[sdiy] On cloning, and the Oberheim VCO

[mark verbos]

Aaron,

I'd suggest you look at the Kobol oscillator on my page

http://www.simple-answer.com/DIY.html

It's basically the same one that's in an SEM, but with OPamps instead  
of those transistors for that buffer. Sadly, you'll notice that it  
uses wider supply rails than +/-15, like the SEM. They also used a  
3086 for the NPNs in the core.

Incidentally, Q4 does not invert the signal. Q5 and Q6 are a  
comparator, R29 is adding positive feedback for hysteresis, Q8 and Q9  
are switching not buffering.

My guess is that they used transistors because they are small and  
cheap. It's probably a fatter, driftier sounding oscillator for it.  
RSF used SSM chips in every part of the Kobol except the oscillator,  
it's a GREAT sounding box.

Mark




On Apr 14, 2008, at 4:49 PM, mike ruberto wrote:
> 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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