Feedback in audio esp wrt op-amps.

[Eeyore]

MooseFET wrote:

addressing individual points here

Since the transistors used in power stages are usually slower than the
others in the design. The output is almost always where the pole you
didn't design in lives.

In a power amplifier design this is pretty much invariably true (although power devices from the
likes of Toshiba tend to be pretty fast) but for a 'discrete op-amp' the output devices certainly
need not have such a limitation. I'd expect to be using parts with a 100MHz fT.

As an example of designing around the problem where you do need some watts of dissipation, where I
once needed to provide a highish current drive stage to drive some Mosfet gates I used several
reasonably fast TO-92 parts in 'parallel' rather than go for a slower TO-220 device.

Graham

 

[Phil Allison]

" Graham Stevenson Total Wanker "

You can (and people do) argue forever about what is or isn't subjectively
audible. The '990C' discrete op-amp was mentioned in another thread for
example.
With THD of 0.06% (-64dB) under some conditions it strikes me that those
distortion products could easily be audible yet ppl leapt to its defence.

** Shame how the incorrigible, self aggrandising Graham Stevenson charlatan
deliberately did not provide a link to this obscure product from the audio
wanker's brigade.

Here it is:

http://www.johnhardyco.com/pdf/990.pdf

The 0.06% figure is there in the specs.

If refers to operation at 20 kHz, with 40 dB of gain and 19 volts peak into
a 75 ohm load - a power level of 2.5 watts !!!

Naturally, the THD figures improve dramatically at lower frequencies, power
levels and with common load impedances.

WHAT a CROCK OF SHIT !!!


This link has some actual test results with range of popular audio op-amps.

http://www.dself.dsl.pipex.com/ampins/webbop/opamp.htm




....... Phil

 

[MooseFET]

On Aug 19, 5:49 pm, Eeyore <rabbitsfriendsandrelati...@hotmail.com>
wrote:
[....]

You do need merely "high gain" however. This high gain needs to be
true at the frequencies of interest so the GBP does have to be at
least some reasonable amount.

10MHz seems to work reasonably well but 120dB gain at LF is not a requirement.

I quite agree. The point that matters more is the gain at the top of
the band. At the low end you almost always have more than enough
gain.

Besides, gain is cheap these days. I have no objection to the
introduction of another gain stage for example. I'd rather have a sensible amount of very
linear and well defined gain than oodles of 'poor quality' gain.

Adding stages adds to the phase shifts.

Needn't be a very significant phase shift. Plus, if the 'natural' phase shift of the existing
stages is reduced through degeneration, that's all fine.

The local feedback is lowering the gain and thus shifts the gain cross
over downwards. When you add the stage, you never get quite the full
gain increase. At least, this is generally true for gains greater
than about e.

 

[MooseFET]

On Aug 19, 5:57 pm, Eeyore <rabbitsfriendsandrelati...@hotmail.com>
wrote:

MooseFET wrote:
addressing individual points here

Since the transistors used in power stages are usually slower than the
others in the design. The output is almost always where the pole you
didn't design in lives.

In a power amplifier design this is pretty much invariably true (although power devices from the
likes of Toshiba tend to be pretty fast) but for a 'discrete op-amp' the output devices certainly
need not have such a limitation. I'd expect to be using parts with a 100MHz fT.

As an example of designing around the problem where you do need some watts of dissipation, where I
once needed to provide a highish current drive stage to drive some Mosfet gates I used several
reasonably fast TO-92 parts in 'parallel' rather than go for a slower TO-220 device.

No cursor again damit!

Zetex makes the 2N2222 is a SOT223 package. Each one is good for
about 0.5A of gate drive and are quite fast. I've used them as RF
devices at 90MHZ.


> Graham

 

[Eeyore]

MooseFET wrote:

Eeyore wrote:
MooseFET wrote:
addressing individual points here

Since the transistors used in power stages are usually slower than the
others in the design. The output is almost always where the pole you
didn't design in lives.

