Sent Tue, Jan 7th 1997, 21:20
Hello again! A happy new year to all of you! I had the time to build some things on veroboard during the last weeks. But before I talk about the thing I proudly announced in the topic line, let me report about a failure first. I tried to build a 16-stage phaser around 4 CEM3320 filter chips, because I thought it would be so easy. The idea was to build something like the Moog phaser, with variable stages and so on. I wanted to use chips with a good SNR (compared to the CA3094) and so I would not need a compander circuit. Unfortunately, I ran into serious stability problems. *One* chip (i.e. 4 stages, like in Barry Klein's book) works perfectly, but if I cascade several 3320's, the whole thing turns into a radio receiver at certain control voltages. Even without feedback applied. Must be some feedback over the supplies, I guess. I put a *lot* of decoupling capacitors into the circuit, but it wouldn't help. So I gave it up, and I will use 3080's or LDRs if I will build a phaser again one time. That was the bad news. Now to the VCF, which was just the contrary. But let me tell it one by one. For my polyphonic project, I always wanted to have a 2pole filter and a 4pole. 2pole should be a SEM-type state variable, no doubt. But the 4pole wasn't that clear. First I wanted to use my beloved SSM2040 chips ("Best filter chip ever") to get this famous Prophet5/rev2 or Kobol sound. But then I thought that someone else might want to build this synth as well, and using ulta rare chips is very problematic then. So I was almost sure that I would use a Moog ladder instead - my second-favorite 4pole filter. I even had already designed this part of the pcb. But then I tried something better, something I always wanted to try, but never found the time so far: Building a "clone" of the SSM2040 with standard components. Well, I must be careful with the word "clone", because this would mean a one by one reproduction, which it surely isn't. I don't even know all details of the original circuit. But at least I know the circuit of the signal path (Dave Rossum was so kind to answer a letter about this question), and here are some details that I think are important for the SSM2040 sound: (1) A minimum (for a ota/buffer filter) of pn junctions in the signal path, due to a very minimalistic ota topology: the ota just consists of a differential pair and one single current mirror. (2) The very special distortion scheme, due to the same minimalistic ota topology: The ota output can only swing into the positive direction. The buffer is a npn darlington, so levels are shifted by approx. -1.4V, so you get a very unsymmetrical clipping at the output. This, together with the frequency-selective distortion of the ota-inputs, gives a very unique overdrive charactersitic. Now this is how I build my discrete filter: I selected 8 transistor pairs by the Moog method to +/-1mV: 4 pairs of BC550C (npn, for the differential pairs), and 4 pairs of BC560C (pnp, for the current mirrors). The bases of the npn's are connected to gnd by 200 Ohm resistors. The pnp's form a current mirror for this pair, to the positive supply. (The 2040 uses 3-transistor mirrors which compensate for base currents. But with discrete high-beta pnp's, 2-transistor mirrors are fine.) This is the whole ota: 4 transistors ! At the ota output there is a 1nF capacitor to gnd, and a darlington buffer made from two BC550C transistors. The darlington works into a 500uA current source which is made from a 7th transistor (again BC550C). The feedback network consists of the 200 Ohm base resistors and two 10k resistors, from the input and from the buffer output to the base of the transistor that forms the inverting input of the ota. Four of these stages are connected in series. The currents into the emitters comes from an exponential converter, built around a CA3086 array and a dual opamp. One transistor for temperature compensation, 4 transistors for the 4 otas. Emitters all tied together. I chose a reference voltage of -1V for the expo converter, to allow some voltage drop over the transistor array. This results in a -1V instead of 0V summing node for control voltages. This is no problem in my PolySynth, where the CVs will be mixed with otas anyway. For the usual applications, you'd need another opamp for level shift. One thing I had worried about were the offset voltages. But I only have approx. 100mV offset at the output, and the best thing is that this offset only changes by a few millivolts over the entire control voltage range. So it turns out that selected transistors are better than the monolithic transistors of the original in some respect. Temperature drift is worse, of course. If I touch one transistor of a pair with my finger, the offset increases to one volt. With the limited output swing, this might be a problem. But heating one transistor to 37 degrees and leaving the other at room temperature should be the exception, I assume. So I am quite optimistic that I won't need copper clamps or the like. I also added a voltage controlled feedback loop (which is not part of the SSM chip). I tried a 3080 which works fine, but I finally used another discrete differential pair. Call me purist. The sound? Unbelievable. A good and clean 4pole sound with regular input levels, and full power brute force with overdrive. All I ever wanted from a 4-pole filter. Without overdrive, the VCF's own frequency dominates when Resonance is set to self-oscillation. With higher input levels, the input signal dominates. I think this is also a result of the ota-output clipping, and again it's exactly what I wanted. I also added a trimpot and a resistor from the filter input to the feedback differential pair. This way the usual bass loss with increasing resonance can be compensated for. I am not yet sure which amount of compensation is the best, but it's nice to have it programmable with a single resistor value. As I plan to make crossfades from 2pole to 4pole filters, and as the SEM-style 2pole hasn't got the bass loss, I think some compensation will be fine. Hmmm - a long mail that was, and probably all you want to see is the schematics. I'll send it to Kevin and ask him to put it on the web site. JH.