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

[Harry Bissell]

Potting for thermal issues is certainly useful, however
I think in the case of ~most~ ARP designs there are other factors

1) ARP sold its modules as a sepatate product line to users or OEMs.

2) Potted modules tend to "hide" the circuit details, to prevent cloning.

Potting a module with "large thermal mass" guarantees that if you take the synth
in from a cold storage (car trunk in winter) to a warm stage, it will drift
for hours, albeit slowly.

If you want to reverse engineer a potted module bad enough, it can still be
done. I reverse engineered the Gentle Electric Pitch Follower module when I bought
a unit that was DOA.  If it had worked I probably would never have bothered.

Proof of #2 might be that later ARP VCOs (such as used on the "Little Brother"
were not potted.

H^) harry
----- Original Message -----
From: thx1138 <thx1138 at earthlink.net>
To: rob at emulatorarchive.com, SDIY List <synth-diy at dropmix.xs4all.nl>
Sent: Sun, 18 Apr 2010 10:55:30 -0400 (EDT)
Subject: Re: [sdiy] Potting ARP VCOs, was: VCO reference voltages

On 4/17/10 3:31 PM, "rob at emulatorarchive.com" <rob at emulatorarchive.com>
wrote:

> Hi,
> In the non-potted ARP 2600 VCO I found that the integrator capacitor and
> HFT resistor were sensitive to temperature, possibly also other
> components. Of course the transistor pair and tempco were...! The
> transistor pair being 2 transistors, at best glued together, is very
> sensitive, and not "buried" in a 8 pin DIL "matched transistor pair".
> 
> So I simply potted the complete VCO core, using the same layout as the
> 4027-1 but with 2x 5pin SIP plugs/sockets.
> Modern thermal potting compound is very good. A very simple test is to
> blow over your VCO and see if its frequency changes.
> With a potted module it does not and there is lots of thermal mass.
> 
> Whether its worth potting other modules is a mute point.
> For frequency accuracy like the E-mu Systems VCC  - yes.
> For VCF's I really don't think so.
> I do plan to build future Roland VCO's with a potted core.
> Just the matched pair and tempco.
> 
> Regards
> Rob
> www.amsynths.co.uk
> 
> 
> thx1138 wrote:
>>> 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?
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Hi Rob,

The reason we potted several of the VCF - UAF was to stabilize the Filter
trackin when we turned up the Q and it became an oscillator. We wanted it to
be able to track like an osc as well as possible.

Regards,

Terry

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-- 
Harry Bissell & Nora Abdullah 4eva