Ampeg Based Piezo Preamp

[blown240]

I am on vacation for a week and I am starting to plan my next build.  I play an upright bass that has a dual piezo pickup.  It came with a preamp that sounded so bad I literally threw it away.  What I want to build is an Ampeg b15n based dual channel preamp that I can use to plug into house sound, my regular bass amp, or use to record direct.  

My bass has a stereo jack on it, so the preamp will have a single stereo input.  It will have separate volume, treble, and bass controls for each channel, and I will use push/pull tone pots for the bass and treble boost.

I want to make the preamp circuits as close as possible to the original B15n but obviously setup for a piezo.  Also, when playing at louder volumes, or when the acoustics of a room are terrible, my bass feedsback at about 100hz.  So I want to do a notch filter on each channel that drops 100hz, but leaves everything above about 120hz and below 80hz. I will have a separate notch for each channel that is switched with a small micro switch.

Here is a link to the pickup I am using:
http://www.kksound.com/bassmasterrb.html

Here are some questions I have:

1.  Input Resistor – the pickups input resistance is 1m per channel.  Should I match that, or should I got with something like a 4.7m input resistor like a lot of the peizo preamps I see online.

2.  Rectifier – I want to use a tube rectifier, but I am not sure if the tranny I am using will let me, or what rectifier tube to use. Here is a pic of the small trans that I will be using (tranny on the left):

[IMG:640:480]http://i268.photobucket.com/albums/jj26 ... 185049.jpg[/img]

3.  Notch Filter – No idea how to do this.  Any ideas?

4.  Is there anything else I need to do to the basic b15n preamp circuit to better work with a peizo?

5.  I want to make sure I have loads of clean headroom, so anything that will help that...


Thanks for any input!


Here is the b15n schematic for reference:

[IMG571]http://i268.photobucket.com/albums/jj26 ... g_B15N.jpg[/img]

[wicker]

Notch Filter -> google for Twin T filter, good luck.

[blown240]

Here is a schematic I did to start with. I still need to make sure the input impedance is correct for a piezo. And I need to figure out where to put the notch filter(s) and exactly how to do them.

[IMG608]http://i268.photobucket.com/albums/jj26 ... Preamp.jpg[/img]

[xtian]

Have you actually played your bass thru a B15?

Piezos are such different fish from magnetic pickups, they need their own style of EQ. See, for example, Fishman preamps purpose built for piezos.

[blown240]

not my exact bass, but I have played others with piezos thru a b15 and they sound great!

[blown240]

Here is an updated schematic:

I have added a cathode follower to boost my output in case there are long runs to house sound, also I have changed to a SS rectifier since there really is no reason for a tube rectifier in this. I still need to figure out the notch filter and I may add a trans and XLR out for a DI.

[IMG581]http://i268.photobucket.com/albums/jj26 ... pdate2.jpg[/img]

[blown240]

Here is where I am at now.

I have added:

1. Ground Switch to isolate the grounds after the standby switch in case there is a ground loop issue. (shown in green)
2. Added a DI out
3. Added a variable notch filter with a DPDT on/off
4. A couple other minor tweeks

[IMG581]http://i268.photobucket.com/albums/jj26 ... pdate4.jpg[/img]