I have posted some bits about this project to DIY every now and then, and I have discussed some aspects of the circuits with several people privately. It is still not built into an enclosure, and it still lacks interfacing such as a mic preamp, but the core is finished, and it works to my full content.
I had started with a BFO design like the Moog / Bode and Electronotes shifters, but I ran into severe problems when I tried to get very low shift (fractions of one Hz). Now, thru-zero operation was the reason why I tried the BFO method, but several people convinced me that the slightest jitter in either of the HF oscillators would result in severe phase errors near zero Hz Beat frequency.
So I designed a base band Sin / Cos VCO that can go thru zero. This is achieved by full wave rectifying the frequency control voltage, and changing the “spinning direction” of the sin / cos oscillator with the Sign of the same control voltage. It took me some time to figure a practical circuit out, but in the end it was simply combining an idea from Electronotes (for a “simple”, i.e. not quadrature, VCO), with a tri-shaped quadrature oscillator from Tietze/Schenk, “Halbleiterschaltungstechnik”. The VCO runs from -20kHz to +20kHz, and it is no problem to set it to something like 0.1 Hz as well. There are linear (thru-zero) inputs and exponential (V/Oct) Inputs.
Another circuit detail is the use of one cheap chip, the MC1496, to get both, low-distortion tri-to-sine-shaping (degenerate emitter method), _and_ multiplying audio signal and carrier, at the same time.
Carrier suppression of this system wasn't that bad (never measured it, but I think it's way below -60dB), but I thought a compander system would be nice to reduce the noise of the multipliers and to quiet the carrier completly when there is no input signal. (I don't like the idea of noise gate thresholds, and there's a hint in the Serge catalog that they do it in a similar way.) So I took the compander circuit of the Roland VP-330, adjusted the time constants for an application where input- and output waveforms aren't similar anymore, and here is the resulting circuit:
The Hilbert Transform approximation (or “dome filter” in Moog speak) is a direct copy of the Electronotes 12-pole design, and so I have not redrawn this part of the circuit. Simply a pair of 6 phase shifter stages, with high tolerance capacitors from the box, but then measured exactly with a DMM, the resistor value calculated for an exact RC product, and then implemented in the form of 2 resistors in series. (One slightly smaller than the required - odd - value, taken from E-24 1% box, measured, and complemented by the right small resistor to fill the difference.)
Juergen Haible
Contact the author with any questions about this article:
