Audio Hardware

1. Dead Kings bass effects pedal

I’m starting this new project on Github, heavily inspided by my recent discovery of the band Royal Blood

This is the first post in the Dead Kings preamp series.

This preamp / effects pedal isn’t supposed to emulate the sound of Mike Kerr, I’ve just got some ideas I would like to try 🙂

Just for the kicks and as a challenge, I’ll be building this using only (or at least mostly) germanium transistors. I’ve been compulsively buying germaniums for quite some time, need to put them to good use!

The design will be licensed under Open Hardware permissive license, the sources will go here:

The main idea is to have two or three mixable channels, one clean and two effects – an overdrive/distortion and an octave up distortion.

Overall block diagram so far:

The buffer / gain stage in its first iteration: so far only as circuit simulation to set the experimentation baseline. This circuit has the following characteristics:

  • input voltage up to 500 mV
  • input impedance 900 KOhm
  • total gain: variable between 0 dB and 20 dB
  • bandwidth 20 to 20000 Hz

3 PNP low noise germanium transistors – MP39B and one NPN germanium transistor MP37.

I’m using the transistors Q1 and Q2 as a “bootstrapped complimentary pair” input stage, this allows relatively high input impedance even when using low gain bipolar junction transistors. The circuits like this aren’t used at all anymore, instead using the field effect transistors that have very high impedance without the complex schematics, but here I’m note using those, sticking to my self-imposed “all-germanium” limitation 🙂

The gain and buffer stage are done using two directly coupled PNP transistors Q3 and Q4. This simulates fine in LTSpice, but I’m a little suspicious of R9 value that I feel too high. I’ll test all of this soon and tweak the values if necessary.

A tentative layout for perfboard – yes ON PAPER again. I like paper workflows for some of the work I’m doing. The final PCB designs will be done in Kicad though, I like the final product to have professional quality 🙂