Cgs synth

> On 4/13/07, contact@... put forth:
> >
> >I'll clear up the confusion. For the daughter board, the schematic
> >is correct. The - and + pins on one op amp are swapped, basically. I
> >use a CGS simple mixer PCB, which needs to have those physically
> >swapped, as the ground trace is a bit too big there to simply cut.
> >You then run jumpers to the points those pins need to go to. Those
> >jumpers are wired directly to the pins of the TL072. If doing it on
> >your own, you could use protoboard or a Tellun MUUB-2 if my solution
> >seems inelegant :) It works well with my bracket, though, and is
> >not hard to do.
>
> I'm sorry, I still don't get it. The schematic shows an LM358 op
> amp, so I went and looked that up. Pin 8 is V+ and pin 4 is V-, which
> is the same as a TL072.

Oops... I meant the - and + _inputs_, sorry. That is, the inverting and
non-inverting inputs to the opamp. The power supplies are indeed the
same, and although I use 072, the 358 should also work fine there.

>
> It's my understanding that a a 1V/Oct input and exponential FM input
> would be the same thing.

They are indeed. I almost left off the expo input in favor of linear,
but I think most people usually use the expo in addition to 1v/oct, so
there would be more patching.

>
> I had thought the "Ext Excite" was an input for an external VCO to
> clock the filter (although that might not be a bad feature). If
> that's true, then a 1U expansion module would seem unwise as that 1U
> could be used for an MOTM-310 or some other 1U VCO.

The Ext Excite is a really just a second input to the filter.
An external VCO to clock the filter would be superfluous, I think, as
the Bintic has its own VCO built in.
The expansion would be for the Linear CV fanatics, to give a variable
pulse output to the VCO, and to give VC Bandpass, which I think would
be very useful, as that is the knob I am always tweaking on this
filter.

>
> Now I'm thinking that it might be an additional audio input for the
> filter. Is this the case?? So if the saw output is normalled to the
> "Ext Excite" jack, following the chickenhead selector:
>
> EXT -- saw
> 1 -- square
> 2 -- square, F/2
> 3 -- square, F/4
> 4 -- square, F/8
>
> are the inputs to the filter, is that correct?? So one way of
> looking at is that the "Input" and "Excite" knobs have the same
> function for their corresponding inputs??

That's it, though the squares are divided lower, I believe. I don't
have the 4024 datasheet here, but I'm pretty sure they are.

>
> I took another look at the schematic for the VCO output
> daughterboard. I'm thinking that it taps the sawtooth going into the
> CMOS counter, and the first op-amp servers as a follower. Where does
> the 560K with the down-pointing arrow go?? Then I'm thinking that a
> square wave is generated by the massive gain (2M/2K) of the second op
> amp by driving it into the rails. Although, afaik, a TL072, unlike
> the LM358, doesn't like going to rails. I'm not sure about positive
> outputs, but the common mode voltage getting too close to the
> negative rail of a TL072 can cause phase inversion and latch up. I
> apologize if I sound overly critical, but for educational purposes,
> I'm not sure why another comparator circuit was not used.

Maybe the 358 would be better. I just had 072s, and they've worked on
around 10 Bintics I've built.
Follow the Electronotes link on Ken's VCO page to get a full
description. My mod is from there.

Oh, and critical is good :)

Cheers,
Scott

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