Nasty radio-sounding noises at Zero Beat?

Got a Moog Etherwave Standard in Walnut, and it has a nice clean timber while I play...

But when I'm at Zero Beat, and not playing it, the Amp produces a range of sounds that seem to emulate soft radio-noise while I'm near it-- but not while playing a note.

The Wavefront Classic I have, does not do this at all; using the same AD30VT Amp, in the same environment.

Any ideas what might be making the softer radio-squeals at Zero Beat?

Thanks,
Rowsby

Possible things to try:

Ferite isolators (cylindrical clamshell device) on the audio, and power cables of the Etherwave. (doesn't allways work).

Ground lifting adaptor. (the 3prong to 2prong adaptor) connected to Etherwave power cord. Sometimes helps on the amp. (mostly for groundloop humm) But, to be safer, use a hum eliminator. Don't know how that would work on the radio like sounds.

It's also possible that internal tuning (may) be needed.

I'd check with Moog Music as well to be sure.

At zero beat:
both the reference and pitch oscillators are at the same frequency, so any sounds are likely to be due to modulation with some other frequency (some HF can leak to the audio output stage - this can itself beat with other external HF signals )

Your amplifier would be the most likely suspect if it was class D - but your amplifier is linear..

I suggest:
1.) Look for any sources of radiating signals in range 200kHz to 1MHz - This can be almost any electronic device - LED displays, LCD displays, unshielded telephone wiring carrying broadband, mains signal transmission systems.. Move the Theremin to different locationsand see if the problem goes away, or changes.

2.) Use good quality audio connectors and wiring. Fitting Ferrite filters could be helpful - there are clip-over filters available - put these on audio and power leads..
As Thomas says, these sometimes improve things.

3.) Load the Theremin audio output - Try putting a 10k resistor across the output - sometimes his is enough to stabalise an unstable output stage.. If this fixes the problem, you need to look at your Etherwave output stage some time n the future, as it has a stability problem.. This is particularly likely if there is a high capacitance loading the output.

Sounds like noise to me...

Hello,
I have experienced this same 'soft radio' noise in just the same way as described by Rowsby, with my There-Max. This noise seems to be intermittent and is not location dependent, as I always play this Theremin in exactly the same place. Curious!
I shall try the 10K coupling fix that was suggested by FredM, and report back.
Good Luck!

teslatheremin

[i]"But when I'm at Zero Beat, and not playing it"[/i]

Am I correct in believing that at this time the volume is not being reduced - hand is away from the volume antenna - and that you are relying on zero-beat to mute the output ?

If the above is true, then the answer could relate more to stray fields within the Theremin than to external fields.. The volume amplifier (VCA) will be 'wide open' (maximum gain / minimum attenuation) - there will be interactions between the HF oscillators which, due to non-linearities in the amplifiers etc, could hetrodyne to produce low level beat frequencies - these may not be easy to hear when there is an (intentional) audio beat frequency being produced (in which case also, the volume is likely to be reduced, so proportionally also reducing these unwanted audio signals).

I have found many unexpected sources of unwanted audio 'noise' or 'interference' within my Theremin prototypes (another reason it is taking me so long to get my Theremin to market) and have been forced to concentrate on extensive shielding of radiating components, and to take EXTREME care with power distribution within the unit..

One 'simple' fix would be to have a filter and envelope follower tracking the pre-VCA signal .. If the beat frequency then dropped below (say) 20Hz, the VCA could be 'closed' (maximum attenuation) automatically.. something like a compressor, but without the latency, pumping, or other problems compressors and noise gates can cause.

Hi Tesla..

Sorry, but the 10k fix is not likely to work with the ThereMax.. it has a different output topology which is unlikely to go unstable.
Even the EW output stage is unlikely to go unstable - but my reading of the schematic makes me think that it has a slight possibility of doing so.

“But when I'm at Zero Beat, and not playing it, the Amp produces a range of sounds that seem to emulate soft radio-noise while I'm near it-- but not while playing a note.”

If you could capture this quiet noise in a little sound byte it would be fun to hear.

I suspect it is what I call “birdies in the background”. Re-tuning could solve the problem as said but that’s a little scary. Odd as it may sound I would try a ferrite bead or collar or donut on the wire between the pc board and the antenna. I believe this noise is caused from the antenna picking up the boards own oscillator magnetic fields and regurgitating them in the pitch oscillator.

If you use a ferrite collar as mentioned in the earlier good suggestions, put two or three wraps of the antenna connecting wire on it. You might find one on your digital camera cords.

Christopher

Non Linear Pitch (http://www.oldtemecula.com/theremin/library/danielmccauley/html/drsstc03_01.htm)
.

“birdies in the background” - Thats a great way to describe the sort of sounds some prototypes of mine have produced!

I believe the frequencies of the volume and pitch oscillators with respect to each other can cause this problem (certainly has been identified on my prototypes) - The pitch / reference oscillator relationship is (in my expierience) not the cause.

If the volume and pitch oscillator frequencies are too close (or have harmonics which are close), then pick-up (via the pitch antenna) can be fed to the mixer, and produce audio output from hetrodyning with the reference oscillator - As you suggest, attenuation (by a ferrite) of induced signals will (or might) reduce the problem.

My expierience is that high purity sine waves on both volume and pitch antennas greatly reduces the problem - which leads me to believe that it is the harmonics rather than the fundamental frequencies which are responsible.. If it is pitch and volume HF which is interacting to cause the problem, perhaps a ferrite on both antenna leads would be worth a try.

Mine does that too, and I have even noticed that the area around the volume antenna's zero beat (or point of activation) will actually produce and allow for control of the squeaks and squelches.

Someone on here disproved me, but I thought that I was onto the "hidden tones" that I thought that I learned that Lev was working on, which was a duophonic theremin, the second pitch coming from the volume antenna. (If my memory of this is accurate.)

I noticed that when I boosted the signal, like with the Moog Delay or with the ELectro Harmonix Bass balls, that I really could listen very closely to these sounds. I think that this is just an inherant part of this particular model theremin, just like hearing the carrier frequency is typical of their Ring Modulator. I don't know anything about other models of theremins, and I could be wrong.

What are you running the theremin through and are you boosting the signal with anything?


here is a link to the post that I made on their forum (I don't know where the same post is on this forum)
http://moogmusic.com/forum/viewtopic.php?t=4874

Eric