[Eeyore]

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

[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 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.

[MooseFET]

On Aug 19, 8:39 am, 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 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

Degeneration is NFB. It is just applied locally. What you really
want is to go with a topology that is naturally more linear,

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 ?

One huge problem with including a lot of local NFB is that it makes
the overall system harder to close. Local feedback often creates 2
pole systems with modest Q values within the system. When you go to
close the loop, you have to keep a good phase margin so you are forced
to use a lower overall loop gain.

Try spice modeling a thing like this:


Vcc
---------------+------------
!
\
/
Vbias \
! !
/ +--------------
\ ! !
! !/ e !
---!!--+------! PNP ---
!\ ---
+--Out !
\ !
/ !
\ !
! !
+--------------
!
V D1
---
!
GND

Change D1 to be a resistor and back and you will see quite a
difference in the amount of degeneration needed to get the same
distortion values for a modest signal of lets say 10mV in.

[Eeyore]

MooseFET 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 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

Degeneration is NFB. It is just applied locally. What you really
want is to go with a topology that is naturally more linear,

Sorry I didn't make that clearer.

Yes, I'm referring to the reduction of overall loop feedback.

Graham

[Eeyore]

MooseFET wrote:

One huge problem with including a lot of local NFB is that it makes
the overall system harder to close.

That's not my experience. Quite the reverse actually. But then I do tend to
incorporate internal lead-lag compensation. This results in a far BETTER phase margin.

Graham

[D from BC]

On Sun, 19 Aug 2007 16:39:55 +0100, 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 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. 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.

D from BC

[Eeyore]

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

[Eeyore]

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 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.

There's more than a little discussion about what level that is, and indeed it's
known that audibility varies according to harmonic number.


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.

Graham

[MooseFET]

On Aug 19, 9:57 am, Eeyore
wrote:
MooseFET wrote:
One huge problem with including a lot of local NFB is that it makes
the overall system harder to close.

That's not my experience. Quite the reverse actually. But then I do tend to
incorporate internal lead-lag compensation. This results in a far BETTER phase margin.

This means that you have lowered the outter loop gain in the process.
If the internal part looks kind of like this:

--!!-/\/\--
! !
---/\/\-+--!-\ !
! ------+---
!+/

The amplification stage you are placing the NFB around must have a
great enough bandwidth to make the feedback determine the responce.

The local feedback has all of the problems a global feedback has with
creating upper harmonics. Your global feedback is now at a lower gain
and thus can't remove them. This is just a case of the lack of a free
lunch.

The pole and zero inside the loop is a good thing to do to improve the
phase margin when you have other poles in the system. It allows you
to determine where the gain crossover happens and the phase at the
crossover. It is a method of lowering the overall loop gain. It
doesn't however get rid of the harmonics issue. It also is something
that you can only do a few times inside the loop. When the system
starts to look like 3 of those in series, you are back in trouble.

In audio stuff, you generally want to put the pole-zero thing near the
output, ideally enclosing the output. This makes the system apply a
low pass filter to any distortion products that the feedback can't get
rid of.

[MooseFET]

On Aug 19, 9:43 am, "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 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.

[MooseFET]

On Aug 19, 10:03 am, D from BC wrote:
On Sun, 19 Aug 2007 16:39:55 +0100, 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 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

[MooseFET]

On Aug 19, 10:03 am, D from BC 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.

[whit3rd]

On Aug 19, 8:39 am, Eeyore
wrote:
...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.

All amplifiers have characteristic curves; the gain isn't completely
linear.
Feedback components can be (very linear) resistors. So you get
some combination of amplification and negative feedback in most useful
low-distortion amplifiers.

A single transistor can have power gain of 10,000; a single vacuum
tube
or MOSFET can have more. Giving up gain for linearity is a good
trade.
It's never perfect (even resistors are distortion sources, if you have
signals
at 2 Hz and the self-heating of the resistors isn't insignificant),
but it's good
enough. Listen. Enjoy.

[William Sommerwerck]

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?

[MooseFET]

On Aug 19, 2:42 pm, "William Sommerwerck"
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 also has to work for a reasonable time, be easy to manufacture and
look good.

