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194 Bandpass Filter

All discrete transistor bandpass filter (not voltage controlled), based on the Buchla 194 circuit. One audio input, 4 filtered outputs: LP >200hz, BP 200-900hz, BP 900hz-4khz, and HP <4khz. In conjunction w/ a 4-channel mixer, can be used as a comb filter or graphic EQ.

PCBs are $15.00 @ — ORDER HERE

Bill of Materials

  • Resistors
    • 100k x5
    • 10k x10
    • 15k x6
    • 1k5 x5
    • 220r x4
    • 2k2 x1
    • 3k3 x2
    • 4k7 x3
    • 68k x4
  • Capacitors
    • 100uf x2
    • 10uf x1
    • 1uf x3
    • 10n x3
    • 220n x1
    • 22n x1
    • 47n x3
    • 2n2 x2
    • 4n7 x1
    • 910pf x1 (mouser #598-CD15FA911JO3F)
  • Transistors
    • BC550 x7
    • BC560 x4
  • Misc.
    • .156" power header x1
    • ferrite bead x1
    • in/out jacks x5
    • 100k pot (optional input atten.) x1

Build Notes

  • Component values are marked on the board. Components are all pretty standard w/ the exception of the 910pf cap—a mouser part number is included in the BOM; or you could sub a .001uf and probably not notice the difference.
  • The original Buchla 194 had one in and 4 outs; later versions added a dedicated mixer. This PCB does not implement a mixer, but you could easily breadboard one or use the basic mixer available from Ken Stone (CGS04).
  • Buchla modules are designed for 1.2v audio signal levels. Feeding 5v signals typical of other modular systems may result in some distortion. This particular module "degrades gracefully", and the distortion may or may not be objectionable to you. If you're primarily using this with non-Buchla modular gear, you could add an attenuator pot on the input (50k or 100k wired as a voltage divider) to compensate. The module has some inherent gain, so the input attenuator may still leave you w/ enough useable signal, or you could add 4 simple makeup amps using a quad opamp chip or similar.
  • There is no voltage control of the frequency bands or levels. For voltage control of the later, you could run the outputs to 4 VCAs, or to a mixer for a kind of VCable 4band graphic EQ. Personally, I've gotten a lot of mileage out of just the barebones core, without additions