Crossover distortion and NFB

[Tam/WB2TT]

"ChrisGibboGibson" <chrisgibbogibson@aol.com> wrote in message
news:20041024000852.22732.00002555@mb-m15.aol.com...

Rich Grise wrote:

Summat....

You're close!

Gibbo

Why not measure the THD of your signal source, using the same
procedure.Might be interesting.

Tam

 

[Joerg]

Hi Kevin,

... The effect he documents is like "14b 0.00103%, and for 14c 0.00153%". A difference of 0.0005%,
with the speaker generating, say 1% is hardly a good argument for not overbiasing at all. The main reason for not significantly overbising is simply to avoid wasting power.


That's exactly the point. Besides the speaker, which source of audio can

truly claim 0.001%?

Regards, Joerg

http://www.analogconsultants.com

 

[Rich Grise]

On Sun, 24 Oct 2004 03:43:23 +0000, ChrisGibboGibson wrote:

Rich Grise wrote:

[snip]

It's because they eat goats


And was it the beer that made me laugh or was it Rich?

What does it matter which? Isn't the laughter the goal, after all? ;-)

I'll never forget Kermit the Frog, one of my heroes, in a uniquely
poignant(sp) moment, in "The Muppet Movie." The line was, "Millions
Of People Happy..." And I swear on my Mother's grave, that sock puppet
had stars in his eyes, theatrically speaking.

But that has always resonated with me as a pretty cool life goal:
"Millions of people happy."

So, I try to aspire to "class clown", while also attempting to promote
my own particular brand of trans-meta-para-dyno-infra-philosopho-ergono-
radio-femto-gefilte-preposto-scientific quasiknowledge, that can only be
found within, underneath the noise of the voices that are already
directing you, _especially vehemently,_ to resist your own awakeness,
which means the uncreation of the evil, which to [ERRRNT - LOST CARRIER]

And I betcha "not get killed by some wacko empire builder's armies" is
only the tip of the iceberg as to possibilities for happiness worldwide.

I know it's right up there in the top thing of my agenda. %-}

Cheers!
Rich

 

[Joerg]

Hi Chris,

I have an old radio (circa 1930) that I keep because it fits in with the
surroundings (I live in a narrowboat with a vintage style stern and like to
keep some of the tradition) but the sound is beautiful. But hardly HiFi.


Well, how could that be HiFi? In 1930 there was no FM so your radio will

only feature the AM band, since you live in Europe probably also long
wave and then maybe a portion of short wave. I have one of those as
well, an old 'Sachsenwerk'. It states the dial position in meters
instead of kilohertz. Most of the stations listed on that dial are long
gone. The main problem is that the black paint of which the lettering
behind the dial glass consists is starting to flake off.

Living on a boat must be pretty romantic. If that is within an old
marina I am sure there are also plenty of nice pubs around.

Regards, Joerg

http://www.analogconsultants.com

 

[Ken Smith]

In article <KFUed.18276$nj.6532@newssvr13.news.prodigy.com>,
Joerg <notthisjoergsch@removethispacbell.net> wrote:
[...]

Besides the speaker, which source of audio can
truly claim 0.001%?

How about an SRS DS360? Its about in there. Listening to it would get
boring quickly though.

--
--
kensmith@rahul.net forging knowledge

 

[Joerg]

Hi Ken,

Besides the speaker, which source of audio can
truly claim 0.001%?



How about an SRS DS360? Its about in there. Listening to it would get
boring quickly though.


That sure is a nice generator. I don't know what it costs, probably in

the vicinity of a nicely equipped car.

Yes, listening would be boring but maybe it has a fast enough interface
to use it as a synthesizer for a rendition of a Led Zeppelin piece.
There you'd have to add some distortion back in to make it all sound
good ;-)

Regards, Joerg

http://www.analogconsultants.com

 

[Mac]

On Sun, 24 Oct 2004 05:13:55 +0000, Ken Smith wrote:

In article <pan.2004.10.24.04.21.01.963053@bar.net>, Mac <foo@bar.net> wrote:
On Sun, 24 Oct 2004 01:00:56 +0000, Joerg wrote:
[...]
betas are on, the driving stage (op-amp or other) experiences lower load
impedance than when one of the transistors is off. This non-linearity is
bound to have repercussions. But if you can get the bias just exactly
right, you would never have both transistors on, and you wouldn't have a
dead zone, either.

