[sdiy] LM3900

[Grant Richter]

The LM3900 is not the first choice for audio, because the slew rate is no
better than a 741. Unless you are using it for a waveshaper where the slow
slew is good for smoothing diode transients or other glitches with high
frequency content.

There's four useful circuit topologies that come to mind that, as far as I
know, can only be done with a CDA.

The first is a full wave rectifier that uses only a single op-amp stage.
Shown in the ap-notes as a frequency doubling tachometer, it forms the basis
for the Serge Waveshaper and NTO waveshaper.

The second is a current controlled integrator. Current into the negative
terminal makes it ramp down, current into the positive terminal makes it
ramp up. With no control current and proper trimming, it holds the output
voltage, ramp and hold. Hit either input with a fixed width pulse, and the
output "steps" up or down. Works great for a linear envelope generator. The
step generator makes a useable voltage controlled frequency divider.

The third is minimizing parts count for voltage comparators. They eliminate
several resistors, while providing a robust interface. Bernie Hutchins used
them as input comparators for the Electronotes ADSRs. One thing he didn't
explore was the use of a single LM3900 for latching the trigger pulse and
acting as it's own voltage peak comparator for reseting the latch. Works
good.

And if you want to make an analog neuron, they are essential:
http://www.musicsynthesizer.com/Neurons/Neurons1.html

It's an amazingly versatile chip for those who genuinely enjoy the challenge
of getting the absolute most from the fewest number of parts.

Of course, the advantage to low parts count is low noise, as every active
device adds some noise to a circuit.

>> Imagine
>> my surprise to hear Bob Pease slam the chip as a piece of sh!t (my original
>> opinion
>> btw).
>> 
>> I guess the principle and practice of the chip do not match well... and it is
>> indeed a
>> crappy
>> "opamp" if you call it that.  The ARP filter is really brilliant, imho.
>> 
>> There is a "Dual Norton"  (LM359 iirc) which I have not tried, Bob Pease did
>> not have
>> an opinion on that one. Its not real cheap like the 3900.
>>