VCOs overview and sanity check
gstopp at fibermux.com
gstopp at fibermux.com
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
tracking.
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
reasons:
1. It's actually used in a commercial synth
2. The schematic is very similar to the EN sawtooth design, which works
well
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
part?
TTFN,
Gene
Hey I finally found a sig -
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\ \
\ "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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