VCOs overview and sanity check

gstopp at gstopp at
Fri Aug 18 00:37:35 CEST 1995

     Hey all,
     Warning! - another long post!
     All these posts I've been doing about making various VCOs in the lab 
     have been pretty much nuts 'n bolts. I keep wanting to add comments 
     along the way on just what the heck it is that I'm after (aside from 
     the sheer fun of manipulating electrons in musical directions), but I 
     can never seem to switch tracks in mid-email. So here's some thoughts 
     about direction:
     Number one: come up with good VCO design suggestions for all the people 
     on AH and Synth-DIY who have been looking to build a quality system 
     themselves. Number two: I want more VCOs for my own use. Please 
     understand that I cannot claim that these are the best possible designs 
     but they're the best I know about, so keep those alternate ideas coming!
     Assuming that the source list for CEM3340's has totally dried up and those 
     parts are few and far between, the best option left is to come up with a 
     discrete design made from the least amount of those pesky non-Radio Shack 
     components. Well, sorry to say, you may have noticed that recently Radio 
     Shack has trimmed dowm their offering so that they are now more detestable 
     than ever. Forget that route. That leaves three sources for parts:
     Source 1 - The local "serious" electronics stores:
     This is the easiest. If we can come up with a design that uses parts that 
     you can get by a single visit to ITC or Electronic City or whatever you 
     have, then that would be perfect. Kinda pricey though.
     Source 2 - Mail-order distributors:
     For those parts that you didn't get during your single visit to the local 
     store, you can probably go to the mail-order guys. This includes things 
     that the local stores don't carry at all as well as things they were just 
     plain out of stock on.
     Source 3 - The big boys themselves - the industry distributors:
     For the real exotic parts you will have to go visit the local industry 
     distributors. Call them first - they may or may not cater to "the man on 
     the street". If they do then you will probably have to satisfy a minimum 
     order dollar amount and pick your stuff up in person at the will-call desk.
     Okay now let's relate those to the VCO designs I've been working on:
     National App Note #299 VCO:
     Parts: CA3046, LF412, 2N2907, 2N2222A, LM329
     The parts for this circuit are all Source 1 parts except for the LF412 and 
     the LM329 which are Source 2 parts. The main advantage to this design is 
     that there are no Source 3 parts. However I personally don't like this 
     design because:
     1. I don't like the way it performs. I want wider range and better
     2. The parts are not arranged in a familiar configuration so it's harder
        for me to figure out what values to change to cause some desired 
        effect. This is more a shortcoming of my EE abilities but hey, I still 
        don't like it.
     Like I said these are my hunches and I still haven't tried the LF412 and 
     who knows, that may alleviate my performance gripe.
     Electronotes sawtooth VCO:
     Parts: TL082, CA3046, CA3130, LM311, 2N4856, tempco resistor
     The parts for this one are all Source 1 parts except for the tempco 
     resistor which is a Source 3 part. However this part may be left off or 
     socketed for eventual installation (see note below). The CA3130 can be 
     replaced with the TL082 which will limit the low end (the local stores has 
     3130's but they were out of stock). An AD818 can be used instead of the 
     CA3046 which will get rid of the high-frequency track trimmer, but it is a 
     Source 3 part. The performance is very good and the parts count is low. I 
     would choose this circuit for a saw/pulse VCO and leave off the other 
     waveforms because the tri-square VCO is better for those. Of course 
     additional waveshapers can be added to make this a complete full-featured 
     VCO if desired. A triangle wave can be produced from the sawtooth by 
     full-wave rectification of a level-shifted sawtooth. A sine wave is easily 
     derived from the triangle. A pulse wave can be derived from a comparator, 
     with a summer for PWM. A square wave is available at the pulse output when 
     you're not PWM'ing, or you can add still another comparator for a dedicated 
     square. Linear FM and sync inputs are available. 
     Electronotes tri-square VCO:
     Parts: TL082, LM748, CA3080, MPF102, MAT-03 or AD821
     The TL082, 748, and MPF102 FET are Source 1 parts. The CA3080 is a Source 
     2 part. The MAT-03/AD821 is a Source 3 part (and is rather expensive too 
     at $8.50 each). The performance of this VCO is excellent, probably thanks 
     to the expensive matched PNP pair. Two waveforms (triangle and square) are 
     available right from the get-go, however they need buffering before they 
     can go to output jacks in modular systems but that's easy. A sawtooth wave 
     is made by combining the triangle and square in a circuit which is where 
     the FET is used. A pulse wave can be made from the triangle, with a 
     comparator and PWM summer. A sine wave can be made from the triangle. 
     Linear FM and sync inputs are available. The sawtooth generation method 
     lends itself to Symmetrized Ramp Modultion (voltage controlled sawtooth 
     shape) which is not present in any commercial synth (far as I know) but 
     sounds to me like PWM based on my trimpot-tweaking.
     The as-yet-still-unbuilt Rhodes Chroma 4151 Charge-Pump VCO:
     Parts: 4558, TL082, CA3046, XR4151, tempco reisitor
     Thanks to you guys who have brought this to my attention!
     The 4558, TL082, and CA3046 are Source 1 parts (the 4558 can probably be 
     replaced by another TL082). The XR4151 I found in Digi-Key as the 
     NJM4151D-ND for $1.23 in singles so it's a Source 2 part. The tempco 
     resistor is a Source 3 part. This VCO design has promise for three 
     1. It's actually used in a commercial synth
     2. The schematic is very similar to the EN sawtooth design, which works
     3. The parts count is the lowest yet
     Looking forward to trying this one.
     About Source 3 parts:
     Tempco resistor - okay guys there is no way around getting these from a 
     Source 3 supplier. However as posted earlier I found that you don't really 
     need this in a controlled environment studio. If you are building a 
     machine to tour with you'd better use them, though.
     MAT-03/AD821 - expensive but worth it if you want Emu/Moog modular 
     quality stuff out of your workbench.
     Idea - we can do a group buy! Any thoughts? Who wants how many of what 
     Hey I finally found a sig -
     \                                                      \ 
     \  "While it is possible to still build analog music   \ 
     \   synthesizers, we can not recommend that anyone do  \ 
     \   it, except for research and educational purposes." \ 
     \                                                      \ 
     \                       - Bernie Hutchins,             \ 
     \                         1995 Electronotes Catalog    \ 
     \                                                      \ 

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