Crossover distortion and NFB

[ChrisGibboGibson]

A few years ago I posted a similar question and got...

A) Told to stop trolling.

B) Told to stop being a cock

Or

C) Told to go back to school

I'm still no wiser. I did all three and I'm still no wiser.

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.

Now I know about logarithmic ears etc. But why was I told to stop trolling?

I'm only posting this because everyone else is talking about Dubbya or Kerry
which I suppose means that, technically, it *is* trolling but surely it's more
interesting. Especially for some of us outside USA who don't give a flying fu*k
who you get lumbered with. Whoever it is, our kissass PM will be stuck right up
him anyway.

Gibbo

 

[Joerg]

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 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, 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.

Regards, Joerg

http://www.analogconsultants.com

 

[ChrisGibboGibson]

(John S. Dyson) wrote:

[snip]

My guess is that the term 'trolling' comes from a fishing technique
where (AFAIR), the boat is moving slowly and the hook/bait is moving
by attempting to tempt fish to bite. The 'trolling' metaphor is
probably meant like throwing out some 'bait' for the fish to bite.

The 'bait' in this case is likely some kind of outrageous or
controversial comment or claim. When someone responds to the
'baited' claim, then that is equivalent to the fish taking the bait.

How the slow movement of the boat, with the fishing pole hanging
over the edge (loaded with hook and bait), trying to entice a fish
to bite is associated with the term 'trolling' is something that
is apparently not directly related to the mythical creature. The
term that describes the slow movement probably comes from the
Germanic word 'trollen.'

All that makes perfect sense.

Gibbo

 

[ChrisGibboGibson]

Joerg wrote:

[snip]

Tubes? Well, they just sound great. Then there is that glow, a little
hum, a crackle now and then. Anyway, feedback also works great with
tubes. For some reason it wasn't done a lot in their days.

No doubt whatsoever. But it isn't because of their fidelity. It's because of
their distortion.

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.

Gibbo

 

[Joerg]

Hi John,

My guess is that the term 'trolling' comes from a fishing technique
where (AFAIR), the boat is moving slowly and the hook/bait is moving
by attempting to tempt fish to bite. The 'trolling' metaphor is
probably meant like throwing out some 'bait' for the fish to bite.


That's it I guess. Even some small outboard motors in marine stores are

listed as trolling motors. A few are electric to keep the noise level down.

... The term that describes the slow movement probably comes from the Germanic word 'trollen.'


In German it's a bit of a slang word, not in a bad sense. When someone

gets tired late in the evening and says 'ich trolle mich' that means 'I
am turning in for the night'.

Regards, Joerg

http://www.analogconsultants.com

 

[ChrisGibboGibson]

Rich Grise wrote:

[snip]

It's because they eat goats

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

Gibbo

 

[Rich Grise]

On Sun, 24 Oct 2004 02:36:11 +0000, ChrisGibboGibson wrote:

(John S. Dyson) wrote:

[snip]

My guess is that the term 'trolling' comes from a fishing technique where
(AFAIR), the boat is moving slowly and the hook/bait is moving by
attempting to tempt fish to bite. The 'trolling' metaphor is probably
meant like throwing out some 'bait' for the fish to bite.

The 'bait' in this case is likely some kind of outrageous or
controversial comment or claim. When someone responds to the 'baited'
claim, then that is equivalent to the fish taking the bait.

How the slow movement of the boat, with the fishing pole hanging over the
edge (loaded with hook and bait), trying to entice a fish to bite is
associated with the term 'trolling' is something that is apparently not
directly related to the mythical creature. The term that describes the
slow movement probably comes from the Germanic word 'trollen.'


All that makes perfect sense.

And plus, not only that, "trollen" sounds a lot like "trollin'", so

it's only natural to turn it into a verb.

But I do think the noun form is intended to have unpleasant connotations,
as in a monster who lives under a bridge and terrorizes passersby, and
whose intent is to eat the unsuspecting.

