Here’s some experimentation. I set attack fairly fast, sustain 100%, release fastest and tried:
- 1 µF aluminum electrolytic (ChengX?, Tayda A-4505)
- 1 µF tantalum electrolytic (AVX, Tayda A-265)
- 1 µF PET film (WIMA, Tayda A-4168)
- 10 µF aluminum electrolytic (JB, Tayda A-4349)
- 10 µF tantalum electrolytic (AVX, Tayda A-5226)
with a ~2.5 second gate. Results:
Interestingly, for 1 µF the tanty sags less than the aluminum, but for 10 µF the aluminum is better and the tanty is horrible. For 1 µF the film cap is pretty much perfect. This is only a sample of 1 for each value and type, of course, and any of these parts could be anomalous, but for now I think 10 µF Al and 1 µF film is the way to go.
I was shocked enough by the 10 µF tanty that I decided to look at two others. One behaved about the same. The other did stuff like this:
Seriously. I switched the two in the socket and the behavior followed the capacitor.
Maybe I can market this thing as a capacitor tester.
After a while, without changing anything I’m aware of, the crazy behavior stopped, but it lost voltage even faster than the other two.
I’ve discovered looping stops if the decay time is turned up beyond some point, about 2/3 or a little less of the way up on the fast setting or maybe 20% of the way up on the slow setting (which is about the same decay time). The whole attack time range is available and the decay time limit doesn’t seem to depend on attack time. Sometimes it’ll go for 2 or 3 pulses before stopping. I have no idea why unless it has to do with the folly of using a TL071 as a comparator.
Added still more:
For the heck of it, I swapped the TL071 for an LM741. Because I had one. The looping still stopped, at about the same decay time — actually maybe slightly lower, but only slightly.
And yet more
OK, think I see what’s happening. I breadboarded the looping mod to make it easier to look at it. Here’s a scope picture at a fairly fast decay time, triggered by the comparator when the envelope falls below threshold:
The yellow trace is the envelope. It’s decaying slowly at the start and then the next attack begins. Purple is the comparator output, low when the envelope is above threshold and high when it’s below. Cyan trace is the trigger pulse made from the rising edge of the comparator.
That rising edge is slow, much slower than the falling edge. So the trigger is relatively wide and not that high.
Here it is with a longer decay time:
Now the rising edge is slower, so the trigger is wider and shorter. (And there’s a longer delay between the trigger and the start of the next attack, for some reason.) At some point the rising edge becomes so slow the trigger becomes too small to trigger the 555 and looping stops.
Seems the op amp does not swing abruptly when the input crosses threshold slowly.
I guess it’d be possible to condition the comparator output, to make a faster rising edge before making the trigger pulse from it. Not something I’d want to try kludging with the present PCB, though, and not clear how many components it’d be worth adding to a later version to fix it.
One more thing!
Fixed it! Just had to bump up the value of the capacitor in the gate to trigger. Now it loops even at the slowest decay speed.