[sdiy] Microphonic PCB

[Richie Burnett]

> I had a cap from the middle of the resistive divider to ground
> and now that I've removed that the microphonics are gone.

Any capacitor with a DC bias across it is a candidate for microphony.  High K
multi-layer ceramic SMDs are notorious.  The explanation comes from the formula
Q=CV

In supply decoupling or DC blocking applications you have a constant V across
the capacitor.  Mechanical shock to the PCB varies the seperation of the
capacitor plates slightly and so varies C.  This change in capacitance causes
charge to flow in or out of the capacitor in order for the voltage to stay
constant.  If there is enough amplification in the stages that follow you will
see the result of this displacement current as microphony.

-Richie,