Change buffer size from 128 to 150 to further reduce effect of mains hum

[kevinjwalters]

Many of the samples are using a 128 sample buffer at 500sps. This does not equate to an exact number of mains electricity cycles. A small alteration to 150 equates to 15 cycles at 60Hz or 18 cycles at 50Hz. Empirically this reduces oscillations from the IIR filtering.