EHX micro synth, guitar synth


this is what the mail box brought me few days ago. Another cheap faulty little thing.  A nice micro synth. Bought for a nice price because of a broken trimpot (and other things that I haven’t been told….). Not a lot to say about it because the vendor didn’t include the  24V power supply ! and I don’t have a 24V power supply available on my messy workbench. Really boring all these different power supplies at EHX ! Beside the plug is a 2.5mm with a reverse polarity (positive at the center) . Argghhh !

I took pictures , guts of the circuit seem rare on the internet .


the pcb supporting the sliders also has few parts as well but they only do the power supply circuit and the mixing of the signals. The main interest in on the bottom board pictured.

The circuit is interesting , waveshaping are used in different manners to save space and components and acheive different fonction to make the circuit work proprelly.

here’s the schematic :

Note the use of  CA3094 and really commun ICs and transistors. No specialized IC or micro-controller chip. Also all op amps are 4558 , you know this IC that all ingnorant gearhead love in their tubescreamer. Thinking that if a 4558 (JRC is a manufacter )  is present the distortion circuit  will inevitably sound “good”. Sorry , but 4558 is one of the cheapest IC available (and still today ! ) on the market. It have been used for decades in cheap electronic circuit , effect pedal , Hi-Fi , consumer electronics and all sort of craps….Open up and cheap electronic device from the 80’s and you’ll see load of those !

Nothing magic or mojo in here. It for sure has its own characteristics due its techonology (in regard to today op amps) which particape to the sound (bandwidth , input impedance ect…) but It’s certainly not “hi-quality” , it’s one of the crappiest once again (and quality has nothing to do with sound) And I saw people proud to have spend 150euros on a vintage JRC4558 , yes some are dumb enough to buy that price ….

We can also think that replacing them for higher quality ones the sound could be improved in termes of noise at least. But I won’t do it .

the Name “synthesizer” is a bit funny because there’s no oscillator into this pedal , the guitar signal is treated as an oscillator would be in a moog or ARP synth. It’s filtered by a filter controlled by an attack/decay circuit (enveloppe generator) . The different timbres is obtained by modifying the guitar signal into different circuits. Seems really funny thing (I didn’t play it yet because of the missing power supply). One could complain about the missing patch save  or things like that, remember that it was designed in the 70’s !

It’s one of David Cokerell design for EHX . If you don’t know who David Cockerell is , scroll back a bit on this page to land on the youtube EMS documentary .

thanks for reading



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2 responses to “EHX micro synth, guitar synth

  1. Martin

    this is my next proyect thanks so much!

    • Hi !
      you want to build one ? nice project !
      Maybe you could try to find some improvements with faster op-amp for the triggering process and improve the Signal/noise ratio of the signal path. I know that it exists a working PCB avalaible on diy webiste for this particular circuit. Beside , as the schematic should mention there’re only 4 caps to change to transform it to the “bass model” .

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