Site updated Oct. 14, 2010
Full design and test documentation
Web Order Form
HPF-Pre is a simple preamp for piezo upright bass pickups, with high input impedance, an adjustable high-pass filter, and a phase switch. It is designed to solve the most important problems that arise when plugging a piezo pickup into a mainstream bass amp or PA system.
HPF-Pre now comes in two versions. Series 1 is the original preamp, as shown in the picture above. Series 2 is the same basic audio circuit, with addition of a volume control and a LED battery indicator. The LED gives a brief “flash” when the circuit is turned on by plugging into the output jack, if the battery is good.
Features and Specifications
Input impedance / capacitance: 10 Megohms / 220 pF
Active high pass filter with 12 dB/octave slope
Filter cutoff adjustable from 35 to 140 Hz
Phase (polarity reversal) switch
Series 2: Output volume control
Input level: Up to 1 V p-p with musically forgiving overload response
Runs on single 9-V battery
Series 2: Battery indicator LED
The wide dynamic range of the preamp allows it to be used in an effects loop, if you are happy with an existing preamp at the front end of your system. The preamp exhibits a musically forgiving "soft clipping" effect that makes it reasonably tolerant of overload signals.
HPF-Pre is an "open" design, with more specs and a detailed technical report available online. You are welcome to build your own.
HPF-Pre is available from my part-time home business to customers in the United States. I will maintain a modest stock, and orders received during the week will typically be packaged over the next weekend.
Please visit my Web Order Form to order.
Unfortunately, the HPF-Pre is not available outside the US.
The HPF-Pre concept emerged from discussions on the TalkBass Forum. I am indebted to the collective intelligence of TB members who have provided me with valuable suggestions and feedback. The following thread contains user comments about the HPF-Pre device:
TalkBass thread
An article by Dan Berkowitz for Premier Guitar Magazine provides a good explanation of the need for a high-pass filter.
Premier Guitar Magazine article
I am making the HPF-Pre available through a part-time home business. My goal is to provide simple products with uncompromising quality and service. I also encourage DIYers to use my designs in your own projects!
Francis Deck
