Jul 8, 2018

Transistor ladder filter

When I started with synth DIY, I knew little about how transistors work, and the famous Moog transistor ladder filter just looked like magic. Intrigued, I stared long at the schematics posted by Yusynth, and worked my way to the very detailed and mathematical analysis by Tim Stinchcombe.

Eventually the transistor ladder filter was the very first synth module I built, initially in a cardboard box and later transplanted to fit behind a eurorack-format panel. My implementation was a kludge on prototyping board, with later modifications bodged in and not-quite-quality components. While the circuit still sounds fine, it now has intermittent issues which are simply not worth tracking down and fixing anymore. Instead, I designed a proper PCB for the circuit and built a few new ones. The PCB fits behind an 8HP eurorack panel with all through-hole components and no wiring required. I sometimes have some of these PCBs available for sale, and the build documentation can be found at the bottom of this page.


My version of the filter is not intended to be an exact replica of the Moog filter. While it uses the traditional transistor ladder core and exponential converter, I used an opamp instrumentation amplifier (inspired by Rick Jansen) as output stage replacing the original discrete outputs. I added a CV input with attenuverter and a Drive control, allowing to adjust the input saturation that is a significant part of the filters characteristic sound.

Build documentation

I offer the PCB for this module for sale. The build documentation below contains everything you need to know to complete a successful module. Please read it through carefully before starting, and especially check the notes in the Bill of Materials when ordering parts.

7 comments:

  1. A warning to all: if you intend to use 2N3904/2N3906 for the transistors, the pinouts are NOT THE SAME as the BC547 and BC557. I repeat: pinouts NOT THE SAME.

    ReplyDelete
  2. No problem, you must it rotate at 180°

    ReplyDelete
  3. hi there...do you still have a pcb available, please? how much does it cost along with postage to Malta, EU? thanks. Mike

    ReplyDelete
    Replies
    1. Hi! I have PCBs available. You can see the prices on the page "PCBs for sale" on top!

      Delete
  4. The sound of this Ladder Filter is phenomenal! What an outstanding value-priced PCB + an easy build. At this price I should have bought some more ;^)

    ReplyDelete
  5. I'm scratching my head about a few points in the schematics.
    1. Where's C5? I can't see it in the pics of the board.
    2. I want to use SMD pairs. Does that mean that BIAS1 is not connected to transistors?

    ReplyDelete
    Replies
    1. Hi!
      1. C5 is on the bottom side of the board
      2. BIAS1 connects to R6 and R7, and then the netlabels IN_L and IN_R connect to the transistors.
      Best,
      Caspar

      Delete