[sdiy] 2164 Expo correction

[ASSI]

On Friday 25 March 2011, David G. Dixon wrote:
> For the particular 2164 chip I measured, the exponential
> response (ignoring any delay effect of the comparator reset) was roughly
> 180mV per octave at the bottom of the audio range (at about VC =
> 1470mV), and increased to roughly 190mV per octave at the top of the
> range (at about VC = -360mV).  This is in contrast to the constant 200mV
> per octave one would expect from the stated 33mV/dB of the datasheet. 
> The fact that the gain is not constant is the direct manifestation of
> the poor log conformance of 2164.

Well, this more or less exactly is the gain constant that I would expect if 
the input divider really was the 9:1 that the datasheet schematic shows 
(I've been posting about this discrepancy before).  Are you talking about an 
SSM2164 or a V2164?  If the latter, Coolaudio might have fallen into a 
copycat trap there (unless the gain constant in the SSM datasheet has been 
wrong all that time, which is possible, but hard to believe).
 
> Anyway, based on the numbers I measured, I found that I could model the
> exponential response (which should be linear on a log-linear plot) more
> or less perfectly with a quadratic function.

You should also find that the residual error after that correction is a 
cubic... :-)

Without seeing your numbers and doing some calculations I can't be sure, but 
I think the effect you see might be consistent with a loading of the input 
resistors by the base currents.  Only for unity gain this effect is 
compensated by the replication of the input network to tie the inner bases 
of the input pairs to reference ground.  From your two datapoints the base 
current should change by ~0.2µA (that's for two transistors) over the range 
you measured, which is at least a plausible number (depending on what your 
input current is, of course).  I've not done any further checks...

> Has anyone else gotten up to similar mischief?

Not really, but it looks I should really get a handful of 2164.  Smallbear 
still carries the V2164, right?

What you could also do is make a better external expo and use something very 
much like the Mike Irwin linearization to servo one fourth of an 2164 to 
that and use the control voltage to feed the other gain cell(s).  That gets 
you back to square on on expo temperature compensation, though.


Achim.
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