[sdiy] Modified ASR working perfectly!

[David G. Dixon]

Well, the saga of the modified CGS Analog Shift Register has come to a
satisfying conclusion.  I built and tested my revised board tonight, and it
is perfect, as far as I can tell.  This baby has an input buffer (which
shares a TL072 with the clock comparator) with input protection resistor and
diode clamps, a modified clock which can be triggered with any wave which
crosses 2V (not just edgy ones), two DG409 dual 4x1 analog muxes instead of
CD4052s to avoid having to shift the input up and down, a 78L05 to provide
5V for the CD4024 ripple counter and the mux logic and enable inputs,
metallized polypropylene sampling caps, and a single LF444 instead of four
individual LF356s for the S&H buffers, with no additional output buffer
stages (none are required since the input signal is processed as is), just
1k output resistors.  I even managed to fit in guard traces which completely
surround the high-impedance S&H traces, and to fit the whole circuit onto a
board which is only 2.1" x 3.7".

The samples hold rock steady for as long as you care to measure them, the
transitions are very sharp, and the unit can run quite nicely with the clock
in the audio range.

The only thing is, I'm not sure now whether my first attempt wasn't also
adequate, because what I've discovered is that the CVs generated by my
keyboard scanning circuit (a slightly modified MFOS design) fluctuate,
particularly when keys are held down, and I was mistaking this as error in
the ASR.  In any case, it's too late to find out because I desoldered the
original board and threw it in the trash.  In any case, the new one has
incorporated all of the excellent advice I have received over the last few
days, so thanks to all who contributed ideas and suggestions.  Now I have to
figure out why my keyboard CV is fluctuating.