Feedback in audio esp wrt op-amps.

[Eeyore]

"Scott Dorsey Notorious Charlatan"

path.

** Huh ??

A few HUNDRED times ???????

The colossal fool must be on LSD.

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

Graham

 

[Phil Allison]

" Graham Stevenson Mentally Deranged Pile of Autistic Shit."

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

** **** OFF YOU ASININE TROLLING MORON !!




....... Phil

 

[Scott Dorsey]

Something like The Second Coming?

No, more like something out of Revelations.
--scott

 

[MooseFET]

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

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.

 

[Eeyore]

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 sounds pretty extreme certainly.

Graham

 

[MooseFET]

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.

 

[MooseFET]

That's because of the falling loop gain with frequency of the amplifier. Not what I was
referring to.

..... plus the increasing phase lag. Must not forget that nasty
detail :>

You've missed the point I was making entirely. Other posters have explained it better than
myself however.

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

 

[[email protected]]

I guess I think phase for repeating waveforms.
Audio is like noise.
I haven't heard someone say "That noise is lagging by 40 degrees."
2 sine waves out of sync can be expressed by degrees or time delay.

Any complex signal can be broken down into a combination of sine
waves. So you can still use the notion of phase shift.

 

[[email protected]]

Sure it can. Just put something that creates harmonics in the feedback
path.

Negative feedback relies on the feedback path being linear and having
low group delay. If these aren't the case, bad things can happen.
--scott

I'm not sure I'd bring group delay into the discussion. What you need
is phase margin.

 

[Les Cargill]

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.



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?

 

[D from BC]

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.

[snip]

Cool...
Maybe call it a distortion loop.

+-<<<--------------------------------------<+
| |
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...

Significant magnitudes???

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

D from BC

 

[Mr.T]

What's the K1 like then ?

Like most other good amps, so good you can forget about it being the
problem, unless it's broken of course

MrT.

 

[Mr.T]

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.

MrT.

 

[Robert Latest]

["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

 

[Robert Latest]

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

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

 

[Eeyore]

["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.

Oh, I reckon the latest fad, the single ended tube output satge (preferably with
no feedback) can achieve that !

Graham

 

[William Sommerwerck]

It does, because a stage which is audibly blameless

** 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

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.