Insturmenting the amplifier for sampling (Was:Re: [linux-audio-dev] Re: Cheby amp code?)

[Lamar Owen]

On Wednesday 06 November 2002 14:47, Mark Rages wrote:
> On Wed, Nov 06, 2002 at 02:09:50PM -0500, Lamar Owen wrote:
> > cap already, in which case you have real problems, because the effective
> > capacitance or two series caps is equal to the reciprocal of the sums of
> > the reciprocals of the individual caps (the same as parallel resistors).

> Ahh, but like parallel resistors, if one is "big" the other resistance will
> set the value.  So choose a "big" balue for the external cap and you'll be
> fine.

Which means the soundcard's cap would be the upper asymptote.  Which is where 
I was driving towards -- unless you have a good soundcard, that is.  Typical 
soundcards don't have really good input coupling caps, and that would 'color' 
the waveform seen by the A/D.

> > Photflash electrolytics would be the next best thing, configured in a
> > nonpolar back-to-back arrangement, as they have extremely low ESR and
> > inductance, meaning they should be pretty linear.

> But you don't need nonpolar caps... Just get the polarity right.  Assuming
> your soundcard is going to be grounded with the amp (grounding in tube amps
> is itself... interesting), you will, in almost every circumstance, want the
> + side to go to the amp.

Ah, but did you see the schematic of the amp in question?  'Ground' could be 
the 120VAC LINE if you have the grounding switch set wrong.  Best be safe, 
since the amp's ground and the soundcard's ground could have 120VAC 
differential.

Besides, a nonpolar coupling cap would be more useful in the generic case, so 
that you could trace through both a negative and positive supply voltage 
situation.

> I've seen schematics for the Bassman on the web. .

Yeah, I'll google around for it.  I just have to see which flavor of Bassman 
this one is (twin 12's with separate head). It is very old, that much I know.

> > And I'm still wondering whether a SPICE model of the amplifier in
> > question would tell us anything about the waveforms.... :-)

> Probably not much more than careful thought and intuitive mathematical
> modeling... besides, SPICE probably doesn't consider things like capacitor
> soakage and transformer hysteresis, which can give an amp some 'flavor'.

You can model those things easily enough with the port parameters SPICE 
allows.  And that's why I suggested such.  SPICE is incredibly powerful -- I 
have seen the pspice commercial flavor used for modeling HID lamp ballasts, 
which operate on magnetic circuit principles.
-- 
Lamar Owen
WGCR Internet Radio
1 Peter 4:11