[sdiy] LM3900 Sub ( more info then you'd ever wanted! )

[Tom Arnold]

Because I'm bored, I scrapped my 40k email mailbox for LM3900 emails.
TI is still making the LM3900
Jameco.com ( and I'm sure others ) still stock the LM3900.
NTE has a cross reference that is probably from TI's "maginally defective"
pile.

And here are the comment cream of the crop :
( I also need to note that LM3900's seem to come up about every 6 months
amusingly enough! )

From: "Steve Ridley" <spr at spridley.freeserve.co.uk>
Date: Sat, 2 Mar 2002 14:43:52 -0000

You could try the LM2900, or the MC3301 or MC3302.                    


From: Ian Fritz <ijfritz at earthlink.net>                         
Date: Sat, 02 Mar 2002 10:19:45 -0700                                  

An upgraded Norton amplifier is the LM359. It is much faster than the 3900,
which it is intended to replace. DigiKey has them. The downside is that
they are duals rather than quads, and they are a bit pricey. Has anyone
tried these for DIY?


From: "John Loffink" <jloffink at austin.rr.com>
Date: Sat, 2 Mar 2002 13:06:49 -0600                                        

Note that some of these equivalents have a more limited supply voltage
range, 5-18V, not 4-32V like the LM3900s.   Also, I bought some RCA
3401s from a discount supplier (All Electronics) and the matching of the
current mirror appears to exceed the specification, causing some
circuits to fail unless you tweak them.  One clue about these parts was
that RCA hasn't badged ICs in 10 or 15 years, having gone through a
series of buyouts by GE, Harris and Intersil. 


From: "Paul Schreiber" <synth1 at airmail.net>
Subject: [sdiy] LM3900 slamming
Date: Sun, 8 Feb 2004 12:40:48 -0600 

The *original* intent of the device was to control variable-delay windshield
wipers in cars.


From: Grant Richter <grichter at asapnet.net>
Date: Sun, 08 Feb 2004 13:20:09 -0600
Subject: [sdiy] LM3900

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.




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