Some build pics of the Silicon Chip Thermin.
This kit has been sitting in the bottom of a draw for about a decade and I only dug it out
after a friend asked for advice re building a thermin from the old Moog designs.
Though this kit isn't the same I thought I'd finally put it together to learn about thermins in general.
I think this old Jaycar kit is discontinued.
This looks like a very easy project that
is great for beginners.
is great for beginners.
All through hole components.
This is part 2 of my Thermin notes.
The first part of the build notes are here:
I'll discuss the circuit as I build the synth.
All Thermins start with this same basic circuit.
The basic circuit comprises 3 oscillators (reference, pitch & volume).
In this case they all operate at 455kHz.
Each oscillator is essentially identical.
They comprise a JEFT (junction field effect transistor) and a pre-wound IF (Intermediate Frequency) transformer.
The JFETs are all 2N5485.
In this case the Reference oscillator = Q1 & T1
The Pitch Osc = Q2 & T2
Vol Osc = Q3 & T3
The original Moog circuit used 2N 168 & 2N1086 transistors.
These are old GE intermediate- and high-frequency transistors intended for use in radio receiver circuits.
I reckon these would be difficult to find today.
T1,T2 & T3 are the pre-wound IF (Intermediate Frequency) transformers.
These are a modern substitute for the inductors used in the vintage circuits.
Looks like you can buy these on Aliexpress, etc
The Tone Oscillators (ref & pitch)
The Reference & Pitch Oscillators are mixed in IC1 (called the product detector) to produce a audible tone.
IC1 = MC1496
It's a balanced modulator/mixer.
You won't find this in the original Moog circuit where the two oscillators are mixed through two capacitors and rectified through a diode.
Pretty old ... dates I think from the 1970s. Easy to find on Ebay.
The MC1496 is used here as a mixer.
The signal inputs are on pins 1 & 4 .
Carrier inputs are pins 8 & 10.
Outputs are pins 6 & 12.
The deeper I compare this circuit with the original Moog circuit, the more I realise
how components have changed over the last 70 years.
I guess this is to be expected.
I wonder if its practical finding all those old vintage parts if you wished to stay true
to the original schematics.
Anyway, back to the build.
The VR2 trimmer looks like an new addition to the original circuit..
I think this is a basic wave shaper ???
& adds some waveform distortion.
The addition of this shaper circuit changes the final timbre.
It can now be varied from a pure sinewave (which is not characteristic of a Thermin) to a much deeper sounding square/ramp tone, which I think has a lot more harmonic content.
The output from IC1 is fed into IC2.
IC2 is a LM358 op amp.
It acts like a low pass filter.
Again, you won't find this in the vintage Moog Thermin.
That circuit has no ICs. Its all transistor based.
The output from the filter is applied to the power amp (IC3, LM386)
and this drives the speaker.
IC 3 = LM 386
IC 2 = Lm 358
IC 1= MC 1496
The Volume Oscillator.
This is made up of T3 & Q3
The secondary winding of T3 is connected to the base of Q4 (BC548)
The collector of Q4 is attached to T4 (and its capacitor).
T4 is another IF coil.
To be continued ......
Thermin- part 3 build - completion
PS: do let me know if there are any mistakes or omissions. Cheers Jono
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