Theremax Tone - Mixer Output

Posted: 6/25/2012 3:50:19 PM

From: Northern Virginia, USA

Joined: 5/10/2012


I realize that there has been a lot of discussion on the Theremax in terms of tuning and also many suggestions for different oscillator coupling mods to get different difference waveforms and have read and I think I understand what I have found.

I finally finished building an enclosure and installing 3/8" supply tubing antennas to the specs posted on the RCA theremin.  The theremax is working well in terms of pitch and volume response so I think I basically have it tuned acceptably so now I am ready to start working on its tone.

One thing about the current 9505D kit that may have changed from previous kits is all four oscillator NPO caps are 100pF, not a pair of 68 and 100 that I have seen in other posts.  I have 68pF NPO caps on order but for now I was successful in preventing major interaction between the pitch and the volume by simply tuning the volume frequency to around 720kHz as opposed to the pitch that I set at 798kHz.

The specific problem I am having right now is the output difference waveform from the pitch mixer is squashed in that it is asymmetric with the “top” appearing to be soft clipped.  So the one side (depending on inversion) is wide.  I can send a scope picture if that would be helpful.  The waveshape appears to be independent of the difference frequency from 100Hz to 5kHz or so so I don't think that oscillator coupling is at play here and I don't want it to be for now.  I would like to see an undistorted sine wave without coupling and then experiment with coupling from there.

The oscillator p-p amplitude at the transistor collector is about 350mV.  I have seen varying numbers on the expected amplitude.  I haven't seen any mods that suggest changing the mixer component values any my LTspice simulation to the extent that it approximates reality does output a sinusoidal looking difference output.  I was wondering if I should experiment with the mixer component values or if I should concentrate on the oscillators themselves.

Thanks in advance,


Posted: 6/26/2012 7:19:03 AM

From: Northern Virginia, USA

Joined: 5/10/2012

I'm glad that no one tried to answer my question because it turns out that the reference oscillator amplitude is over 13.5 volts and is distorted at the bottom.  That might be the cause of the distorted mixer output. 

I have posted some pics of the build and scope plots here.

I guess I would like to do the following things:

1) Reduce the reference oscillator amplitudes - maybe reduce the value for
R8 and R13.  I need to crack open my old transistor text book and refresh my
memory on biasing...

2) See if I can reduce the reference oscillator coupling - first try moving
the front panel wires around and then maybe put in isolation walls - might
that work?

3) Try to reduce the noise on the output waveform.  Maybe build a higher
than second order LPF or do something to reduce the noise?

Any help or advice would be greatly appreciated!




Posted: 6/26/2012 3:39:00 PM

From: Northern NJ, USA

Joined: 2/17/2012

"I have posted some pics of the build and scope plots here."

Nice build!  I can't help you with the oscillator amplitude, but others here almost certainly can.

After a peek at the schematic, I think I'd re-do the supply using three terminal regulators.  PAIA tends to cut too many corners IMO.

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