[sdiy] Little help with S and H circuit

[harrybissell]

Hi Mike

mike:prophecysound wrote:

> Hey all.....
>
> I'm struggling to get working a S&H circuit that is mostly based on one
> of Tom G's early LFOs:
> http://prophecysound.com/temphost/sandhschematic.pdf
> (Adobe v6 required)
> I lack the understanding to debug the CA3080 control part - what I have
> is that there is +/- approx 12V waveform at L2 (I'm running this from a
> +/-15V power supply) but I'm not getting a changing CV at the output ...
> a few questions:
> 1) what is the purpose of the C3/R12/C5/D1 components?

C3 and R12 make a differentiator that makes a very narrow pulse on the
rise and fall of the LFO  (square wave at link 2)  This pulse will go
positive when
the output at link 2 goes high, and negative when it goes low.

When applied to the control pin of the CA3080... it will turn the CA3080 on
when
the LFO goes high... but the negative voltage will go LOWER than the
negative
supply, possibly damaging the CA3080.  D1 will prevent this by clamping the
negative spike to the negative rail

C5 is wack, IMHO.  I see no reason this should be in the circuit.  I'd
recommend two
changes.  One... add a 10K resistor in series with 3080 pin 5.  This will
prevent death
should the circuit go wrong.

3080 will DIE if too large a current goes into pin 5.  Since pin 5 is just
above the negative
supply, a short to ground or positive supply is instant death.

Another thing I would do is add a resistor from pin 5 to the negative
supply. Probably one
megohm would be fine.  This will assure that the 3080 turns ALL the way off
(it must!)

The FET should be OK if you have the pinout correct... it is a simple
voltage buffer

One way to troubleshoot would be to remove link 2 temporarily... and use a
switch to
apply either the negative supply, or ground... to that 10K resistor I
insisted you add :^P
With the switch to negative supply, it should be holding. With the switch to
ground... it
should have noise from the noise generator at the output.

>
> 2) should the collector of Q1 be unconnected or connected to ground?

unconnected would be best

>
> What purpose does C1 serve?

AC coupling of the noise transistor -  it has some DC level and you don't
want to
amplify that.  Making this bigger (to a point) would increase the low freq.
response
of the circuit... probably a non-issue in your generator

H^) harry

>
> Oh, and I'm using a 2N5484 instead of the MPF102 - I guess I might need
> to tweak the circuit to allow for a different switching voltage but I
> don't think that's the trouble for now, I think the CA3080 switching
> part isn't working.
>
> Any hints or tips GREATLY appreciated. I can provide more diagnostic
> voltages if need be.
>
> Cheers
>
> MikeB