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

[thx1138]

On 4/17/10 6:23 AM, "cheater cheater" <cheater00 at gmail.com> wrote:

> 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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Hi Damian and Rob,

There were several reasons for potting the Sub-modules we built at E-Mu.

1st was securing the design so people could not copy the module, but this
was not the main reason.

2nd was temperature stability. Our VCO's, VCF, Voltage Controlled Clock were
potted to keep thermal drift to a minimum. The wave shaping portions of the
VCO's were less of a problem.

I have had several people ask as to why the potting as it made for difficult
repair.

One major flaw in the design of the Modular system was the use of the DIP
cable power supply connection. If the cable connection  was inserted 180
degrees off it usually destroyed the chips on the Module on the board.

I would have used a more robust connector in the design but many of these
modules were already in design.

Dave may have been a good design person, but he lacked experience in
mechanical areas. Dave's brother, John built the wood cabinet's for E-Mu and
he did other odd jobs on occasion.

When we built the VCO submodule for example we put the Tempco, diode and
Matched transistors all together and epoxied them en masse for thermal
transfer connection.

Btw, Concentrated Nitric acid will remove the Epoxy but I would not
recommend it due to the hazard aspect. I took all of my dead modules apart
in the LAB at work and depotted them and repaired them. I don't like working
with Nictric acid much and hope that nothing fails in the near future.

Hope that story was of some interest at least.

Regards,

Terry