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A-140 Envelope Generator Self-Cycling Modification
It was recently pointed out to me (Jan 2014) that I didn't have any detail of this mod here on my website. Several reasons caused this: the mod was worked out a long time ago (in 2003), before I had any means of obtaining a hardcopy output from an oscilloscope, so I wasn't able to present any output showing what the oscillating A-140 looked like; but mostly it was because I'm not that convinced that it is actually a very useful modification to do—to me it feels a bit 'second rate', but I guess I should let others judge for themselves how useful they might find it. (I often do things just for the sake of seeing if it can be done, or to see if I can actually do it, and this mod is very much in this camp.)
The mod makes use of the op amp used to generate the inverted envelope out, and so that particular function is sacrificed: additional components turn the op amp into a comparator which generates a 're-trigger' signal once the envelope has decayed down to the 'sustain' level. To make the modified 140 self-cycle the 'gate' must be held 'open' (with a sufficiently high voltage), and the 'inverse output' jack is patched back into the 'retrig' input. It sometimes won't re-trigger if the sustain level is too low (approx. knob settings causing this are given below), and to get it going again the sustain level needs to be increased a little, and then the gate signal needs to be taken low and then high again for the oscillation to get going once more. Since it re-cycles once the sustain level is hit, the output always contains a DC offset (varying with the sustain level), as is readily seen in the traces that follow.
The mod was originally posted to the Doepfer A100 Yahoo group (links below), and I have merely copy-pasted the instructions for doing the mod below as well. But first let's look at some traces of the waveforms that it produces. For each switch setting of 'L', 'M' and 'H' (effectively: fast, medium and slow) I used three different pot settings, with all pots, attack 'A', decay 'D' and release 'R' , set together at one of the same three knob positions, 0, 5 and 10 (again: fast, medium and slow), and then within this ran the unit with the sustain level 'S' at three levels (about) 2 (or 2.5), 5, and 8 (low, medium and high). All vertical scales are the same, with the 0V level being at the bottom of the grid. The annotations show both the vertical scaling, with all being '1V' per (major) division, and so the envelopes swing from just above ground at low sustain levels, up to around 7.5V, and the timebase scaling, again as so many microseconds/milliseconds/seconds per division. Care will be needed when reading the horizontal scales as there is so much variation, and where the settings result in a really fast envelope, but with a comparatively large gap between repeats, I have 'zoomed in' on the horizontal timebase in order to better see the shape and timings of the major part of the envelope. Hence many of the plots below are pretty 'busy', and it will involve a little effort to 'see the wood for the trees'!
It became impractical to capture the longer repeats using my main scope, on which most traces were taken, and so I had to revert to logging the voltage levels at regular intervals (using my older Picoscope) and then plotting the envelopes in Excel—for these plots the horizontal axis actually just shows the elapsed time. In a few of the plots I have illustrated the effect of the big negative spike that results from the envelope resetting to zero (mentioned in the instructions below), but which can be circumvented by ensuring some release is dialled-in—these are annotated as 'S x + R 0', where the 'R 0' shows that the R setting goes against that used for the rest of the traces on that plot.
On the 'H' switch setting, it is possible to get some really skinny envelopes, but with a huge gap between the cycles—the spike of the envelope does go to the same height as others (as seen in the next but one trace), it is just that the logging is not fast enough to capture it:
Here we zoom-in on the spike (using my main scope again), and we can see that the spikes really are quite fast, but which do go to the full height of the envelope (about 7.5V):
Here is a verbatim copy of the instructions for doing the mod, as originally posted at message #3276 of the Doepfer Yahoo group. The photos were originally uploaded to the photos section. Note that there is one big assumption that the A-140 PCB hasn't changed after all these years!
The bottom of the PCB:
Closer detail on the bottom:
[Page last updated: 25 Jan 2014]