[sdiy] Help with lm339

[J C]

I think I am missing something easy.  I have put a rough sketch here:
http://home.cfl.rr.com/entropy/comp.jpg
What I have is a sustain voltage selected by the pot, buffered through
the TL072 and then presented to the comparitor to be compared to the
envelope being generated.  What I want to do is to add a slight
voltage to the + input so that the comparitor will trip as the
envelope decays and approaches the sustain level.  I do not need
hysteresis per se because I am only looking at this when the envelope
is decaying to the sustain level.  The sustain voltage is also being
used by another part of the circuit and so the injected voltage can
not change that - this circuit is to only add state LEDs to the EG. 
Does this make any better sense?
On 4/1/06, harrybissell <harrybissell at prodigy.net> wrote:
> without seeing your schematic, its tough to give exact advice...
> but the usual solution to your problem is to add hysteresis
> to the circuit... you need a series resistor in the positive input
> and a large value resistor from the output to the positive input.
>
> Hysteresis moves the positive voltage a little bit... so that when the
> trip point is reached the signal is suddently moved WELL into the
> region and there is no oscillation.  Use a small amount, or you will
> create a latch situation.
>
> I did an envelop gen years ago where I chose your proposed solution...
> it will work but it is a little complex.
>
> H^) harry
>
> J C wrote:
> >
> > I am trying to use a lm339 comparitor to determine when an ADSR signal
> > reaches the sustain level.  What I want to do is add a small voltage
> > to the sustain level voltage so that the comparator fires just above
> > when the envelope reaches it (if I don't do this, it just waffles
> > around when the two voltages are equal ie:when the envelope is
> > sustaining).  Is there a simple way to do this (without adding a
> > summing opamp)?
>