High Pass Filter for Bass

[FranVarin]

Hi!
I have a bunch of confusion of the use of an HPF to remove subsonic frequencies in order to protect and increase the efficiency of speakers in a live performance. I've researched the two front running HPF devices, the Micro Thumpinator and the fdeck hpf. I've also read a lot of articles on the various filters active vs. passive. It seems to me this should be fairly simple but, then again it seems like I must be missing something. I took a look at the fdeck schematic and there is a lot more going on in there since it is also a preamp. Recently, I saw a picture of the board in the Thumpinator and that is much more than just a simple passive RC circuit. It seems the RC circuit should work just fine from what I've read as long as you match up the cap and resistor to target about 40hz and lower. Can somebody clear this up for me? Is a passive RC circuit enough? I've built a couple of stompboxes but, I'm no expert for sure so, a layman's explanation would work best for me to understand what's required.

Thanks in advance!
FRan

[samhay]

Hi,
Welcome to the forum.

These devices are active as you want a multi-pole filter to give a steep enough roll-off to preserve the lowest fundamental of interest (~30 Hz for a 5-string bass) yet get decent filtering of lower frequencies.

Rod Elliot has a fairly detailed write up of a bass preamp he designed recently. It contains a switchable high pass filter, which uses 2 op-amps and has some discussion of his design choices, which are always worth a read.
Note, it will need a buffer in front of it. Look for 'High Pass Filter' here:
http://sound.whsites.net/project152-1.htm

Edit. The Thumpinator has balanced input and output, which explains the extra 2 dual op-amps you can see in the gut shots.

The manual for the HPF Pre contains the schematic, which says a lot of good things about the company ethos.
It only uses a 2-pole filter, but has adjustable frequency, which some may find useful.
https://sites.google.com/site/hpftechllc/home/technical-diy-stuff

[FranVarin]

thanks for the reply!

I took a quick look at the article you were kind enough to share. It looks like some great information and I intend to study it more thoroughly!

Fran