Feedback in audio esp wrt op-amps.

[MooseFET]

On Aug 20, 11:08 pm, D from BC <myrealaddr...@comic.com> wrote:
[.....]

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...

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 <kensm...@rahul.net> wrote:

On Aug 20, 11:08 pm, D from BC <myrealaddr...@comic.com> wrote:
[.....]



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...

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 <makolber@yahoo.com> wrote:

On Aug 21, 9:37 am, MooseFET <kensm...@rahul.net> wrote:
On Aug 20, 11:08 pm, D from BC <myrealaddr...@comic.com> wrote:
[.....]



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...

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 <makolber@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.
--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 here:

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" <grizzledgeezer@comcast.net> wrote in message
news:Y4CdnfSdS8XamVfbnZ2dnUVZ_o6knZ2d@comcast.com...

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]

Mark wrote:

On Aug 20, 2:30 pm, Eeyore <rabbitsfriendsandrelati...@hotmail.com
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
ka@kevinaylward.co.uk

 

[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 <makolber@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.

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" <grizzledgeezer@comcast.net> 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" <rabbitsfriendsandrelations@hotmail.com> wrote in message
news:46C8644B.2EB28465@hotmail.com...

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 here:
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 <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.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."