Feedback in audio esp wrt op-amps.

"Eeyore" <rabbitsfriendsandrelations@hotmail.com> wrote in message
news:46CB8EC1.EA5B806C@hotmail.com...
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.
Click to expand...
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
 
Scott Dorsey wrote:

Eeyore <rabbitsfriendsandrelations@hotmail.com> 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.
Click to expand...
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 Ł1 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
 
In article <5ivg1eF3opbb0U1@mid.dfncis.de>,
Robert Latest <boblatest@yahoo.com> wrote:

Eeyore wrote:

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.
Click to expand...
If they really were, they'd break when you tried to bend them 8^}

Isaac
 
isw wrote:

Robert Latest <boblatest@yahoo.com> wrote:
Eeyore wrote:

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.

If they really were, they'd break when you tried to bend them 8^}
Click to expand...
Indeed. 'Ordinary' copper is pretty oxygen free too. It's just a load of nonsense
about how many 9s you want after the decimal point.

Graham
 
"Scott Dorsey" <kludge@panix.com> wrote in message
news:faeobm$ir6$1@panix2.panix.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...
Click to expand...
Yes, but do you understand what I said, or are you just choosing to ignore
the difference between parallel circuits and series circuits?

MrT.
 
"Eeyore" <rabbitsfriendsandrelations@hotmail.com> wrote in message
news:46CB8BBF.72CB8457@hotmail.com...
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 ?
Click to expand...
Electronic devices seem to tend to have exponential characteristics.
 
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.
Click to expand...
I first read this rule in the '60s.
 
"Eeyore" <rabbitsfriendsandrelations@hotmail.com> wrote in message
news:46CB90AF.9C14C4D1@hotmail.com...
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.
Click to expand...
The spectrum is important, in fact it is critical. In many cases, 2nd order
distortion is hard to hear, particularly when listening to solo instruments.
This is due to concurrent spectral masking in the human ear.

Higher order harmonics are not so well masked as 2nd, so the ear can pick
them out, even when they are relatively small.

Complex music may be more critical of lower order distortion, because
audible IM products are more likely to be generated by complex sounds.

One of the worst cases for IM is music with lots of energy at high
frequencies, and less energy around 4 KHz. Any IM products that are
generated by the energetic highs fall in the midrange where the ear is most
sensitive, and there is very little masking due to the lack of midrange.
Thus, recordings of keychains jangling are better for judging nonlinear
distortion than say a full big band.

When I was at HE2005, I found that many SET demos were based on simple
music - solos and the like. I did hear some SETs being demoed with more
complex musical sounds, and frankly it could get ugly.

It is pretty atypical for feedback to fail to reduce even high harmonics,
perhaps with less improvement than lower harmonics. As long as you keep
feedback factors >= 10 (20 dB) across the audio band, inverse feedback is
very much your friend.

Of course there are exceptions, such as mic preamps based on cheap op-amps
that don't have enough bandwidth. A mic preamp with a 40 dB gain stage
needs to be based on an op amp with a GBW of at least 20 MHz, for example.
Thus, there is 60 dB open loop gain at 20 KHz, and 20 dB feedback factor
with a closed loop stage gain of 40 dB.

Even the hot new op amps like the LM4562 have only 55 MHz GBW, so 60 dB gain
in one stage remains elusive. The old Neve favorite 5532/4 had only 10 MHz
GBW. But, Neve wasn't shy about cascading them to reduce stage gain
requirements.

I've seen cheap mic preamps built with op amps whose GBW was well below 10
MHz. This is no doubt one reason why cheap mic preamps tend to slack off on
maximum available gain.
 
On Aug 21, 6:17 pm, Eeyore <rabbitsfriendsandrelati...@hotmail.com>
wrote:
Scott Dorsey wrote:
Mark <makol...@yahoo.com> 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.
Click to expand...
No, that is the wrong way to go. You want to apply buckets of NFB to
a stage that is already perfectly linear.

You want to push the gain crossover up to high frequencies and have
enough NFB to ensure that none of the harmonics made by any
nonlinearities can have significant amplitudes. "Moderate" may not
mean enough for this. One mans "moderate" is another mans "extreme"
so perhaps you should have used a number.
 
In article <46cbddfa$0$12802$afc38c87@news.optusnet.com.au>,
Mr.T <MrT@home> wrote:
"Scott Dorsey" <kludge@panix.com> wrote in message
news:faeobm$ir6$1@panix2.panix.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...

Yes, but do you understand what I said, or are you just choosing to ignore
the difference between parallel circuits and series circuits?
Click to expand...
I understand what you said, but it isn't really relevant except in the
case of the equalizer mentioned by another person in this thread, or if
you want to handwave about the number of individual op-amps in all the
channel strips on that SSL.

