Hi everyone. I'm a relative newcomer to the A-100 scene - bought mine
last September and its getting bigger with alarming rapidity! I was
going to post this on the Doepfer website forum, but (at last) found
the exact address of this one by accident, and decided it would be
better here. It relates to the compatibility of the A111/115/117, as
discussed in some old messages in the other forum ('A117 with A111',
'Most hated...'), so apologies in advance if I'm going over old ground
(I've not had chance to spend that much time looking back at old stuff
here...)

In both the 117 and the 115 the input signal is merely switched
through a transistor in order to produce a 'squared up' pulse which is
then fed into the appropriate chip (4006 shift register in 117, 4024
binary counter in 115). The 'rise time' of the leading edge of each
pulse depends on the 'quality' of the input signal, and is quite
critical to the correct operation of the module. The recommended rise
time for the 4000 series family chips is 4 microsec at 10V supply down
to 1 at 15V - we have 12 volt supplies so our target is somewhere in
between. On my 117 if I feed a low frequency sine wave in it is quite
possible not to get any output at all (i.e. similar to problem
reported by 'jim' I guess, with A111, 11/28/01) - investigation shows
that the shift register chip just doesn't run at all (it's designed to
run at frequencies up to about 30MHz, so we're just toying with it
really!). Slowly cranking the frequency of the sine wave up has the
effect of decreasing the rise time, and when it gets to about 3 or 4
microsec, lo and behold, it will burst back into life! (To be fair it
must be said it is obviously much happier with a square wave as
input!) The effect on the 115 seems similar - the binary counter seems
to 'miss a beat' now and again, which you can hear as clearly 'a not
properly divided' signal. The square wave coming off the CEM3340 chip
in my A111 is pretty awful - square is hardly the correct word, it is
well rounded, and becomes more so with increasing frequency, so it
doesn't really drive the 117 particularly well either.

Two solutions: there are 2 modules that can help get around this - the
idea with both is the same: stick the signal through some nice CMOS
logic that'll give a lovely 'sharp' pulse out. The A150 is probably
the simplest - square wave into CV input;+5V, -5V into the two inputs
(I use a 129/3 as a simple CV source, plus an inverter - a 176 or 174
would do just as well);then patch output into the 117 (or 115). This
certainly gives the 'playable noise' effect with the 117 using the 111
on my system. Alternatively the divided output from the A163 probably
comes off a chip (I've not checked in detail), but that is nice and
square - just set it to divide by 1.

Overall I'm having great fun with my A100 - learning lots about sound,
music (I'm not a musician) and electronics - I love it! I'm expecting
to get a copy of Chowning's paper on Frequency Modulation soon, and
then I intend to really start exploring the linear FM input on the
A111 (and it will be interesting to see what limitations analogue
places on it, rather than doing it digitally!).

I also have a question of sorts - do people find their questions
eventually get answered through this medium I only ask as looking
back at some old posts here I noticed a query about the A-150/A-151
inability to switch clock pulses. The answer I think is in the manual
- they will only cope with signals less than 8 volts (shown in a
diagram somewhere - this is from memory!), and generally clock pulses
go from ground to supply, i.e. 12 volts, and this will upset the
circuitry somewhat, and it allows some of the signal through.
Depending on where you're trying to send them, simply attenuating the
signal (e.g. A-138 linear mixer) might do the trick.