Feedback in audio esp wrt op-amps.

[Eeyore]

<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 <[email protected]>
wrote:
[....]

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.

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]

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.

 

[Eeyore]

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]

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.


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.

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

 

[Scott Dorsey]

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

 

[John Larkin]

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]

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]

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

 

[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

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]

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.

 

[Scott Dorsey]

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

 

[Arny Krueger]

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

30 years of experience with bias-controlled listening tests says that
*seems* and *is* can be two different things. Intellectual logic has this
interesting tendency to rule, once the comparison is based on just
listening.

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

The fun begins when you level-match, time-synch and eliminate other
non-audible cues.

 

[Arny Krueger]

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.

Something to do with reality intruding on fantasy. Normally, a reduction of
the overall
distortion level is accompanied by a similar but possibly smaller decrease
in higher-order harmonics. A decrease in all forms of distortion is the
primary effect. The shift towards larger percentages (but not larger
amounts) of higher order distortion is a secondary effect.

One possible exception was described by Don - relating to marginal
stability.

Another common situation is where the open-loop gain of the amplifier inside
the loop simply falls with increasing frequency. Very common, particularly
with op amps. The higher harmonics are still reduced, but they may be
reduced by a smaller amount than the lower harmonics. This leads to the
higher harmonics being a bigger slice of a significantly smaller pie. The
smaller pie is the stronger effect, so the size of all harmonics is still
reduced.

When the pie is as sour-tasting as nonlinear distortion is in reproduction
equipment, I'm always in favor of significantly smaller pies!

The source of this myth is the mistaken idea that negative feedback
regenerates the audio signal, and the nonlinearity of the amplifier leads to
higher order products of the regenerated harmonics and the nonlinearity of
the amp. This ignores the fact that the regenerated signal is brought back
in out-of-phase, and has the primary result of reducing the high order
harmonics.

 

[Arny Krueger]

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.

Let's put this into a real-world perspective. The LM 4562 has a typical GBW
of 55 MHz. If it is a typical compensated op amp, that means that its gain
at 1 KHz is 55,000. The feedback factor at 60 dB gain (x1,000) and 1 KHz is
thus 55. Based on its specs, its open-loop nonlinear distortion at 1 KHz is
55,000 times its unity gain distortion @ 1KHz is less than 2%. At 60 dB
gain, negative feedback drops this to about 0.035 %. Let's assume the worst
case - the nonlinear distortion is all second harmonic. Then, when fed back,
0.035% or less of the 0.035% second order distortion undergoes conversion to
third harmonic.

IOW with feedback, there is now 0.00001225% or less third harmonic in
addition to the 0.035% second. I wouldn't expect anybody to hear the 0.035%
second order nonlinear distortion, and definitely not the 0.00001225% third.

 

[Eeyore]

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

 

[Eeyore]

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

Absolutely true.

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

Graham

 

[Eeyore]

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.

The irony being that it sounds 'worse' on 'good' speakers.

This is why domestic hi-fi tends to have little in common with studio monitors.
Horses for courses and all that.

Graham