Unwanted harmonics

Several years ago, I purchased a used theremin. It says "Etherwave Theremin by BigBriar" on the front, and it looks like it was assembled from a kit. I was never able to get it to sound like I wanted, which I assumed it was my own ineptitude, and mostly forgot about it. But yesterday, I tried it again and realized that (in addition to my ineptitude) it sounded like there were some harmonics in the output that would account for it always sounding out of tune.

I did some searching and found that others have had this problem and that it's usually an interference or grounding issue. So, I tried using different outlets in different rooms, moving it away from any electronics, etc., but it didn't make a difference. This sound has been the same in the last two apartments I've lived in, so I don't think it's something peculiar to the wiring of my current apartment.

For the grounding issue, I used a multimeter to verify that there was a good connection between the ground pin on the power plug of the transformer and the contact labeled GND on the circuit board inside the box.

I don't really know what else to check or try, so let me know if you have any ideas.

I made a recording (http://soundcloud.com/fjord41/theremin-test) of what it currently sounds like. That link also has a picture of the theremin. Let me know if you need any more details.

Thanks!

What are you using for an amplifier and speaker? Do you have anything in your signal path other than a theremin, a patch cord, and an amplifier? What was the signal path of your recording? What did you record with?

For the recording, it went through an M-Audio AudioBuddy (http://www.m-audio.com/products/en_us/AudioBuddy.html) preamp and then to a Layla24 (http://www.echoaudio.com/Products/Discontinued/Layla24/index.php) audio interface for my computer. But it also sounds the same without the preamp (direct to the Layla24) or just plugging headphones in.

I could hear 2 problems:
a) there are spurious very weak ghost tones which are aliasing interferences between remaining RF signals in the theremin's output audio signal and harmonics of the A/D converter's sampling frequency. Either you use a very strong high cut filter (7kHz) before sampling or better you do as Peter Pringle does or BBC did when recording Celia Sheen for example: You make a nice audio setup with effects, equalization, good speaker(s) and so on and you record all this through a dynamic microphone. The result will be much closer to what the audience would hear than a directly sampled and recorded theremin output.

b) EVERY unmodified Big Briar or Moog Etherwave Standard or Plus theremin sounds like a dying old motorbike in the lower register. This is due to a rather poor mixer stage design and can be cured by adding a ESPE01 add-on module to your theremin.

In addition to the above I want to add that the timbre of the Etherwave theremins is not very pleasant in general when both waveform and brightness knobs are in default (12 o'clock) position. Try to set the waveform to half past ten and the brightness to three o'clock and you should get a much more pleasant, string instrument-like sound. A woodwind-like timbre can be obtained setting the waveform to 3 o'clock and the brightness to half past one. This are approximate values since there are lots of tolerances in the used components, don't hesitate to experiment.

Finally I allow to remark that if a theremin sounds out of tune, it is because the player plays out of tune... The human ear registers the pitch errors the more, the more harmonics the signal contains. That means that the same false tone will sound more false with a brilliant timbre than with a muffled one.

Filtered through my pc speakers and the tiny microphone on my ipod...

spectrum plot link (http://i1184.photobucket.com/albums/z328/ChainsawWillie/Snapbucket/ADA6F533-orig.jpg)


My spectrum analizer app shows a fairly big odd-order harmonic. This is a snapshot of the primary signal when it was about 1kHz. The little peaks at 2 and 4kHz are the type of harmonics that most people sound "rich and full". That peak at 3kHz is the ugly sound.

Have you tried playing the theremin into something other than the pc interface?

Interesting. It's not super high quality, but here's a recording (http://soundcloud.com/fjord41/theremin-test-2) made by plugging the theremin directly into some speakers and recording that with my cell phone.

The first sweep is with the settings Thierry suggested: "Try to set the waveform to half past ten and the brightness to three o'clock." The second sweep is with the same settings as the first recording: both knobs all the way to the left.