Distortion... why/how is it created?

[[email protected]]

While so many people run around with their hands in the air talking
about this amp and that amp, their distortion measurements,
tube-vs-transistor, yadda-yadda-yadda, I'm concerned with something
much more fundamental.

If the phrase is true "all amplifiers cause distortion," which I
believe is the case, my question is simple...

How is it created?

For instance, let's take the scenario of an all-analog, all pure
class-A staged amplifier.. My *assumption* is that in an ideal model,
this scenario would generate no distortion, but in using real-world
components, distortion is still generated.

I understand that there exist what are called "nonlinearities" in the
amplifier, where at some input levels, a change of the input voltage
causes a particular change in the output voltage, but at some other
input level voltage, the same change in voltage (just offset from the
original) would cause a different amount of change in the output.

So is distortion's root this nonlinearity?

And if so, why does this nonlinearity always manifest itself as n-order
harmonics?

And how does clipping come into the picture?

I've read that class-A tube distortion is "more pleasing" because most
of its generated harmonic content are low-order fundamentals with a
steep rolloff (n < 5), but push-pull (transistor-based but even
apparent in push-pull tube) topologies have a less-steep rolloff, with
harmonics still of decent amplitude even with the higher-order
harmonics (n>10).

Even if that is assumed to be true, what causes the tube to have a
steeper harmonic rolloff? One article I read seemed to imply it had to
do with a tube being a "high impedance" amplifier. Not sure what that
means, if you compare a 30W tube amp to a 30W transistor amp, what's
different? Could you adapt a transistor-based circuit topology to act
as a higher impedance amplifier?

Some quick background-- I've got an EE degree in electrical and
computer engineering with emhpasis on the digital realm of circuit
design. But I've been trying to go back and "fill in the details" in
the analog world due to my heavy interest in audio. So while I easily
understand some EE topics, others I may not have as fundamental a grasp
on.

Any and all input would be greatly appreciated!

...dane

[Harry Lavo]

> wrote in message
ps.com...
> While so many people run around with their hands in the air talking
> about this amp and that amp, their distortion measurements,
> tube-vs-transistor, yadda-yadda-yadda, I'm concerned with something
> much more fundamental.
>
> If the phrase is true "all amplifiers cause distortion," which I
> believe is the case, my question is simple...
>
> How is it created?
>
> For instance, let's take the scenario of an all-analog, all pure
> class-A staged amplifier.. My *assumption* is that in an ideal model,
> this scenario would generate no distortion, but in using real-world
> components, distortion is still generated.
>
> I understand that there exist what are called "nonlinearities" in the
> amplifier, where at some input levels, a change of the input voltage
> causes a particular change in the output voltage, but at some other
> input level voltage, the same change in voltage (just offset from the
> original) would cause a different amount of change in the output.
>
> So is distortion's root this nonlinearity?
>
> And if so, why does this nonlinearity always manifest itself as n-order
> harmonics?
>
> And how does clipping come into the picture?
>
> I've read that class-A tube distortion is "more pleasing" because most
> of its generated harmonic content are low-order fundamentals with a
> steep rolloff (n < 5), but push-pull (transistor-based but even
> apparent in push-pull tube) topologies have a less-steep rolloff, with
> harmonics still of decent amplitude even with the higher-order
> harmonics (n>10).
>
> Even if that is assumed to be true, what causes the tube to have a
> steeper harmonic rolloff? One article I read seemed to imply it had to
> do with a tube being a "high impedance" amplifier. Not sure what that
> means, if you compare a 30W tube amp to a 30W transistor amp, what's
> different? Could you adapt a transistor-based circuit topology to act
> as a higher impedance amplifier?
>
> Some quick background-- I've got an EE degree in electrical and
> computer engineering with emhpasis on the digital realm of circuit
> design. But I've been trying to go back and "fill in the details" in
> the analog world due to my heavy interest in audio. So while I easily
> understand some EE topics, others I may not have as fundamental a grasp
> on.
>
> Any and all input would be greatly appreciated!
>
> ..dane

Dane, nothing wrong with posting this here. But you may get additional
input and a higher signal-to-noise ration by posting it on rec.audio.tech as
well.

