Adding a gain trim pot to an onboard preamp?

[fourstringbliss]

I have a Tone Monster SBK-3AD preamp installed in one of my basses. I like the preamp fine, but the Bart pickups are lower output than my other bass.

Here's the preamp wiring diagram:

and how it looks with the added (and useless) active/passive switch:

What could I do to add an internal trim pot to boost the output without changing tone or adding distortion?

I'm considering removing the A/P switch and giving the pickups separate single/series/single switches as well, so the A/P switch isn't necessarily permanent.

Thanks!

 

[Crater]

What could I do to add an internal trim pot to boost the output without changing tone or adding distortion?

Basically, you can't. A pot can only cut levels, as it's a passive component. It would take modifying the preamp itself, or installing a second on-board booster preamp.

 

[fig]

Yeah, fourstringbliss, unfortunately everything Crater said is true.

Just curious, is this bass alot lower than your other one, or only a little? Just wondering why you can't compensate with your amp.

There are outboard booster/pedals available, but it doesn't sound like you want that.

Bartolini pickups are known to be low output. I have two in my bass and had to add a Bartolini onboard adjustable booster. Absolutely transparent. Since you like the sound you have, you can adjust it as loud as you want. Half the size of a 9v battery, adjustable from 0 to 12db. 0db (built in trimpot pot fully off) will get you the volume you have now. Start cranking it up from there.

In the forum there are some other ones talked about, even some do-it-yourself builds.

Here's the Bartolini page offering a single channel (the one I have) and double channel (the one you should look at if you're gonna keep the balance function).: Invalid Link Removed

Connection diagrams (pdf) for passive or active basses are towards the bottom of the page.

If you do go with one, don't buy it from them, they're too expensive. Much less everywhere else.

 

[kikaida03]

Bartolini pickups are known to be low output. I have two in my bass and had to add a Bartolini onboard adjustable booster. Absolutely transparent. Since you like the sound you have, you can adjust it as loud as you want. Half the size of a 9v battery, adjustable from 0 to 12db. 0db (built in trimpot pot fully off) will get you the volume you have now. Start cranking it up from there.

Fig,

Thanks for this! Sorry for breaking in here, but will this work for an MTD Andre Gouche bass? I'm not very tech inclined, but I'm presuming since the bass has Barts on it, this thing should work.

Benny

 

[fig]

Thanks for this! Sorry for breaking in here, but will this work for an MTD Andre Gouche bass? I'm not very tech inclined, but I'm presuming since the bass has Barts on it, this thing should work.

I wasn't familiar with your bass, but did a quick look online. In addition to the pickups, it looks like the preamp is Bartolini also, so it should work fine. Actually, the Bart Adjustable Gain Buffer I referenced, and use, should work with any brand.

Here's a pdf document from the Bartolini site showing a hookup using the Gain Buffer (the dual AGDB model) with one of their own 3 band preamps:
https://bartolini.net/wp-content/uploads/Docs/Electronics/Buffers/AGDB-918-2_Wiring.pdf

Double check with Bartolini for any questions/concerns, just to be sure, but I don't foresee an issue at all. I've emailed them twice with questions and they got back with me fast.

 

[kikaida03]

Oops, sorry for the late reply (gotta change my email preference!) But thanks much!

 

[incommunicado]

Are you guys saying a trim pot cannot raise the volume of the pickups, but can only cut it? I'm really confused here, isn't the trim pot on preamps that have it used exactly for that? (and lowering the volume too). From the comments I understand that in case I wanna raise the volume of my pickups (Bartolini MK-1) I can't do it by adding a trim pot (gain, volume? - are those different?), but by adding an adjustable gain buffer like the one suggested? The onboard preamp I have on my bass is a Markbass MB INSTRUMENT PRE P. Initially I thought I should replace the pickups with original MK-5CBC but from what I've been reading - that won't guarantee me louder output, so I thought adding a gain (volume?) trim pot was a better solution. That or an adjustable buffer?

