[sdiy] Buffering Busses?

[Scott Gravenhorst]

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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-- Scott Gravenhorst | LegoManiac / Lego Trains / RIS 1.5
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