In a power amplifier design this is pretty much invariably true (although power devices from the
likes of Toshiba tend to be pretty fast) but for a 'discrete op-amp' the output devices certainly
need not have such a limitation. I'd expect to be using parts with a 100MHz fT.

As an example of designing around the problem where you do need some watts of dissipation, where I
once needed to provide a highish current drive stage to drive some Mosfet gates I used several
reasonably fast TO-92 parts in 'parallel' rather than go for a slower TO-220 device.

No cursor again damit!

Zetex makes the 2N2222 is a SOT223 package. Each one is good for
about 0.5A of gate drive and are quite fast. I've used them as RF
devices at 90MHZ.

In my case quiescent dissipation was a factor so SOT223 wouldn't have been helpful. Also I was using
+/- 105V supplies. MPSA42 and 92 did the job.

Graham

 

[D from BC]

On Sun, 19 Aug 2007 11:38:53 -0700, MooseFET <kensmith@rahul.net>
wrote:

On Aug 19, 10:03 am, D from BC <myrealaddr...@comic.com> wrote:
On Sun, 19 Aug 2007 16:39:55 +0100, Eeyore



rabbitsfriendsandrelati...@hotmail.com> 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 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

Just speaky from some audio hobby work....

*Like with most things in electronics, there are frequency limits. I
think feedback decreases with frequency.
Yes it typically does generally decrease. It also has a phase shift.
If you add feedforward, you can have a band in which the feedback
increases with frequency.

The harmonic distortion
becomes an ultrasonic problem.

One problem is that ultasonic things can interact on any nonlinear
part of the system. This can lead to frequencies that are things like
7*F1 - 9*F2 in the circuit. It is like someone injected a signal at
that frequency into that point in the circuit. How the system
responds to it determines whether it will be heard or not.

*Feedback is a correction signal.. If nothing messes up this process
then all's well.
*For large signals, doesn't every semiconductor naturally distort?
Developing the best linear open loop design may not be enough.

D from BC

I have to wonder how often BW limiting (say cutoff at 20khz) is
practiced in audio electronics design to filter out ultrasonic
harmonics produced by op amp stages.
For example: Active crossovers, sound cards, mixing boards...
D from BC

 

[D from BC]

On Sun, 19 Aug 2007 11:49:37 -0700, MooseFET <kensmith@rahul.net>
wrote:

On Aug 19, 10:03 am, D from BC <myrealaddr...@comic.com> wrote:
[...]
*For large signals, doesn't every semiconductor naturally distort?
Developing the best linear open loop design may not be enough.



For some reason my cursor went away. This makes it harder to edit
what I'm typing.

Making the "best linear open loop" is for all practical purposes never
enough.
You need a very linear open loop design with a low enough phase shift
to make the NFB work and ideally to have a lowpass effect applied to
any distortion that is created.

You also have to trade off performance against water cooling. A
simple class A power MOSFET common source stage can be used as an
example. If you use about 10 power MOSFETs in parallel, have each one
passing about 0.5 Amps, and run with a 50V supply, you will have a
circuit that is darn linear for a 1mV input signal.

Lowpass effect??
First time I read it that way... but I think I know what you mean.
It's the increasing distortion with increasing frequency.
All happening due to decreasing open loop gain with increasing
frequency.
(With a open loop phase(f) such that the amp is stable.)

Speaking of phase... Here's something I find fuzzy..

For an amplifier.. the input signal is summed with the output signal.
The result of the summation is the input signal + call it an
anti-distortion signal. The more fed back the more the gain goes down
but the more linear the amp acts..
Great if it all happens instantly..
But I can't imagine it does.
Electronics have time delays.
Feedback kinda looks like a late arrival.

It's just amazing the amplifier can keep up and fix its own
nonlinearity with chaotic audio jumping around at all differant rates.

D from BC

 

[Eeyore]

D from BC wrote:

I have to wonder how often BW limiting (say cutoff at 20khz) is
practiced in audio electronics design to filter out ultrasonic
harmonics produced by op amp stages.
For example: Active crossovers, sound cards, mixing boards...