Anyway, he concludes that class AB is not a good idea. Either go fully
class A, or just go class B (again, with his definition of class B).

I disagree with this suggestion. What you want is for the output stage's
voltage to remain a linear function of the driver transistor's current. If
the current in one transistor stops exactly at the point where the current
in the other starts, there is a voltage at which the current in the
transistor is 0.3mA. Assuming an 8 ohm load, the voltage gain of the
output section will be 0.5 at that point. Obviously this can't be true
and have the system linear. This implies that some idle current must be
flowing so the stage must be AB.
--

Well, really what you want, I think, is to adjust the quiescent current of
the open loop circuit for lowest THD. At any rate, this was Self's
approach.

The system will not be perfectly linear no matter what you do, so your
argument is weak. Or at least, it isn't any more compelling than Self's
argument.

--Mac

 

[Kevin Aylward]

Mac wrote:

On Sun, 24 Oct 2004 09:48:50 +0000, Kevin Aylward wrote:

Mac wrote:
On Sun, 24 Oct 2004 01:00:56 +0000, Joerg wrote:

[snip]

This is why all linear audio amps
need a certain amount of quiescent current. Class D doesn't, of
course, but that's a whole other topic.

Have you ever read Douglas Self's book, _Audio Power Amplifier
Design Handbook_?

Some of this is apparantly here,
http://www.dself.dsl.pipex.com/ampins/dipa/dipa.htm


I scanned the article, and it looks very much like the book. I think
many of the figures are exactly the same.

[snip]

But the interesting thing is that he measures the THD and residuals
for class B (his definition) and Class AB open loop output stages,
and the class B stage has lower THD than class AB.

Maybe. Why Self claims that this possibility may be a "vital fact is
little known" is beyond me:

5.3 DISTORTION 3.
"THD increases as the bias advances into AB operation. This is due to
so-called "gm-doubling" (ie the voltage-gain increase caused by both
devices conducting simultaneously in the centre of the output-voltage
range, in the Class-A region) putting edges into the distortion
residual that generate high-order harmonics much as under-biasing
does. This vital fact is little known, presumably because
gm-doubling distortion is at a relatively low level and is obscured
in most amplifiers by other distortions"

This is very well known in the industry. For example, it's a
fundamental issue in designing op-amps with rail to rail inputs.
However, it a bit more complicated than this. The increase in gm due
to bias increase, generally gives better linearity and can produce,
in principle, a *lower* THD. Its not clear from Self's paper how he
actually determined that the optimal point occurred when the
transistors were actually on the edge of switch over, which infact
they cant do in such a manner anyway. These measurements should
really have been performed open loop to make the effect more
noticeable. For example, even if the thd was initially higher, the
higher gm might possibly make the system more stable (less effect of
re.Cload), allowing for more feedback which could reduce the
distortion. You simply don't want to operate output devices at too
low a current. Everything collapses.

Secondly, I have done quite a few measurements myself on this, and I
can state that for mosfets, the x-over distortion, by and large, can
keep going down as you move more and more into AB->A. In general,
there is no general claim that can be made here on the "best" bias
point.


Self takes a dim view of MOSFET output stages for power amps.

Yes I know, but he is misguided on this.

The basic argument against mosfets is that they are not as linear in an
emitter/source follower configuration because of their much lower gm.
This is correct. However, this is compensated by other advantages.
Mosfets are much faster with much lower capacitances. This results in a
simpler driver with less phase shift, and a allows for much larger
feedback. This larger feedback gets the distortion very low. At the
larger powers, say 500 watts, these aspects are very significant.
Typically you need 3 stages of emitter follower drivers, for the
bipolar, against only one driver stage for the mosfet. For example, a
modern high performance 50Mhz ft power transistor (MJL4281A) would have
an effective input capacitance of 1.5nf with a 4ohm load, a mosfet might
be only 250pf. In addition mosfets are much more resilient to overload.

