[sdiy] LM13700/LM13600-based filters

[David G. Dixon]

> Hmmm... i seem to remember some simulations that ~work~ and the real
> circuits
> won't. One more embassasing one was a dual clamp I designed using two
> opamps
> (741).  A second look made me think... 'what limits the current source
> from feeding
> unlimited current to the current sink ??'
> 
> answer, nothing. Standing current was about 250A ....   :^)
> 
> OTOH Spice says that my guitar pitch converter is unworkable. After two
> weeks I built
> it and it worked great !

Yeah, it ain't perfekt.  On the other hand, it will almost always catch
simple mistakes, like opamps wired up backwards!

Funny you should mention that thing about current.  I breadboarded a
keyboard scanning circuit I found on the web one time, and the gate signal
wasn't coming up to the required voltage.  Also, a couple of transistors
were getting uncomfortably warm.  Upon simulation with Multisim, I realized
that there was an unimpeded current path through the C-E junctions of the
two transistors, passing about 65mA directly from one rail to the other.  I
simply didn't see it when I breadboarded it, and neither did the person who
published the circuit on his website.  Simulation saved me from building a
faulty circuit.