Never IME. Flat to 100kHz is the order of the day.

Graham

 

[Eeyore]

D from BC wrote:

It's the increasing distortion with increasing frequency.

Open loop gain reduces with frequency (Miller Effect). Reduced open loop gain at
higher frequencies means less negative feedback available. Less 'correcting' NFB

increased THD.

Graham

 

[Eeyore]

D from BC wrote:

Electronics have time delays.

Switching circuits have time delays ( Ton - Toff - Tstg etc ) . Amplifier
circuits are not normally hard switching. It's more useful to look at phase
shift with them.

Graham

 

[D from BC]

On Sun, 19 Aug 2007 18:41:42 +0100, Eeyore
<rabbitsfriendsandrelations@hotmail.com> wrote:

D from BC wrote:

Just speaky from some audio hobby work....

*Like with most things in electronics, there are frequency limits. I
think feedback decreases with frequency. The harmonic distortion
becomes an ultrasonic problem.
*Feedback is a correction signal.. If nothing messes up this process
then all's well.
*For large signals, doesn't every semiconductor naturally distort?
Developing the best linear open loop design may not be enough.

You need to learn more.

I appreciate your interest but your grasp of the issues is beginner level.

Graham

Learn more..No wayy...
I gave up audio electronics in 98.
Is there still money to be made in that area?

D from BC

 

[D from BC]

On Mon, 20 Aug 2007 05:19:10 +0100, Eeyore
<rabbitsfriendsandrelations@hotmail.com> wrote:

D from BC wrote:

Electronics have time delays.

Switching circuits have time delays ( Ton - Toff - Tstg etc ) . Amplifier
circuits are not normally hard switching. It's more useful to look at phase
shift with them.

Graham

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.

But yeah... when it comes to feedback, time delay within a 1/2 cycle
is of concern..so I guess that's why phase is the better term.

I mentioned time delay to express the time it takes for a signal to
pass through x amount of transistors in an op amp.
After that, feeding back the signal kinda doesn't look like
instantaneous correction.

In some ways feedback is seems like continuously breaking wine glasses
on the floor.. If the clean up is done fast enough...it doesn't look
like any glasses are being broken.
Well...that's probably a crappy analogy but best I can think of...
D from BC

 

[Scott Dorsey]

In article <3qudnVgLqt60JFXbnZ2dnUVZ_oytnZ2d@comcast.com>,
William Sommerwerck <grizzledgeezer@comcast.net> wrote:

A good op amp can be used as a buffer and be sonically transparent,
its output indistinguishable from its input.

As a buffer it has 100% NFB and I hope that's the case. As a gain stage
with say 40dB of voltage gain that isn't the case however.

Really, part of what I'm saying is that the classic op-amp isn't really
the ideal gain stage for audio circuits if you want to produce totally
"technically blameless" performance.

That's certainly true. But does it matter what type of circuit or components
you use if the performance is audibly blameless?

It does, because a stage which is audibly blameless by itself may turn into
a sonic disaster when it appears a few hundred times in the signal path.
--scott

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

 

[John Larkin]

On Sun, 19 Aug 2007 11:29:26 -0700, MooseFET <kensmith@rahul.net>
wrote:

On Aug 19, 9:43 am, "William Sommerwerck" <grizzledgee...@comcast.net
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 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 ?

That negative feedback linearizes the transfer function at the expensive of
adding higher-order harmonics has been long-known. What you say is perfectly
logical.

However, the presence of higher-order harmonics is not the only factor, but
their amplitude. Below a certain percentage (I'm sure Arny will be able to
tell us what that is), they're inaudible.

A good op amp can be used as a buffer and be sonically transparent, its
output indistinguishable from its input.


Even at reasonable gains, there are many that will perform well enough
that nobody will hear the difference. Power amplifiers are the place
where it gets very hard to keep distortion low at reasonable
efficiencies.

