ARP 4014 Ring modulator clone

[rutgerv]

Dear synth enthousiasts,

I've been playing around with a Spice model I made of the ARP 4014, following this schematic: http://www.arptech.synth.net/pdf/4014.pdf. Instead of the LM301A opamps I've used TL072 and all transistors are either BC550C or BC560C. I made both inputs DC coupled with a 47K impedance.

Mostly, the model behaves as I would expect from a ring modulator. When I use a 10kHz sine as carrier and a 1.5kHz modulator I see them nicely multiplied together at the output. However, there appears to be an artifact in the output as well, which occurs every time the modulator signal swings from positive to negative, or the other way around. This causes the carrier wave to be modulated 'through zero', but adds a short burst of high frequencies on top. The attached picture shows the output of the ring modulator in red and the voltage at the output of A1 (the op-amp at the modulator's signal path) in yellow.

I was wondering if this artifact contributes to the desired "ARP sound", or if ARP was avoiding it by providing a raised VCO signal at the modulator input (which never crosses zero). Or does it come from the presumably much faster responding TL072s, as opposed to the original LM301A?

Finally and most importantly, would there be a way to get rid of this artifact?

Regards,

Rutger

---

test.pdf
Description:	ARP 4014 ring modulator in Spice. Carrier is a 10kHz sine wave, modulator is a 1,5kHz sine wave.	Download (listen)
Filename:	test.pdf
Filesize:	102.03 KB
Downloaded:	288 Time(s)

---

[Doopie]

I believe this is because the original PDF only covered the schematic of the 4014 potted ring modulator module. The 4014 was made up of 2 two quadrant multipliers: one for the positive phase of the waveform and one for the negative phase. Only one multiplier was active on any given phase. So what I think you are seeing is typical crossover distortion.

When installed in the ARP 2600 there were a number of ancillary components connected around the potted module. So for example there were 2 off 50K potentiometers (R33 & R34) connected between ±15V to supply a DC null offset so that the two separate halves of the multiplier could be provided with a DC bias on pin 10 and pin 18 of the module respectively to get rid of the cross-over distortion.

Do a google search for the "ARP 2600 Service Manual"and look for "Section 3.3 Board 1 Adjustments" for details of how to adjust R33 and R34. A copy can currently be found here: http://guitarfool.com/ARP2600.html

[andrewF]

looks like you just need to do some setting up to get it working properly
Bergofotron gives some instructions for this - http://hem.bredband.net/bersyn/VCA/ringmod%20ARP4014.htm