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

Mark Rages markrages at
Wed Nov 6 16:24:00 UTC 2002

On Wed, Nov 06, 2002 at 03:57:05PM -0500, Lamar Owen wrote:
> 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.

I'll disagree here, because the input coupling caps are just going to gave 
a 6dB/oct rolloff below frequencies we're interested in, anyway.  If you're 
speaking only about capacitor quality, then specifics are necessary to draw 
a good conclusion.  

> > > 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.

Yeah, this is what I was getting at.  If the amp's ground is not at the 
soundcard ground, there are several choices:

1) Audio transformer coupling.  Will color signal to some degree.
2) Capactor coupling, but just flip the amp's ground switch till you 
   measure 0V between computer and amp. 
3) Capacitor coupling, computer or amp plugged into isolation transformer.
4) Capacitor coupling, ground lift on computer or amp... not recommended. 

In any case, a capacitor by itself is probably not going to be enough to 
couple the signal.
> 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.

True.  My point is that voltages are generally positive in tube equipment. 
Nonpolars are harder to find in high voltages.
> > 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.


> 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.

Glad that's not my job.  It's hard to get convergence with hysteresis in
the system.  I did some simulation of electirc motors on computer, but we
always linearized the equations first.

I'm afraid I've made a digression of topic here. But if we can figure a way 
to sample the signals from a tube amp without killing the people writing 
plugins, we have contributed to the Linux audio effort.

> -- 
> Lamar Owen
> WGCR Internet Radio
> 1 Peter 4:11

Mark Rages
markrages at

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