Air cored inductors

Story so far...

I have assembled, but not yet powered up a SC/Jaycar Mk 2 theremin kit.

Rather than use the small plastic box sent with the kit I have made a plywood chassis to mount the theremin in somewhat like the design of the EM theremin case. The chassis is 18" long, 6" back to front and the working depth is about 2 3/4".

I have made a hairpin loop of 10 mm diameter aluminium alloy solid bar. the parallel legs of the hairpin are 4" apart and the distance between the end of the radius and the case is about 9"

The volume antenna is mounted on the lefthand end of the case using an aluminium collar and grubscrew to set the angle to the horizontal.

I have made up a 12 VAC power supply from a 3VA mains transformer (old Radio Spare item) It has two 0 - 12 V windings that I have wired in parallel. The transformer is housed in an earthed aluminium case and fitted with a 12V filament indicator lamp. The no load output measured on the AC range of my digital multimeter is 13.5 V

The reasons that I have decided to deviate from the SC/Jaycar plans is to make a better looking device and by increasing the separation of the pitch and volume antennae reduce any interaction between the two. It also gives me room to add linearisation inductors between the circuit board and the antennae.

Looking at the EM and EW designs I see that Moog used 40 uH split into 4 separate ferrite cored inductors. From other postings on this forum I understand the reason for splitting the inductance up is to reduce the effect of the parasitic capacitance associated with the inductor. I assume that by putting the four inductor in series it has the effect of reducing the effective parasitic capacitance resulting in the small changes in the Earth to Antenna capacitance having a greater effect on the oscillator frequency.

I would like to try winding my own air cored coils. Using the formula L = 0.001 N^2 r^2 / (228r+254l) I have calculated that 110 turns of 1 mm diameter wire close wound on a 60 mm diameter former would give me an inductance of 41 uH.

Would such a coil improve the performance of the theremin?

If I were to halve the diameter of the wire, wind twice as many turns using two lengths of wire, and then remove one set of windings to leave 110 turns but with a gap between each turn and the next would this reduce the parasitic capacitance of the coil?

Bob

a) The linearization inductors are 4 x 10mH, not uH!
b) The needed inductance for better linearity depends from several factors: The pitch oscillator's frequency, the static pitch antenna capacitance, and the L/C ratio of the pitch oscillator's tank circuit.

The EPE/Jaycar theremin has not been designed for the use of linearization coils. The tank circuit (the white IF transformer) has an inductance of 820uH and a capacitance of 150pF which gives a resonant frequency around 460kHz. In the EM theremin which runs at about 285kHz, the inductance is smaller though (100uH) and the capacitance much bigger (3300pF).

So you will be able to "linearize" somewhat your theremin with the use of an inductor of 9 to 11mH (wind the 11mH and make taps at 9 and 10mH) but the linear octave spacing will only be about somewhat less than the half of the Etherwawe/EM's spacing. I'm not sure if it will be better playable with this.

Thanks for pointing out my error.

Redesigning the coil to match your suggested values it becomes clear that a single layer coil is probably impractical. An air cored coils matching the Brooks pattern seems possible. My proposed coils would be wound on a 32 mm daimeter former, 16 mm wide using 0.65 mm (probably 22AWG) wire. 470 turns gives an inductance of 9 mH, 500 turns gives an inductance of 10.1 mH and 520 turns results in 11.02 mH. These figures assume a 15% allowance for 'random' laying of the wire.

I think such a coil would be manageable in my workshop.

I have powered up the SC/Jaycar circuit and can report that it works insofar as the volume circuit works as predicted and the pitch circuit can be adjusted to give high pitches with the hand close to the antenna and the pitch falls away as the hand is moved further away. However the whole set up seems very unstable and especially sensitive to the extension of the pitch antenna. The article in Everyday Practical Electronics suggest using the length of the pitch antenna to tune the theremin. I am going to try replacing the telescopic antenna with an 18" length of 10 mm diameter aluminium rod to see if this improves the 'predicatability' of the device.

Bob

You will kill the variable inductors if you use them every day for fine tuning...

My Jaycar came with a 6-element telescopic antenna. I tuned the if transformers once with 4.5 elements of the antenna pulled out. Since then I have only to lengthen or shorten the antenna by a few millimeters in order to tune this toy, sorry, theremin correctly.