Doepfer A-128 fixed filter bank MOD

Extending the Doepfer A-128 fixed filter bank eurorack module with 4 bypass switches.

Who is it for?

If you are the owner of a Doepfer A-128 fixed filter bank this project might be useful for you.

With a few bucks for material and 1 hour of work you can create an extension module that adds bypass functionality to your filter bank.

What does it do?

• Switch 1: bypass the whole filter module and send the incoming signal directly to the output
• Switch 2: mute all high frequencies (greater ~1500 Hz)
• Switch 3: mute all mid frequencies (~200 - 1500 Hz)
• Switch 4: mute all low frequencies (below ~200 Hz)

Will my filter bank still be functional without the extension module after modding?

Yes, simply place 4 jumpers in the breakout connector. The signal flow will be exactly the same like without the mod.

Step by step build instructions

1. Desolder audio input wire that connects the 3 PCB's

Find and desolder the wire on the upper half of the module that connects the 3 PCB's with label AUDIO INPUT.

2. Modifications on PCB1 (low frequencies)

Drill a 1.5mm hole into left PCB (with in-/output jacks and potentiometers for low frequencies) where you desoldered the AUDIO INPUT wire.
Ensure that there is no connectivity between the pins shown in the picture below.

Solder 2 wires into the connection points next to the drilled hole.
I used a red wire for AUDIO INPUT which has connectivity to the tip of the input jack and a black wire which is the audio input for the low frequencies circuit.

Solder another wire next to the OUTPUT-jack.

3. Modifications on PCB2 + PCB3 (mid + high frequencies)

Solder 2 wires into the holes where you removed the AUDIO INPUT connection wire in step 1 on PCB2 and PCB3

4. Ensure the additional cables stay in place

Hot glue is your friend

5. Solder other end of cables to a 10 pin box header

Further you have to connect the 4 pins.
TODO: add picture with circuit schematics of box header

6. Ensure the box header stays in place

Hot glue is your friend - again

7. create cable and front panel

• Create a 2HP aluminium front panel 128.5mm x 9.8mm x 2mm or use a blank panel Doepfer A-100 B2
• drill 4 holes for the switches
• mount the 4 switches
• solder the cables to the switches

TODO: add circuit schematics for the switches

Doepfer Filter Bank Bypass Breakout on modulargrid.net

see my other eurorack DIY projects