HPF-Pre, updated Sept. 30, 2014

Status: There is a new HPF-Pre, the Series 2M. 

Series 2M has the same preamp circuit as Series 2, but is in a metal box like Series 3. I have always considered the plastic box to be pretty rugged, but the metal box is arguably more rugged.

The price will be $80 plus $8 for shipping. I plan on continuing to offer Series 2 as well but if demand shifts to the 2M, then I will probably phase out the plastic box version.

Piezo preamp with adjustable high pass filter for bass amplification

Here's my new e-mail address:

Basic Information

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 three versions:

  1. Series 1 is the original preamp, as shown in the picture above.
  2. Series 2 is the same basic audio circuit, with addition of a volume control and a LED battery indicator.
  3. Series 3 uses the new "12+12" filter circuit, and has an optional DC power input, but retains the original "front panel."

I think that I should mention something about Series 3, and high pass filtering in general. My documentation talks about the benefits of high pass filtering in terms of the behavior of speakers. However, I'm hesitant to say that the HPF-Pre is a speaker protection device. You can still blow your speakers. In fact, if the HPF-Pre enables you to achieve higher volume by limiting overexcursion, you will eventually run into the danger of burning out your speakers by thermal overload instead.

Also, the HPF-Pre won't protect your ears. You still need to exercise appropriate caution to protect both your equipment and your hearing.

HPF-Pre Selection Table

 Series 1
 Series 2
 Series 2M Series 3
General Input impedance 10 MegOhms / 220 pF
Active high pass filter (HPF)
Filter cutoff adjustable from 35 to 140 Hz
Phase (polarity reversal) switch
SameSame, except:
10 MegOhms / 100 pF
Volume Control
No Yes YesYes
Filter Type
12 dB/octave
Same Same"12+12" Curve, see note 4 below
Power 9-V Battery (not provided)
No DC power jack
No DC power jack
No DC power jack
9-V Battery, plus jack for optional DC power supply (not provided)
Battery indicator  
LED "flash" if battery is good
Enclosure Plastic
Plastic, slightly bigger
 Die-cast aluminumSame
Input range
1 V p-p with "musically forgiving" overload response
 Same6 V p-p with hard clipping response
Roughly +2 dB above the corner frequency
 Same0 dB above the corner frequency
Double bass (DB) amplification
DB amplification
 SameDB amplification
Cone control on high performance ported speaker systems

Which one to choose? Here are some pointers:

1. For typical double bass amplification, such as jazz at moderate volume levels, Series 1, 2, and 1M get the job done, and are not obsolete. If you have one of these preamps and are happy with your sound, then you don't need to upgrade. Or, if you don't have extreme cone control problems on an electric bass rig, then Series 1, 2, or 2M are economical choices.

2. The volume control on Series 2, 2M and 3 is handy when you can't reach the volume control on your amp. This gives you the option of moving your amp away from your bass to help control feedback.

3. The plastic boxes are rugged, but it's indisputable that the metal boxes more rugged. Now the following is purely anecdotal, and not based on any kind of statistics or science, but is my impression based on servicing units that have gotten damaged: Electric bass use tends to be tougher on gear than upright. I don't think it's because of the bassists themselves, but because electric bass gigging seems to involve more gear getting bounced around by more people. Or maybe you guys simply get more gigs. So, with rising demand for HPF-Pre in the electric bass market, I'm leaning towards the metal boxes for future development.

4. The "12+12" filter curve is two filters in one. The first filter has a fixed corner frequency of 35 Hz, and 12 dB/octave slope. The second filter is adjustable from 35 to 140 Hz, and also has a 12 dB/oct slope. Thus it always has 24 dB/oct slope below 35 Hz. I designed the Series 1 and 2 filter (same circuit in both) for double bass amplification, to tame the infrasonic "thump" produced by the acoustic instrument, which interacts poorly with the low frequency excursion behavior of ported speakers. Interest in a 24 dB/oct filter came from electric bassists who are experimenting with the latest high-performance speakers, to avoid driving the system at frequencies below the port tuning frequency. I designed the "12+12" filter with the same input conditions and "front panel" as Series 2, so that either could be used for double bass.

5. Maybe you don't need a preamp! Since I consider the HPF-Pre to be a problem solving tool, it's worth investigating whether your rig suffers from the problems that it purports to solve. A bit of research is always worthwhile. There are a number of credible people on the TalkBass and Greenboy forums, including engineers and reps from some of the major gear makers, who regularly provide competent and honest answers to technical questions asked by bassists.

HPF-Pre is an "open" design, with more specs and a detailed technical report available online (coming soon when I finish this site). You are welcome to build your own, and a number of people have done so successfully.

Sales Information

HPF-Pre is available from my part-time home business, HPF Technology LLC, to customers in the United States and Canada. I will maintain a modest stock, and orders received during the week will typically be packaged over the next weekend.

Unfortunately, the HPF-Pre is not available outside the US.

News and reviews

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

About my business

I am making the HPF-Pre available through a part-time home business. My goal is to provide simple products with uncopromising quality and service. I also encourage DIYers to use my designs in your own projects!

Francis Deck