[sdiy] VCO reference voltages

[Simon Brouwer]

Op 16-4-2010 18:47, David G. Dixon schreef:
> Thanks for all the responses.  I think I've formulated a design philosophy.
>
> Here's what I'm thinking:
>
> 1. Use voltage regulators for "critical" on-board voltage references such as
> thresholds, servo currents, and for critical adjustments in shapers, such as
> triangle or saw alignment.
>
> 2. Use rail voltages for coarse and fine tuning pots.
>    
Why not use those voltage regulators that you plan to have anyway?
> My rationale is that the tuning knobs are going to be twiddled constantly,
> so if their end points move slightly as a result of PSU loading, this is not
> important, and will go unnoticed.
That may be different if you use an octave selector switch instead of a 
course tuning knob, but OK.
>    After all, the PSU load is largely fixed
> unless modules are added or removed from the cabinet, and this doesn't
> happen during operation.  Hence, the rail voltages should remain fixed to
> within 1 or 2 mV during actual operation.
That sounds a bit optimistic. The current consumption may not be *that* 
constant (think flashing indicator leds etc.).

>   It is only when modules are added
> that the rail voltages may change measurably, and only then when the PSU is
> at or near its drive capacity.  However, all this will do is change the
> range of the tuning pots very slightly.
>
> If the rail voltages change measurably as a result of just twiddling knobs,
> then the PSU is probably at or near its drive capacity, and I don't think
> that module design should have to compensate for this.  Hence, it is really
> only inexactitude in the rail voltages themselves that should be compensated
> for, and this is accomplished by using references for those voltages which
> should take "exact" values on the board.
>
> The other downside of regulating rail voltages for the tuning pots is that,
> because regulators only regulate to within 1 or 2 volts of the rails, using
> regulated voltages for the rails will limit the range of the pots,
> particularly in a +/-12V system.
If that is really the reason to use the rail voltages you should think 
again. Using the lower voltages you only have to adapt the
value of the summing resistors for those pots to get the same range.

-- 
Vriendelijke groet,
Simon Brouwer.