Fun with appnotes: AN-72 (LM3900 quad Norton amp)

[earthtonesaudio]

A nice result from the "paint by numbers" approach, using only ideas from an appnote:

http://www.national.com/an/AN/AN-72.pdf

Built these in series:
Figure 13 (input buffer with gain)
Figure 35 (low pass)
Figure 66 (comparator)
Figure 80 (frequency doubling tachometer)
Output is an octave-up square wave.  Tracks well, pretty good octave up sound.  Not as obvious as some, but it's there.  Choosing the capacitors and resistors in the tachometer section have a lot of influence on the octave sound.

Cool feature: uses only one diode yet still full-wave rectifies. 

[cpm]

(bump)

nice!
would it be interesting to use as a fundamental extractor?

[earthtonesaudio]

Maybe.  I nearly forgot all about this little experiment.  For a fundamental extractor, there's no particular reason to use this oddball op-amp over other, more common types.

[soggybag]

I have a bunch of these and LM2900 in the pile of chips. I should try this out.

[earthtonesaudio]

They're pretty neat little chips.  To avoid burning chips, always keep in mind that they're not like general purpose op-amps, and can be damaged if you treat their inputs improperly.