Subject: Insturmenting the amplifier for sampling (Was:Re: [linux-audio-dev] Re: Cheby amp code?)
From: Lamar Owen (lamar.owen_AT_wgcr.org)
Date: Wed Nov 06 2002 - 21:09:50 EET
On Wednesday 06 November 2002 12:50, Steve Harris wrote:
> On Wed, Nov 06, 2002 at 12:07:24PM -0500, Lamar Owen wrote:
> > As it also works as a transient recorder and spectrum analyzer (to
> The trick really is get get a sample off of it, 8bit might be enough, but
> I suspect 16 will be easier to deal with.
A series 1 microfarad capacitor in the 1kV breakdown range in series with your
line input should work, but you might want to use an old soundcard and
machine to do it... :-) Paper 1 mic caps in that voltage range are available
-- in a pinch 0.47 or even 0.2 would work, but the lower you go on
capacitance the more the probe will pollute the signal. Assuming the
soundcard's line input doesn't already have a low-voltage series coupling 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).
I have a couple of 50 microfarad oil-filled caps in the 12kV range, but you
don't want me to ship them (all 100 plus pounds) to you.... The capacitors
that are used in mag-reg HID arc ballasts are high value and high voltage,
but I'm not sure what their frequency response will be like. ESR for those
big oil-filled paper caps at 1kHz is probably going to be quite high due to
the inductance of the paper/foil rolls.
It just depends upon how precise you need it to be. A high voltage
nonpolarized electrolytic would be close to ideal -- tweeter caps, for
instance, for high-power speakers would be good, as at the typical input
impedance for a sound card they would pass the lower frequencies just fine
(they ordinarily work with a low impedance driver, thus decreasing the time
constant and increasing the cutoff frequency). Their ESR shouldn't be a
problem, as they're designed for low impedances.
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.
I have a mostly working Fender Bassman available for testing. Does anyone
here have a schematic for that beast? I can get the particulars. It has a
nasty hum, and the bias on the 6L6's is quite bad (they run bright orange
instead of just barely red). There are probably many resistors that have
changed values in it, along with several dried out electolytics.
I also have a Dynaco three-tube kit special (A450 output with twin 8417's)
available for testing.
And I'm still wondering whether a SPICE model of the amplifier in question
would tell us anything about the waveforms.... :-)
-- Lamar Owen WGCR Internet Radio 1 Peter 4:11
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