[sdiy] SSM2164 last time buys

[David G. Dixon]

> I've had similar experiences with CoolAudio v2164 where it overheated a
> lot (and very quickly) when my negative supply was accidentally
> disconnected. BUt in my case, those chips are now worthless. Considering
> that some power supplies have been known to have one rail lock up on
> startup, this is a bad thing for anyone trying to use these chips in a
> module.
> 
> Is there a simple way to test for this "lack of negative supply" condition
> and disconnect the positive rail automatically before the chip becomes
> toast?

I can certainly attest to that.  I've never used SSM2164 (I'm too cheap),
but I've killed half a dozen V2164 with power supply hookup errors.

I've never had a V2164 fail in normal service, and they don't get warm with
normal use.

In a linearized 2164 situation, you should put a resistor on the output of
the linearizing opamp to limit the current to the VC pins of the two 2164
VCAs.  That opamp saturates to the positive rail (i.e., 13.5V in a +/-15V
system) when the VCA is turned off, and this voltage into the 5k input
impedance of the 2164 VC pins can generate 72mW of power dissipation (36mW
per VCA) or 144mW to the whole chip if it is a dual unit.  That could
increase the chip temperature by about 10C or 20F.  I use a 4.7k resistor on
the opamp output, which limits the voltage to about 4.7V, and decreases the
maximum power dissipation to about 18mW.  The chip will show no noticeable
warming under these conditions.