ThereMax - Help Needed!

Posted: 12/18/2006 6:37:41 AM

Joined: 12/18/2006

Dear All,

Firstly, my apologies if any of this has been asked before, but I'm a total newbie where theremins are concerned, and could really benefit from some advice.

Firstly, I should explain my setup:

The Theremin
I have a Paia ThereMax, which I built myself. It has no mods, and all instructions in the associated manual have been followed exactly. The theremax has been built carefully, and no mistakes have been made in the soldering of components. The metal frontpiece (into which the knobs and jacks fit) has been grounded.
The only thing I'm not too sure on is the ariel configuration (which is actually self-contradictory in the theremax manual). I currently have the vertical ariel on the right, controlling volume. I have the horizontal ariel on the left, controlling pitch.

The Case
The only thing I did not get from Paia was the lectern case. My theremax has four nylon feet attached (one in each corner), and sits on a piece of cardboard that is roughly the same size as the theremax PCB. This cardboard has been wrapped - on Paia's advice - in tinfoil, which is grounded.

The Problems
Right, now we come down to the problems I'm having:

Pitch Ariel
I have followed the manual calibration techniques, and found the 'sweet-spot', where a very loud pitch rises to full then falls to null before a second one rises again. The left ariel then increases in pitch as I get closer, and decreases as I pull away.
However, when fully away, the theremin isn't silent. There is a slight 'rumble', very low in pitch, which I don't seem to be able to get rid of either by adjustment of the L2 OC, or with the Pitch Trim knob. As I get closer to the ariel, this rumble increases in pitch but remains rumbly until I get to a certain distance when the tone suddenly changes to a constant clean sound. This clean sound suddenly kicks in - I'd say at about G2 - so I'm missing out on clean lower notes.

Volume Ariel
The volume ariel seems to work, and I can set it to perform either so it gets louder as I get nearer to it, or it gets quieter. the problem is that in the mode I really want it in (where it gets louder the closer I get), the progression is not smooth. Additionally, I hear an increasing pitch - much like the one when I use the pitch ariel - which is loud enough to be very distracting. This is not be just getting close to the pitch ariel by mistake, because if I use the pitch ariel as well I then get two separate pitches; one for the left and one for the right.
If I flip the volume control so getting closer makes it quieter, I do not get this second pitch.

Both Ariels
My area (distance) of controller seems extremely limited. The pitch will go from a rumble to suddenly relatively high (i.e. no 'low notes' are ever played) when I get to a distance of about 10 inches from the pitch ariel. It then continues to pitch up smoothly, if a little quickly, as I get closer.
The volume is a real problem. There is no noticeable difference in volume change until I'm about 4 inches away, then all the volume change happens in the space of about 2 inches.

I really, really want to get this instrument working, so can anybody offer any advice?
Posted: 12/18/2006 10:27:54 AM

From: Kansas City, Mo.

Joined: 8/23/2005

Hello, and welcome to the world of the Theremin and to the joys (or frustrations) of building your own Theremin.

[i]I currently have the vertical ariel on the right, controlling volume. I have the horizontal ariel on the left, controlling pitch.[/i]

Make sure your antenna leads are not reversed. The vertical rod should control pitch and the horizontal loop for volume.

I would suggest that you check the antenna connections and be sure that they are not reversed. If indeed they are reversed, once you switch them, you will have to re-calibrate your oscillators. Here is a procedure that may be useful to you:

After tuning the pitch oscillators, before you remove the jumper, go ahead and turn the adjustment on L4 and you may find a place where L4 interacts with the pitch oscillator. Adjust L4 to a position where the pitch is stable as you move your hand to and from the volume antenna. Then, remove the jumper and tune L3 to achieve your desired volume response.

~~ explanation ~~

If the pitch and volume coils are tuned at or near the same frequency, then the variable frequency oscillator for volume will interact with the pitch circuitry. Try experimenting with different settings for L4 -- one turn may not necessarily work.

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