[sdiy] droop of sustain level in analogue ADSR?

rsdio at audiobanshee.com rsdio at audiobanshee.com
Fri Nov 14 03:42:25 CET 2014

On Nov 14, 2014, at 11:20 AM, Michael Zacherl <sdiy-mz01 at blauwurf.info> wrote:
> Hi, had a rather intense discussion with a colleague today.
> He claimed there are analogue ADSR-designs which suffer from a voltage droop in the sustain stage.
> Much like a sample&hold. I didn't exactly believe that.
> Does anyone know of such a design?

There are many reasons why a Hold stage will droop.

The first is the capacitor itself. Many capacitor chemistries will self-discharge, making it impossible for the remaining circuitry to avoid drooping, since the capacitor is the source of the voltage. Polystyrene and other poly capacitors are the best, but they are expensive. I have also noticed that poly caps are only available in through-hole packages, which may not be a factor for most vintage analog synths, but would certainly be a factor in modern, cost-effective designs. Search for "capacitor self discharge."

Outside the capacitor, assuming it can hold a charge well, there must be a way to distribute the voltage to other circuits. Typically, this is going to be an op-amp voltage follower. Ideally, an op-amp input pulls no current, but rarely do op-amps deliver ideal performance. As soon as some current flows into the non-ideal op-amp input in order for the op-amp to function, this current discharges the capacitor. The best performance is from op-amps with JFET input stages, because these can have tens of tera-Ohms of resistance. But even the best op-amp does not have infinite resistance. Thus, the second contributing factor is the non-infinite resistance to ground that discharges the capacitor.

Beyond the capacitor and voltage follower, there might be aspects of PCB conduction. The resistance across a PCB, which is ideally an insulator, can still be in the giga-Ohms of resistance. I have no idea whether this factor is significant in any known design, but it's theoretically possible. There are also issues of dirt or unexpected material, as pointed out earlier in this thread.

All considered, especially with the cost of good capacitors and the limited availability of advanced op-amp designs in vintage circuits, I can see that there are easily many ADSR designs that could droop.

Sorry, I don't know specific brands, though.

Brian Willoughby
Sound Consulting

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