Stripped down OBA design

Since I’m nearly done with my first synth I’ve got a lot of leftover parts. I wanted to make a small simple circuit on stripboard that will get outside signals of to modular synth voltages kind of like the OBA.

The difference is this one can’t go in the synth because I have no more room so it doesn’t have to be the size of a module (I’ll probably still use eurorack power tho or however else is easy). It doesn’t need to have as many features as the OBA either, it just needs to get a signal to the right voltage for modular. The main reason is because I have the megadrone in my synth and its really quiet otherwise, but I also might want to use other audio.

I haven’t designed off of bare stripboard so I was looking for some help because I feel like this would help a lot of people get audio into their synth without having to buy the OBA especially if they’ve got leftover parts.
Anybody got any ideas? It doesn’t have to be based off of the OBA as long as I can get the signals up to the right levels.


It’s depend of the level of your external signal (guitar, mic, audio…)

There’s maybe some circuit’s idea here

the right part is an amplifier signal part

For piezzo mic

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As @Dud says, it depends what kind of signal you want to put in. I’m not sure I’m following what you said about the Megadrone, whether that’s the external signal you want to put in or what. But standard synth levels at least for Kosmo are usually about 10 Vpp. Most external signals will be smaller, but how much smaller depends on the source. If all you want is to shift the level then you just need an amplifier with enough gain for that source. The Mikrophonie circuit is nothing more than a basic op amp amplifier circuit:

with a gain from 5 to 130 (it says, but it seems to me the first stage is gain 6.6, not 5). If you need a different gain range that’d probably be just changing some resistors.

You might also want an AC coupled input, which involves adding a capacitor.

Again depending on what you’re putting in, you could think about doing something more complex with more functionality, such as an envelope follower in addition to the amplifier. You’ve said you just want something simple and to use up some parts, so maybe not, but as long as you’re processing external signals an envelope follower would be useful. See Envelope followers.

Barton has a guitar input module here

(designed for Eurorack but no reason it can’t be Kosmo or just an external box. I haven’t built it.).


Im looking to get waveforms from my computer into it mainly. Some patches from serum and other stuff coming out of the computer and going into the synth for effects