The proof is in the pudding. All of the plots on Self's amp page show
THD at 20khz for his reference amps at over 0.01%. Without trying to
blow my own trumpet here, a 500W per chan mosfet amp I designed (i.e.
small modifications of standard circuits,
http://www.studiomaster.com/hp5.html) was < 0.005% at similar 8 ohm
loads.

The main argument for bipoler is that they can be cheaper.


Kevin Aylward
salesEXTRACT@anasoft.co.uk
http://www.anasoft.co.uk
SuperSpice, a very affordable Mixed-Mode
Windows Simulator with Schematic Capture,
Waveform Display, FFT's and Filter Design.

 

[Winfield Hill]

Kevin Aylward wrote...

The proof is in the pudding. All of the plots on Self's amp page show
THD at 20khz for his reference amps at over 0.01%. Without trying to
blow my own trumpet here, a 500W per chan mosfet amp I designed (i.e.
small modifications of standard circuits,
http://www.studiomaster.com/hp5.html) was < 0.005% at similar 8 ohm
loads.

Kevin, why don't you create a little web article with the details
of your amplifier design? It's been some time now, so surely the
proprietary aspects of your original design are no longer an issue?


--
Thanks,
- Win

(email: use hill_at_rowland-dotties-org for now)

 

[Kevin Aylward]

Winfield Hill wrote:

Kevin Aylward wrote...

Winfield Hill wrote:

Kevin, why don't you create a little web article with the details
of your amplifier design? It's been some time now, so surely the
proprietary aspects of your original design are no longer an issue?

Its not *really* my design in the sense that anything new is there.
Its what what we all do, a few simple mods to existing, standard
designs. Its my design in the sense that I had full *ownership* for
all electronic parts of the design. I was free to steal whatever I
needed.

The basic design is a diff pair feeding a diff pair with current
mirror, as shown in the Hitachi apps docs for the 2sk135/2sj50 pair.

Not everyone has access to the old Hitachi app notes.

My extra bit is simply adding in a push pull driver between the
second stage and the mosfets. This was an absolute must to get the
distortion down when driving many || fets. The reason for this is
actually a bit subtle. The main capacitance is Cgs, which is about
600p. If we || up, say two devices, the capacitance doubles, but so
does the gm, so the reflected input capacitance gmRl/(1+gmRl), say
100p, should remain constant. However, what one initially neglected
is the Cgd at 40p. This starts to mount up with many devices.

An excellent point.
I have often puzzled at the wimpy MOSFET gate
drive circuits in most hi-fi amps. Hmm, any issue with crossover
distortion in your push pull driver?

No. I ran it in class A at 20ma. Each mosfet (5 in ||) had its own 1k
gate resistor.

I still think you should do a nice writeup of your design: schematic,
discusssion (as above),

It had +/-75V for the main output, and an independent +/-90V supply for
the actual amp. Not only does this avoid the mosfet gate drive issue,
but avoids having volts of triangle ripple on the small signal power
lines.

a good photo of one of your early production
amps still in use someplace, etc.

I have one at home. A bit battered though as it over 20 years old.

Kevin Aylward
salesEXTRACT@anasoft.co.uk
http://www.anasoft.co.uk
SuperSpice, a very affordable Mixed-Mode
Windows Simulator with Schematic Capture,
Waveform Display, FFT's and Filter Design.

 

[Roy McCammon]

ChrisGibboGibson wrote:

On audio, take the output of an opamp and feed it into dual emitter followers
with no overall NFB.

The distortion looks horrendous on a scope and sounds it. Take the NFB from
outside the emitter followers and it looks great on a scope, sounds better, but
the distortion as measured by a THD meter is the same. The harmonics have moved
(much higher in frequency) but the total %age is the same.

My own limited experience is the opposite. I started
with lousy cross-over. Added NFB. The THD got lower,
but it still sounded lousy.


--
local optimization seldom leads to global optimization

my e-mail address is: <my first name> <my last name> AT mmm DOT com

 

[Ken Smith]

In article <417D0853.20003@mmm.com>, Roy McCammon <rbmccammon@mmm.com> wrote:
[...]

