Building the EarSewage quad parallel bandpass filter

Building the EarSewage quad parallel bandpass filter

Got a set of Earsewage PCBs but don't know what to do with them?

Read on ...

This isn't a difficult build, but there are a lot of components required - potentiometers in particular. All of the surface mount parts have been pre-soldered for you.

You will need the following components:

  • 1x 2x5 Eurorack power header. The line on the silkscreen corresponds to the -12V rail - this is J2 on the board.
  • 2x PJ301M-12 jacks (aka. THONKICONN). J1 is the input, J3 is the output
  • C14, C15 - 2x10uF electrolytic capacitors (5mm diameter, 2.5mm lead pitch). These are filtering caps for the power supply.
  • 4x10k linear potentiometers - ATT1-ATT4 (filter input attenuation)
  • 4x1M linear potentiometers - CUT1-CUT4 (filter cutoff frequency)
  • 4x1k linear potentiometers - RES1-RES4 (filter resonance)
  • 4x100k linear potentiometers - LVL1-LVL4 (filter output level - audio taper also works)
  • 1x100k audio potentiometer (GAIN1) - you can use linear taper if you like, but I find the response to be a bit better with audio taper.
  • 1x100nF film capacitor, C1. This is an optional DC-blocking capacitor. If you choose not to use it, solder a wire bridge or zero-ohm link in the C1 position instead. I tend not to use it, but the option is there.
  • Knobs to suit - anything wider than 13-14mm diameter might make controls tricky. I generally use 10mm micro-knobs but ultimately it's up to your tastes and sensibilities.

And then there are the filter capacitors ... Each of the four filters has two capacitors, and these are grouped thus:

  • Filter 1 ('lowest') - C2/C6 (pre-built uses 100nF)
  • Filter 2 - C3/C7 (pre-built uses 47nF)
  • Filter 3 - C4/C8 (pre-built uses 22nF)
  • Filter 4 ('highest) - C5/C9 (pre-built uses 10nF)

Capacitor values don't have to be equal,  but in complete builds I use the following values with the centre frequency calculated using the Okawa Denshi filter calculator (http://sim.okawa-denshi.jp/en/OPtazyuBakeisan.htm) - set R1, R2 and R3 to their mid-point values (5k, 500k and 500ohm, respectively)

You can use ceramic capacitors if you like, but I find that polypropylene caps work best in filters - again, it's up to you. The footprint size is 7.5mm x 2.5mm with a 5mm lead pitch.

CV Jumpers

Eagle-eyed viewers will have noticed 4 jumper footprints labelled CV1-CV4 - these are to allow circuit benders to introduce some manner of voltage control and/or modulation into the circuits. The footprints are sized to fit 2mm pitch JST headers (PH-series) with the rectangular pads connected to ground.

If you plan to use these, solder the connectors in first otherwise once you've fitted the pots they'll be very difficult  to retrofit.

You can use the jumpers however you like, although my intention is to make them usable with my 'expy' exponential converter module.

The suggested build order is:

  • CV1-CV4 - if you plan to use the jumpers. Otherwise you can just leave them unpopulated.
  • C1 - if you want to capacitively couple the input (ie. remove any DC) - fit either a suitable cap (100nF). If you'd prefer resistive coupling, a wire bridge or a zero-ohm link will suffice.
  • C14, C15 - power smoothing capacitors. 10uF, 25V recommended. 2.5mm lead pitch.
  • Power header - remember the line on the silkscreen corresponds to -12V. If you're using keyed sockets, the notch should be adjacent to the upper edge of the board.
  • J1, J2 - input and output jacks
  • Filter caps C2-C9 - remember that although they're grouped in pairs they don't necessarily need to be the same value. Polypropylene capacitors recommended - the footprint is 7.5mm x 2.5mm with a 5mm lead pitch.
  • Potentiometers - any order you like. I usually work from bottom to top but that's just me.

How to use it? Just twiddle those knobs ...

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