[Eeyore]

MooseFET wrote:

Eeyore wrote:
MooseFET wrote:
One huge problem with including a lot of local NFB is that it makes
the overall system harder to close.

That's not my experience. Quite the reverse actually. But then I do tend to
incorporate internal lead-lag compensation. This results in a far BETTER phase margin.

This means that you have lowered the outter loop gain in the process.

Yes. As stated in my introduction, the pursuit of super-fabulously high DC (or AC) open loop
gain makes no sense for audio. 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.


If the internal part looks kind of like this:

--!!-/\/\--
! !
---/\/\-+--!-\ !
! ------+---
!+/

The amplification stage you are placing the NFB around must have a
great enough bandwidth to make the feedback determine the responce.

Yes and yes. Many IC op-amps used for audio have GBPs in the 10MHz region so this isn't too
difficult even if using one of those inside the loop (which I did in a recent design ).
Discrete stages suitably degenerated can have higher GBPs than that.


The local feedback has all of the problems a global feedback has with
creating upper harmonics. Your global feedback is now at a lower gain
and thus can't remove them. This is just a case of the lack of a free
lunch.

I'm not sure I 'get that' entirely. I see where you're coming from and that would lead one
to imagine that pursuit of linearity in individual stages was a pointless pursuit and you
might as well have tons of non-linear gain and I know that's not the case, not least because
the very hugh gain system has to be stable and that tends to lead to rolling off the gain
(and the advantage of NFB) from very low frequencies.


The pole and zero inside the loop is a good thing to do to improve the
phase margin when you have other poles in the system. It allows you
to determine where the gain crossover happens and the phase at the
crossover. It is a method of lowering the overall loop gain. It
doesn't however get rid of the harmonics issue. It also is something
that you can only do a few times inside the loop. When the system
starts to look like 3 of those in series, you are back in trouble.

3 of them would be a bit much. I've not used more than 2 inside the loop in fact.


In audio stuff, you generally want to put the pole-zero thing near the
output, ideally enclosing the output. This makes the system apply a

low pass filter to any distortion products that the feedback can't get
rid of.

Interesting idea.

Graham

[Eeyore]

whit3rd wrote:

Eeyore wrote:

...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.

All amplifiers have characteristic curves; the gain isn't completely
linear.

I know. That's why I said the individual stages should be degenerated to linearise
them. This results in a lower gain but this may not be a problem in practice as
long as GBP is maintained.


Feedback components can be (very linear) resistors. So you get
some combination of amplification and negative feedback in most useful
low-distortion amplifiers.

A single transistor can have power gain of 10,000;

In every audio amplifier stage I know, POWER gain is of little importance. Voltage
gain is what's required. Cuurent gain can be readily added where needed by using
emiiter followers.


a single vacuum tube or MOSFET can have more. Giving up gain for linearity is a
good
trade.

That was indeed my point wrt giving up some of that *open-loop* gain in a gain
block.

Graham

[Eeyore]

William Sommerwerck 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?

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.

If it can be shown that the defects must be inaudible from first principles
(such as distortion below 100dB for example) you're on firmer ground IMHO.

Graham

[MooseFET]

On Aug 19, 4:34 pm, Eeyore
wrote:
MooseFET wrote:
Eeyore wrote:
MooseFET wrote:
One huge problem with including a lot of local NFB is that it makes
the overall system harder to close.

That's not my experience. Quite the reverse actually. But then I do tend to
incorporate internal lead-lag compensation. This results in a far BETTER phase margin.

This means that you have lowered the outter loop gain in the process.

Yes. As stated in my introduction, the pursuit of super-fabulously high DC (or AC) open loop
gain makes no sense for audio.

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.

Very high values of loop gain makes for very large amounts of
reductions in the harmonics within the band. This can argue for much
more gain than it would normally appear you need if you only needed
enough gain to be sure that the feedback resistors were what was
setting the 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. This is another "no free
lunch situation". When you increase the number of stages you also
want to increase the bandwidths of most of the stages to keep the
phase shift near the gain cross over within reason.


Keeping this for reference later:


--!!-/\/\--
! !
---/\/\-+--!-\ !
! ------+---
!+/


[.. stuff we have covered and agree on ...

