[sdiy] droop of sustain level in analogue ADSR?

[Tim Stinchcombe]

> 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? thanks, Michael.

Ages ago I had wondered what the long-term sustain behaviour the Doepfer
A-140 would be, as in sustain the main cap discharges through the decay pot
and the emitter of a PNP transistor whose base is held at the sustain level
(wiper of a pot to the 12V rail), and it always struck me it would take a
long time for the transistor to turn off, as the differential between the
emitter and base very slowly decreases. So without further ado I set mine up
and logged the sustain voltage over about 20 minutes - here's a plot of the
result:

http://www.timstinchcombe.co.uk/synth/140_sustain_droop.gif

This was using the 'H' time setting (main cap is 100u); A, D, R all fully
CCW; sustain set arbitrarily around 3V, and at the resolution of my
Picoscope, it didn't really pick up the initial spike of the envelope.

Green trace is the 'full' envelope (against the right axis), showing that
the voltage is slowly decaying. The red is an expanded vertical scale (left
axis), showing greater detail - at some point I suppose that the envelope
does actually drop below a level which causes the PNP to turn off (possibly
one of the wobbles, but I didn't wait for the 140 to warm up, nor indeed the
Picoscope, so there could be any manner of temperature drift effects in
there...). One contributor to the droop will be the input bias current of
the TL084 OA buffering the cap: the blue line is a straight-line fit to the
end of the curve, and equates to about 5nA per second from the cap - this is
a good deal more than the typical input bias current for the TL084, so other
leakages are apparently present.

Whether this sort of droop is going to cause a problem probably depends on
what time-frame any particular user is interested in.

Tim
__________________________________________________________
Tim Stinchcombe 

Cheltenham, Glos, UK
email: tim102 at timstinchcombe.co.uk
www.timstinchcombe.co.uk