[sdiy] VCO reference voltages

[David G. Dixon]

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.

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.  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.  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.  This is not necessarily a problem on the
upper end, but it could be on the lower end unless the VCO has an LFO range
switch.


> Voltage references can be very cheap and I like using them instead of the
> power rails when I can for critical VCO voltages (threshold for reset,
> expo
> current bias etc.).  In my VCO modules I use an LM4040, 39p for peace of
> mind and independence to the V+ power rail over a range of 12-15V.  You
> can
> use an LM4030 which is even better, but I found it not necessary.
> 
> The module; http://sites.google.com/site/hearnmorley/products/hmvco1b
> 
> 
> -----Original Message-----
> From: synth-diy-bounces at dropmix.xs4all.nl
> [mailto:synth-diy-bounces at dropmix.xs4all.nl] On Behalf Of Simon Brouwer
> Sent: 16 April 2010 11:31
> To: synth-diy at dropmix.xs4all.nl
> Subject: Re: [sdiy] VCO reference voltages
> 
> 
> Oren Leavitt schreef:
> >
> >
> > David G. Dixon wrote:
> >> Does anyone here have any strong views on deriving reference voltages
> >> from
> >> the power supply rails in VCOs for comparator or schmitt trigger
> >> thresholds,
> >> etc (as opposed to using precision references)?  I know that most "DIY"
> >> VCO
> >> designs do so.  I'm wondering whether most commercial VCOs also do so,
> >> and
> >> whether it is considered "kosher", even in more expensive VCOs.
> >>
> >
> >
> > Me-self:
> > I greatly prefer on-board voltage refs for the "critical" voltages - the
> > Schmitt trigger threshold, expo converter reference current,
> 
> Note that if these both are derived from a non-stable reference voltage
> the effect on frequency stability on the one will be compensated by that
> on the other. You will only get a small difference in amplitude.
> 
> > the "rails"
> > for any coarse/fine tuning controls,
> 
> A quick calculation: the voltage regulation of an LM7815 for output
> current variation between 250..750 mA is 50mV (max) which is 0.33% of 15V.
> Assuming 0V=middle of the audible VCO range then this corresponds to ca.
> 20 cents error at the top or bottom.
> 
> Using a 78L12 for local voltage reference could be a cheap option. Lightly
> loaded and supplied by a stabilized voltage, line and load regulation are
> hardly an issue; remains a ca. 0.008%/degC voltage drift.
> 
> --
> Vriendelijke groet,
> 
> Simon Brouwer
> 
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