Hm. I think I've just described "Homeland Security." ;-)

Cheers!
Rich

 

[Tam/WB2TT]

"ChrisGibboGibson" <chrisgibbogibson@aol.com> wrote in message
news:20041023215302.23728.00002316@mb-m12.aol.com...

"Walter Harley" wrote:


"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?


Far better

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;


Exactly my feelings. But measurements don't bear out my gut feelings. And
other
posters proved mathematically (with maths that is way beyond my abilities)
that
NFB cannot reduce the overall crossover distortion, it can only move it
elsewhere.

I've not tried that precise experiment myself, but it
seems counterintuitive.


It does indeed

Before I gave it too much credence I'd want to make
sure that the THD meter was working properly


I did, many times

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,


Indeed. We are talking the 6% THD caused by crossover as compared with
real
equipment with a THD figure of 0.01%

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


But it's during the silent period, and ears are logarithmic. But how
quickly?

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.


Indeed. Which expains why I was puzzled when I was accused of being a
troll
when I first posed this question.

For example why is 0.1% THD as a result of clipping inaudible to most
people
yet 0.1% as a result of crossover distortion is blatanly obvious to anyone
with
even the clothest of ears? They are still odd harmonics.

Having said that. The one who accused me of being a troll has since been
shown
to be a cock.

A sinewave with 5% THD as a result of clipping actually doesn't sound so
bad.
Some people can't even hear it. But add THD as a result of crossover and
it's
blantantly obvious and sounds awful.

I was genuinely asking a real question.

Gibbo

There is basically something wrong here. There are probably 100 million
amplifiers out there that work the way you describe that don't have high
measurable THD. If you built the amp yourself, the op amp probably has
insufficient slew rate, or you have no diode equalizer for the Vbe drops. If
you bought it, it is probably broken.

Tam

 

[ChrisGibboGibson]

"Tam/WB2TT" wrote:

[snip]

There is basically something wrong here.

Obviously

There are probably 100 million
amplifiers out there that work the way you describe

No I doubt it. They all use biasing to get rid of 99.9% of the THD as a result
of crossover.

that don't have high
measurable THD. If you built the amp yourself, the op amp probably has
insufficient slew rate, or you have no diode equalizer for the Vbe drops. If
you bought it, it is probably broken.

I think you misunderstood the question.

The question is why that particular amp with no biasing to the output pair,
sounds almost acceptable with NFB but totally unbearable without it. It
shouldn't really be much different. THD figures are almost identical both ways.

Even spectral analysis bears this out, just that the harmonics are in a
different place (but still. audible)

Gibbo

 

[ChrisGibboGibson]

Rich Grise wrote:

Summat....

You're close!

Gibbo

 

[Rich Grise]

On Sun, 24 Oct 2004 01:53:02 +0000, ChrisGibboGibson wrote:

"Walter Harley" wrote:
"ChrisGibboGibson" <chrisgibbogibson@aol.com> wrote in message
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?
Far better
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;
Exactly my feelings. But measurements don't bear out my gut feelings. And
other posters proved mathematically (with maths that is way beyond my
abilities) that NFB cannot reduce the overall crossover distortion, it can
only move it elsewhere.

I was gonna try to follow this thread a little, to be able to think my
imput here might make some sense.

And bottom line, when I think about in the context that I _think_
ChrisGibboGibson is alluding to, is that the objective numbers and the
subjective sound don't necessarily correlate, although I'm kinda
tending to a discontinuous or lumpy inverse proportional-type thing.

I'm doing a gedankenexperiment here, where I'm imagining what a clipped
sine wave might sound like vs. what a crossover-distorted sine wave might
sound like, and the crossover-distorted one "sounds" worse. ;-)

It's kinda like, "Boooooowwwwwrrrrrrrrrzzzzz" vs. "Boorraaaaaaeeeeeeiii"

Try it some time!

Cheers!
Rich

 

[Mac]

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_?

