[sdiy] Korg KR-55 drum machine query

Mattias Rickardsson mr at analogue.org
Tue Nov 14 07:58:44 CET 2023

Tom Wiltshire <tom at electricdruid.net> skrev:

> Despite what Boss/Roland say about "Interrelations between frequencies are
> so critical that slight deviation of one frequency can cause beat sound or
> distortion" in the service manual, I don't believe the frequencies are
> *that* critical.

Perhaps frequencies are not super critical (different Rolands use different
sets of frequencies, btw), but the resulting levels in different
frequencies (overall spectral content) are somewhat important. The
oscillator mix is followed by a bandpass filter on the way to the nonlinear
dist-VCA that squashes the frequencies while it applies the amplitude
envelope. The signal level coming into this dist-VCA is important to have
within reasonable limits, since the dist behavior depends on it. Depending
on the oscillator frequencies chosen, different amounts of energy is
filtered away by the BPF, with different resulting levels. I imagine that
especially the Korg design could be a bit sensitive to this, since
ringmodulation sometimes creates very low difference frequencies, where
lots of energy would be removed by the BPF. On the other hand Korg uses
less filtering IIRC, especially compared to TR-808. :-)


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