Theremin trouble...

In november I bought a Moog Etherwave to have fun and play from time to time. However I only recently really tried it on for size, and to my horror, it doesn't sound like I hoped. It has this "choppy" sound where I'd expect it to be smooth as long as I would keep the soundwaves soft as well. I've made two short videos with some experiments. It's most noticable in the lower spectrum, just before it goes silent.

The first video (http://www.youtube.com/watch?v=bA5WPb9aHPA) (see "more videos from same user" for the second one if this one isn't enough)

I should note that since I'm located in Denmark, I have the 220V version of the theremin. This apparently means that the transformer is without the grounded plug. I can imagine this will have a somewhat significant impact on the sound, but I wouldn't expect it to damage the theremin in any way. I'm recording the sound with a labtop which at the time of recording and playing is connected to nothing except the theremin (so the noise isn't caused by the computer). I've tried connecting the computer to it's powersupply, and it wreaks havoc with the theremin response...

Thomas Grillo was so kind to refer me to this forum, as he thought some of you might have an idea on how to deal with it?

I'm no stranger to do-it-yourself electronics, so if you have solutions involving soldering, I'm game - although I would prefer not to modify the theremin itself...




Regards

Simon T

Hmmm...the 220v grounding question came up not long ago--try the search function. I don't know about the Etherwave, but at least some theremins are grounded through the amplifier.

I have a few of questions and a few thoughts.

Did you buy the theremin brand new, or is it used?

Does it sound like this when you plug it into something other than the computer? Have you plugged it into an amp with the same results?

Is the theremin (and computer) plugged directly into the wall receptacle, or are you using some kind of power strip/surge protector/UPS (uninterruptible power supply)?

First just let me say, I am NOT an electronics expert, but I have had some experience in this area.

It sounds to me like your signal is being modulated. It’s anybody’s guess what the source might be.

I have a few theremins and a very old house with NO grounded receptacles. I have never had this problem with my theremins plugged directly into the wall receptacle.

However, I did have a similar problem using a power strip/surge protector. A surge protector does contain electronic components that can alter the quality of the electrical current, especially if it is not working properly.

Another thing to be aware of is having the theremin, or the power supply, near any electronic units that can cause interference, such as fluorescent lighting, televisions, unshielded loudspeakers, motors, etc…

If you are living in an apartment in a multi-unit dwelling, it is possible one of the other tenants, or a system somewhere else in the building, is introducing distortion into the building’s electrical power.

It is also possible there is a problem with the theremin’s power supply as well.

If you’ve eliminated all of the simple things, you might need to find someone who can do a power quality test for you. You could do it yourself, but the instruments required are quite expensive. Perhaps a rental business might have one?

Good luck!

Hmmm....on second thought...

Since the modulation rises and falls pretty much (but not exactly) with the pitch, I'm thinking it might be your power supply.

Perhaps the smoothing capaciter(s) have gone bad and you are getting excessive ripple? It's a possibility.

Do you know someone else, or a possibly a musical instrument store, that might have another power supply you can try?

Hmmm....Now that I've looked at the schematic and photos of the PCB it looks like the power supply is in two parts.

The "wall wart" only drops the AC voltage down to a harmless 14VAC. The rectification to 12VDC is done on the PCB. The two large 2200uF, 35V electrolytic capacitors on the PCB are part of the power supply. It's possible one of them is not up to spec.

I had this problem once, and it turned out to be dirty pods. I didn't play my standard for about a year and the moister affected the sound similar to what I heard.
I'm also an electronic idiot, so don't count on my advice, it's just what happened to me once.

Andy

Hmmm, I'm more likely to believe in faulty electronics than dirty pods, since it's pretty much been left in the same spot for 3 months (no disassembly or anything). But as far as I'm informed it's important that the power supply is grounded which mine isn't. If that's the case can I patch something together to make an artificial grounding?

Hi Jeff.

I was in quite a hurry when I replied yesterday, so I didn't see your first post. I have tried all the "simple things" (in the video you will even see it balanced on the wooden legs of an upside down chair, because i suspected the mic stand might be involved :-)

The reason I'm suspecting it's an outside disturbance, is that every time I plug the labtop in the sound gets even worse (hmm, I'll probably have to make another video so you can see/hear it).

The theremin is brand new (well was), and I will try to ask where I bought it. It's an "old fashioned" guitarstore as in "they know their way around a soldering iron". So I suspect they could help me try another power supply - or help me repair it...

It seems that there is a 50 Hz modulation. To be shure that this does not come from an external source (are you living below a high-voltage power line? Do you have neon lamps?) just carry it i.E. to a music shop and ask them if you could connect it on one of their keyboard amps (guitar amps are not suitable because of the different frequency response) and see if the problem persists. If you find out that it is really inside your Etherwave, I would ask for warranty since it's only 3 months old. Or you will have to find someone with an oscilloscope, a webcam and internet access in the same place, then I could help you with diagnostics.

gadgetguydk,

“I should note that since I'm located in Denmark, I have the 220V version of the theremin. This apparently means that the transformer is without the grounded plug.”


Don’t loose site of your ground idea on the lowest frequencies in your original post. Your theremin sounds good with only this minor issue. A theremin using a LC pitch oscillator will not operate properly without a true earth ground. Distortion will more likely sneak in with the lower audio frequencies along with other erratic behaviors.

As a test, connect a long wire to your theremins ground. You could use a Y-connector cable plugged into the audio-out of your theremin to tap into the instruments ground if there is not an easier method. Then connect the other side to your indoor metal water pipe somewhere. There are other methods of improvising the ground but for now you just want to isolate the problem. This is your easiest first test.

I welcome you Thierry - your knowledge is on target and refreshing.

Christopher
.

Well, I finally got it checked out at the guitar shop where I bought it, and surprise - no problems. It turns out that it was a problem with the shielding of the cables leading from the Theremin to the computer. So all I have to do is just buy a cable of proper grade, and I should be okay.

Thanx for the suggestions :-)