Theremax, audio output noise and distortion

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Theremax, audio output noise and distortion

Post by PAiA-Scott »

If the Pitch heterodyne signal is weak, the tendency is to run the audio output level control towards maximum which makes it more likely for a "feedback" condition to result with pulsing of the f-v circuit section getting into the audio output, stronger than the real pitch signal.

Maybe L2 is stopped short of the real pitch tone and at a weaker harmonic. The method in the tips/suggestions make this less likely as the volume cv isn't exaggerated as it can be when tuning using the alligator clip. Try going more inwards with L2 for a louder Pitch Tone. If none encountered, try to find a more inward initial setting of the slug for L1 that will make a tone with the range with L2. Try 1/2 turn increments for L1. The more the slug is in, the stronger the oscillation from the oscillator circuit.

Wires laying against or too near the capacitors, resistors, and transistor in the oscillators for the pitch pair might be loading and weakening one of these which combine for the pitch tone (circuits including L1 and L2), or the pitch antenna cable ends. The pitch antenna should not be too near an appliance such as a lamp or bench equipment, or metal rod or pole of a shelf or table leg running down to the floor.

Check the Pitch Trim control connections on the panel. R79 should have the same code marked on back as R80 (the 1k marking is on the front of the brown layer towards the panel. The soldering for the red-vio-brown-gold 270ohm R85 must be solid and for wires B (upper terminal) and A (middle terminal) and the brown-green-red-gold 1500ohm that links down to the ground circuit at J5-S(leeve) below. Nudge each of the bare wire ground links from sleeve terminal to sleeve terminal to be sure one isn't loose.

V+ being 8.2v (within 0.1 v or so) is important too for the biasing of the transistors in the oscillator circuits and the Q8 and Q9 amplifier stages.

The next area for for a second look is area between the oscillators and the vca/IC2amp. Inspect and confirm the printed-circuit side of the board to be sure the path from the pitch pair of oscillators through yellow-violet-black-gold 470ohm Rs 5, 6, and brown-black-orange-gold 10k R11 and on to Diodes D3 and D5 is correct. Yellow-violet-red-gold 4700ohm R26 and 221 marked 220pF C22 join the pitch heterodyne over to the Q8 amp stage through capacitor C23. Inspect the solders for this part and it's polarity (these taller parts can work like a lever against joint and printed-circuit. C29 is another, after the Q8 transistor amp stage for the pitch heterodyne signal. A yellow-violet-yellow-gold 470k at R29 and an orange-white-orange-gold 39k at R31 connect from the V+ supply to ground to bias the transistor so it's collector is a dc level in the range of about 2-6 volts. The voltage for the collector is from yellow-violet-orange-gold 47k R30. The audio signal into the base of Q8 runs towards orange-orange-brown-gold 330ohm R32 and 100uF C42 and the transistor amplifies this to be a stronger audio voltage on the collector. Frequencies from the high audio range and beyond go back through C27 and cancel but the desired audio goes on through C29 towards the Timbre control which is really just a variable shunt. It steers either the near-sine Q8 signal or the pulse train from IC1A away from the vca/audio-output-amp section that follows.

Example of the Theremax tone:

Theremax Sound Example

Tips for installation in a custom case:

Containing the Theremax Kit

If after checking through all this and the trouble persists, a solution might be the addition of a 0.1uF capacitor from V+ to Ground at transistor Q6 as detailed here:

Volume CV Circuit-section Filtering
Scott Lee