[Arny Krueger]

> wrote in message
ps.com...
> While so many people run around with their hands in the air talking
> about this amp and that amp, their distortion measurements,
> tube-vs-transistor, yadda-yadda-yadda, I'm concerned with something
> much more fundamental.
>
> If the phrase is true "all amplifiers cause distortion," which I
> believe is the case, my question is simple...
>
> How is it created?
>
> For instance, let's take the scenario of an all-analog, all pure
> class-A staged amplifier.. My *assumption* is that in an ideal model,
> this scenario would generate no distortion, but in using real-world
> components, distortion is still generated.

When you say "In an ideal model, this scenario would generate no
distortion" you may not mean what you said can be interpreted. A reasonably
detailed but ideal math model of an amplifier would show that it generates
distortion.

> I understand that there exist what are called "nonlinearities" in the
> amplifier, where at some input levels, a change of the input voltage
> causes a particular change in the output voltage, but at some other
> input level voltage, the same change in voltage (just offset from the
> original) would cause a different amount of change in the output.

> So is distortion's root this nonlinearity?

Yes.

> And if so, why does this nonlinearity always manifest itself as n-order
> harmonics?

Not always. Consider an amplifier that is nonlinear but also acts like a
bandpass filter. Any harmonics that might be generated fall outside the
bandpass, so they never show up in the real world outside the amplifier.
That's a pathological example, but amplifiers like this do exist,
particularly in radio receivers and transmitters.

In the general case, an amplifier can be charactized as having an
input-output plot associated with it. An ideal amp would plot a straight
line. A real-world amp will show some curving and maybe even some looping.

If the line is curved but smooth, it can be represented by a polynomial of
X. Applying a signal to the amplifier can be modelled as plugging in
various values for X. If the input is a sine wave, and the polynomial has
non-zero coefficients for orders 2 and greater, then the output will contain
harmonics in accordance with the trig identities for sine squared, etc.

> And how does clipping come into the picture?

Clipping is like a really sharp turn in that line we hoped was straight.

> I've read that class-A tube distortion is "more pleasing" because most
> of its generated harmonic content are low-order fundamentals with a
> steep rolloff (n < 5), but push-pull (transistor-based but even
> apparent in push-pull tube) topologies have a less-steep rolloff, with
> harmonics still of decent amplitude even with the higher-order
> harmonics (n>10).

Like all generalities, this is false. Pleasing can be a complex value
judgement. More audible is something that can be quantified more simply. It
is true that higher order harmonics are more audible all other things being
equal. It is also true that a perfectly balanced and symmetrical push-pull
circuit can produce no even order distortion. Therefore it can't produce
second-order distortion, and all distortion is third or higher (and odd) and
this being higher order than two is more audible, again all other things
being equal. But, in the real world all other things never are equal! ;-)

> Even if that is assumed to be true, what causes the tube to have a
> steeper harmonic rolloff?

Fantasies.

The most important thing about distortion is keeping it low enough so that
it isn't heard, if you goal is clean reproduction.

> One article I read seemed to imply it had to
> do with a tube being a "high impedance" amplifier. Not sure what that
> means, if you compare a 30W tube amp to a 30W transistor amp, what's
> different?

One thing that is different is the internals of the parts and how they
respond to inputs and outputs. For example, some tubes may under some
conditions have an input/output characteristic that follows a 3/2 power law.
Some transistors, under some conditions may have an input/output
characteristic that follows an exponential law. But, there are lots of ways
to use both tubes and transistors. There are lots of different types of
tubes and transistors, as well.

> Could you adapt a transistor-based circuit topology to act
> as a higher impedance amplifier?

Yes for what good that will do you.

> Some quick background-- I've got an EE degree in electrical and
> computer engineering with emphasis on the digital realm of circuit
> design. But I've been trying to go back and "fill in the details" in
> the analog world due to my heavy interest in audio. So while I easily
> understand some EE topics, others I may not have as fundamental a grasp
> on.

Take a look at "The Art Of Electronics" by Horwitz et al. It ain't cheap,
but it might make up a good starting point for you to stary learning about
the wonderful world of (more) linear electronics.

If you want a good classic look at tubed audio electronics, you can find
copies of the Radiotron Designer's Handbook (RDH4) in PDF format online.

[Trevor Wilson]

> wrote in message
ps.com...
> While so many people run around with their hands in the air talking
> about this amp and that amp, their distortion measurements,
> tube-vs-transistor, yadda-yadda-yadda, I'm concerned with something
> much more fundamental.
>
> If the phrase is true "all amplifiers cause distortion," which I
> believe is the case, my question is simple...
>
> How is it created?

