Every source needs an outlet. After I build about 10 drum modules, it was time to connect them and make some sounds. The requirements were simple:
- a lot of inputs without level control,
- some amplified inputs with level control for low output drum modules,
- output with level adjustment,
- output level indicator LED.
Schematics & PCB
I checked a lot of mixer schematics and designed one that suited my needs. There are six no-amplified inputs and two with level control with maximum amplification of 2x. I used only one amplified in Ponyo because of limited space – I wanted to build two units anyway.
The circuit is build around standard mixer circuit (IC1A) and additional inverter (IC1B) to keep input signal non-inverted. The amplification of mixing stage is equal to one. A simple LED driving circuit gives some information about presence of output signal – a very handy feature. R15 is 470R to give LED current ~20 mA, but you can make it greater (f.e. 1 – 2k) to make module more power efficient. Synths primary goal is to make sound, not to blink. Two mod points (MOD1 and MOD2) near IC1A input let you add more inputs, just with two resistors and a jack socket. The power connector is standard eurorack 2×5 raster pin header, but MOTM 4-pin can also be used. The device accepts +/- 12 and 15 V power supplies.
Not all components were used in my build. As I needed only one amplified input, IC2B is terminated – R18 is shorted, R17 and R19 not used and pin 5 of IC2 connected to ground. The opamps are generic TL072 – cheap and low-noise. Pots are Alpha Taiwan bought from Tayda Electronics. All audio path resistors are 1% tolerance metallized.
Bended aluminum sheet is mounted under the jack socket and acts as a holder for PCB – this way front panel has no visible screws and looks very clean. The ground connections between jacks (and 100k resistors) were made with uninsulated wire.
The front panel was designed in Inkscape, then printed on water decal, protected with transparent acrylic paint and put on 2 mm, 8 HP aluminum sheet. Some may say it is a waste of space – only 8 jacks in 8 HP, while some designers are able to put those in 2 HP. My answer is: I am not a hamster, but an adult human.
Mixer was tested and works fine. It is not as universal as other designs, but meets my drum eurorack modular needs very well.