EHX Bass micro synth mods

My ‘broken’ EHX Bass Micro Synth was gathering a ton of dust. After bringing it back to life I noticed all the sliders were far from performing well and should be replaced. Sourcing 10 100K linear sliders with the right dimensions turned out to be rather difficult and costly. So I decided to transplant the pedal’s guts into a 19″ rack casing from an old and broken M-Audio firewire audio interface and at the same time see how I could make it more synth/modular friendly. So, here’s a brief list of things I ended up doing to it:

  • Switchable filter type (BP / LP)
  • Added a square sub oscillator
  • Added an external input
  • Changed input/output gain behaviour
  • Extended the filter range
  • Added and ‘extra’ resonance switch
  • Added external trigger input
  • Added filter CV input
  • Added Envelop follower CV output
  • Added trigger output
  • Added trigger/filterCV leds (the thing could use more leds)
  • Added a sort of distortion/clipper
  • Replaced the internal power supply with an external adapter (I really didn’t trust that thing)

So, it’s quite a list. But I’m pretty happy with the results so far. Things could do with a bit of tweaking here or there, but that’s up to personal taste. You’ll can find the schematics of the EHX Bass Micro Synth easily enough on the web, but for the lazy, click here. I’ll try to address the implementation of some mods in this article.

Audio demo

I’ve recorded a simple audio demo. It starts with a clean version of a saw wave riff generated with a Korg Gadget app.  I recorded some tweaking and went through more or less all the settings except the external inputs and triggers. It should give an idea what the unit is capable of when applied to synths. The filter really can get some 303-like action going on, especially when you use the square wave generator.

Switchable filter type (BP / LP)

This is a really simple mod to perform and opens up a whole new range of sounds. I don’t know whether it’s useful for a bass guitar, but it sure is for running synths through the thing. As you can see in the pic, output 6 from A11 provides a band pass output. Just install a switch between this and the filter output and you’re good to go.


Extended filter range

This one is a bit more invasive to the machine, so it might not be for everyone. I found the filter a bit lacking in range when used for synths. I did like the quirky filter but it just hadn’t enough ‘Oomph’ to it. Sending white noise through it I found out it’s range was about 700Hz-18KHz. IMHO it really needed to go down a lot deeper, especially if you want to use it with synths.

The filter consists of 3 identical stages, changing capacitors c25, c26, c27 from 33nF to 68nF shifted down the range, however, not only the bottom end, also the top end. To counter that I used an opamp (non-inverting) to amplify the control voltage going to A10,A11,A12 over the 3.3k resistors and coming from A13. That fixed the issue but now the ‘rate’ control also has an influence on the filter range. At short rates the filters drops lower then on long rates. I didn’t pursued this issue any further as I didn’t really found it that big of a problem, and I feared trying to solve this would really be quite an undertaking.

Be careful with the EH1040 IC’s though. They tend to die on you when mistreated. I managed to kill one two while working on the filter. Luckily it can be replaced (at least in the filter) by an LM13700 and work fine. I’ve ordered a few CA3094’s to replace it, we’ll see whether that works or not.

Beef up the resonance

This one is again a fairly simple one to implement and extends the resonance control up to the point of oscillation. It has the tendency to start going ape-shit and turn into an overdriven mess, so a bit of subtlety is at it’s place to find the right resistor. What I’ve did was simply connect the output of A12 (pin6) back to the input of A10 (pin 2) over a 56K resistor. I also added a switch to disable this mod. You should try out a bit of other resistor values, 47K will allow for a bit more extreme sounds. I also suspect different units to behave slightly different here so you might need to adjust to taste.


Square Sub Oscillator and external input

These 2 are similar. It might not be that obvious from the schematics, but the voice mixer is set up as a simple mixer where A10 is the summing opamp. So you could easily add an external input to the mixer by making an input over a 10K resistor to the summing point on the mixer. I’ve added a level control as well so one could blend, say some noise, into the signal before it enters the filter.

The Square Sub oscillator works on the same principle. Pin 2 of the CD4013 (A17) output a square wave an octave below the guitar input. The CD4013 used to divide the incoming frequency and thus achieve a sub octave. I simply took that signal, put it through an opamp configured as a voltage follower, so It wouldn’t interfere with whatever was going on in the sub octave modulator. Whether that was necessary or not, I have nu freaking clue. I added that signal to the mixing section with it’s own level control. It’s a nice addition to the voice mixer and certainly a useful mod.

Filter CV output

I wanted to make the whole thing Eurorack friendly, so it would interact with the rest of the Eurorack system. Adding an output for the Filter CV isn’t that much of an issue. Internally there is a voltage controlling the filter. The voltage from the A13 pin 6 that drives the 3 individual filter stages and swings somewhere around -9v to -7v. So, we need an offset to get it up to modular levels (0v – 5v). I’ve added a trimmer on the 9V offset so it’s pretty easy to calibrate. R3 is there to amplify the CV voltage a bit, if it doesn’t rise high enough for your taste you can put a higher value here. There’s also an optional status led to show CV activity, this thing needs more leds anyway and it’s even useful.

Output gain / input gain

Changing resistor R98 from 47K to 4,7K will significantly boost the output voltage, bringing it more in line with the higher levels typically used in synths. Don’t do this if you still want to use it with a bass or guitar amp. For the input gain, I simply change the trimmer (R3) in the pre-amp with a 10k pot.

1 Comment

  1. hello, I was tweaking around with the micro synth bass section and I found out that R89 is anoted 4.7k instead of 47k and you should try to replace this by a pot of about the same value to see how it can change the resonance behaviour too. Better than the switch mod maybe. nyways thanks a lot for this page full of informations.

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