noise on the power rails as a cure for locking VCOs ??
Juergen.Haible at nbgm.siemens.de
Mon Mar 23 14:58:45 CET 1998
Ok - before someone sues me for creating a myth, note the two question
marks in the subject line.
I don't know if it's true, and it may be pure speculation, but I'd like you
to think about it and tell me what you think.
I spoke to some very experienced service tech recently, and after exchanging
the usual vintage synth trivia he mentioned casually that the Minimoog's
VCOs simply do not lock to each other because of the noise on the power
I never heard of this before, but it sounds very resonable.
With several VCOs physically close together, you always fight against a tiny
amount of soft sync that limits the minimum beat rate between oscilators.
This unwanted coupling takes place over GND and supply connections,
little spikes caused by the discharge of one VCO, wandering along the lines
and reseting another VCO as well, if it happens to be so close to its own
threshold voltage that the minimum spike lifts it over the threshold.
Common means to minimize it is capacitive bypassing of each VCO, or even
having separate supply voltage cables from the PSU to each VCO.
I always wondered how the Minimoog could sound so "rich", though there
are several different voltage rails that are shared by all 3 oscillators.
Maybe some amount of noise on the supply rails "masks" the spikes ?
Dithering comes to mind ...
does this all sound reasonable?
Perhaps I must withdraw my statement that 78xx and other noisy stuff
would be no good for serious use.
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