[sdiy] SNICster page up

[Ian Fritz]

Hi Scott et al. --

>I have a question about the schematic: Is R11 and R13 supplying linearizing
>diode bias to the LM13600?  I'm also curious about why 6.9 volts was
>chosen.  Is this necessary or can I use a higher voltage and larger resistors?

Right, R11 and R13 feed the linearizing diodes (Id), and I2 and I3 are the 
Iabc's.  I have generally used the LM329 super-zener as an inexpensive way 
to get a reference voltage, although 6.9V is a bit of an odd number.  Using 
a reference voltage is just a way to keep the design independent of supply 
voltage.  You can use some other reference or just run R11 and R13, scaled 
appropriately, to the positive rail.

Same thing for the 6.9V connections at the "Shape" and "Mix" 
controls.  Just increase R30 and R41 as needed.  For the R37, 38, 39 
network, the junction needs to be at about 4 V, so set the divider accordingly.

Oooo -- I just noticed that the -6.9V reference is not used.  Leave out R47 
and Z3!  (That was a cut-n-paste job.)

You could also do without the clamp on A2 and just let A2 self-limit.  This 
requires measuring the Vs of A2 and recalculating a bunch of components, so 
I'd recommend staying with the Z1 clamp.

>Geez, I can't wait for this stupid economy to right itself (so I can get a
>freakin' job).  I really want to build stuff again, and this is among the
>stuff...

Yes, very frustrating.  The economy looks like it is starting to move a 
bit, but jobs aren't picking up as fast as expected.  Hang in there, it 
won't last too much longer.

   Ian