Ring modulator

A ring modulator is a device that is able to multiply two signals. Therefore 
it is also known as a four-quadrant-multiplier. An ordinary VCA is actually 
a kind of ring modulator. But 1: A VCA is normally incapable of working with 
audio signal on the control voltage input, and 2: A real ring modilator also 
deals with negative input signals reversing the polarity of the output.
The schematic I bring you here (see file ringmod) is actually two VCAs 
coupled as positive and negative responding units.
When the signal on "In2" goes positive the signal on "In1" is carried out 
normally. When the "In2" goes negative the signal on "In1" is carried out 
with reversed polarity.
The construction of the ring modulator is quity straight forward. I'll just 
say a little bit about the adjustment of Trim 1&2. The FETs are to be 
considered as voltage controlled resistors. The more the gate voltage goes 
negative - with respect to the source - the higher resistanse. When there is 
no signal on "In2" there is not supposed to pass any sound from "In1" to the 
output. The right adjustment of the trims are the point where the signal is 
just cut of whith nothing on "In2". Connect an oscilloscope to the output of 
the amplifier connected to trim1. Adjust trim1 to just no signal. Do the 
same with the other amplifier/trim.
Now you should be ready to use the ring modulator.
The only thing you should care about is not to overload the "In2" input. If 
the level on the gates of the FET exceeds +5 Volt the gate voltage will leak 
through the source and cause the opamps to pump out DC. That sounds awfull. 
On the one I have build I have used the unused opamps in the two LM324s two 
make a little VU meter. That way I can check the volume.
Finally a word about the components: The OpAmps are not critical. I have 
used ordinary LM324's. The FETs are not critical either. Just get the JFET 
types. I have used BF256 which is a European type. but I think any JFET wil 
do.
