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

Sat Apr 17 21:47:25 CEST 2010

Gotcha. Thanks!

On Sat, Apr 17, 2010 at 19:53, thx1138 <thx1138 at earthlink.net> wrote:
> On 4/17/10 8:55 AM, "cheater cheater" <cheater00 at gmail.com> wrote:
>
>> 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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>>>>> Synth-diy at dropmix.xs4all.nl
>>>>> http://dropmix.xs4all.nl/mailman/listinfo/synth-diy
>>>>>
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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
>>>
>>>
> Hi Damian,
>
> Actually no, as direct thermal coupling and a plastic box work better.
>
> Terry
>
>