[sdiy] More on Xpander modes was Re: HP from LP ?

Neil Johnson neil.johnson71 at gmail.com
Tue Aug 1 10:15:57 CEST 2017


"David Moylan" wrote:


This is due to the fact that achieving HP responses relies on nulling out
terms in the transfer function via mixing the different poles.  If you're
not exact you don't get a full null and the attenuation of low frequencies
suffers (looks more like a shelf instead of continuing roll off of low
frequencies and usually has a little dip in the middle).  I'm using 1%
tolerance resistors, but am about to sub in some 33.2k for 33k to get even
closer to the theoretically perfect values.

Even if you had perfect resistors I would imagine any mismatching between
the OTA stages would also cause the attenuation to suffer, but I haven't
modeled that.


The integrator capacitor tolerance will likely swamp any improvements in
resistor tolerance.  For example the 2164 is quoted as having worst gain
matching of 0.2dB which is just over 2%, yet capacitors will likely be 5% -
you can get C0G down to 1% if you pay for them. So 1% resistors are already
better than anything else in the signal path.

Neil
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