single-supply filter questions - pot crackle

[Eddododo]

For a variable-freq sallen-key Low-pass filter.. when using Op-amps with a single supply and a Vcc/2 reference; where would one insert the Vbias? since the resistors would be pots for a variable filter, wouldnt having the DC from the Vbias on the pots cause crackle? Could I just insert it AFTER the filter resistor/pots?
http://www.schematica.com/active_filter_resources/images/sallen-key_lp_single_supply_capacitor_couple.jpg
 

For an Op-amp baxandall http://www.albertkreuzer.com/pics/el/pre/schem/preamp_sch21.gif
would converting this to single supply cause the same issue of bias-voltage causing pot crackle?

[PRR]

> DC from the Vbias on the pots cause crackle?

Crackle happens when there are *different* DC voltages on the pot.

Typical is an emitter resistor with +1V at one end and zero at the other end, and wiper rubbing all voltages between.

In the plan you link, both ends (of both pots) are the *same* DC voltage. (This assumes that opamp input current is small; TL072 are a blessing.)

> insert it AFTER the filter resistor/pots?

Loads-down the filter, makes the action wonky.

I suppose it could work with 50K pots and 10Meg-22Meg bias resistor (which a TL072 will tolerate fine). But the way shown is fine and un-complicated.

> For an Op-amp baxandall

Cap-couple the in and out, wire the "-" input to Vbias.