This is yet another of my projects. For some reason, I thought it'd be a fun thing to learn to play. Alas, I couldn't find one for sale (at a price I could afford!) so I decided to make my own.

A Theremin is really a very simple instrument. As I know from my GEC radio, when I place a hand next to its earthing point, the volume is raised. This slight capacitance difference influences the audio circuit. The same principle can be used to design something that reacts to the movement of someone's hands. A Theremin is essentially an audio-signal oscillator, where both the pitch of the signal and the loudness of the signal are controlled by the proximity of your hand to two different aerials.

I started off by asking around on the RAT newsgroup, and got all sorts of helpful replies. Some people suggested good component sources, others web sites, and some people were kind enough to send me some scans and photocopies. I had ended up swamped with information! I eventually chose to build a Theremin by following a DIY article in an American magazine from the '50s - it used standard valves, seemed simple enough, and best of all, told you how to wind your own coils. The valves I needed for this were two 6C5s, one 6SN7, and one 6SA7. I'd always meant to try ordering from a surplus warehous in Canada, so I ordered these as part of a large batch.

Getting hold of the parts

After a week or so, the package arrived from Canada, and I really started getting to grips with the design in earnest. I had the valves, and could easily buy the components I needed from RS.

I decided to use a larger case than I'd done for my pre-amp (RS to the rescue as ever), and found a decent sealed transformer to drive it. I'd had this one lying around for ages. As you can see from the picture below, the RS case had runs for a chassis plate, so I contacted a local company to get one made up from 3mm aluminium. My little addition here included adding an internal power supply with a choke and 6X5 rectifier valve. This might provide enough power to drive a small speaker, too.

Thankfully, the instructions I was following included coil winding details. I could have worked something out, or used standard coils, but this simplified matters considerably. I improvised here, as you can see from an example of a home-made coil below.

There were one or two small modifications I chose to make to the circuit. As well as having its own power supply, I wanted a small speaker. Since I didn't have enough power to run a 6V6 (the transformer was only rated for 40mA, and even with its margin of error, I felt I must be pushing things!), I played around with the idea of doing something rather different - I'd either use a 6J5 to drive a speaker, or if the power supply couldn't handle it, add a mains rectifier to fit a 6V6.

Sources of information

Please note that I have put together a few scans of valve Theremin projects and uploaded them as a .ZIP file in the Downloads section of my site! You can also download the file - Theremin.zip - by clicking on its name here (please note that it is 2.47MB large!).