From ms20@aa.netWed Mar 22 10:16:26 1995 Date: Wed, 22 Mar 1995 06:29:27 -0800 (PST) From: Romeo Fahl To: analogue heaven Subject: VCOs: Was Re: uA726 query On Wed, 22 Mar 1995 MARSHALLR@opsusa.sms.siemens.com wrote: > >I agree, too. The CEM3340's built-in tempco mechanism is very good (though > >not perfect). The only way to achieve similar results would be to use > >those rare heated dual transistors (uA726??), or external tempco circuitry > >based on an analogue multiplier (=$$$$$). Or, perhaps you could do the > >tempco stuff in the digital domain with a DSP chip. Anyone take the > >challenge? > > At the risk of backtracking a bit, could someone explain how the uA726 was > used in a VCO, and what manufacturer/models it is found in? Also, what is the > availibility of this device now, and how much? How does it's implementation > compare with the 3340 and the CA3086/tempco designs w/ regard to temp > stability and linearity? Was it an V/oct or V/hz design? The 726 is a matched pair with a built in heater to keep the IC above ambient temperature. The chip is next to impossible to find now and is pretty spendy if you do. I don't know much about how stable, etc it is, but this was used in VCO's in older ROland gear, old Moog, Elektor Formant, Buchla and I'm sure there are more. Another method requires Q81 tempco resistors (basically a thermistor with a 3300ppm sensitivity). These resistors are put in a type of feedback loop in the exponential converter to stabilize it during temperature fluctuations. Back when the analog synth industry was booming these weren't too hard to come by, but now it's more of a challenge to find them. Another method is to use the CA3046 transistor array. This method was used in the Moog Prodigy and probably some other Moogs as well. RA Penfold uses a similar design in his Electronic Music Projects book (wish I could find my copy). I don't know how stable this design is, maybe someone who owns a prodigy can comment. Last, but not least is the CEM route. The 3340 is a fairly full featured VCO on a chip, very stable and compact. If you're worried about your synth sounding boring by using one of these for the VCO - stop worrying! Make the rest of your modular out of discrete components and let that add some funk to your sound. Also, OnChip makes the 3374 dual vco IC. This IC has been used in the Oberheim Xpander and the Chroma Polaris, plus I swear I saw them being used in the newer OberM006 or whatever it's called. One advantage of it still being used in industry is that there'll be a supply of them for some time to come. Hope that sheds some light on the VCO issue. R>