[sdiy] Buffering Busses?

[harrybissell]

The VCO3d has a 'bug' (feature).

In the original Moog design, the CV inputs were permanently connected
so the impedance into the VCO never varied.

In a modular system, going from open circuit to 100K (the input resistor)
will throw the pitch off everytime a connection is made.

The design does NOT have a true summing junction, its 100K and 100ohms.
Close, no cigar.

I added an opamp voltage follower to each input. Now connecting the CVs
does not change the impedance at the pseudo-summer.

It was late in the design when I found the trouble. Buffers were the easy way
out at that point.

H^) harry

Scott Gravenhorst wrote:

> Dave Kendall <davekendall at ntlworld.com> wrote:
> >Hi Seb.
> >
> >> You should use low output impedance buffers (to avoid voltage drop from
> >> connecting multiple things), but maybe you have them already in the
> >> output stages of your midi to CV module?
> >
> >It's a PAIA Midi2CV8, <http://www.paia.com/midi2sch.pdf> in mono mode with
> >home-made LAG on the MW AT and PB outputs, and offset/scaling on the
> >Pitchbend output to give ±5V. How do I work out what the output impedance of
> >the 1V/OCT is, and what would be a good value to end up with?
> >
> >I got a tracking error when connecting both oscillators in an EFM VCO3D to
> >the 1V/oct CV from the Paia. One was fine, but both threw the pitching way
> >off, that's what started this train of thought. The eventual plan is to have
> >between 4 and 8 VCOs in total, with one set of 4 driven from a CGS37 CV
> >cluster, which I hope will provide a measure of buffering, so I guess I need
> >something that can handle LOTS of VCOs...
>
> I don't know anything about the VCO3D, I assume it is a dual VCO?  You say that
> one is "fine", did you try using just the "bad" VCO alone and measure the pitch
> CV both with and without?  If the CV changes when one VCO is plugged in, but
> doesn't change when the other is plugged in, the "bad" VCO has a low impedance
> input (a shunting resistance to ground?) or perhaps something very bizarre
> happening with the buffer.
>
> I'm sure that there is a way to test the output drive capability of the buffer,
> probably with a simple resistor to ground where if the impedance is good, the
> output voltage of the buffer should not change (significantly).  What the size
> of the test shunt resistor ought to be for the test - ???  I would probably try
> something like 5K to 10K thinking that a VCO pitch CV input should probably be a
> lot higher than 10K.
>
> >>
> >> Remember also that any input voltage offsets in your buffer opamps will
> >> cause errors in your cv voltages - not so bad if the voltage offsets are
> >> static, but changes with temperature will not be good (choose your opamp
> >> buffer for pitch carefully .. I would recommend something like LT/MXL
> >> 1013/1014)
> >
> >OK seems worth the extra money. Would just the one OPA on the 1V/OCT output
> >be sufficient to feed up to 8 VCOs? I haven't yet found a really good buffer
> >design, and was planning on using just a simple voltage follower. Do you
> >know of a schemo anywhere, perhaps with provision for trimming any input
> >offset?
> >
> >Lots of questions, - sorry!......
> >
> >Cheers,
> >
> >Dave
> >.
> >(I REALLY need to get that Horowitz and Hill book..... :-)
> >
> >
> >>
> >> Seb
> >>
> >>
> >> Dave Kendall wrote:
> >>
> >>> Hi All.
> >>>
> >>> Am building a Midi 2 CV module with a perfboarded Blacet MIDI sync IC in it,
> >>> and am thinking about running several Busses  throughout the modular, to
> >>> minimise on panel jacks, and make set-up faster.
> >>>
> >>> 1/ 1V/octave
> >>> 2/ Gate
> >>> 3/ Trigger
> >>> 4/ Clock *1
> >>> 5/ Clock *2 (maybe....)
> >>>
> >>> Question is, should I use voltage followers as buffers on these outputs
> >>> where they come out of the MIDI2CV? I figured that this might help stability
> >>> when feeding several VCOs from the 1V/oct outputs via the busses. Is it a
> >>> good thing to do this with the clocks and gates etc........ ? I know Doepfer
> >>> use a buss system, but I haven't got any of their gear, so don't know how
> >>> they do it.
> >>>
> >>> Any thoughts appreciated.
> >>>
> >>> Dave (saving up for Horowitz + Hill - £50 in the UK - Yikes!)
> >>>
> >>> P.S. thanks Ryk, Harry and Seb for stuff on tempcos :-)
> >>>
> >>>
> >>>
> >>>
> >>>
> >>>
> >>
> >
> >
>
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