"JH-4" is going to be a DIY polysynth. At the moment, it's a 4-Voice Keyboard Scanner (Photo) and one single Voice Module (Front Panel Layout). But I can already play polyphonic with this one voice, plus my Minimoog Expander, plus two additional voices patched on my JH-3 Modular system.

I was always impressed by the old Oberheim Four Voice, with its dedicated set of knobs for each voice, and with its unsurpassed variety of key assign modes. There are some well known shortcomings in these SEM-based synths, such as only one LFO waveform, only a few parameters under voltage control, and maybe a few more.
So I decided to build my own version of a SEM module, plus a clone of the original Oberheim keyboard scanner.
These were my design goals:
* Every parameter must be voltage controlled
* An additional 4-pole filter would be nice
* Generally, a few additional parameters should be added (see below)
* The whole thing should not have too many knobs (remember there's a set of knobs for each voice!)
* Build the whole thing as "discrete" as possible. No integrated circuits in the signal path.

One working Prototype of the JH-4 Voice Module exists, with 4 pcb boards (200cm x 150cm each) sandwiched together. Some of its features:
* VCO circuits are very similar to SEM / OB-X
* VCF is a SEM filter clone and a clone of the SSM2040 in parallel. Crossfading from 2-pole LP to Notch to HP to BP to 4-pole LP is possible. (Manually or with envelope)
* VCA is similar to the Minimoog's, only that it is stereo with voltage controlled panning function.
* LFO waveform can be crossfaded from Sine to Rectangle to Sample&Hold to VCO2-SAW. The Sine Wave can be frequency modulated by an envelope.

There will be a redesign of the VCO board to include the Linear Detuning feature. (Link to Roy Tate's site)

The Keyboard Scanner of JH-4 is very much a clone of the Oberheim 4-voice, without the split mode, but with all this great variety of assignment modes continuous/reset, reassign, etc.
It is build on veroboard, and was tested with the first JH-4 Voice Module and seveal voices from JH-3 Modular.
On the picture you see it connected to the keyboard of a dead Polysix.


The Multimode Filter of JH-4 is a little more advanced than the SEM multimode Filter. There is an additional 4-pole Low Pass Filter, and the filter mode is completely under voltage control. No more extra click-stop BP position, either. With a turn of one knob you have acess to the following modes:
2-pole LP  (8 o'clock position),
2-pole Notch  (10 o'clock position),
2-pole HP  (12 o'clock position)
2-pole BP  (2 o'clock position)
4-pole LP  (4 o'clock position)
These are the "pure" modes. The transition between 2p-LP and 2p-HP is exactly like the SEM, with the Notch in between being a mixture of both (and not really a separate output). There are similar transitions between the other modes. Especially the mixture between 2p-BP and 4p-LP can be very pleasant: A fat "Kobol" filter with a bright resonsnce added.
You sweep the filter modes manually, but you can also use Envelope 1 for that. (Envelope 2 is always routed to the VCF cutoff control, so you have all degrees of freedom to control Cutoff and Mode independently. You can also choose which Envelope controls the VCA.) There are two ways to do the ENV 1 controlled filter sweep. "Positive" will always start at 2p-LP, and the depth is set with the same knob that you use in Manual mode. Set it to the cw end position, and the envelope will sweep thru all modes and back, and will rest at a level set by the Sustain parameter of the envelope. If you set the knob to some other position, the sweep range will be reduced. "Negative" will do just the opposite: Start at 4p-LP and sweep ccw as far as you select the modulation depth.
How does it work ? It is (of course) another application of my Interpolating Scanner. I don't use the general version that you will find elsewhere on these pages, but a dedicated version that is smaller, less precise, only using discrete transistors, and is a little more difficult to calculate. (Don't be frightened by the discrete control network - and don't change any resistor value, either ! )
A Block Diagram will hopefully make clear what's going on in the VCF and VCA section.

You can see the Prototype (Key assign logic, one voice module, keyboard from a dead Polysix) here.

For more information, please contact
Juergen Haible
All drawings copyright J. Haible (C)1998