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

Sat Apr 17 17:55:10 CEST 2010

Terry,
have you tried using something else for thermal coupling? For example,
putting the stuff in a fairly hefty metal box, for example - wouldn't
that work equally well?

Cheers
D.

On Sat, Apr 17, 2010 at 16:05, thx1138 <thx1138 at earthlink.net> wrote:
> 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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>>>
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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
>
>