My girlfriend is a musician (well, for now she's a student) and some days ago she came to me saying that a teacher had told the class about some weird electronic instrument that you played without touching it and that was used to make alien sounds for movies. Being the electronics freak I am I automatically knew she was talking about a theremin. She was quite fascinated with it and told me she would like to try one.
So her birthday is relatively near and I'm determined to build her one as a present. But I wanted it to be a true theremin, fully analog, with all its LC oscillating glory... so I searched a bit and found this:http://www.suonoelettronico.com/downloa ... herwav.pdf
That has an schematic which uses mostly modern components and that uses transconductance amplifiers which should emulate the original valves quite well. Actually that's the schematic of a kit that some folks sell for 400 bucks so you can make your own professional theremin. I also found the schematic it derives from: http://wiki.joanillo.org/images/a/a0/EMTheremin.pdf
which has some info about the required inductors and that.
Finding the components was a pain in the ass. Virtually no distributor sells adjustable inductors anymore, and high-Q fixed inductors of over 10uH are also hard to find nowadays. Finally found some TOKO FDSV series adjustable inductors and some relatively high Q fixed inductors at RS components. And surprisingly they were SMD (hooray!). Ordered them ASAP. Also surprisingly the 10, 5 and 2.5mH chokes required for the antennas (BOURNS 6300 series) were available everywhere at a price I'd never imagined. I was expecting 5 bucks per coil but they are just 1,35€ each.
Next I ordered everything else from digikey. I have chosen quite premium components. Capacitors related to audio are either tantalum or poliphenylene sulphide film capacitors which are quite expensive but that way audiophiles won't say it sounds bad because my capacitors haven't got enough magic powder inside them. Also there are only a few caps that need to be this good and this is a present for someone I love beyond money so... I also threw in some 10uF 16V 0805 X5R capacitors for decoupling pourposes that weren't in the original schematic but that I think they should be neccesary. 100nF's wouldn't do shit in this design as they have large impedances at the frequencies involved in the circuit. All the other caps are also high quality ceramics or similar but those run for pretty cheap.
LM13600M's were found on ebay.
I decided that unlike in the original desing i'd try to make the board as short as possible and assemble the RF chokes for the antennas in series and in line inside a plastic tube instead of mounting them in the board (well, I have to mount one in the board because of the way it is connected but the rest will be outside). That should improve the way the inductors couple to act like a single but "distributed" inductor with reduced parasitic capacitance and such things.
Next stage: design the PCB. I love squeezing circuits in tight layouts but having in mind a couple of good layout rules. I'm pretty happy with the result. It still needs a bit of touching and I've used overly large trace thicknesses just because and because I could. It's meant to be mounted upside down (the wooden case will have three holes in the bottom that align with the tunable coil's screws so I can tune the thing) and have the large 2200uF caps mounted at right angle. I have to correct that silkscreen, for example.
I'll try to finish the board this christmas and maybe populate one just for testing. The wooden box, the front panel an everything else will be made after I finish my finals.
Here's the current board and schematic. I hate doing schematics so forgive me for the lack of cleanliness... The board is much better done.
I may give away some of the boards when they're done if any of you also wants a theremin.