Regular hookup wire, shielded cable or twisted pair?

Posted: 9/12/2013 12:10:34 AM

Joined: 1/14/2011

Hi there,

I posted recently that I've finally got my theremin working, well now I need to fix some of the mistakes I've made along the way. One of these mistakes was using shielded cable everywhere and as a result the pitch increases markedly when the theremin is boxed up because of the extra capacitance.

So, my questions are threefold:

1) Is it ever OK to use shielded cable in a theremin?

2) If so, for which connections?

3) If not, which is preferred, regular hookup wire everywhere or hookup wire for single connections and twisted pairs for grounded connections (e.g. to the speaker or pots)?

Thanks in advance :)

Posted: 9/12/2013 4:16:37 PM

From: Eastleigh, Hampshire, U.K. ................................... Fred Mundell. ................................... Electronics Engineer. (Primarily Analogue) .. CV Synths 1974-1980 .. Theremin developer 2007 to present .. soon to be Developing / Trading as . ...................................

Joined: 12/7/2007

" One of these mistakes was using shielded cable everywhere and as a result the pitch increases markedly when the theremin is boxed up because of the extra capacitance." - patricks

I dont think the above problem is caused by shielded cable - its probably just the box capacitance.. if anything, shielding the antenna lead (a BAD idea!) will probably reduce the effect of the enclosure, so I doubt that putting this 'right' will fix that issue..

That said..

1.) Unless one specifically accounts for screened RF cable capacitance, or designs the theremin in a special way*, Dont use screened cable at the RF side - use thin hookup wire - ideally single conductor ..

1b.) The above is particularly true if you have a equalization coil in the antenna circuit - The better theremins generally have this - and when fitted, the total antenna capacitance must usually not exceed about 15pF (usually between 10pF and 15pF) - screened cable can easily add 5pF.

1c.) Simpler theremins (like the SC/Jaycar) have the antenna in parallel with the tank capacitance - this capacitance is large (180pF for the SC) so the extra capacitance of screened cable is not so significant.... BUT... Its still not a good idea - the capacitance of screened cable is extremely variable and temperature dependant.

2.) Apart from the RF side (antennas and all signals before the mixer) it is my opinion that screened cable is a good idea - particularly on all audio leads and this includes leads to potentiometers.

3b.) I would even advise using screened cable for power supply connections.

3.) IMO, access to the tuning presets without having to open the theremin should be compulsory - It is just insane to have an internal adjustment which is invalidated when one puts the unit back together!  I advise doing whatever is needed (hole in the box or whatever) to facilitate this access


*The only time I have used screened cable for the antenna was with double screened cable, with the inner screen driven by a shielding signal (which adds no capacitance) and the outer screen grounded.. But its a LOT of difficult design and effort, and not worth doing IMO.

Posted: 9/12/2013 4:57:36 PM

Joined: 1/14/2011

Excellent, thanks very much :)

Yup, it's just a cheapie SC/Jaycar theremin, but I'm thrilled to have it working so I'll change the connections to the antennas. The screened cable currently connecting the antennas to the board is actually from a TV antenna (I chose it so I could use the antenna itself and scavenged the cable while I was at it). Thick, solid-core wire with chunky shielding!

Good to know that the screened cable for audio leads is OK, that saves quite a bit of effort. I'll make some measurements and drill some holes for adjustment.



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