[sdiy] 3080/13700 OTA question

[Tim Ressel]

Ah. I tried reversing the 10K and 2.2M with no luck. The OTA just loads down the 1uF cap preventing it from developing significant voltage. From your results I suspect the circuit will work if I juggle some values, but if the performance was that poor then why bother?

I just tried my other drive circuit board, which just happens to closely match the circuit Tim offered, and it works fine. Perhaps I will do a merge-O-matic of the two circuits i.e. put a buffer at the end of the problematic AD circuit.

Thanks to all!

--Tim (no, not that Tim, the other Tim) Ressel


--- On Fri, 1/2/09, Brock Russell <brockr0 at shaw.ca> wrote:

> From: Brock Russell <brockr0 at shaw.ca>
> Subject: Re: [sdiy] 3080/13700 OTA question
> To: synth-diy at dropmix.xs4all.nl
> Date: Friday, January 2, 2009, 1:09 PM
> I built this back in '76, my first significant synth
> project, and 
> from looking at the vca boards and my lack of notes of any
> errors, I 
> think the circuit is correct as drawn. I used the pwm
> attack/decay 
> driver circuits too. The circuit might be correct but the
> performance 
> was poor, very noisy and lots of feed through. Hope you
> have better results.
> 
> Brock
> 
> 
> At 10:03 AM 1/2/2009, Tim Ressel wrote:
> > > Clearly an error on the drawing.  My guess would
> be that
> > > the 10k and 2.2M resistors are swapped.  this
> would give you
> > > envelope time up to a few seconds and 1.5 mA of
> Iabc.
> > >
> > > Ian
> >
> >Interesting! I dug up a copy of the June '76
> Popular Electronics 
> >article and it has the same values. Of course they both
> could be 
> >incorrect. I will try swapping the 10K and 2.2M right
> away.
> >
> >--TimR
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