[sdiy] Newbie JFET-switch problem

[Richard Arntzen]

Thanks, Tony! Will try out the other tranny. I think the purpose of C17 is
to generate the negative voltage (when pulled down by Q1).

The purpose of the circuit is due to the interface; I have one set of
switches that controls semitones and another that controls octaves. When I
play across octaves, it is hard to be accurate in operating both sets of
switches simultaneously, so I want to introduce a lag whenever anything is
switched, so that I can move into the next note without the loud pops :)

Richard

> -----Original Message-----
> From: Tony Clark [mailto:clark at andrews.edu] 
> Sent: Monday, August 08, 2005 19:08
> To: Richard Arntzen
> Cc: 'synth-diy'
> Subject: RE: [sdiy] Newbie JFET-switch problem
> 
> > I have uploaded the schematic at
> > http://home.chello.no/~richarnt/ewicv-v1.pdf and the 
> measurements at 
> > http://home.chello.no/~richarnt/JFET-meas.jpg - now if I 
> add let's say 
> > an opamp with strong positive amplification between 
> R24/D3/C17 and the 
> > base of Q2, will that move the JFET operation to the 
> extremes of the 
> > transfer function?
> 
>    I really don't know what that would do, but having looked 
> at the circuit, I think I can understand your problem.  The 
> diode should be turned around.  What it is doing is clamping 
> the voltage at the base of the JFET to 0.7V or so.  This 
> isn't enough to turn the JFET on.
> 
>    Instead, here's what I would do:
> 
>    First remove D3.  Second, lower R24 down to 100k or so.  
> Then remove
> C17 and connect the collector of Q1 straight to the base of 
> the JFET.  
> Then remove R23.
>    Now what you should have is a transistor that, when it 
> conducts, clamps the base of the JFET straight to ground.  
> Actually, if you are trying to use the JFET as a sample-hold, 
> you probably want to pull the base negative, since as I 
> recall, their maximum non-conductance (highest off 
> resistivity) is at a fairly negative voltage.
>    Anyway, you can then adjust the timing of the 555 timer to 
> adjust the on/off time period of the transistor/JFET.
> 
>    Hope that helps!
> 
>    Tony
> 
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