commercial synth circuits

[Debby and Gene Stopp]

How the heck did I miss the original posts? Ah well, new baby, new job, no
sleep, no time, the once beloved email list has become the occaisional
diversion for lurking...

So - "Terry Mikulic" now? Along with Ian Fritz? My gosh, who's next, Jan
Hall? Lovin' it. Where's Dave Rossum when you need him. Hey it's high school
all over.

As some of you archive-divers may know the KE4859 VCO made it to the top of
my test heap a couple years back, mainly because of its combination of range
and accuracy. Other deciding factors included the availability of the FET
still (a.k.a. the NTE 466), the fact that Harris still makes the CA3130/3140
(still as static sensitive as ever), the fact that the parts drawers at the
old job contained oodles of LM311's, and last but not least the fact that
2N3904's could be used in the expo converters. All this and not a lot of
parts! Thanks, Terry. Very nice, to-the-point VCO.

Close second still is the 3080-based tri-square VCO. Bernie's 3080
schmidt-trigger version from the PCC works great as well. No FETs to worry
about, just 3080's, but I can still get 'em and the current Harris parts
sure seem to act a lot better than the RCA crap from the 70's (many of which
still survive in my original modular somehow).

I guess I like the sawtooth-based VCO better philosophically - the thought
of piecing together the saw from an alternatingly-inverted tri plus
one-sixth the square just doesn't seem to me to be capable of being as
"pure" as a fast-reset ramp generator. I admit it, a nice 3-VCO sawtooth
minimoog lead solo still makes me wake up the neighbors. Call me biased.
Maybe someday I'll learn to appreciate the beauty of a perfect triangle, and
the tri-square will become my favorite. Come to think of it, a clean 50%
square wave in headphones can be inspiring...

Anyway, while I do appreciate the concept that perfect operational
parameters should override any parts-count considerations, the fact is that
the 3130-FET-LM311 VCO core is both accurate and simple, a tribute to the
designer. Interestingly, the Roland 700 modular has pretty much the same
design.

- Gene



-----Original Message-----
From: terry michaels <104065.2340 at compuserve.com>
To: synth <synth-diy at mailhost.bpa.nl>
Date: Sunday, August 09, 1998 4:47 PM
Subject: Re: commercial synth circuits


>
>
>-------------Forwarded Message-----------------
>
>From:   terry michaels,
>To:     INTERNET:ijfritz at earthlink.net, INTERNET:ijfritz at earthlink.net
>
>Date:   8/9/98  5:45 PM
>
>RE:     Re: commercial synth circuits
>
>Hi Ian
>
>I chose the KE4859 primarily because of its low on resistance. FET leakage
>is also important.   At the time I designed this circuit (October 1973),
>this part was the best choice I could find that was easily available.  It
>was sold by National Semiconductor, and I could buy National parts over the
>counter here in Milwaukee.  The combination of this part, the .0024 cap,
>and the CA3130 slew rate and settling time yields a sawtooth flyback time
>of less than 2 microseconds, as recently measured on modern test equipment.
> Since the flyback time is a source of frequency error at the higher ranges
>of the VCO, I sought to minimize it by using the KE4859.  I am currently
>redesigning all of my synth circuits, and I will get to this one soon.  I
>will look at currently available parts, and I will be happy to let you know
>what I find as a modern replacement.
>
>I do not prefer the resistance in series with the timing cap method of
>high-end compensation.  The steady state voltage drop across the resistor
>at high frequencies alters the end points of the sawtooth waveform. This in
>turn affects the purity of the other waveforms usually derived from the
>sawtooth.  However, it has the benefit of accomplishing high-end
>compensation very simply, with only one component. This comes down to
>personal design philosophy and goals.  Mine is generally to achieve the
>best performing, most precise circuits, with only a secondary consideration
>to the number of components needed.  I recognize the desire of others to
>create simple, inexpensive designs, and there is clearly a need out there
>for that design approach.  The great value of the DIY list is providing a
>forum for all types of designs
>to be shared, so everyone can find what they want.
>
>Using the Franco method doesn't alter the design considerations with
>regards to FET switching time.  The goal is the same - discharge the
>integrating cap as fast as possible.
>