[sdiy] new schematic posted

[harrybissell]

Hi Ingo...  (inline)


> Nice circuit. I must confess, it took me a while to find out why the
> output of AR3 is tapped off *after* the diodes and not directly from
> the opamp's output. This is to compensate for the diode voltage drop,
> right?

right.

>
> But doesn't this cause some interaction between the attack and the
> decay pot if the shape pot is not in one of its end positions? As I
> understand it, one one of the diodes conducts at a time. Say, the
> attack diode conducts and the attack pot is set to some high
> resistance. Isn't there some voltage fed into the integrator via R9 and
> R10 and the decay pot? Unless the shape pot is in an end position, in
> this case this path would be shorted to ground or virtual ground. But
> if the shape pot is in center position, R9 and the two halves of this
> pot form a voltage divider. So the decay pot would have an influence on
> attack too.

Hmmm. I have not noticed any interaction.  Both sides of the shape pot are
at ground... The worst case would have to be with the pot at center
rotation...
each side would be 5K... in parallel that is 2.5K to ground.  The 20K /
2.5K
divider would limit the effect of feeding back through one 20K... past the
2.5K to ground, through the other 20K... and through whatever remains of
the decay pot.

You could bring the resistor levels up (proportionally) and the effect
would be still
less... but it may be that they do interact a little. I'll try and look for
it.

> Another question, as I see it, in linear mode (wiper of shape pot at
> ground) AR3 is overdriven (no local feedback). Can't it latch up?

I suppose it could... but there is always the feedback from the integrator
as well.  I've built a log of linear lag circuits with this technique...
and have not seen
a latch up.  If you were going to have one, it would probably be at some
negative
input voltage (imho).

Choosing a different opamp could cure that if it was found to be a
problem...

Thanks for the comments !

H^) harry