Status: Things are back to normal, and I am taking orders of all Series of preamps.
Piezo preamp with adjustable high pass
filter for bass amplification
Site updated Sept. 12, 2012
Thanks to everybody for your heroic patience! Series 3 is now ready for orders and shipping! I am ready to take orders from anybody in the US. Folks who have already expressed an interest via e-mail or PM will receive preferential treatment. In practical terms, this will probably not affect shipments.
Sometime around the end of August, I expect that HPF Technology will take a brief hiatus while I revamp my production area, catch up on building preamps, and move my entire website over here. This will include all of the design documentation that I have always provided for my products.
To contact me, here's my new e-mail address:
And welcome to the HPF-Pre family:
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
HPF-Pre now comes in three 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
- 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 3
||Input impedance 10 MegOhms / 220 pF
Active high pass filter (HPF)
Filter cutoff adjustable from 35 to 140 Hz
Phase (polarity reversal) switch
10 MegOhms / 100 pF
||9-V Battery (not provided)
||9-V Battery, plus jack for optional DC power supply (not provided)
|LED "flash" if battery is good
|Plastic, slightly bigger
|Die cast aluminum
|1 V p-p with "musically forgiving" overload response
|6 V p-p with hard clipping response
|Roughly +2 dB above the corner frequency
|0 dB above the corner frequency
|Double bass (DB) 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 and 2 both 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 or 2 are an economical choice.
2. The volume control on Series 2 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 case is rugged, but it's indisputable that the metal case is more rugged.
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.
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:
An article by Dan Berkowitz for Premier Guitar Magazine provides a
good explanation of the need for a high-pass filter.
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!