My own limited experience is the opposite. I started
with lousy cross-over. Added NFB. The THD got lower,
but it still sounded lousy.

There are posible reasons:

If you only suppress the 3rd harmonic, the THD goes down but the result
doesn't sound much better.

If you have too little phase margin in the feedback loop, the peaking at
the high end can make for some funny artifacts. If the gain crossover
point is too close to the human hearing range, these artifacts can be
heard.

Intermodulation distortiong and THD don't always track each other. If you
really work at it, you can make a circuit where increasing the NFB leaves
a high IM distortion. Perhaps you (un)lucked onto one of those
situations.

--
--
kensmith@rahul.net forging knowledge

 

[John Larkin]

I build amps that need a few PPM noise+distortion up to, say, 50 KHz
bw and peak powers up to 17 KW. My first rule is to make the amp as
linear as possible *before* applying overall nfb. The issue is that,
near the bw limit, there's not a lot of excess gain available, so the
feedback doesn't help much.

John

 

[Rich Grise]

On Tue, 26 Oct 2004 14:38:32 -0700, Tim Shoppa wrote:

Mac <foo@bar.net> wrote in message news:<pan.2004.10.24.04.21.01.963053@bar.net>...
In it he argues that if the output transistors are not conducting half the
time for a sine wave, then it shouldn't be called "class B." He reserves
the term "class B" for when there is an essentially seamless transition
from the upper transistor to the lower one, but no period where they
both conduct. So if the bases of the output transistors are connected
together, it is NOT class B, according to Self. This makes some sense.

It's more of a tautology. If you define class B that exact way, then
no real circuit is ever class B. It's like asking for all people who
are exactly 6 feet tall: nobody is exactly 6 feet tall. There will be
some people who are between 5.99999 and 6.00001 feet tall, though.

Most will be 6' 1/2" in the morning and 5' 11 1/2" in the evening.

;-)
Rich

 

[Terry Given]

Rich Grise wrote:

On Tue, 26 Oct 2004 14:38:32 -0700, Tim Shoppa wrote:


Mac <foo@bar.net> wrote in message news:<pan.2004.10.24.04.21.01.963053@bar.net>...

In it he argues that if the output transistors are not conducting half the
time for a sine wave, then it shouldn't be called "class B." He reserves
the term "class B" for when there is an essentially seamless transition
from the upper transistor to the lower one, but no period where they
both conduct. So if the bases of the output transistors are connected
together, it is NOT class B, according to Self. This makes some sense.

It's more of a tautology. If you define class B that exact way, then
no real circuit is ever class B. It's like asking for all people who
are exactly 6 feet tall: nobody is exactly 6 feet tall. There will be
some people who are between 5.99999 and 6.00001 feet tall, though.


Most will be 6' 1/2" in the morning and 5' 11 1/2" in the evening.

;-)
Rich

and lets not forget that yoga practitioners tend to get taller with age,
as opposed to the rest of the population, who shrink (many try to
compensate for this by expanding sideways)

Cheers
Terry

 

[Walter Harley]

"ChrisGibboGibson" <chrisgibbogibson@aol.com> wrote in message
news:20041023204151.02596.00002525@mb-m13.aol.com...

Even if you only count the distortion products within the audio band
(which
gets lower with each passing year for me) the overall THD is only slightly
lower. Yet the difference in sound quality is dramatic.

OK, you've got a scope and a distortion analyzer. So, what do the residuals
look (and sound) like? If your question is answerable, that's where the
answer will lie.

Personally, I confess to some skepticism about the assertion that the THD is
only slightly lower; I've not tried that precise experiment myself, but it
seems counterintuitive. Before I gave it too much credence I'd want to make
sure that the THD meter was working properly and there wasn't some other
oddness going on. For instance, if your sine wave source was distorted to
begin with (in some visually innocuous way, like slightly flattened peaks)
that could provide enough THD to mask a large change in xover distortion,
without being particularly audible. Looking at the residuals would quickly
tell you the difference.

