[sdiy] Coolaudio VCAs: V2162 noisier than V2164?

[Neil Johnson]

Mattias,

> Those considerations should be cool, but one thing that I've *not*
> done like the datasheet is the input resistor of 20k. Since I used a
> soundcard directly as the test tone generator, with a signal of just a
> couple of volts, I put 2k5 as input resistance and recalculated the
> voltage values to give me the same input currents as in the datasheet.

Ummm, no, don't do that.  I'm pretty sure the THAT guys chose the
value of 20k carefully.

> Interestingly, that page says
> "For best noise performance with the 2162, we recommend 20 kΩ for the
> input resistor (Rin) and for the output (Rout) resistor in the
> current-to-voltage converter."
> This could either mean just that you should avoid higher resistances
> as they give a lower signal current and worse noise due to lower
> dynamic range, *or* they could mean that the input resistor actually
> affects the behaviour of the VCA. I'm not sure! Are you? :-)

Yes it can.  For example they might have current injection systems to
minimise noise optimised for a 20k input resistor.

Remember: data sheets are also a marketing tool, so they are going to
specify things to give the best possible performance.

Neil
--
http://www.njohnson.co.uk