Pop the cover on an SSL 4000 and count the number of individual gain stages
from the front to the back, then into the tape mchine and back again. There
are lots.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."
 
The spectrum is important, in fact it is critical. In many cases, 2nd
order
distortion is hard to hear, particularly when listening to solo
instruments.
This is due to concurrent spectral masking in the human ear.

Higher order harmonics are not so well masked as 2nd, so the ear can
pick them out, even when they are relatively small.
Click to expand...
The audibility of harmonic distortion might also depend on tuning. In a
system using whole-number ratios, harmonic distortion of all orders might be
less audible than in a tempered system.
 
"William Sommerwerck" <grizzledgeezer@comcast.net> wrote in message
news:1bednZh-GfJoxVHbnZ2dnUVZ_qmlnZ2d@comcast.com...

The spectrum is important, in fact it is critical. In many cases, 2nd
order
distortion is hard to hear, particularly when listening to solo
instruments.
This is due to concurrent spectral masking in the human ear.

Higher order harmonics are not so well masked as 2nd, so the ear can
pick them out, even when they are relatively small.

The audibility of harmonic distortion might also depend on tuning. In a
system using whole-number ratios, harmonic distortion of all orders might
be
less audible than in a tempered system.
Click to expand...
Why might that be?
 
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 ?
Click to expand...
Yes, the sim I posted is local feedback around a single stage.

This is an interesting math excersize but of little practical value.
It tells us that in the case of an ideal second order device that
otherwise would only create 2nd order distrotion and NO 3rd 4th etc.
adding neg feedback will reduce the 2nd as desired but will also
create a small amount of third and forth etc that otherwise would not
be there.

Any practical non ideal real amplifier stage will create its own 3rd
4th etc and the neg feedback will reduce them as well.

So neg feedback (done correctly) is always a good thing.
 
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.
Click to expand...
Well, there are too many of them...

It is an interesting observation but not very relevant to real world
audio amplifier circuits that are not ideal square law devices.
Click to expand...
Ho hummm...it illustrates the principle mate, so is very relevant, I just
cant be bothered to include all the terms. If you want to see some more
terms of distorion thingies, try
http://www.kevinaylward.co.uk/ee/bipolardesign2/bipolardesign2.html

or even http://www.kevinaylward.co.uk/ee/tapebias/tapebias.html might be
interesting as well.



--
Kevin Aylward
kaEXtraCt@kevinaylward.co.uk
 
The audibility of harmonic distortion might also depend on tuning.
In a system using whole-number ratios, harmonic distortion of all
orders might be less audible than in a tempered system.

Why might that be?
Click to expand...
Masking effects -- or lack thereof. For example, in such a system, the third
harmonic would fall exactly on a scale note, whereas under our current
system, the third harmonic comes _very_ close, but not exactly.

This is speculation, of course. It's unlikely anyone has researched this.
 
"William Sommerwerck" <grizzledgeezer@comcast.net> wrote in message
news:ptydnbFbio9jFlHbnZ2dnUVZ_gOdnZ2d@comcast.com...
The audibility of harmonic distortion might also depend on tuning.
In a system using whole-number ratios, harmonic distortion of all
orders might be less audible than in a tempered system.

Why might that be?

Masking effects -- or lack thereof. For example, in such a system, the
third
harmonic would fall exactly on a scale note, whereas under our current
system, the third harmonic comes _very_ close, but not exactly.
Click to expand...
Masking is not *that* frequency-sensitive.

This is speculation, of course. It's unlikely anyone has researched this.
Click to expand...
A lot of masking curves have been developed, based on measurements. They are
relatively broad affairs. So there has been a lot of research into masking,
and how it works is fairly well-known.

Besides, lots of musical instruments generate overtones that are not exact
integer multiples of the fundamental.
 
William Sommerwerck wrote:

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.

I first read this rule in the '60s.
Click to expand...
Where did you originally come across it ?

Graham
 
William Sommerwerck wrote:

William Sommerwerck wrote:

When I worked at Rupert Neve..

When and where was this ?

Neve Connecticut, USA. 1983, I think.
Click to expand...
Bethel presumably ? Did you ever meet Robin Porter. I was told he'd spent some
time at RN Inc.

Graham
 
William Sommerwerck wrote:

The spectrum is important, in fact it is critical. In many cases, 2nd
order distortion is hard to hear, particularly when listening to solo
instruments. This is due to concurrent spectral masking in the human ear.

Higher order harmonics are not so well masked as 2nd, so the ear can
pick them out, even when they are relatively small.

The audibility of harmonic distortion might also depend on tuning. In a
system using whole-number ratios, harmonic distortion of all orders might be
less audible than in a tempered system.
Click to expand...
But who's still using those old tunings ?

I've heard the difference it makes to how the music sounds btw (at college 30
yrs ago) and it's quite remarkable.

Graham
 
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