Due to several things I’ve been wanting to try out requiring it, I’ve decided to break out the +/-12V power supply I scavanged from my dead ESQ-1 again, and resume work on more modules for Phoenix. My initial thought was to re-house the current 9V modules, and replace them with new stuff- however, I realized that I would soon need more space than that, so I’ve decided to start a second chassis instead.
The first circuit I built for the second Phoenix chassis was a 12dB/oct Sallen-Key state-variable filter (SVF)- there are several variations on this basic design (including the MS-20 and others), the one I built is based on a design by Nyle A. Steiner (original schematic at Experimentalists Anonymous, Philip Baljeu’s mod/redraw with LDR’s in this e-m thread). I made a few changes, including changing the gain on the output buffer (it’s now set at 100x gain, well into overdrive territory- but the volume pot allows you to attenuate the signal), and changing the res pot’s value. I think it sounds pretty good, but the LDR’s I have don’t react very quickly, so audio-rate modulation is not possible.
Although I do have several modulation sources available, I decided to build another to go with this filter, just in case they’re all in use elsewhere. I went with a simple LFO found at the MFOS site- it is essentially the “Super-Simple” LFO, with the diodes and associated resistor removed, and a square output taken from the output of IC1-b. I also decided to go with a single speed range- I went with a 470nF cap there, which gives a pretty good range. There was enough room left on the perfboard I used for the filter for me to build one of these on the same board. I’ll probably build at least one more, possibly with a bigger cap for ultra-slow rates.
Another nifty-looking thing I had recently come across was the Simple Saw VCO (and several other cool circuits) from Nicolas Woollaston at the e-m forums. It works great, so I decided to try to combine it with this saw to pulse converter from the EA archives. This also worked quite well, so I’m thinking I’ll build a pair for this chassis. As of this writing, the VCO/waveshaper is still on breadboard here.
Since it had worked so well with Loid, I decided to try building a +/-12V version of the WP-20 VCA I posted about a short time ago. As it turns out, all I had to change was the op amp- I used an RC4560.This is also still on the breadboard.
What follows is a simple recording of the VCO’s pulse output, being processed with the lowpass portion of the VCF, which in turn is then fed into the VCA. Modulation routing is as follows: the MFOS LFO’s square output is controlling the VCA, and the triangle output of the same LFO is controlling the PWM of the VCO. Two triangle LFOs from Loid are then used to modulate VCO pitch and VCF cutoff.Partway through, the MFOS LFO’s triangle out is re-routed to the filter CV, and the triangle LFO from Loid which was on the filter is in turn re-routed to the PWM.
Here’s a recording of the VCO’s saw processed with the bandpass portion of the VCF- this one also includes some filter overdrive sounds:
When I get the VCO and VCA off the breadboard, some AREG’s will follow.