[sdiy] ARP VCO drift

[Ray Wilson]

My two cents. Try an integrated matched pair of transistors (something like 
LM394 or SSM2210). And as you have already done put the tempco in contact 
with the package. Worth a try after everything else you've done.

----- Original Message ----- 
From: "rob" <rob at emulatorarchive.com>
To: <synth-diy at dropmix.xs4all.nl>
Sent: Sunday, August 10, 2008 4:49 AM
Subject: [sdiy] ARP VCO drift


> Hi folks,
> I need some expert guidance and ideas, a rather boring subject but this 
> one
> has me stumped.
> I recently built a couple of ARP style VCO's (Odyssey/2600) with new
> components but the same design.
>
> The VCO waveshapes and frequency range are excellent, but the frequency
> drifts a lot more than I expected or have measured with a real ARP 2600.
> At 200Hz the ARP clone VCO varies by 1-2Hz, whilst my E-mu Systems VCO
> varies by less than 0.1Hz, as does the original ARP.
>
> I have tried - but with no impropvements:
>
> Different makes of 1K87 3500ppm Tempcos
> Adjusting the Tempco ppm with a series resistor
> Odyssey and ARP2600 front end designs before the expo pair
> 2N3906/3904 transistors
> BC550C/560C transistors
> Integrator capacitor is 1% polystyrene
> Comparator voltage is stable to at least 0.5mV
> Frequency adjustment voltage rails come from a voltage reference chip, at
> least better than 0.5mV
>
> The transistor pairs are not matched, but are in contact with the Tempco.
> There is some improvement by getting this contact.
> The original ARP 2600 VCO's are of course encapsulated....
>
> Where am I going wrong?! Any ideas?
>
> Regards and Many Thanks for any help...I really would like to get these
> VCO's to be usable.
> Rob
> www.emulatorarchive.com
>
>
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