[sdiy] On cloning, and the Oberheim VCO

[mike ruberto]

Aaron,

This VCO isn't an easy project for a student IMO. I have worked on
making a clone of it and have had many problems. Some of which I still
can't solve.

Q2 and Q3 are 2N4302 devices which have some squirrly behaivior traits
and really bad matching between devices. I may be wrong about this but
I think Q4 is simply there to invert the wave.

Replacing this buffer stage with an opamp might be tricky because the
comparator stage after D1 is very sensitive to the pp voltage level
applied to it. This is also the reason for the 18.5V supply voltage.
However, the trigger threshold for the comparator can be changed with
R24 and R28.
It's probably much less work to just try a comparator chip after an
opamp buffer. But then you would be working with something more like
the ASM1 VCO than the SEM.

Q5 and Q6 are the comparator. R29 I'm not sure about. I originally
thought it was for resetting the comparator but I'm probably wrong
there.

Q7 works the way you described it.

I actually do not recommend upgrading any of the opamps. Nothing else
I've tried works properly for A4. Something particular about the 301
seems to be at play here. Also, I found oddly enough that modern 741s
do not work so well for A2 and A3. I tried some NOS 70s era 741s and
got much better performance in my clone circuit.

I'm still stuck on how to get more than 2 octaves of range out of my
clone. I got frustrated and shelved the project after not being able
to solve the problem.
I really should go back and rebuild it from scratch. I think I still
have two of the PCBs I made for it I can work with.

Maybe if I start over again after all this time the problem will be
more apparent...

Good luck if you do try it.

Mike





On Mon, Apr 14, 2008 at 4:08 PM, Aaron Lanterman
<lanterma at ece.gatech.edu> wrote:
> On Apr 14, 2008, at 2:48 PM, Tim Parkhurst wrote:
>
>
> > Tim (of course, a CLONE would be the ultimate DIY project...) Servo
> >
>
>  I know some manufacturers do build several prototypes at once. It's not
> clear that the customer actually ordered more than one, but sometimes some
> extras get thrown in. I spoke to one guy at the hospital whose wife had just
> had triplets - he had three little notes on his wrist. I also spoke with a
> woman who as having triplets who had to be in the hospital the entire last
> month of her pregnancy due to complications.
>
>  Being a new dad, I could easily do nothing but talk about diapers and
> breastfeeding and things like that all day. But since this is SDIY and not
> BDIY (baby do-it-your-self), I now force myself back to synths...
>
>  My students are starting work on their final projects. One of them is Van
> Halen obsessed, so I suggest he build the VCO from the Oberheim SEM. (Yes, I
> know Jump wasn't done on and SEM, and yes I know that custom ICs were used
> in the synth used on Jump, and yes I know that any sawtooth will do, and yes
> I know more modern VCO designs will be "better" - but any "hook" I can find
> to get the students interested works for me). One of the parameters of the
> project is that the students can't build something "as is" - they must make
> some kind of twist on the design. Depending on the particular project, can
> be as simple as updating the components (which sometimes turns out to be
> trickier than you'd thing!)
>
>  So I pointed him here:
> http://www.crowncity.net/ratcave/Electro/Oberheim/SEM-1A.pdf and told him to
> look
>  at VCO1.  I suggested updating the op amps with TL08xs or TL07xs, and using
> a CA3046 (which I happen to have a lot of on hand) instead of the CA3086.
>
>  Then I started looking for discrete transistors to update. Q2 and Q3 are
> just a straight dual FET voltage buffer, and Q4 is a PNP emitter follower. I
> recognize the dual FET voltage buffer as a good thing because of its high
> input impedance.
>
>  a) Why add the the BJT emitter follower buffer? Is it because it can
> provide more current that the dual FET voltage buffer?
>
>  b) Could we safely replace the FET buffer and BJT buffer with say a
> TL08x/TL07x buffer? If so... when would it NOT be safe to make such
> replacements? When would one really want to stick with the dual FET? (I do
> recognize that the dual FET would be faster than an op amp).
>
>  c) This is related to part (b)... the emitter follower (Q4) will have an
> output that will ride one "diode drop" above the input. If we replaced the
> buffers with op amps, would we need to tweak any of the rest of the circuit
> to compensate for the fact that the 0.7-ish V difference is no longer there?
>
>  d) This is related to (c) - any insights on what D1, Q5, Q6, and Q7 are
> doing? Clearly they must be doing some combination of acting as a comparator
> and charging up the 1000 pf integrating cap when the comparator trips.
>
>  e) Q9 and Q8 look like output and input buffers, respective. Are they ripe
> for replacement with op-amps?
>
>  f) Question for the old-timers - why so many discrete transistors? Is it
> that the op amps were expensive at the time? Op amps seem so much easier to
> use (although they have their dangers).
>
>  My guess is that Q7 is acting as some sort of switch, shorting the top of
> the cap to +15 volts (hence "charging up") the cap when the sawtooth needs
> reset. The rest of the time Q7 is switched "off" and the expo converter
> drains the charge from the cap. Does that sound like a good guess?
>
>  If so, I'm still puzzled on Q5 and Q6... but I must admit, I suck at
> discrete transistors. I just recognize patterns I've seen in The Art of
> Electronics. ;)
>
>  Maybe R29 is providing some "positive feedback" to make the comparator trip
> faster?
>
>  I'm asking these questions since (a) it will provide debugging insights
> when we get to that stage and (b) I want to make sure I don't lead my
> student down the wrong path.
>
>  - Aaron, really a DSP/radar/image processing guy who got into analog synths
> because they looked like fun
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