HPF-Pre Series 1 and 2 Technical Manual
Francis Deck, Mar. 15, 2012
HPF Technology, LLC
Overview
HPF-Pre is a preamp with adjustable high pass filter and phase switch for use with piezoelectric pickups on
the double bass. It is intended to address the primary issues faced by bassists when playing through
mainstream amps and PA systems.
Foremost, HPF-Pre is a preamp. Its high input impedance ensures proper buffering of the signal from piezo
pickups. It is battery powered and housed in a tough plastic enclosure with internal shielding.
An adjustable high pass filter tunes out the subharmonic “thump” generated by the plucked bass, which
otherwise can drive ported bass speakers to over-excursion. It is also a useful tool for coping with adverse
acoustical environments and PA systems with excessive subwoofer support.
A phase (polarity reversal) switch is provided to help control feedback problems. It could also help when
setting up systems with multiple pickups, or a pickup and a microphone.
Series 2
of the preamp adds an output volume control and an LED battery status indicator. The volume
control is handy when your stage setup requires your amp to be out of arm's reach.
HPF-Pre is an "open" design. I sell the HPF-Pre at a modest price from my part-time home business.
Otherwise, you are welcome to build your own.
Specifications
Here are some preliminary specifications for the HPF-Pre. This report goes into considerable detail on how
these parameters are measured. The results are from initial builds that I expect to be representative.
•
Input resistance / capacitance = 10 MegOhms / 220 pF
•
Overall gain = +2 dB
•
High pass filter corner = Adjustable from 35 to 140 Hz
•
High pass filter slope = 12 dB/octave with minimal “hump”
•
Phase switch = Applies phase inversion to signal
•
Connectors =1/4 inch input and output, power switched by output jack
•
Distortion = 0.1% THD at 1 V p-p input
•
Noise = -107 dB referred to 1 V p-p input
•
Input level = Up to 4 V p-p with tolerable distortion
•
Power requirement = Standard 9 V battery
•
Design topology = Solid state Class-A circuit
