This is an easy build and rather simple circuit, based on the Roland SH-5 BPF. There are a number of bandpass filters for Euro inspired by the Roland, yet I didn’t find any DIY versions that have CV over cutoff frequency. So, I simply took the original schematic and replaced the dual gang cutoff pot with a dual OTA and CV control. While I was at it, I also tapped the High Pass and Low Pass frequency responses that are unused in the old Roland monolith (or any of the BPF clones, for that matter).
That’s what it looks like. Improvement in the version linked below is that the lettering is on the silkscreen layer, i.e. brighter (white) and better readable.
A couple of sounds:
Low Pass
Band Pass
High Pass
Dual Bandpass (two Model 5 in series, one with inverted modulation)
Circuit Description
As you can hear, the filter sound stays mostly sweet in contrast to, say the “can sound beeped up” vibe of the Oberheim SEM. You can push the Model 5 in the latter direction easily, though, if you want to. Simply decrease the input attenuation (replace the 22k resistor R2 with 100k). This is how the “stock” version as per schematic sounds (3340-based VCO going full tilt into the filter):
Another characteristic of the original design is that the maximum cutoff frequency is relatively low at (around 6.5 Kilohertz), which I kept in this module too.
As for the rest, the thing does what it says on the tin. Input with level control, unipolar modulation input, manual resonance control, buffered outputs for low, band, and high pass.
Build and Production Files
There is nothing really complicated here, really. A LM13700, two quad op amps, and a dual op amp. For the 22n capacitors at the filter core, I use Panasonic ECHUX 22N 16, which sound great and fit the footprint well. All resistors are 0805, and no special parts are needed.
Here’s the schematic
And here are the Gerbers (Panel, and main PCB), BOM, and what have you.
And here’s a strip board version, someone kindly sent to me:
