uA726 replacement

[Paul R. Higgins]

I would be very surprised indeed if the "Tom G. style" heated 3046 chip wasn't 
as stable or even more stable than tempco resistor stabilized designs.  There's 
a discussion of exponential converter temperature dependence in Hal Chamberlin's
"Musical Applications of Microprocessors" (as well as many other sources, I'm 
sure) that gives an equation which I can't remember offhand.  He points out that
you can compensate for the temperature dependent factor in the equation by two 
means.  The first is to use a tempco resistor, which unfortunately is only an 
approximation (only valid at room temperature?).  The second approach is to 
actually heat the transistor(s) above ambient temperature.  However, to do this 
most effectively, you need all the expo converter transistors on the same 
substrate, so that they all track together.  Hence the popularity of the 3046 
and other transistor arrays.  By holding the temperature dependent factor 
constant, it effectively drops out of the equation.

The only drawback to the heated chip approach is that additional current is 
required for the heater transistor.  I don't consider this a big price to pay 
for the stability it affords.  

In fact, the much venerated CEM 3340 VCO chip used this heated substrate trick 
with great success.  Warm-up time was < 30 sec, much better than (dare I say it)
tubes.  In fact, Craig Anderton used the 3340 in his "Hyperflange and Chorus" 
design to get an exponential triangle LFO that was stable over a very wide 
frequency range (0.001 Hz to low audio frequencies).  It was surely a case of 
over-engineering, but it worked really well.


PRH


> I'll have to try this with a temperature stabilized 3046 vs the "hot" chip.
> I'm still not convinced that just making it "hot" will really make the tempco
> problem go away. I've still got my prototype... I'll measure the temperature
> of the regulated chip, adjust the "hot" chip to the same temp... and let them
> go head to head....  :^) Harry
> 
> (PS to Tomg... I haven't tried that regulated 3046 with the external tranny
> (prodigy was it???)  That looks good too.)
> 
> Harry (holding his vote for "the way" until I see all the data) Bissell

 
> tomg wrote:
> 
> > >
> > > The question is: Is there a LM3046/MAT-02 thermostating
> > > scheme "blessed" by this group, and can you get it
> > > to run using a single supply op-amp?
> >
> > My standard scheme of collector to +V, base to gnd, and emitter
> > thru resistors to -V can be modified.
> >
> > Collector to +V (say 12V) and emitter 470 ohms from gnd. Then take 2
> > resistors (say 10k) and split the supply( +6V ?).  Apply this to the base.
> > Heats right up.
> >
> > -tg


Paul Higgins
email: higg0008 at tc.umn.edu
University College, University of Minnesota