[sdiy] Simple FM input (and tricks...)

[Justin Owen]

Hey all,

The first of a run of questions this weekend...

I've recently added a simple FM input to my current VCO. It's basically just an op-amp summer - the first input is a pot set up as a voltage divider for Pitch CV and the second input is for an external sound source.

I'm not trying to re-invent the DX7 - I'm just enjoying all the nice metallic tones - and of course all the great space noises!

But... I was wondering if there'd be any advantage to adding a 'stage' of some sort between the external audio/FM input and the summing op-amp? Something like a simple buffer stage - or maybe even gain?

The reason I'm asking is because, while this build will only have an external sound source at something approaching 'line level' for FM - future builds might have sources at different levels/impedances - so I thought it might make sense to try and match the different inputs a little closer - before they enter the summing stage.

Does this make sense? Is there any advantage to adding something?

Cheers much

Juz