From: gstopp@fibermux.com Date: Wed, 08 Nov 95 09:17:56 PDT Encoding: 85 Text Subject: Re: Has anyone used ICL8038 chip? X-Pmflags: 35651712 Using the 8038 as an electronic music VCO was discussed in Electronotes quite a long time ago (early '70s) and I'm turned to that page as I write this. Apparently the problem with starting out with a linear VCO and trying to "exponentialize" it is that there are too many circuit sections required that are subject to error. These are: 1. Temperature in the exponential junction 2. Typical exponential errors in the converter semiconductors 3. Errors in the current-to-voltage converter 4. Errors in the positive supply reference difference amplifier (the 8038 CV is referenced to V+) 5. Errors in the 8038's linearity 6. Errors in the power supply that affect item #4 above These are the type of errors that exist in any design; however, in this case there are too many in a row and the cumulative effect is that the resulting VCO will not be accurate or reliable enough to have "professional" performance specifications. In other words, if you reached out to play the keyboard at any given moment you couldn't be certain that exactly the right note would play. But it is mentioned that for non-critical applications it would probably work fine, like as an LFO or for special effect sounds. In typical Electronotes fashion there follows a "cheater" circuit for the 8038 which bypasses the current-to-voltage problem altogether and injects exponential current directly into what would normally be pins for timing resistors, making the VCO exponential with a 30,000:1 voltage/frequency sweep. The outputs are Sine, Triangle, Sawtooth, Square, and Pulse (with PWM). This circuit uses an NPN matched transistor pair, followed by a PNP matched transistor pair, along with 5 op-amps and a FET. While this sounds attractive, if the performance of this VCO is expected to be as good as possible, then the two matched transistor pairs will add up to about twenty bucks. This combined with the cost of the 8038 itself makes this design less attractive than say one of the later Electronotes designs which use only one matched pair or even a 3046 transistor array, plus a few op-amps, with performance specs as good as anything ever available commercially (and better than most). I ordered some 8038 samples recently to play with, so that will be my first experience with these. I suspect they will end up as cheap and easy VCLFO's. BTW maybe we should continue this thread on the synth-diy list.... but for a first-poster I think you picked the right place! - Gene gstopp@fibermux.com Subject: Has anyone used ICL8038 chip? Author: K.Dunnicliffe@uk22p.bull.co.uk at ccrelayout Date: 11/7/95 12:54 PM Hi all -- first posting! I'm currently scratch-building a VCO module, and messing about with a number of different VCO types - mainly using discrete op-amps and transistors. My *current* prototype is based on an ICL8038 chip (Harris). Data is available at http://www.semi.harris.com/datasheets/lin-tel/icl8038/ It has linear sweep voltage with a 1000-1 range and *seems* to generate good quality square/pulse, triangle (and pseuodo-sine) waves. Looks like PWM should be straightforward too. I'll probably put a moog-style log CV generator with tempco on the front end. Has anyone else tried DIY with this chip? Any hints, tips or warnings? Like to share schematics ?? (I'm currently just following the data sheet examples). Does anyone know of any commercial synth using this chip? My rate of building is *very* slow so I won't get to hear this thing through a VCF/VCA for a while yet. Happy burbling, Kevin.