[sdiy] ARP VCO drift

[mark verbos]

uh, I think the ARP exponential converter uses one NPN and one PNP.

Mark



On Aug 10, 2008, at 8:11 AM, Ray Wilson wrote:

> 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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>
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