Glasgow Theremin (analogue)

Hey there,

I built one the  Volume part of this Theremin to use it as CV - Source for my modular synthesizer.

http://www.physics.gla.ac.uk/~kskeldon/PubSci/exhibits/E9/cir1.gif

I use exactly the same parts as in the circuit but it just doesnt work well.

This means that the sound isnt clear and the hf oscillators dont create real sinewaves, but some distorted sinewaves.

 In addition to that the theremin must be trimmed again when i power it up again.

I built this thing on a matrix board and soldered and old radio Antenna to it via a 1mm thick cable which is about 15cm long.

The Capacities in the circuit must be electrolytic capacitors because their capacity is so high. So I just used this type of capacitors always with the negative pin to the ground... I dont know if this is right.

Another thing is that the oscillators oscillate at about 700kHz instead of 760, therefore i should try some other inductivities (sorry if that word doesnt exist).

I hope someone can understand this text and that somebody can help me out

-Flo

 Hello Flub & Flo,

I am sorry your first attempt did not go as planned but it is the way of the theremin, she has attitude.

I wish to understand more about volume CV, I am not a musician. I have actually designed a volume only board for my own research that is very adaptive.  This will be made public soon once I am happy with pcb layout and have boards made. If you have interest, email me, I think we can learn from one another.

My stand alone volume control can be used with any theremin. Easily convert a left side volume to a right side by plugging into the theremin output jack. It is completely distortion free with up to 12” logarithmic shading in the quiet range.

Also it can give you a proximity controlled 0 – 10v swing as smooth as the audio sample below. This voltage would need an external Op amp source follower buffer, a simple one chip add on postage stamp size board.

I use the radio audio as the best method to demonstrate distortion free

Sample .mp3 103k Subconsciously this is about me and my theremin. lol

Often in theremin design or performance you need to reevaluate the road you’re on and sometimes unfortunately need to backup and take a new direction.

I call my project enigma; this is a completely original project for home builders, how refreshing! The name came from a poster (OregonJim) about a year ago that said the theremin is not special or an enigma. LOL

O my email, I am not hard to find.

Christopher

Hi,

thanks for your answer. CV means Control Voltage - in this case you control a voltage with your hands distance to the antenna. Now you can take that voltage and let it control all kinds of parameters of voltage controlled devices.

almost every synthesizer module is voltage controlled. some examples are (Voltage Controlled Amplifiers=VCA | VoltageControlledFilters=VCF | Ringmodulators | VoltageControlledOscillators | VoltageQuantizers etc)

Now back to my question, does anyone know this circuit ore even knows that it works? i read somewhere here on the forum that also somebody else called Thajan or so had a similar problem with this circuit.

btw: your sound example sounds super!

-FLo

Interesting schematic, one I hadn't seen so thanks for the link to it!

"This means that the sound isnt clear and the hf oscillators dont create real sinewaves, but some distorted sinewaves."

How are you examining the oscillator waves?  If you put a scope probe on a high impedance point you might alter the wave shape and/or frequency.

"In addition to that the theremin must be trimmed again when i power it up again."

Theremin oscillators are quite sensitive to component temperature dependencies and such.  It could be that your coils or capacitors are not very stable, or that your circuit layout is not very good. 

"Another thing is that the oscillators oscillate at about 700kHz instead of 760, therefore i should try some other inductivities (sorry if that word doesnt exist)."

Having it oscillate ~10% lower than you expect is not too surprising, coils assume different values at different operating frequencies, and parasitic capacitance can lower frequencies too.

okay the whole thing is air wired on a matrix board is the frequency range of the circuit to sensible to route it very chaotic?

"okay the whole thing is air wired on a matrix board is the frequency range of the circuit to sensible to route it very chaotic?"

Hard to say without seeing the physical arrangement.  Could you post a picture?

Flub said: "does anyone know this circuit ore even knows that it works?"

If you can handle the truth I think this Glasgow design was a college students finale project that was never intended to work, it was just to look fancy on paper for a grade. This is why there is no author webpage to go to for accountability.

As I always say: "If a webpage does not furnish a sound byte for the design run!"

This design should be purged from all sources, it's on the internet and will haunt us until the end of time! In my ten years of theremin research this same theremin trapped many a young lads and ruined their theremin careers.

Maybe and I hope someone will have answers for you. dewster has good  knowledge, I see it in his statements.

Looking at the schematic, the 0.01uF capacitor coming straight from the antenna looks like it might swamp the 33pF LC tank capacitor. I'd have to breadboard it to know for sure though.  180 degrees from the transistor should add to the 180 degree phase shift from the CLC PI network and oscillate if there is enough loop gain.

The Transistor symbol is for a PNP, but it should be NPN (BFY51 is NPN I believe).

What kind of trimming capacitor are you using (the variable 15pF)?

The intersections of the two wires to the right of the text "output 1 ~760kHz" and "output 2 ~760kHz" should NOT be an electrical connection.  In other words, the wires should cross over each other and not connect to each other.

Hell...These are simple Colpitts oscillators which will never produce a clean and undistorted waveform. The 0.01uF capacitor is only for DC decoupling and has no big impact onto the resonant frequencies. The schematic is erroneous - NPN transistors have to be used. It looks like the 47uF capacitors are connected to the oscillator's outputs, but they are NOT, the horizontal and vertical lines are crossed but not interconnected at these points!

Neither the waveform nor the absolute frequency of the oscillators will have a major impact on the control voltage output since only the sum and the difference frequencies will appear at the output of the ring modulator. The sum frequency and eventual distortion and harmonics in the difference signal will be filtered out by the low pass filters between the ring modulator and the active rectifier. 

Always make sure to fully understand a circuit before building and/or discussing it!

    Ouch!     Ont de miséricorde.