A few months ago at band practice Bill mentioned ring modulators, and asked if I knew how to make one. I had seem some circuits and knew they were pretty easy to construct so I said yeah, no problem, give me some time. Since my life is basically just going from project to project, I had some other stuff to clear out before I could get to this one. I wrapped up some other stuff and started fleshing out the project after Christmas.
A ring modulator is a device that multiplies two audio signals together. Usually you have a carrier frequency, a sine wave or something like that, and another audio signal like a guitar or piano or something. Bill sent me a really sick example of the Mahavishnu Orchestra playing live, with one hell of a ring modulated Rhodes solo somewhere in it. Most modern ring modulators are actually balanced modulators, but the effect is basically the same. The ring in ring modulator comes from the shape of the diode configuration.
I knew of a couple of circuits off the top of my head, but after a little research settled on this design: which consists of a couple audio transformers, four diodes, and a signal generator. With the exception of the signal generator, the circuit is passive, and if you use an incoming carrier signal instead of a generated sine or square wave, it requires absolutely no power. I could have used the much more complicated “modern” circuit, but decided to stick to the analog approach. Here is the parts list:
- 2 x TM018 Audio Transformers
- 4 x 1N4002 Diodes (I used matched pair germanium diodes instead).
- 3 x Mono Audio Jacks
I assembled the circuit on breadboard, using an atari punk console as my signal generator (since it happened to be sitting on the breadboard already), and ran a few tests. To be honest it sounded like shit. At first I was testing with a guitar and just couldn’t get a good sound out of it, it was scratchy at best, and the apc was overpowering the guitar. I figured the output level on the guitar must be too low, then after a few minutes the atari punk console (signal generator) gave out, so I walked away for a while out of frustration.
A day or so later I came back to it with a new strategy. Rather than test with a guitar and the apc, I could just test with two synthesizers, one generating some simple carrier tones, and the other controlling the melody. The XP-50 was already hooked up, and the Casio SK-1 had some batteries in it, so what the hell, I gave it a shot. This was working much better, and after some tweaking I was starting to get some quality sound out of it. Here is an example of using the Roland to play some stupid melodic lines with the SK-1 ‘flute’ tone, and a lot of portamento:
Not too bad, sounds like a ring modulated keyboard. Surely a more talented ivory-tickler could do better. I thought about it for a few minutes and realized that the signal levels of the carrier and source signal would have to be at the same level (not volume) for the output to sound correct. This mismatch in level is why the guitar APC combo didn’t work, but this did, since the keyboards were both at line level. When I build the final circuit I’ll have to remember that. There is also some leakage in the carrier (nobody wants to hear the modulating frequency) which I’ll have to address, its probably due to the voltage drop across the diodes (1.6 volts?), but that should be no big deal. I might also through an opamp on the output to drag the signal level back up.
On the other hand, since I had the thing up and running now, I screwed around with it for a while. The SK-1 is a terrible little synth from the 80’s that samples. So I recorded a ‘whooping’ noise and looped it, then used that as the carrier:
Since the carrier signal was now a vocal sample instead of a sine wave, I could make pretty much any stupid noise I wanted and it would modulate the XP-50 synth…So I made some siren sounds:
and finally, I put the keyboard in polyphonic mode and hit several keys at once. Since the SK-1 changes the speed of the sample to modify the pitch, you end up with the same sample played back at different speeds. When used as a carrier frequency you get this:
Once I box this thing up for Bill I’ll have to build one for myself…