[sdiy] Mysterious transistor on one of the ASM-1 VCO versions

[Pete Swarbrick]

Aaron et al,

Isn't that (NPN!) transistor just the open collector o/p stage of the
311 drawn separately? It's not annotated with a part no. and since there
are no pin numbers either this seems the most likely answer. I don't see
any other explanation but can readily see why it is confusing !
Incidentally, I have just built three 3rd generation Minimoog VCOs for(
yet another) mini clone project requested by a friend. Although the
parts count is higher they are really sweet to build, align and listen
to! Tony Allgood's Oakley Sound 'One-of Three' VCO used the same core
with some updated components and additional waveshaping/buffering; a
great update of an old design.

Just my twopennorth
Best
Pete
PETER SWARBRICK
Head of Digital Imaging
 
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-----Original Message-----
From: synth-diy-bounces at dropmix.xs4all.nl
[mailto:synth-diy-bounces at dropmix.xs4all.nl] On Behalf Of Aaron
Lanterman
Sent: 02 May 2008 08:52
To: SDIY DIY
Cc: Greg Hartl
Subject: [sdiy] Mysterious transistor on one of the ASM-1 VCO versions

OK, so I'm throwing yet a third oscillator at my student Greg. The  
first two I threw at him were datasheet ones. I know people got the  
AN-299 working, but Greg was getting frustrated, as was I, so I  
figured I'd throw at him the ASM-1 VCO, which is one of the most  
proven, often built, and best understood designs out there.

There are two versions of the ASM-1:

1996: http://home.swipnet.se/cfmd/synths/friends/stopp/asm1vco.new.pdf

2003: http://home.swipnet.se/cfmd/synths/friends/stopp/asm1vco-1.1.pdf

The main difference between the two is an NPN transistor at the output  
of the LM311, which appears in the 2003 version but not the 1996  
version.

I reasonably understand the 1996 version - I even did a lecture on it,  
or actually the version that appears in Hal Chamberlin's book.

But this 2003 version with the PNP bewilders me. Alas, that's the one  
I happened to send the link to Greg to build, and the PNP has been  
vexing us today.

He built the oscillator, but it appears the gate is stuck at around 0  
V, so the JFET is conducting, and as we change the CV, we get a DC  
change at the triangle output, but no actual oscillation. The level at  
the + input of the comparitor is less than the 5 V threshold, so the  
output of the comparator is at -15 V; my understanding of BJTs is that  
the transistor at the output is then "off", so no current is flowing  
through the collector, and the gate is more or less hooked to ground.  
And then I'm confused what would happen if the LM311 turns "on" - as  
an open collector output, it should disconnect the base, and then I'm  
not sure what the BJT would do - would it set the gate to the -15  
rail? And if so... isn't that BJT doing the opposite of what we want?  
Like it's almost acting as a logic inverter on the LM311 output.

I get more confused reading the ASM-1 webpage:

"When output of the CA3140 goes above Vref, the comparator LM311 will  
pull its output transistor collector (pin 7) to -15 V since its  
emitter (pin 1) is hooked to
-15V. This will bias the JFET (Q3) into conducting range and reset the  
capacitor C2 through the Rds(ON) of the JFET."

I though setting the gate to -15 V should pinch the JFET "off," not  
put it into conducting range... this is suggested by p. 190 of  
chamberlin, which says: "as ong as the integrator output is less than  
Vref, the comparitor output is negative, which keeps the FET switch  
across the integratind compacitor off, allowing it to charge. As soon  
as the integrator voltage reaches Vref... The comparator output is  
constrained to rise no further than ground but this is enough to fully  
turn on the high-thrshold-voltage FET switch."

So the ASM-1 webpage seems to say that -15 V at the gate does the  
reset, and the Chamberlin book seems to say that 0 V at the gate does  
the reset.

*confused Aaron is confused*

- Aaron
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