problem with metal mic stand

Posted: 2/27/2009 1:53:30 AM

From: Seattle, WA

Joined: 2/21/2009

i wanted to put my etherwave on a mic stand so i went out and bought one from guitar center. after i mounted it, i turned on the theremin and it just made a constant pitch even when i was standing no where near it. i don't have that problem when the theremin is sitting on a table or a keyboard stand. i'm thinking it must have to do with the mic stand being metal. is this a common problem? do i just need to find a plastic stand or what? suggestions? thanks
Posted: 2/27/2009 2:29:03 AM

From: Colmar, France

Joined: 12/31/2007

It's not a bug, it's a feature...

Welcome in the world of the theremin!

It's normal that a theremin shows this behavior when additional metal which acts as virtual grounding is brought into its pitch range. That's why the theremin has a pitch tuning knob in order to compensate this effect. If the pitch tuning range is not sufficient you will have to re-tune the instrument internally as described in this service manual (
Posted: 2/27/2009 2:39:15 AM

From: Seattle, WA

Joined: 2/21/2009

well, turning the pitch knob makes the pitch higher, but it no matter what position it's in, i still get a constant sound. it's only when i have it on the mic stand. when it's on other surfaces, it works just fine.

my concern with this internal tuning you speak of is that if i do that, and get it to work on the mic stand, is it still going to work fine on other surfaces like it does now, or will it have to be on the mic stand all the time?
Posted: 2/27/2009 2:02:17 PM

From: Escondido, CA

Joined: 2/6/2008

There is at least one other TW member who lives in Washington state. They can probably help you with your tuning problems.

Posted: 2/27/2009 2:12:59 PM

From: Croxley Green, Hertfordshire, UK

Joined: 10/5/2005

Hi extragabe.

A metal mic stand should not have the effect described.

I have a vague suspicion about what the problem is, but I need to ask a few questions.

The keyboard stand you were using, is that metal, and was a part of it close to the volume loop? When you placed it on a table, was it a wooden table and did you place the theremin with the loop hanging over the edge of the table or not?

(The presence of a large metal or wooden mass (or generally any conductive surface that could act as a capacitor plate) very close to the volume loop will significantly affect its behaviour. If the volume circuit is tuned to operate correctly in such circumstances it may well behave as you describe.)

Also, when you walk away from the theremin does the sound go down to a low growl, become silent and then rise very very slowly, or does it remain silent, or show some other behaviour?

(I am wondering if the pitch circuit is tuned correctly. It should stay silent when you walk away from the theremin.)

Posted: 2/27/2009 2:48:00 PM

From: Redmond, WA

Joined: 9/1/2007

I am in Kirkland, Washington and would be glad to take a look at it if you'd like. Email me at carport888 (at) hotmail dot com.

Posted: 2/27/2009 4:19:04 PM

From: Seattle, WA

Joined: 2/21/2009

To answer your questions:

Yes the keyboard stand is some type of metal, and yes a part of it was near the volume loop. The table was a wooden table with metal fold-out legs. I don't remember where the loop was positioned on the table. (Come to think of it though, I believe I was having the same problem on that particular table as I am with the mic stand. The keyboard stand however works great.)

On the mic stand, the theremin produces a constant high pitched sound unless I have my hand near the volume loop. When I move my hand close to the antenna, it seems to operate normally, changing from low to high pitches as I move my hand as it should. BUT, when I put my hands down and walk away, it starts producing a constant high pitch again. As I am moving away from it, it goes down to a low pitch, then when I am no where near it, the pitch becomes high again.

On the keyboard stand, it is silent when i walk away as it should be; the pitch and volume control work great and everything seems to be fine. I just have issues when it's on the mic stand.

(And thanks for the offer carport, i'll probably shoot you an email sometime in the next week or so)
Posted: 2/27/2009 5:18:02 PM

From: Croxley Green, Hertfordshire, UK

Joined: 10/5/2005

OK. This is what I think. When you built your etherwave you tweaked the coils to get it working while it was sat on a table, without having the loop sticking out over the edge of the table. Now it is tuned to want a large capacitive mass near to it.

You need to open it up and retune the coils while it is on the mic stand.

As a temporary fix you might try sitting a little stack of CDs on the top of the etherwave, near the volume loop.

That may not be the end of your problems, though. I am unable to explain the very high pitched tone, and will leave that to the electrical experts.
Posted: 2/27/2009 6:39:41 PM

From: Seattle, WA

Joined: 2/21/2009

hey so i got it working decently on the mic stand by opening it up and adjusting the L5 and L6 slugs like Thierry suggested. But I am still having problems:

After making adjustments so that the theremin is silent upon walking away, it works just fine with the cover off. However, when I shut it off, unhook it, replace the cover, hook it back up, and turn it back on, the problem starts again. it makes a constant high pitch even when i'm no where near it.

so i am getting it to work when the cover is off, then the problem comes back when the cover is back on. then i thought 'so maybe if i take the cover off again, it will work again', seems logical to me, but no. taking the cover off again, the problem still occurs as if putting the cover back on erases all the adjustments i made. if i re-adjust, it works again on the stand with the cover off but screws up again when the cover goes back on. it's a horrible endless cycle.

this is so frustrating. and what's worse is that it appears that the slug inside of L6 is breaking apart or something. as i was turning it, i heard cracking and i could see that a chunk of it was breaking off. i was not using excessive force; it just started cracking. i'm afraid to fiddle with it anymore because i don't want to break it. i'm at a total loss :(
Posted: 2/27/2009 6:41:29 PM

From: Seattle, WA

Joined: 2/21/2009

oh, and when i was building it, i didn't tweak the coils at all. i didn't even know you could until now. i think i may have made it worse though because now it won't even work properly on the keyboard stand anymore.

oh and another thing: before building my theremin, i read about making sure not to paint it with anything containing conductive metals. i did spraypaint the case red, but i before buying the paint i called the manufactuerer and made sure that there were no metals in it. so that shouldn't be the problem with putting the cover back on. i don't know how else the replacing the cover could effect it like this. makes no sense to me at all.

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