitry in a CD player is basically a stereo DAC, a pair
of op amps, and maybe an analog switch for muting.

But you're right, the way it's written, Fig 1 could be
based on some measurements coupled with a mix of theory,
experiences and speculations (or superstition, it you
will).

I'm thinking it has a lot of the latter, because the thresholds shown are
below the generally agreed-upon state of the human ear, and by several
orders of magnitude.

The article has a link to Part 1 somewhere near the top.
Both papers have a nof subjective listening comments, so
YMMV..

Exactly. It's highly speculative stuff. If someone said that they took the
current understandings of the sensitivity of the ear, and added 2-3 orders
of magnitude "safety factor", I might understand.

The reason I put some faith in the papers is that most
all comments on subjective sonic performance seems to fit
with what I read elsewhere, which of cause doesn't
nessesarily make it technically valid.

There's a lot of people who say this kind of stuff, and it appears to rock
their cradle. Doesn't make it the least bit true.

 

[Mogens V.]

The analog domain circuitry in a CD player is basically a stereo DAC, a pair
of op amps, and maybe an analog switch for muting.

Basically, yes. Sometimes I wonder about all that electronics in some
devices. The reference thingy (an SACD, it seems) in said article seems
to be loaded with electronics. YMMV..

There's a lot of people who say this kind of stuff, and it appears to rock
their cradle. Doesn't make it the least bit true.

Nod. As I said, I was searching for tonal references more than tech
facts. It's for modding the analog part of guitar processors, where a
nof devices will all serve equally well, from a tech POW.
Not having the time nor the test gear, I was going more for a matching
range of subjective sonic comments. The tonal/sonic 'findings' in the
referred article mostly seem to match what's been commented in here, so
at least subjectively, it's not totally off.
I fully agree with your comments on scale et al.., though
Haven't completely lost my ability to think and read between lines..

 

[Arny Krueger]

message

Basically, yes. Sometimes I wonder about all that
electronics in some devices. The reference thingy (an
SACD, it seems) in said article seems to be loaded with
electronics. YMMV..

This is ironic, because one of the alleged benfits of DSD is supposed to be
a simplified player.

However, there are two other explanations for the apparently complexity of a
SACD player:

(1) It's a high end audio product, and the oversized collection of hardware
is supposed to convey perceived value.

(2) It's a low-volume or early production design, and has very few of the
advantages of component integration, which requires more development time,
and/or technology and expenses that only high volumes can make economically
justified.

Nod. As I said, I was searching for tonal references more
than tech facts. It's for modding the analog part of
guitar processors, where a nof devices will all serve
equally well, from a tech POW.

Regrettably, the meaning of the previous paragraph seems to be hindered by a
few misspellings.

Not having the time nor the test gear, I was going more
for a matching range of subjective sonic comments.

Subjective comments are like - individual and subjective.

The
tonal/sonic 'findings' in the referred article mostly
seem to match what's been commented in here, so at least
subjectively, it's not totally off.

I fully agree with your comments on scale et al.., though
Haven't completely lost my ability to think and read
between lines..

Just trying to throw a flag on a play that seems to be way out of bounds...

Good luck with your project.

 

[Mark]

I believe there is a fundamental emotional problem some people have
with audio equipment in that they believe that simple, easy, common,
and cheap means it cannot also be good.

But that is exactly what advances in technology (like negative
feedback) strive for. With today's technology, a simple, cheap,
common, easy to design in op-amp can indeed be VERY good.

I think it is a stoicism thing.

Mark

 

[Arny Krueger]

I believe there is a fundamental emotional problem some
people have with audio equipment in that they believe
that simple, easy, common, and cheap means it cannot also
be good.

Agreed, and there are plenty of people who are attempting to profit by
fanning these kinds of fears.

But that is exactly what advances in technology (like
negative feedback) strive for. With today's technology,
a simple, cheap, common, easy to design in op-amp can
indeed be VERY good.

Again agreed. BTW, after looking at

http://members.cox.net/alexhardware/opa.htm

Then I noticed a reference to the author's previous article:

http://tangentsoft.net/audio/opamps.html

In this article, a wide selection of audio op amps that are generally
designed to drive normal line level circuitry were evaluated as headphone
amplifiers. <!!!!>. The tests involved loads as low as 33 ohms. The
apparent test criteria was the power supply voltage required to deliver
signals on the order of 0.5 volts into 33 ohm loads. <head shaking>.

I'm not knocking the need, but I've got a lot of questions about the
approach to testing, and the choice of parts tested for the stated purpose.

There are such thing as op-amp chips that are very appropriate for driving
headphones, none of which I see being tried.

I think it is a stoicism thing.

When I started out in audio, it took a half-a rack or so of epxensive
equipment (based on octal tubes) with a farily thick power cord to do a what
then passed for a high quality job of recording two tracks from condenser
microphones. Today, a Microtrak fits in my hand, runs off a built-in
battery for an hour or more, and does a far better technical job of doing
the same basic thing - recording and playing audio. Not everybody seems to
be able to get their head around such dramatic changes.

 

[Mogens V.]

message



Regrettably, the meaning of the previous paragraph seems to be hindered by a
few misspellings.

Que? not sure I understand..

Good luck with your project.

Don't worry, I'm not about substiting all opamps just because I _think_
I can get better tone. It's mostly for things like programmable
compressors build on a NE570+TL082, to be substitutd for a NE572+NE5534.
I'd like to adopt a better, more natural sounding (sideband) compressor
design without breathing artefacts, though I'd have to expect facing
some problems integrating it into this existing programmable design.
Wouldn't mind references to a good sideband compressor, though..

 

[Arny Krueger]

message

Que? not sure I understand..


Don't worry, I'm not about substiting all opamps just
because I _think_ I can get better tone. It's mostly for
things like programmable compressors build on a
NE570+TL082, to be substitutd for a NE572+NE5534. I'd
like to adopt a better, more natural sounding (sideband)
compressor design without breathing artefacts, though I'd
have to expect facing some problems integrating it into
this existing programmable design. Wouldn't mind
references to a good sideband compressor, though..

The NE570 variable gain element seems to be far more likely to create
audible effects than even just a fair op amps.

You might want to check That Corp semiconductor for their latest-greatest
VCA chips.

http://www.thatcorp.com/vcas.html