[sdiy] TR-909 undersized DC blocking caps

[Richie Burnett]

> I simulated this in LTSpice and got the same results as you. I was
> obviously wrong. Please accept my apologies for doubting you...

No apology needed.  I raised the subject to promote some discussion here as I
thought the design was odd.  All discussion is good!

> At least
> I've learnt something, and it's interesting to know.

I think Roland probably thought it worked the same way as you when they designed
it, so you're in good company :-)

> I've always put a
> separate coupling cap on the input end of each summing resistor so have
> never had to calculate or test such a circuit.

That is the usual way to design an AC summing amplifier.  That way each coupling
capacitor only sees the impedance of each individual source.

Best regards,

-Richie,