[sdiy] Potting ARP VCOs, was: VCO reference voltages

[cheater cheater]

On Sat, Apr 17, 2010 at 12:10, rob at emulatorarchive.com
<rob at emulatorarchive.com> wrote:
> Hi
> The implementation of precision voltage regulation for the "critical"
> on-board voltages of a VCO, as per Tony's recommendations, does make a
> worthwhile improvement to VCO stability. I have reworked a number of legacy
> designs from ARP, E-mu Systems and Roland with 10V and 5V precision
> regulation - with positive and measurable outcomes. I have also experimented
> with potting the VCO core and found this to have advantages as well,
> especiallty for the ARP design.

Hi Rob! Very interesting. I have read your page on the ARP VCO - very,
very cool! You mention that "the VCO core has a number of temperature
sensitive components as well as the transistor pair; the integerator
capacitor and a few resistors". What were those resistors? Why did
they need to be potted with the transistors?

> In terms of reduced voltage swing for Frequency, it is easy anough to
> compensate for this in the frequency voltage summer. Or in the case of the
> E-mu Systems VCO, who needs 60kHz anyway...I also like dedicated LFO designs
> where the waveform is more accurate at low frequencies.
>
> There are a number of re-worked VCO design on my new web site at
> www.amsynths.co.uk
> I plan to load the Eagle CAD files  so anyone can download the schematics
> and boards and make them.

Wow, great! Please do so - I'm sure I wouldn't be the only person here
anticipating this!

I've had a longer look at your site, it's full of interesting info!

One wish I also have with your articles on the ARP stuff is if you
could comment whether or not certain moduels need to be potted. For
example I noticed your clone of the moog style 2600 LPF is not potted,
whereas the original module is potted - do you think this was
necessary at all? You generally don't see moog filters getting potted,
but maybe the ARP adaptation had a special design that needed this?

I really enjoyed the write-ups on the history of filters and generally
all sorts of modules.

Really cool E-mu front panels btw :-)

Cheers,
D.

> I usually use REF01 type voltage regulators with line regulation of 0.006%/V
> Line regulation on a 78L12 is 18mV.
>
> Regards
> Rob
> www.amsynths.co.uk
>
> David G. Dixon wrote:
>>
>> 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.
>
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