ARP 4014 ring modulator clone (AMORE)

The prototype version of the board. 

A ring modulator multiplies the voltages on two inputs and sends the result to the output. Ring modulators have been done in several different ways over the years. The ARP 2600 is renown for its clean sounding ring modulator. This was probably due to its somewhat unconventional circuit design, with the positive and negative parts of the signal split between two two-quadrant multipliers. It has a pre-distortion circuit to minimize the deadband between the two multipliers but it still works a little bit like it has a built-in noise gate. Both the the ARP 2500 and the 2600 used the 4014 ring modulator module. This is a clone of that circuit.
This module is based on discrete transistors for the multiplication effect.  These need to be carefully matched and kept at the same temperature. You should pot the transistor pairs in some isolating material, like a blob of Araldite or something similar. This is not yet done on the prototype board in the picture above. I used ordinary BC550C and BC560C transistors, which works very well if you match them carefully and couple them thermally. The only other difference from a vintage ARP unit is the modern op-amps used. The ARP 4014 used LM301 op-amps which had external frequency compensation, whereas modern op-amps have internal frquency compensation. So there are a couple of capacitors missing in my version. I suppose the different op-amps could make a difference in the sound, but probably not much.
There are no voltage controlled parameters on this module. There are two voltage controlled switch functions though. One mutes the output and the other sends a mix of the inputs to the output instead of the ring modulator output.
Note that this ring modulator is DC-coupled, so you can use it to process control voltages as well as audio signals. When you integrate this board in a module, you should add external DC-blocking capacitors that can be bypassed with a switch, just like it is done in the AMORE exerciser (and in the ARP 2600). Note that you can even mix audio signals and low frequency signals because the latter makes a difference in the multiplied output. A ring modulator is very versatile and more useful than many people realize. If you for instance multiply an audio signal with an LFO signal, it works like a VCA and creates a tremolo effect. But with the difference that the modulation can go through zero.  
 

Trimming

There are three trimmers on this board:

Pos. null, Neg. null
With full signal on the Y input and no signal on the X input, these should be adjusted for at little signal as possible at the output. Let the module be powered up for at least ten minutes before you do the adjustment. 

Gain trim
Adjust this so that the output signal will be 10 volts when both inputs have 5 volt signals.

Skill level required: MEDIUM

The only real difficulty with this module is matching the transistors. You could use matched pairs like MAT01 and MAT03 if you want to make things simple. This comes with a price though – the MAT pairs are very expensive.

Circuit board layout (PDF-file)

Component placement (PDF-file)

Schematics from Arptech