Double Bass Bartolini AGMPB preamp for magnetic and piezo blending (modified FDECK?)

[JBarz]

Has anyone used this particular preamp, the Bartolini AGMPB, to blend their Krivo and Full Circle on double bass?

https://bartolini.net/wp-content/uploads/Docs/Electronics/Buffers/AGMPB-918-Color-Instructions.pdf

I just installed one in an electric bass that had a K&K piezo under the top and it works pretty great. 3.6kohm input impedance for the piezo end - wondering if this would be a happy place for the full circle?

My other thought is to modify an FDECK series 1 to tie this little preamp in. I’m thinking what I’m going to try to do is have a simple dual RCA input for the two pickups. The piezo end might could go through the HPF-PRE circuit first, and then get fed into the Bartolini for volume balancing and, most importantly, the blend knob. I’m wondering if the same single 9V battery in the HPF PRE can also run this Bartolini preamp. I’m pretty sure I can still fit everything inside the FDECK’s enclosure.

As far as mounting is concerned, I’m thinking about velcro’n the box to under the fingerboard, maybe. I might try to fit a master volume knob in if I have room, but that’s icing on the cake.

Thoughts/comments/feelings/opinions?

 

[bassically_eli]

3.6kohm input impedance for the piezo end

3.6kOhm is the output impedance. The piezo input impedance is listed as 3.3M-Ohm, which is a very good thing.

 

[JBarz]

Oh cool! That’s kinda what I was wondering - if it was within the ‘sweet spot’ without needing the HPF PRE as a buffer. I’m going to go for it. Looks like I can get everything I need (enclosure/pots/jacks/preamp) for around $100 flat.

 

[fdeck]

At first glance, this should work for the Krivo and Full Circle. The Full Circle will be happy with the 3.6 Meg resistance of the piezo input. It won't need additional buffering.

 

[JBarz]

Works pretty dang good! Sweet spot seems to be with the internal volume trim pots balanced and the blend knob set right in the middle. The signal runs a little hotter than I’d like, but that makes the volume knob useful. The tone knob doesn’t seem to do anything, maybe I need a different capacitor in there, I don’t know. Hot glue’d for a temporary solution, just need to find some small fiddly knobs. 1.5” enclosure was the perfect size to accommodate.

 

[JBarz]

One more small modification. The humbucker wasn’t grounding right sometimes (dirty power?) and giving a small buzz noticeable at (way too) loud volume. I just took the ground and wired it to one of my kids’ coins you flatten at the zoo, hot glue’d as a thumb rest. Problem solved - simple.

Here’s an internal shot of how I fit everything into the enclosure in case it helps someone in the future:

 

[Matthijs]

Did you try shielding the enclosure?

 

[JBarz]

I knew I forgot something! I’m pretty novice at this kind of thing - thanks for the heads up!

 

[fdeck]

When I had a magnetic, I grounded the strings with a bit of fine wire looped around the ball ends of the strings, to the metal body of the jack.

And the piezo input should be shielded... and shorten the piezo input wiring.

 

[JBarz]

Thank ye! - I’ll absolutely make those changes! I’m not in love with the hot glue mounting ‘solution’. Will try to attach with dual lock Velcro onto the Krivo’s metal L bracket for optional removal. The weight of it and the krivo on the fingerboard really eat into the acoustic tone’s bloom. It’d be nice to take it off when not using it…

I wonder why no one (that I know of) in the upright bass world has incorporated internal wiring and ‘built in’ electronics into the side of the body like you see on all these acoustic/electric guitars… if ever I get a bolt on neck conversion with an access hole, I’ll be all over that like flies on schite.

 

[fdeck]

I have to share an anecdote. I got an acoustic bass guitar, and its built-in electronics started to cut in and out intermittently. And the battery rattled a little bit. I yanked all of the guts, and wired the pickup directly to the jack. I might design something myself -- gotta be me after all -- but for now at least the instrument plays fine.