Consider how the sound got onto a CD or a slab of vinyl: microphones,
preamps, mixers, equalizers, time synchronizers, echo adders,
synthesizers, fake drums, distortion adders, digitizers. All this
supervised by some egomaniac producer who has his own opinion about
what sounds good and what the public wants to hear on whatever
equipment they are likely use, like a Panasonic receiver or a boom
box.

And somehow, magically, the golden-ear boys (is's almost always boys)
think that it matters that what they do to the signal that comes *off*
the CD makes so much difference that they can hear the difference in
the oxygen content of the interconnect wiring, or 0.06 percent
distortion when the producer added 30% of his own, because he liked
the effect.

Ludicrous.

John

 

[Mark]

On Aug 19, 11:39 am, Eeyore <rabbitsfriendsandrelati...@hotmail.com>
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.

Mark

 

[Don Pearce]

On Mon, 20 Aug 2007 09:32:21 -0700, Mark <makolber@yahoo.com> wrote:

On Aug 19, 11:39 am, Eeyore <rabbitsfriendsandrelati...@hotmail.com
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.

Mark



The way poorly implemented overall feedback can increase the level of

higher order harmonics is by permitting a marginal stability margin at
the top end. This is usually a result of a misguided attempt to
extract maximum possible bandwidth by not using a dominant pole at a
low enough frequency. Instead of a smooth top-end roll-off you get a
dip, then a rise. It is in the frequency range of this rise that the
feedback is tending towards positive rather than negative, and can
result in increased harmonic levels. Hopefully this (if it happens at
all) is well beyond the audible range.

d

--
Pearce Consulting
http://www.pearce.uk.com

 

[William Sommerwerck]

And somehow, magically, the golden-ear boys (is's almost always
boys) think that it matters that what they do to the signal that
comes *off* the CD makes so much difference that they can hear
the difference in the oxygen content of the interconnect wiring,
or 0.06 percent distortion when the producer added 30% of his
own, because he liked the effect.

What you say is intellectually logical, but it seems that post-recording
distortions can be plainly audible, regardless of the quality of the
recording.

When I reviewed, I made final judgements with my own live, undoctored
recordings.

 

[William Sommerwerck]

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'm sorry, Mark, but this has been known for decades, and was not
established by audiophile reviewers -- the reduction of the overall
distortion level is accompanied by an increase in higher-order harmonics.

I apologize for not having a reference.

 

[Kevin Aylward]

William Sommerwerck 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'm sorry, Mark, but this has been known for decades, and was not
established by audiophile reviewers -- the reduction of the overall
distortion level is accompanied by an increase in higher-order
harmonics.

I apologize for not having a reference.

Well, it is trivially obvious that a pure square law device, with a *small*
amount of feedback will generate 3rd harmonic distortion, that was never
orginally there, from the mixing of the second and the fundamental. It is
also true that for such low levels of feedback, although the total thd is
less, the new 3rd component may sound more objectionable to those goldern
ears. However, assuming *sufficient* feedback is applied, the final
distortion will be audiable less noticable.

--
Kevin Aylward
kevinEXtract@kevinaylward.co.uk

 

[Scott Dorsey]

William Sommerwerck <grizzledgeezer@comcast.net> wrote:

And somehow, magically, the golden-ear boys (is's almost always
boys) think that it matters that what they do to the signal that
comes *off* the CD makes so much difference that they can hear
the difference in the oxygen content of the interconnect wiring,
or 0.06 percent distortion when the producer added 30% of his
own, because he liked the effect.

What you say is intellectually logical, but it seems that post-recording
distortions can be plainly audible, regardless of the quality of the
recording.

Oh, absolutely, but sometimes that's because of what the distortions do
to the artifacts in the original recording.

I like to use a particular track from Hair for listening to speaker systems...
something in the vocal chain on that track (2-4-0-0) is right on the edge
of clipping and the problem is much more audible on good speakers than bad
ones.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."