[sdiy] problems going from breadbaord to pcb/solder

[David G. Dixon]

Two comments about this expo/VCLFO/breadboard issue:

1.  I've built lots of circuits on breadboards, and they have all worked
more or less exactly the same once committed to PCB.  I don't think this is
an issue.

2.  I haven't analyzed Ray's circuit in detail, but I must say I don't
really understand why the expo converter is configured that way, with one
3904 of the pair sinking current from the base of a 3906 which sources
current to the OTA.  One would typically just source current directly from
one 3906 of a pair, with no 3904s at all.  An excellent example of this
simpler, more straightforward plan, which I have used in several VCOs and
(altered slightly for higher source current) in a 4-pole VCLPF, is the expo
converter from Thomas Henry's VCO-1.  The schematic can be found on Scott
Stites's site:

http://mypeoplepc.com/members/scottnoanh/buildingthevco1/

In fact, this is an excellent circuit to analyze in order to completely
understand just exactly what an expo converter is doing, and why.  This is
an activity which I think we all must eventually undertake at some point in
our synth-diy careers.


> I've played with LM13700 based LFO cores some and have seen the same
> problems. Maybe the cases are the same ones that I've seen. I'm assuming
> there is an expo converter in there too. If not what is said applies to
> whatever is supplying the current to the OTA Iabc pin.
> 
> The overall problem sounds like the current that controls the OTA gain (ie
> freq) is going down to the point where it is shutting off the OTA
> completely
> or to the point there isn't enough signal or slew to trigger the
> comparator.
> Also when the current gets really low (ie low freq) the offset of the OTA
> becomes a factor and that effects the symmetry. Some people had symmetry
> problems when the op-amp they used in the ASM1 LFO had a large offset. So
> I'd check the expo converter out and make sure that it was working
> properly
> and all the right resistor values are in there. If there is a current
> limiting resistor in between the expo and Iabc pin check and make sure
> that
> is the correct value. For a LFO I wouldn't suspect there would be any
> issues
> signal feed through in the wiring effecting the circuit. Might also add an
> offset trim to the OTA integrator for more dynamic range if it is an expo
> controlled LFO.
> 
> Best of luck.
> Jay S.
> 
> > -----Original Message-----
> > From: synth-diy-bounces at dropmix.xs4all.nl
> > [mailto:synth-diy-bounces at dropmix.xs4all.nl]On Behalf Of Dan Snazelle
> > Sent: Saturday, August 15, 2009 3:36 PM
> > To: synth-diy at dropmix.xs4all.nl
> > Subject: [sdiy] problems going from breadbaord to pcb/solder
> >
> >
> >
> > a friend of mine has been working on a vclfo partially based on
> > the ray wilson VCLFO.
> >
> > it uses a triangle core with a lm13700, an integrator, and a
> > tl072 comprator
> >
> > it worked without ANY problem on the breadboard
> >
> > however, when he soldered it up on perfboard the following problems
> arose:
> >
> > 1. the symetry is off. mainly as the oscillator gets slower and
> > slower, the symmetry falls out of line. it is fine at higher speeds
> >
> > 2. at lower speeds, the oscillation dies. (at speeds which work
> > on the breadboard)
> >
> >
> > I told him to try changing the hysteresis resistor value and cap
> > on the comparator and this DID help somewhat (it kept the osc from
> dying)
> >
> > however, things are still not quite right.
> >
> > IS THIS due to the difference between the capacitance on a
> > breadboard and the capacictance on a soldered board?
> >
> > or could it be something else?
> >
> > anyone have any ideas on how to save this?
> >
> > i am very interested in the results as I will soon be moving some
> > of my projects from breadboard
> > to finished pcbs
> >
> >
> > thanks so much
> >
> >
> >
> > --------------------------------------------
> > check out various dan music at:
> >
> >  http://www.myspace.com/lossnyc
> >
> > (updated monthly)
> >
> > http://www.soundclick.com/lossnyc.htm
> >
> >
> >
> > http://www.indie911.com/dan-snazelle
> >
> > (or for techno) http://www.myspace.com/snazelle
> >
> > ALSO check out Dan synth/Fx projects:
> >
> > AUDIO ARK:
> >
> > www.youtube.com/watch?v=TJRpvaOcUic
> >
> > www.youtube.com/watch?v=BqIa_lXQNTA&feature=channel_page
> >
> > www.youtube.com/watch?v=V4nJPjGgOcU&feature=channel_page
> >
> > and soundtrack/design work:
> >
> > NEW: check out Dan's sound design from the 1998 award winning
> > film SAFARI by catherine chalmers
> >
> > http://www.catherinechalmers.com/videos.cfm
> >
> >
> >
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