 

[fig]

...so I thought adding a gain (volume?) trim pot was a better solution. That or an adjustable buffer?

Short version at bottom.

(gain, volume? - are those different?)

Conceptual difference: Normally, 'Gain' is the process that takes a small signal (fed to preamp input) and raises it to something useable (exit from preamp output). 'Volume' takes that raised useable signal, after it's left the preamp, and controls it.

Physical difference: A trim pot and volume pot are both just potentiometers. 'Gain' or 'Volume' is just a name to describe how the pot is being used (as a gain pot or volume pot).

Are you guys saying a trim pot cannot raise the volume of the pickups, but can only cut it?

Unfortunately, yes, it'll only cut if installed anywhere outside of the Mark Bass preamp. It must be connected to the preamp's innards, the circuit board, to control the preamp's gain. Raising preamp gain will raise output volume.

I'm really confused here, isn't the trim pot on preamps that have it used exactly for that? (and lowering the volume too).

Yes, because the trim pots are already connected to the preamp's circuit board, by the manufacturer, to adjust the gain.

I haven't seen the back of the Mark Bass preamp. Are the electronics totally covered? If so, you can't add a trim to it anyway.

Short version: An adjustable buffer.

Hope that helped.

 

[incommunicado]

Thanks a bunch for the detailed answer! Yes, the electronics of the preamp are totally covered (see attached photo). So what I should be looking for is an adjustable buffer and not a trim pot, I get it...

 

[Jefenator]

Bartolini offers an adjustable gain buffer module with the appropriate model designation: AGB. (With that you do indeed get a trim pot which can be very handy for matching levels with other now-softer instruments.)
Some Bartoloini pickup designs are low in output - technically passive, but really intended for use with an active onboard gain boost. IME they can sound great with the right preamp - and be kind of a drag without it.
Bartolini offers many options for such pickups - I believe Nordstrand also has some onboard preamps that boost gain. There may be others, but one should probably assume there is no gain boost, unless it is specifically mentioned in the features or spec's.

 

[incommunicado]

Thanks for the reply! I realize a new preamp with a gain boost might be the solution to low output of the MK-1s, but I was just thinking to keep the original preamp and wire an AGB (50-60 USD or so), rather than replacing the preamp (a new Bartolini preamp is around 260 USD or so - which is half of the bass's price in my case, so are the original Bartolini pickups, which I've decided not to buy just yet)

 

[dwizum]

Have you adjusted the pickup height? If you want more volume and are looking at money-saving techniques, raising the pickup height is free.

 

[incommunicado]

Yep, that's the first thing I did...The Bartolini MK-1 pickups are just quiet I guess (from what I read in various forums)

 

[Jefenator]

Thanks for the reply! I realize a new preamp with a gain boost might be the solution to low output of the MK-1s, but I was just thinking to keep the original preamp and wire an AGB (50-60 USD or so), rather than replacing the preamp (a new Bartolini preamp is around 260 USD or so - which is half of the bass's price in my case, so are the original Bartolini pickups, which I've decided not to buy just yet)

I feel you - the cost of a full active electronic makeover often stops me.
I was looking at the AGB for a passive bass that had been modified w/ low output Barts. Alas, the control cavity was so cramped, I would have needed to rout another chamber for a battery.
That seemed like more trouble than it was worth, so I went another direction. I bought a used set of normal-output passive pickups & sold the Barts here in the classifieds. Problem solved for minimal $$.
Just putting that out there as another potential option for anyone in a similar situation. (If I'd had just a little more room - or woodworking skills - I might have gone with the AGB. IME those Barts can sing pretty sweetly with the proper implementation.)

 

[incommunicado]

I see...But what exactly do you mean by "normal-output" passive pickups? You have a suggestion for a specific brand? (that can replace the MK-1 Barts and originally have higher output?)