A good ring-modulator is a “must have” in any well equipped synth.
I made this little design using four carefully selected germanium diodes as the exponential element. Germanium seems like a good choice since the “knee-point” is soft and long. The diodes are coupled in the traditional two-transformer setup. The symmetri is OK, and the sound is good.
The transformers are of equal type, and both from a bag of old transformer, that I bought in “Radio Twenthe” (Stille Veerkade 112512BE Den Haag). The transformers are probably form old Philips pocket radios, and Radio Twenthe is really a place to visit for any serious hardware hacker. This is how the prototype looks:
The transformers are quite small, and this causes an unintentional roll-off at low frequencies, so a compensation is introduced in the signal-input amplifier. The modulation input is not going through any transformer, so no problem here. Input voltages are 10Vpp for signal, and 6Vpp for modulation (usually a low frequency signal).
The direct modulator output is 10Vpp, and in addition to this I have made a mixed output (-potentiometer) between modulator output and signal input. The pictures at the top and below shows various degrees of mixing between these two signals (blue signal is modulated, and yellow is mixed).
NOTE: A true amplitude modulated (AM) signal appears when the two signals are equally mixed (top picture). This is actually quite obvious, since the signal input is the carrier missing in the modulated signal, also called a “double sideband signal with suppressed carrier” (DSBSC). When they are mixed, the AM signal containing both the sidebands AND the carrier will appear 🙂
The diagram is shown here: Ringmodulator