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.

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. This makes a certain amount
of sense, too, because when both output transistors with their finite
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).


[snip]

Regards, Joerg

http://www.analogconsultants.com

--Mac

 

[Ken Smith]

In article <bpEed.17820$nj.950@newssvr13.news.prodigy.com>,
Joerg <notthisjoergsch@removethispacbell.net> wrote:
[...]

Tubes? Well, they just sound great. Then there is that glow, a little
hum, a crackle now and then. Anyway, feedback also works great with
tubes. For some reason it wasn't done a lot in their days.

When you have to pay an hours wages for a gain of 10, you are a lot less
willing to give it up than when it costs you less than 1 seconds wages.
Tubes cost a lot to make even back then.


--
--
kensmith@rahul.net forging knowledge

 

[Ken Smith]

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.
--
--
kensmith@rahul.net forging knowledge

 

[Walter Harley]

"ChrisGibboGibson" <chrisgibbogibson@aol.com> wrote in message
news:20041023215302.23728.00002316@mb-m12.aol.com...

OK, you've got a scope and a distortion analyzer. So, what do the
residuals
look (and sound) like?

Far better

I'm wondering what they look like, not how they look. That is, if the
primary distortion mode is indeed crossover, I would expect to see the
residual be close to zero for most of the wave, with a pair of spikes at
each zero crossing. When corrected with NFB, the spikes should be steeper
and shorter in time. Is that what you see? What does the residual sound
like, in the two cases?

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,

Indeed. We are talking the 6% THD caused by crossover as compared with
real
equipment with a THD figure of 0.01%

Sorry, I didn't quite follow your answer there. Can you clarify?

Indeed. Which expains why I was puzzled when I was accused of being a
troll
when I first posed this question.

Blame the goat fanciers.

For example why is 0.1% THD as a result of clipping inaudible to most
people
yet 0.1% as a result of crossover distortion is blatanly obvious to anyone
with
even the clothest of ears? They are still odd harmonics.

Quite so. At this point, you can look to several sorts of explanation, none
very satisfying: for instance, explanations from the standpoint of
evolutionary biology, from the standpoint of psychoacoustics, or from the
standpoint of sensory biology.

I was genuinely asking a real question.

I'm interested in the answer too. Personally I've always just gone for
bias, because the idea of asking an amp to slew as rapidly as it can just
seems like asking for trouble; if nothing else, there's going to be a spike
on the power supply that will be hard to keep out of the sensitive
circuitry. But I agree that your findings are counterintuitive and, if
experiments and math bear them out (which you assert they have done), that
is always fascinating.

I still feel like the answer is in the residuals.

 

[Kevin Aylward]

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

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.

Yes. There is some inconsistent terminology used here. The classic
non-biased push-pull pair might better be class C. No one does though.

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.

Anyway, he seems to make a habit of these "little known phenomena"
claims, like on the same paper,

"It seems to be little-known that electrolytic capacitors generate
distortion when they have a significant AC voltage across them. It is
even less well known that non-electrolytic show a similar effect in
applications like Sallen & Key high-pass filters."

Again, this is a fundamental issue in designing speaker x-overs. That's
why one often uses polypropylene caps. I can't imagine anyone engaged in
such matters not being aware of this.

This makes a certain
amount of sense, too, because when both output transistors with their
finite betas are on, the driving stage (op-amp or other) experiences
lower load impedance than when one of the transistors is off.

The effect is fundamentally due to gm (transconductance) doubling, not
hfe.

The voltage gain is:

Av = gm.Rl/(1 + gm.RL)

Change is gm effect the voltage gain.

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.

I dont see that he actually makes that conclusion in that paper.

Either go fully
class A, or just go class B (again, with his definition of class B).

I dont see this as being practical really. In general, you can't have
production amps tweaked up like this. For starters, bias current
temperature compensation never tracks perfectly. 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.


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.