Note that crossover distortion, although it is sonically ugly, only lasts a
short while and happens at close to zero volts, and therefore does not have
much energy. Thus it doesn't actually contribute that much THD to begin
with, in percentage terms. This is just one reason why THD, as a raw
number, is not very informative about audio quality.

 

[Ken Smith]

In article <a7076635.0410280448.67c2bb18@posting.google.com>,
N. Thornton <bigcat@meeow.co.uk> wrote:
[...]

would normally cost well over a weeks wages, and those valves didnt
last forever. Pfb was big business, just about no-one took nfb
seriously. Hence the development of circuits that amplified the rf,
detected it, then fed it back as audio through the same set of valves
a 2nd time round

In my younger days, I built a transistor version of a refex super regen
receiver. It worked surprisingly well. The audio could drive a small
speaker with just 2 transistors and a modest antenna.

--
--
kensmith@rahul.net forging knowledge

 

[ChrisGibboGibson]

kensmith wrote:

In article <a7076635.0410281601.51fa8e82@posting.google.com>,
N. Thornton <bigcat@meeow.co.uk> wrote:
[...]
Lot of sets back then were 2 valvers - and valve gain was pretty tame
in 1930. I built a reaction set with 2 trs that ran a speaker, with
only one rf tr it needed a big long aerial and only got 2 stations,
but sounded surprisingly good. But stabilty... jeez.

One of the advantages of super-regen. is that it tends to self servo to
swinging just above and below the point of oscillation. Such designs have
two time constants in the self-squegging circuit. The short TC sets the
frequency of the sqegging action and the longer one rides along at the
point of oscillation.

IIRC:
In the one I made, the first transistor was a RF amplifier and also served
as the audio output amplifier. The second transistor was the regen stage.
The first stage is the one that provides almost all of the selectivity.
The second stage is basically sampling the RF at a 30KHz rate.

Is it coincidence that we have another thread along the same lines or has
something got mixed up in the threads ?

Gibbo

 

[Ken Smith]

In article <20041028213818.18151.00005452@mb-m03.aol.com>,
ChrisGibboGibson <chrisgibbogibson@aol.com> wrote:
[...]

Is it coincidence that we have another thread along the same lines or has
something got mixed up in the threads ?

The same people, I'm sure, read both threads. This one was more about the
history and not the current use.

--
--
kensmith@rahul.net forging knowledge

 

[ChrisGibboGibson]

Joerg wrote:

Hi Chris,

On audio, take the output of an opamp and feed it into dual emitter
followers
with no overall NFB.

The distortion looks horrendous on a scope and sounds it. Take the NFB from
outside the emitter followers and it looks great on a scope, sounds better,
but
the distortion as measured by a THD meter is the same. The harmonics have
moved
(much higher in frequency) but the total %age is the same.

Even if you only count the distortion products within the audio band (which
gets lower with each passing year for me) the overall THD is only slightly
lower. Yet the difference in sound quality is dramatic.


If by emitter followers you mean a push-pull stage with an npn/pnp pair
but no or nearly no quiescent current

Yes that's exactly what I mean. No bias whatsoever.

this might be explained by the
fact that this power stage can't swing it fast enough despite the
feedback. It goes through a brief phase where its input voltage changes
but the output voltage doesn't since the driving amp is not infinitely
fast,

Correct

neither are power transistors. Usually that is the range to reach
the respective Vbe. With feedback it tries hard to "regulate out" this
gap which is why you hear an improvement. Leftover distortion products
move higher in frequency because the drive amp speed is kind of ok to
transit through that non-linear range reasonably fast for a few hundred
Hertz of audio but maybe not for 5KHz. This is why all linear audio amps
need a certain amount of quiescent current. Class D doesn't, of course,
but that's a whole other topic.

As to trolling, I have no clue why somone would have said that. AFAIK
trolls are characters in Scandinavian fairy tales and I don't know why
this word got such a bad rap.

I assume you are scandinavian?

Apparently it's old chestnut in HiFi circles.

THD remains the same but it sounds better. But the same ones who argue that the
NFB outside the emitter pair is better because the frequency of the distortion
products have moved are the same ones who argue that tube s can't be better
than transistors for the same reason. Like I said, it's an old HiFi chestnut.

Gibbo