Archive of the former Yahoo!Groups mailing list: Yamaha CS80

As much as I like the sound of the CS-80 Chorus, even the "darkness"
of it, I was wondering why is sounds ∗so∗ dark (low pass filtered),
compared to your usual BBD-based chorus devices.

All right, you need a anti-aliasing filter and a restauration filter
for any good BBD-based effect, but usually you only need dramatic low
pass filtering of ∗long∗ delays (echo devices), where you find corner
frequencies of 5kHz, or even 3kHz, sometimes. But on a chorus, you
normally get away with "almost" full audio bandwidth; the sound may
get ∗slighty∗ dark, but not much.

I've been curious about what dramatic filtering would go on in the
CS80 chorus, and what I've seen so far was a big surprise:

∗ Anti-Aliasing Filter: 16kHz, 2nd order
∗ Restauration filter: 16kHz, 2nd order

Hey - that's pretty much "full audio bandwidth" !
Even the two combined may result in something like a 12kHz corner
frequency, and 24dB/Oct slope - not explaining the dark sound at all.

Curiously looking further, I found a combination of 27k shunt
resistor and 15nF shunt capacitor at the output of each VCA that
passes the signal from the BBD for panning effects. Together with a
100k resistor that feeds an opamp summing node from the VCAs, we can
calculate the corner frequency of the resulting low pass filter
function:

R = 27k // 100k = 21k
(The VCA has a current output which doesn't contribute an R)
C = 15n
f = 1 / (2 ∗ Pi ∗ R ∗ C) = 500 Hz

That means, the signal is conditioned for the BBDs with two 16kHz
filters, only to be filtered with 500 Hz (!) in the VCAs ?!

Very strange. Ok, it's just a one-pole filter (6dB / octave), but
500Hz is damn low.

Maybe there's a typo in the schemos, and it's 1.5nF capacitors, not
15nF. Then we'd have a 5kHz LPF instead. But the same "0.015" is
printed at each of the four capacitors on the four VCAs.

Or is this a deemphase circuit? But I can't spot any preemphase part
anywhere ...

JH.