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The first part I had built was the VCF. I wanted to get that Moog Modular Filter sound which I'd heard on several music albums, and wich cannot be reproduced with the VCF of a Minimoog, nor with any other Moog filter I've heard. (Moog used a lot of different variations of his basic transistor ladder idea in his various synthesizers. The multitude of Moog filter clones add even more variations ...)
I did some very extensive Spice simulation work to find the reason for this different behaviour. My goal was to find the theory behing the various parasitic effects of the Moog Modular filter, which are responsible for its unique sound. Just building the old circuit with modern components does not give the desired result. And any modern clone that doesn't take the effects of the Range Switch into account will fail to reproduce the original, because the Range switch doesn't just transpose the cutoff frequency up and down - it also changes other parameters which are important for the sound.
While I was making these experiments, Bob Moog himself returned to the synthesizer market and introduced his "Minimoog Voyager". This will have a pair of LPF filters instead of a single LPF or a HPF / LPF combination. As I am writing this, I have not seen a Minimoog Voyager "in the flesh", so I have no idea how this Double LPF is implemented. But I've made a few simulations and experiments with various Dual LPF structures and I found that running two LPF's in parallel and either adding or subtracting their output signals gives some interesting sonic results. So I built a second Moog Modular Style LPF and combined it to a Dual LPF for what was to become my JH-5A panel. I doubt that Bob Moog would go back to his very first VCF circuit for the Voyager, so I expect my JH-5A VCF to be quite different, even if parts of it were inspired by the Voyager. With the "2nd Filter" switch in "off" position, it has the typical old Moog Modular filter sound. With "2nd Filter" in "on" position, it shows the phaser-like double resonance effects which are verbally described on the Big Briar page (without any claim to be the same). With "2nd Filter" in "INV" position, BPF-like effects are possible. The sound is quite different from a classic BPF, however.
I already have a lot of different VCOs, from the ultra precise MOTM-300 to the lush, but not so well-tracking VCOs of my EMS-Synthi clone. Each VCO has it's benefits, the CEM3340's (partly under autotune control) are the "workhorses" in the OB-8, the Prophet 5 and my own JH-3, the Yamaha CS-oscillators are more on the "temperamental" side, but still playable without autotune (the 4 ones in my CS-50 stay better in tune than the 8 ones in my CS-60 ...). The precision of the MOTM VCO is invaluable for complex audio range modulation patches. I've never heard better drones than from three EMS VCOs running at almost the same frequency, and beating against each other in an ever changing pattern.
My goal was to build a set of VCOs that have the untamed bass range power of early EMS and Moog VCOs, but which are tracking a keyboard voltage over 5 or more octaves nevertheless. I found that "untamed" Beating in the bass range and controlled beating in higher octaves is not possible with standard exponential 1V/Oct oscillators. A good part of that special sound of early Moog and EMS oscillators is not because of any "randomness", "unstability", "instability" or "noisyness", as so often is said. A good deal of their behavior is because of that, but it is not the whole story. There are also some very deterministic factors in these old circuits which have been unpleasant side effects for the designers back then, but which are worth a closer analysis when we're designing a musical VCO today. This is implemented in form of three "linear detune" potentiometers on the JH-5A VCOs.
for JH-5A VCOs will be availablable in my "Living VCOs" project.
Variable Slope HPF
A HPF to complement the JH-5A's Moog-style LPF.
As the LPF is very close to the Moog 904A, a 904B clone would have been an obvious choice for the HPF. But the 904B was limited to 24dB/Oct slope, and it did not have a Resonance function.
The HPF of the JH-5B will have variable Slope, even under voltage control, and a voltage controlled Resonance.
The two functions are combined - they share one front panel potentiometer and one CV input.
With the potentiometer in center position ("12 o'clock"), the filter is very much like a Moog 904B HPF: 24dB slope, no feedback.
Turning potentiometer to the left will gradually decrease the filter slope. At the ccw end position the slope reaches approx. 6dB/Oct.
Turning the potentiometer to the right will keep the 24dB/Oct slope, but gradually add feedback or resonance.
The following simulation plot will describe this behaviour better than a lot of words.
Copyright J. Haible (C) 1996 - 2001