**Non linearities in the active elements. Non linearities in the passive
elements.

>
> For instance, let's take the scenario of an all-analog, all pure
> class-A staged amplifier.. My *assumption* is that in an ideal model,
> this scenario would generate no distortion, but in using real-world
> components, distortion is still generated.

**Your assumption is incorrect. No active device is perfectly linear.
Therefore distortion will always be produced.

>
> I understand that there exist what are called "nonlinearities" in the
> amplifier, where at some input levels, a change of the input voltage
> causes a particular change in the output voltage, but at some other
> input level voltage, the same change in voltage (just offset from the
> original) would cause a different amount of change in the output.
>
> So is distortion's root this nonlinearity?

**One of them. The major one.

>
> And if so, why does this nonlinearity always manifest itself as n-order
> harmonics?

**Pretty much.

>
> And how does clipping come into the picture?

**Take an amplifier with a Voltage gain of (say) 20 and a sensitivity of 1
Volt input for 20 Volts output. Apply 1.1 Volts input and the result will be
clipping, since the amplifier runs out of supply Volts.

>
> I've read that class-A tube distortion is "more pleasing" because most
> of its generated harmonic content are low-order fundamentals with a
> steep rolloff (n < 5), but push-pull (transistor-based but even
> apparent in push-pull tube) topologies have a less-steep rolloff, with
> harmonics still of decent amplitude even with the higher-order
> harmonics (n>10).

**You've never read that in any publictaion which has any technical
credence. Because it is complete bull****. ANY amplifier which employs
similar topology will produce similar types and levels of distortion,
regardless of the active devices.
I.E: Take a push pull Triode tube amp and compare it to a push pull BJT amp,
which uses an output transformer.
OR
Take a SE Triode Class A amplifier and compare it to a SE BJT Class A
amplifier (with output transformer) and the resulting distortion products
will be virtually identical.

TOPOLOGY is what matters.

>
> Even if that is assumed to be true, what causes the tube to have a
> steeper harmonic rolloff?

**Output transformer.

One article I read seemed to imply it had to
> do with a tube being a "high impedance" amplifier. Not sure what that
> means, if you compare a 30W tube amp to a 30W transistor amp, what's
> different?

**Depends on the topology.

Could you adapt a transistor-based circuit topology to act
> as a higher impedance amplifier?

**Sure.

>
> Some quick background-- I've got an EE degree in electrical

**Which begs the question.......

and
> computer engineering with emhpasis on the digital realm of circuit
> design. But I've been trying to go back and "fill in the details" in
> the analog world due to my heavy interest in audio. So while I easily
> understand some EE topics, others I may not have as fundamental a grasp
> on.
>
> Any and all input would be greatly appreciated!

**As has been mentioned elsewhere, buy Self's book.


--
Trevor Wilson
www.rageaudio.com.au



--
Posted via a free Usenet account from http://www.teranews.com

[[email protected]]

Trevor Wilson wrote:

> > So is distortion's root this nonlinearity?
>
> **One of them. The major one.

For the sake of completeness, then, what are some other causes?

> > And if so, why does this nonlinearity always manifest itself as n-order
> > harmonics?
>
> **Pretty much.

?? not sure what that means; I asked a why question, not a yes/no?

> > And how does clipping come into the picture?
>
> **Take an amplifier with a Voltage gain of (say) 20 and a sensitivity of 1
> Volt input for 20 Volts output. Apply 1.1 Volts input and the result will be
> clipping, since the amplifier runs out of supply Volts.

Yes, that's true, I understand that concept. Wasn't my intended
question though. Based on another reply in rec.audio.tech, I've
adapted my question here to be:

Why is distortion always manifested in whole-order harmonics of the
fundamental, rather than decimal-order harmonics? IOW, if inputting a
1 kHz tone, why doesn't some 1.05kHz multiple come out, if it's all due
to nonlinearities, certainly there can exist a nonlinear element that
would produce a decimal-multiple distortion coefficient?

> I.E: Take a push pull Triode tube amp and compare it to a push pull BJT amp,
> which uses an output transformer.
> OR
> Take a SE Triode Class A amplifier and compare it to a SE BJT Class A
> amplifier (with output transformer) and the resulting distortion products
> will be virtually identical.
>
> TOPOLOGY is what matters.

I'll agree with this statement. Mine was a bit shortsighted, and I've
since done some more reading online (a good one is
http://www.silcom.com/~aludwig/Amplifier_distortion.htm) and agree that
it's the topology that matters.

