[sdiy] On cloning, and the Oberheim VCO

[Aaron Lanterman]

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