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

[rob at emulatorarchive.com]

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?