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-   -   transconductance vs. current (https://www.diyaudio.com/forums/pass-labs/10823-transconductance-vs-current.html)

RobPhill33 6th February 2003 07:47 PM

transconductance vs. current
 
Is there an ideal transconductance vs. current curve that represents the nature of air? Is it linear?

Sch3mat1c 6th February 2003 07:50 PM

Nature of air???

Generally it goes up exponentially.

Tim

RobPhill33 6th February 2003 08:12 PM

Quote:

nature of air
In that air being compressed produces a slightly larger pressure level than when it expands. The gain device should have a slightly larger gain at higher currents, as Mr.Pass explains.

Nelson Pass 6th February 2003 08:47 PM

What we imagine we're looking for here is to mimic
the distortion of air on the presumption that this
will be less offensive than some other form of
distortion. As such we want the second harmonic
character of the output to give slightly higher pressure
variation on compression (+) than rarefaction (-).



You don't like this theory? I've got others.

Sch3mat1c 6th February 2003 09:40 PM

So that's why they put + and - on speaker terminals...

Actually I heard it's so you get phasing right, so the common-mode bass doesn't cancel in air. But I've tried swapping polarity while listening, and this doesn't work. Go figure.

Tim

RobPhill33 6th February 2003 09:50 PM

It makes sense to mimic air for the second harmonic, but you are also making the fundemental signal mimic air. Is it important that the fundamental signal reflect the characteristics of air (besides the fact that it allows the 2nd har. to follow air)?

Hard to word.

Joe Berry 7th February 2003 02:48 AM

Quote:

Originally posted by Nelson Pass
You don't like this theory? I've got others.
It's always seemed to me like a good theory in that it agrees with
the well established observation that even fairly large doses of
second harmonic distortion are pretty innocuous.

Still, I wonder if the addition (or at least non-elimination from an
active amplifying system) of in-phase (additive or expansive)
third harmonic distortion might also be desirable. I'm supposing
here that, on their way from the microphone to the speaker,
audio signals undergo a systemic distortion consisting mainly of
anti-phase (subtractive or compressive) third harmonic as a result
of limited diaphragm elasticities and an accumulation of small
nonlinear conductive losses.

As far as I know, these passive sources of distortion don't
otherwise get compensated for or corrected out of the system.

Nelson Pass 7th February 2003 04:32 AM

Quote:

Originally posted by RobPhill33
Is it important that the fundamental signal reflect the characteristics of air (besides the fact that it allows the 2nd har. to follow air)?
I don't know what fundamental characteristic (besides the
even harmonic nature) of air to which you make reference,
unless it is absolute phase.

I would say that absolute phase is certainly an easy tweak,
regardless of what you might imagine of its audibility.

There are other things, such as dispersion (different frequencies
propagating at different velocities) and also high frequency
attenuation (the sonic difference in top end between front
row and 15 rows back)

One of the most interesting things about dispersion is that it
seems to be something you can mimic a bit in a circuit, so that
the bright top end of close-mic'd recordings can be taken down
a notch, and it improves the perception of the recording. Of
course it doesn't do a thing for something recorded from the
back of the hall....


Quote:

Originally posted by Joe Berry
Still, I wonder if the addition (or at least non-elimination from an active amplifying system) of in-phase (additive or expansive)
third harmonic distortion might also be desirable. I'm supposing
here that, on their way from the microphone to the speaker,
audio signals undergo a systemic distortion consisting mainly of
anti-phase (subtractive or compressive) third harmonic as a result
of limited diaphragm elasticities and an accumulation of small
nonlinear conductive losses.

Unfortunately, I don't know of any ordinary form of distortion
which reverses the odd order compression effect. You can,
however, avoid it by amplifying with single-ended Class A
circuits.

:cool:

RobPhill33 7th February 2003 04:15 PM

Quote:

I don't know what fundamental characteristic (besides the even harmonic nature) of air to which you make reference,
unless it is absolute phase
Perhaps I have misunderstood, you said earlier

Quote:

we want the second harmonic
character of the output to give slightly higher pressure
variation on compression (+) than rarefaction (-).
Whatever is done to the second harmonic (larger gain at higher current) must also be done to the fundemental, correct? Unless I am way off, which is likely, the fundemental would exhibit slightly higher pressure on compression, as would 2nd,3rd,etc. harmonics. I was wondering if this is beneficial to the fundemental alone, or is the subsequent improvement in the 2nd harmonic distortion the primary audable difference?

Nelson Pass 7th February 2003 05:44 PM

The second harmonic simply reflects the gain variation
of the fundamental over its cyle. Higher gain as the
output goes positive (or negative) results in a 2nd
harmonic.


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