The local feedback has all of the problems a global feedback has with
creating upper harmonics. Your global feedback is now at a lower gain
and thus can't remove them. This is just a case of the lack of a free
lunch.

I'm not sure I 'get that' entirely. I see where you're coming from and that would lead one
to imagine that pursuit of linearity in individual stages was a pointless pursuit and you
might as well have tons of non-linear gain and I know that's not the case, not least because
the very hugh gain system has to be stable and that tends to lead to rolling off the gain
(and the advantage of NFB) from very low frequencies.

No what I am pointing out is that local feedback is not a good
substitute for a naturally more linear stage. Consider this sort of a
situation:

---------- ------ ---------- ------
Signal---! Subtract !---! Gain !---! Subtract !---! Bad !--+----

---------- ------ ---------- ! gain ! !
^ ^ ------ !
! ! !
------------------------+---------------------

You can trade back and forth how much subtracting you do in the two
subtraction circuits but you can't really fix the "bad gain" section.
I am for not leaving out the idea that the good gain has a limited
bandwidth and assuming all of the bandwidth limiting happens in the
"bad gain". I think this makes the idea obvious in it simple form.
To take a bit of a real example, consider a dreadful output stage that
works like this:

----+--------+---
! !
\ \
R1 / / R2
\ \
! !
! !/e
+-------! PNP
! !\
!/ !
! NPN !
!\e ! R3
----------+----/\/\---
--- mirror for other half

R3 is providing a measure of local feedback. R2 also is doing so.
This stage will still be a horror story. Adding a diode in series
with R1 to match to the E-B drop of the PNP makes it much less so.
The diode makes the PNP act much more like a linear current mirror and
thus reduces the natural distortion.

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.


The pole and zero inside the loop is a good thing to do to improve the
phase margin when you have other poles in the system. It allows you
to determine where the gain crossover happens and the phase at the
crossover. It is a method of lowering the overall loop gain. It
doesn't however get rid of the harmonics issue. It also is something
that you can only do a few times inside the loop. When the system
starts to look like 3 of those in series, you are back in trouble.

3 of them would be a bit much. I've not used more than 2 inside the loop in fact.

Trust me on this: Don't put three inside the loop. Reconsider the
design if you find yourself going there. Two is ok. One plus a
feedforwards is ok but three always seems to mean trouble.



In audio stuff, you generally want to put the pole-zero thing near the
output, ideally enclosing the output. This makes the system apply a
low pass filter to any distortion products that the feedback can't get
rid of.

Interesting idea.

It only works up to a point. It also requires largish (mechanically)
parts be involved. You have a capacitor and a resistor with fairly
large swings on them and are working at lowish impedances.

[Eeyore]

MooseFET wrote:

Eeyore wrote:
MooseFET wrote:
Eeyore wrote:
MooseFET wrote:
One huge problem with including a lot of local NFB is that it makes
the overall system harder to close.

That's not my experience. Quite the reverse actually. But then I do tend to
incorporate internal lead-lag compensation. This results in a far BETTER phase margin.

This means that you have lowered the outter loop gain in the process.

Yes. As stated in my introduction, the pursuit of super-fabulously high DC (or AC) open loop
gain makes no sense for audio.

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.


Very high values of loop gain makes for very large amounts of
reductions in the harmonics within the band. This can argue for much
more gain than it would normally appear you need if you only needed
enough gain to be sure that the feedback resistors were what was
setting the gain.

That's sort of what I'm after.


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.


This is another "no free lunch situation". When you increase the number of stages you also
want to increase the bandwidths of most of the stages to keep the
phase shift near the gain cross over within reason.

Oh yes and degeneration will do that of course.

Graham

[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 ****** "


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
******'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 **** !!!


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
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
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
wrote:

On Aug 19, 10:03 am, D from BC wrote:
On Sun, 19 Aug 2007 16:39:55 +0100, 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 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
wrote:

On Aug 19, 10:03 am, D from BC 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
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... :P
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
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 ,
William Sommerwerck 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
wrote:

On Aug 19, 9:43 am, "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 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
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 wrote:

On Aug 19, 11:39 am, 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.

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[_2_]]

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

[Scott Dorsey]

William Sommerwerck 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."