RK5 Dual Lowpass Gate
Ever wondered if someone had invented the lowpass gate before Don Buchla did in 1970, what it might have been like? Especially in the 1950s when there were no “integrated circuits” or such? No one had the nerve or the hallucinatory drugs to think such a thing up. But it would have been easy. A cathode follower using the right tube, plus two linear resistive optocouplers (in those days they existed but used incandescent lamps and responded very slowly), and a few capacitors and resistors. Simple, yet it was never done. It was a miracle they had limiters in the early days; and they weren’t very “good” limiters either. Tubes limit your choices but give good sound effects along the way.
IF they are applied properly.
Since part of our remit is to explore an electronic-music history that didn’t actually happen, one inevitable product would have been a lowpass gate. Our version is a dual unit, dead simple Sallen-Key filter with just input, output and CV input for each channel. The tube used is a “high-perveance” dissimilar dual triode, the 17JK8. It was apparently intended for use as a “cascode” RF front-end amplifier in cheap consumer FM radios. It must have been moderately popular because although we’ve never seen an example of its actual use, it is a commonplace NOS leftover item. We found it was ideal for use directly on the +-12 volt rails in a Euro modular as the plate supply, thanks to its high perveance and low plate resistance. Each channel uses one triode and two Silonex LED-based linear optoresistors. Because of the dissimilar triodes and differences in the opto devices, the two channels will sound slightly different and won’t match or track perfectly. If such a device had existed before Buchla, it would have been like that. So it has “forgiving” distortion and a very slow decay time due to the high circuit impedances. This is exactly what you would have gotten in the 1950s--if it had existed in the 1950s. It’s brute force, baby.
All specifications are approximate and may vary from sample to sample. Tube used: 17JK8 dual triode. The RK5 uses only 100 mA (200 mA briefly during cold power-up) from +12v and 2 mA from the -12v supply rail. We deliberately run it colder than usual for maximum lifetime. It should work with most available Euro power supplies, even the ones too small to run other Metasonix RK modules. Input impedance (audio) is 100k ohms and output impedance is 3k ohms. (Do NOT attempt to drive a 600-ohm load.) The CV input requires a 3 mA driver; voltage needed is +2v or less for minimum gain and +5v for maximum gain (about 0.95).