884 - jh_polykorg_clone

I have started to build a (slightly) updated version of the Korg PS-3200 synthesizer. The PS-3200 was the last of three fully polyphonic, semi-modular analogue synthesizers offered by Korg in the late 70's. (See Ben Ward's excellent Korg PS site for detailed information, including user manuals.) The concept of the PS-Synthesizers was different from other manufacturer's early polyphonic instruments. Instead of using a small number of voices and a clever keyboard assigning circuit, the "PolyKorgs" had a complete synthesizer circuit, hard wired to each key. That makes a total of 48 VCFs, 48 VCAs and 48 voltage controlled ADSRs even for the smallest of the range, the PS-3100. The largest of the range, PS-3300, even had 144 of these circuits. The sheer number of synthesizer circuits called for an extremly economic circuit design, and it's a joy to look at Korg's design ideas which led to building blocks that almost did the same as in the better known "classic" synthesizers. And after many years of engineering and reverse-engineering electronic music circuits, I have learned to look at odd solutions not as "substandard", but as a source of creativity an individual character. Here's a list of some highlights: Function Implementation Side effects Single-Transistor Waveform Converter creates triangle, saw, pulse and PWM from saw input, using one (!) transistor, one diode and two resistors per voice, plus two global control voltages Pulse height also changes with pulse width 5-Transistor-VCF (Korg-35) A Voltage controlled 2-pole (Sallen&Key) LPF built from 5 transistors rather high CV feedthru Single-Diode VC Resonance The dynamic resistance of a simple diode is used to alter the feedback gain of the VCF limited range of Q "Expand" function instead of VCF Envelope modulation depth Instead of scaling down the ADSR with a VCA, the a variable portion of the Envelope is just clipped with a single diode. It's so remarkably close to ordinary VCA function that apparently nobody takes notice. At least I have not read about it anywhere. At slow Atack times, the Envelope appears delayed at the VCF (no effect until th eclipping point is reached). Usefull for Brass sounds, and not easy to emulate with conventional synthesizers. Minimum parts count Voltage Controlled ADSR Three transistors, 1/2 of a LM324 and one CD4007 per voice. Plus some more involved control circuit, shared by several voices Transistors must be selected in 13-tuples, not just in pairs. ADSR detail (1): One-opamp control logic 1/4 LM324 is used as Flipflop, which is dynamically set by Gate-ON, dynamically reset by Gate-OFF, statically reset when the attack peak voltage is reached, and whose set/reset sensitivity is altered by a CV Very odd "Hold" function, depending on the "Attack"-value. But very useful in practise. ADSR detail (2): Single-Transistor, exponential slope VC-Decay Using a single transistor per voice for VC Attack and Release is remarkable already, even though the A and R slopes are linerar. But the Decay slope is exponential, and this is achieved with a single transistor and two resistors per voice! The Decay time range is rather limited. No ultra fast Decay, and no ultra slow Decay either. Single-Transistor VCA That's the "Korg standard" VCA, well known from other instruments like the MS-10.

2011-01-21 17:32:35

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