VCO7a

tomg vco at mindspring.com
Sun Jan 24 12:50:33 CET 1999


Over the weekend I was transfering drawings to the pc when I ran across an
old idea I had of merging the 2A and 3A vco designs. I though I might flesh
it out a little at first I used a 2N4391 but on a whim I plugged a 2N3819
in and the oscillator scaled! I couldn't beleave it so I tore it down and
built it  again. Again it worked. It's very odd for a 2N3819 to work.

I replaced the 2A exp-conv. with the 3A current source. 
Replaced the 2A's 1458s with TLO74 (goodby old op-amps!)
Changed a few resistors to make everything happy.
Replaced the 2N4391 with a 2N3819.  You can still use a 2N4391/92 if you
want.

Why is this a big deal? Well the 2N4391 cost $1.30 - $1.50 ( it's not that
hard to find, but it's a mail-order item in most places) and the 2N3819
cost about $.50 sometimes a lot less (It may be the most common fet on the
planet, you can buy 'em at rs). You can go to any radioshack and buy the
parts for this vco. The fet switched ramp wave is lovely to look at! Sounds
very warm and fat. It sounds like the 2A but cost is in the 3A range.

There is an extra op-amp in the design. I though I would change the descret
NPNs to a LM3046 and use it to build a chip heater. Maybe next time I  move
it.....

http://www.mindspring.com/~vco/misc/pics/vco7a_sd.gif

-tg
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