[sdiy] Buffering Busses?

[Seb Francis]

>>.... a VCO pitch CV input should probably be a lot higher than 10K.
>>    
>>
> The  1V/oct input for each VCO goes via a 50K trimmer and a series 22K
>resistor to a summing node, that also receives the Pitch mod input via a
>100K resistor, a 50K INITIAL trimpot followed by a 47K resistor, and the
>coarse and fine tune pots which are both 50K pots connected to +V and -V
>with wipers going via series 1M and 330K resistors respectively. This
>summing node connects to the base of one transistor in a CA3046 (part of the
>expo pair?...) and also goes via a 1K tempco to GND....
>
>Some designs I've seen have standardised on 100K input resistors for all
>inputs.....I'm a bit confused by the different values in the VCO3D...
>
>  
>
Don't be over concerned about it, input impedance is not critical as 
long as it's high enough .. even with the trimmer turned all the way up 
you still have 22K input impedance at the 1V/oct input which will be 
fine.  Worst case if you had 6 VCOs all with the trimmers set like this 
you would have an overall input impedance of 3.7K which means that 2.7mA 
will flow at 10V.  Any opamp follower will easily be able to supply this.

Pitch errors can occur when the output impedance of something is not 
very low (e.g. if you have a 100R series resistor on the output, outside 
of the feedback loop of the output buffer opamp), but any well designed 
pitch CV source should not have this.

>>>I got a tracking error when connecting both oscillators in an EFM VCO3D to
>>>the 1V/oct CV from the Paia.
>>>      
>>>
>>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?
>>    
>>
>
>No - the 1V/oct was planned to go straight to a voltage follower, then to
>the buss.
>  
>
If the MIDI to CV converter has a voltage follower on its output (which 
it does as far as I can see from the schematic) you don't need another one.

As someone else suggested, you should check your VCO for faults if it is 
causing a noticable drop in pitch when you hook it up.

Seb