> > Even if that is assumed to be true, what causes the tube to have a
> > steeper harmonic rolloff?
>
> **Output transformer.

Is that due solely to the frequency response (higher freq. rolloff) of
the output transformer?

IOW, if you used an "ideal transformer" with a flat FR from 0-infinity,
would the tube amp's harmonic content be the same as a transistor amp
(with no xfmr) of the same topology?

> > Some quick background-- I've got an EE degree in electrical
>
> **Which begs the question.......

and what question might that be? Seemed to have a disapproving flare
to your comment, hopefully I'm mistaken?

> > Any and all input would be greatly appreciated!
>
> **As has been mentioned elsewhere, buy Self's book.

Hmmm, don't recall seeing that (not a regular here). What's the title?

thanks,
...dane

[Trevor Wilson]

> wrote in message
oups.com...
>
> Trevor Wilson wrote:
>
>> > So is distortion's root this nonlinearity?
>>
>> **One of them. The major one.
>
> For the sake of completeness, then, what are some other causes?

**That would depend on the topology. The other possible causes are many.

>
>> > And if so, why does this nonlinearity always manifest itself as n-order
>> > harmonics?
>>
>> **Pretty much.
>
> ?? not sure what that means; I asked a why question, not a yes/no?

**Because there are other causes and effects to consider. Noise, for
instance. IMD for instance.

>
>> > And how does clipping come into the picture?
>>
>> **Take an amplifier with a Voltage gain of (say) 20 and a sensitivity of
>> 1
>> Volt input for 20 Volts output. Apply 1.1 Volts input and the result will
>> be
>> clipping, since the amplifier runs out of supply Volts.
>
> Yes, that's true, I understand that concept. Wasn't my intended
> question though. Based on another reply in rec.audio.tech, I've
> adapted my question here to be:
>
> Why is distortion always manifested in whole-order harmonics of the
> fundamental, rather than decimal-order harmonics?

**It isn't.

IOW, if inputting a
> 1 kHz tone, why doesn't some 1.05kHz multiple come out, if it's all due
> to nonlinearities, certainly there can exist a nonlinear element that
> would produce a decimal-multiple distortion coefficient?

**IF you only feed a 1kHz tone in (which never happens in real life), then
you can only get multiples of the harmonics out. In real life, however,
amplifiers are hardly ever presented with pure sine waves.

>
>> I.E: Take a push pull Triode tube amp and compare it to a push pull BJT
>> amp,
>> which uses an output transformer.
>> OR
>> Take a SE Triode Class A amplifier and compare it to a SE BJT Class A
>> amplifier (with output transformer) and the resulting distortion products
>> will be virtually identical.
>>
>> TOPOLOGY is what matters.
>
> I'll agree with this statement. Mine was a bit shortsighted, and I've
> since done some more reading online (a good one is
> http://www.silcom.com/~aludwig/Amplifier_distortion.htm) and agree that
> it's the topology that matters.

**Good. It is a fact completely overlooked by novices.

>
>> > Even if that is assumed to be true, what causes the tube to have a
>> > steeper harmonic rolloff?
>>
>> **Output transformer.
>
> Is that due solely to the frequency response (higher freq. rolloff) of
> the output transformer?

**Pretty much. Almost all tubes possess a frequency response which is vastly
in excess of any audio frequency.

>
> IOW, if you used an "ideal transformer" with a flat FR from 0-infinity,
> would the tube amp's harmonic content be the same as a transistor amp
> (with no xfmr) of the same topology?

**That would be like asking: "What if you can pilot a space ship faster than
light?" There is no such thing as a perfect transformer, nor can there ever
be, so it is not a question worth bothering with. However, the answer is
yes.

>
>> > Some quick background-- I've got an EE degree in electrical
>>
>> **Which begs the question.......
>
> and what question might that be? Seemed to have a disapproving flare
> to your comment, hopefully I'm mistaken?

**What do they teach EE's where you come from?

>
>> > Any and all input would be greatly appreciated!
>>
>> **As has been mentioned elsewhere, buy Self's book.
>
> Hmmm, don't recall seeing that (not a regular here). What's the title?

**Start here:

http://www.dself.dsl.pipex.com/index.htm


--
Trevor Wilson
www.rageaudio.com.au



--
Posted via a free Usenet account from http://www.teranews.com