Miscellaneous Stuff

John Speth johns at oei.com
Fri Feb 27 21:22:53 CET 1998

I bought and built a Theremax a long time ago and mine had problems along the same lines as yours.  My biggest complaint was that the range of the antennae was way too short.  Mine was about 8 inches.  Last week I replaced the flimsy copper antennae with 1/2 inch copper plumbers pipe and fittings all soldered together.  The dimensions and shape were roughly the same as before and only slightly larger.  My range actually doubled!

As for VCA bleed through, I theorized that the 339 comparator is not truly slew limited when it turns on (despite the cap on the 339 output which I beleive is a no-no) and hence it tends to draw a nice little current spike that gets picked up by the VCA.  My solution was to replace the LM339 with a TI's LINMOS version (TLC339) hoping it was a little slower.  It was a sample I've had for years and it didn't eliminate the problem, just reduce it significantly.  PAIA has mods available to improve VCA noise which I found only works a little.

Anemic CV - I think there's no way to avoid this because of the anemic PS voltages in the circuit (+/- 4V !!).  I'm planning to redo the PS section and use a split PS at +/- 12V.  This might result in smoke so I'll really study the circuit hard to assess the danger.  I plan to add opamps to boost the CV.

After comparing notes with several other owners, I have a hunch that the Theremaxes suffer from poor manufacturing repeatability.  And with kits, the variation is even greater.  I think the black magic aspect of RF and the antennae is the root of this problem but it's just a hunch

John Speth
Object Engineering, Inc.
johns at oei.com

-----Original Message-----
From:	media at mail1.nai.net [SMTP:media at mail1.nai.net]
Sent:	Friday, February 27, 1998 12:02 PM
To:	'synth-diy mail list'
Subject:	Miscellaneous Stuff

	Over a year ago I built a Paia Theremax.  It doesn't work right --
the CV outs are extremely anemic (0-1.5V), the carrier bleeds through the
entire circuit, and the VCA will not close all of the way.  I've modified
it a bit to clean up the noise (the output amp, grounding and voltage
regulator), but it still sucks as a controller.  What I would like to do is
keep the front panel controls, lectern case, antennae and inductors, and
throw the rest out.  Basically, want to swap out the PCB for a better
design.  My current assumption is that one could build a better circuit
using op-amps.  Any suggestions??

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