[sdiy] Buffering Busses?

[Seb Francis]

Dave Kendall 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>
>
 From a quick glance at the schematic it looks like it already has opamp 
voltage follower buffers on the output stages, so you wouldn't need to 
add another one.  I assume the transistor bit is not used in 'mono' mode.

>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?
>
>  
>
There's no working out to be done.  You just want it as low as possible 
so changes in load (e.g. more VCOs) have negligable effect on the 
voltage.  An opamp voltage follower buffer has a very low output 
impedance (exactly what you want).

>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.
>
This error shouldn't happen if you are running the VCOs direct from (for 
example) pin 1 of IC11:A.  Do you have some kind of LAG or other CV 
processor circuit in between?

>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...
>  
>
Just a normal opamp voltage follower buffer should be fine, provided the 
input impedance of your VCOs is quite high (e.g. 100K) - look at the 
schematic for your VCOs and see what value the resistor between the CV 
input and opamp input buffer is.

>>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?
>
>  
>
First, I would say with a PAIA MIDI 2 CV you are wasting your money 
using a precision opamp like the LT 1013.  The voltage accuracy is only 
going to be as good as everything else in the converter itself.  
Probably a TL072 will be good enough as long as the temperature doesn't 
change too much.

You are right that a simple voltage follower is what you need for a 
buffer, but as I said the MIDI 2 CV schematic seems to have these 
already.  So you only need to add one if you have included some extra CV 
processing circuitry between the MIDI 2 CV and VCOs.

The only thing with using a simple voltage follower is that you have no 
current limiting (e.g. in case of accident short circuit) except for the 
opamp limit itself.  There are ways to work around this, but most opamps 
don't mind too much being shorted, so I wouldn't worry.

>.
>(I REALLY need to get that Horowitz and Hill book..... :-)
>  
>
Definitely recommended :)

Try looking 2nd hand from abebooks.co.uk
I've seen it for around £5 here.


Seb