Doepfer

Hi list,
So, here is my modification to the A-140 so that it can be made 
to `auto-repeat', i.e. re-trigger when the sustain level is reached.

It turned out to be a little more complicated than I was at first 
hoping for because in its first incarnation it didn't perform that 
well, and so I added a few more bits to improve it. The basic idea is 
to turn the op amp which currently inverts the envelope (so you lose 
that function!) into a comparator which switches when the sustain 
level is reached, and thus feeding this signal back into 
the `Retrig.' input makes the envelope continually cycle. To 
retrigger properly, the switching action of the comparator needs to 
be crisp, but this tends not to be case when the envelope 
is `levelling out' at the end of a long decay, especially if the 
sustain level is low. Hence an extra resistor and capacitor are used 
to `speed up' the comparator switching action.

It works quite well, but it will not repeat if the sustain level is 
too low. With the component values below, I measured these levels to 
be approximately:

Time      Sustain     (Sustain
Range     knob         voltage)
L          1.5          200mV
M          2            500mV
H          2.5          1V

(all at short to medium delay: they increase gradually with much 
longer delays, but note the envelope goes up to around 7V). It also 
(obviously) stops re-triggering when the sustain level is very close 
to the top of envelope. Another thing is that since the `Re-trigger' 
function (as designed in by Doepfer) re-starts the envelope from the 
_beginning_ of the attack phase, when it re-cycles you can get a 
sharp negative-going `glitch', but dialling in some `Release' slows 
down the capacitor discharging to the point whereby to all intents 
and purposes it recycles from the sustain level itself, which looks 
much nicer.

In spite of all this, there is still lots of room for manoeuvre and 
experimentation!

The last part of mod (step 8) drops the re-trigger output down to 
within +/- 8V, so that there will be no problems switching it with an 
(unmodified) A-150 if you want voltage control of re-trigger off/on. 
If you are not bothered by this, just leave out this last step and 
the re-trigger output will be approx +/-11V or so.

New components needed:
10pF ceramic capacitor
1k, 22k, 47k resistors (22k & 47k not needed if last step not to be 
done)
1N4148 diode

("Top, bottom, left, right": for the component side, when viewed with 
the face-plate downwards; for the solder side, face-plate upwards.)

1) Immediately below the TL084 chip find two sets of a 47k resistor 
(yellow, violet, orange, gold) and a diode, one set above "D6", the 
other above "D7". Remove both resistors – the top one is seen to have 
been "R13", the bottom one "R12".

2) Cut the track immediately next to where the left end of R13 was 
soldered, so that it is isolated from pin 13 of the IC (second pin in 
from the right) and what was the left end of R12.

3) Locate pin 12 of the IC (third in from the right end, and directly 
below the left end of R13. Cut the track connected to this (which is 
actually 'ground') - I did it just below pin 14, the right most pin 
of the IC.

4) Solder the 1k resistor between the middle connection on the 
Sustain pot (just below the middle of the middle jack socket) and pin 
12 of the IC (third in from right, now disconnected from ground by 
above cut). (Take care not to get IC too hot...!)

5) Solder the 10pF capacitor where R13 was - bend the left end over 
the bottom end of the 1k resistor and solder, so that resistor & 
capacitor are both electrically connected to pin 12 of the IC. Double 
check that solder has not bridged the cut in the track made in step 2!

6) Solder a plain wire link where R12 was.

7) Locate the 47k resistor above the Decay pot, just above 
capacitor "C1" (looks like this is actually "R2"). Solder the diode 
across this resistor, black stripe to the left.

8) Locate the 1k resistor (brown, black, red, gold) between 
capacitors "C2" and "C3", and remove it (revealing it was "R14"). 
Solder the 22k resistor in its place. Solder the 47k resistor between 
the top end of the 22k one (connected to the 'inverse output' 
socket), and the top pin (negative terminal) of C4, which is actually 
ground.

Re-label the 'inverse output' socket so you know module _is_ 
modified! (I called it 'Retrig Output'.)

To test, I suggest patching the envelope output into an A-110 pitch 
CV input so you can hear what is happening, gate it with a slow-
running LFO, and patch the bottom socket (the new retrig output 
signal) into the re-trig socket.

I shall put some photos (thanks again to Mike) of it in the photo 
section straight away!

Cheers,
Tim