 

[Kevin Aylward]

ChrisGibboGibson wrote:

"Walter Harley" wrote:


"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?


Far better

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;


Exactly my feelings. But measurements don't bear out my gut feelings.
And other posters proved mathematically (with maths that is way
beyond my abilities) that NFB cannot reduce the overall crossover
distortion, it can only move it elsewhere.

I've not tried that precise experiment myself, but it
seems counterintuitive.


It does indeed

Before I gave it too much credence I'd want to make
sure that the THD meter was working properly


I did, many times

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,


Indeed. We are talking the 6% THD caused by crossover as compared
with real equipment with a THD figure of 0.01%

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


But it's during the silent period, and ears are logarithmic. But how
quickly?

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.


Indeed. Which expains why I was puzzled when I was accused of being a
troll when I first posed this question.

For example why is 0.1% THD as a result of clipping inaudible to most
people yet 0.1% as a result of crossover distortion is blatanly
obvious to anyone with even the clothest of ears? They are still odd
harmonics.

Having said that. The one who accused me of being a troll has since
been shown to be a cock.

A sinewave with 5% THD as a result of clipping actually doesn't sound
so bad. Some people can't even hear it. But add THD as a result of
crossover and it's blantantly obvious and sounds awful.

I was genuinely asking a real question.

THD is an average measurement, i.e. its the (root) mean square error. It
takes no account of the instantaneous error. Near the 0-xing point the
relative error is huge, e.g.the output should be 0.5, yet is producing
0. This is a 100% instantaneous error. At clipping say, 10V output, with
desired output of 10.5 (similar THD) the instantaneous error is only
0.5/10 = 5%.

One might thus conclude that the ear/brain can detect instantaneous
errors in the time domain. Even if this is only by means of analysing
the structure in the frequency domain, why should one spectrum of
distortion sound the same as another one that is completely different?
Not, why should all rms measurements sound the same just becase the
value is the same.

Suppose we have a group of people with heights, gassusain distributed.
Now have another group where everyone is the same, except for a few
giants and dwarfs. Such groups can easily have the same standard
deviation, yet obviously be very different and easily distinguished.

I don't see any rational reason why an average measurement *should* be
enough to determine the effect of the details of the what is being
measured. If the ear/brain was a thermal sensor, sure, but it isn't.


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.

 

[Ken Smith]

In article <hSKed.11188$i02.1305@fe1.news.blueyonder.co.uk>,
Kevin Aylward <salesEXTRACT@anasoft.co.uk> wrote:

Ken Smith wrote:
[...]
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.

Not exactly. The goal is for the output voltage to follow the input
voltage linearly. The driver current is not a linear function of the
base emitter voltage, its exponential.

In the designs I was thinking of, the compensation is done at the driver
stage. This makes the driver's transfer quite linear at high frequencies.



--
--
kensmith@rahul.net forging knowledge

 

[john jardine]

Harmonics should read "rms" not "pp"

 

[Tam/WB2TT]

"ChrisGibboGibson" <chrisgibbogibson@aol.com> wrote in message
news:20041023235651.22732.00002553@mb-m15.aol.com...

"Tam/WB2TT" wrote:

[snip]

There is basically something wrong here.

Obviously

There are probably 100 million
amplifiers out there that work the way you describe


No I doubt it. They all use biasing to get rid of 99.9% of the THD as a
result
of crossover.

I mentioned biasing diodes. Why would you not use them? Run SWCAD on your
design, and do an FFT on the output.

Tam

that don't have high
measurable THD. If you built the amp yourself, the op amp probably has
insufficient slew rate, or you have no diode equalizer for the Vbe drops.
If
you bought it, it is probably broken.


I think you misunderstood the question.

The question is why that particular amp with no biasing to the output
pair,
sounds almost acceptable with NFB but totally unbearable without it. It
shouldn't really be much different. THD figures are almost identical both
ways.

Even spectral analysis bears this out, just that the harmonics are in a
different place (but still. audible)

Gibbo