Baja Music Man guitar and bass onboard preamps  [documentation]

Original effects with schematics, layouts and instructions, freely contributed by members or found in publications. Cannot be used for commercial purposes without the consent of the owners of the copyright.
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iq01221
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Post by iq01221 »

Victor Nery wrote:I replaced the TL071 with a low consumption LF351 but... the hum is still there. All the ground connections are ok; includind the one that goes to the bridge. I‘m thinking of reversing the wires of the second cap and the pole of the magnet. How can I do it? Is it really necessary or just turning it 180 degrees will solve the problem? I tried connecting the pickups straight to the output jack letting the circuit out of chain so now I‘m pretty sure the annoying hum comes from the pickups. Not sure about this but is it a question for another topic or it can be discussed here? I just can‘t find the LM4250 here in Brazil but as I told you the problem is the pickups. It‘s driving me crazy. Thanks
Hey! I'm from Argentina and I can't get 4250 :hug: we are 2... But, hey! Wy don't try to put it on a stomp to use it with 9V regulated power supply and let the 071 on the work? :hmmm:
Just a detail: I've used UA741CN, and it's great with my fernandez gravity!
Anyway, I think the problem is in the bass part, if you can try with another bass, coud be great for solve your doubts...
Best regards!!!

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deltafred
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Post by deltafred »

Jazz basses (and others with single coil pickups) can be a real PITA for humming as they are only hum cancelling if you get the 2 volume controls set to exactly the same level, usually full on or off.

This is assuming that they are wired correctly with the correct pickups. One pickup has the North magnet pole up, the other South magnet pole up. The output leads then need to be phased correctly so the reversed magnet polarity does not cause signal cancelling.

If the hum disappears when both volumes are at max then it is a single coil problem. No amount of shielding will cure it as it is caused by magnetic interference to the pickups. Shielding will only improve the hum if it is being picked up by the wiring.

Someone I know had such a problem with his Jazz that the only way he could get rid of the hum (without putting new noiseless pickups on) was to rewire it to get rid of the separate volume pots and have the pickups switched as Series or Parallel. This way they are hum cancelling, acting like a wide spaced humbucker.

Hope this helps
fred
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Post by DaveKerr »

Victor Nery wrote: I‘m thinking of reversing the wires of the second cap and the pole of the magnet. How can I do it?
Check first to see if the pickups are RWRP (reverse wound/reverse polarity). The easiest way to check the polarity of the magnets is to put a cheap camping compass near the top poles of each pickup - if one attracts the North seeking (red, usually) end of the needle and the other the South seeking end, then you're already set up for humbucking (assuming that you don;t have a thin, reedy out-of-phase sound when each pickup is fully on.)

If each pickup attracts the same end of the compass then it gets a little tougher. Jazz bass pickups are often wound with the wire laid directly onto the magnets, so removing/flipping the magnets and reversing the hot/ground leads will very likely bugger the pickup to the point of requiring a rewind. I know some of the cheaper Strat pickups use steel slugs inserted into a plastic bobbin and have ceramic magnets glued onto the bottom - if you've got something similar just flip those magnets around 180 degrees and swap the hot/ground wires.

edit - or, what Fred said directly above - I'll learn to read ahead before responding one of these days
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skrumian
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Post by skrumian »

hi guys, do you also have 3-band bass preamp layout and schematics? something that has bass-mid-treble knobs. I really need the mid knob as i prefer middy sound. Thank you. :D

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Post by jase128 »

would it be easy to mod this to take a pickup balance/blend pot to? for a 2 pickup bass ( not 2 volume controls) if so how could we do it please?

have we finally settled on which resistors etc is best to use? can we have a final parts list please.

many many thanks to all involved, what a fantastic post (mega thanks to baja)

Jason

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Post by deltafred »

jase128 wrote:would it be easy to mod this to take a pickup balance/blend pot to? for a 2 pickup bass ( not 2 volume controls) if so how could we do it please?

have we finally settled on which resistors etc is best to use? can we have a final parts list please.

many many thanks to all involved, what a fantastic post (mega thanks to baja)

Jason
Balance/blend pot - Any resistance between the pickup and the input affects the performance as the circuit is unbuffered. The effect is loss of treble so unless you are prepared to compromise the sound it is not really practical. Try it if you want but the more resistance there is the more treble is lost. For a fixed resistance this can be compensated for with a capacitor in parallel with the resistor but for a pot I cannot see a way of getting round it. Maybe a simple buffer between the pot/s and the preamp would work but I haven't had need to try this.

Component values - The values that Baja listed are correct for the bass that this preamp came from. They also hold good for the one I reversed, a MM SUB4 circa 2003 IIRC. It would appear that other models used differing values. Try them all and use what you like best.
Politics is the art of so plucking the goose as to obtain the most feathers with the least squawking. - R.G. 2011
Jeez, she's an ugly bastard, she makes my socks hurt. I hope it's no ones missus here. - Ice-9 2012

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Post by jase128 »

Thanks deltafred,

if you had a twin mm pickup bass would you just switch the pickups? ie on-on, off-on,on-off ??
where do you order components from too (as we are in a similar neck of the woods)

thanks for you time

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Post by deltafred »

jase128 wrote:Thanks deltafred,

if you had a twin mm pickup bass would you just switch the pickups? ie on-on, off-on,on-off ??
where do you order components from too (as we are in a similar neck of the woods)

thanks for you time
I would first have a temporary switching scheme where I could try out all permutations, then select what I liked the sound of. Some people like to have every possible permutation available all the time. I prefer the ones I am going to actually use on stage.

For MMs I like series, It seems to work best in my band settling, but that is my preference, yours may be something else.

I don't have a twin humbucker MM but I do have a G&L, I tend to use both pups, series most of the time with single coil outers for a couple of songs.

The only problem with series is there is a substantial difference in volume compared to parallel or single coil due to the pickup voltage being approximately double (but not double the volume). I have overcome this by adding a series resistor in series mode. On the MM preamp you need to add a capacitor in parallel with this to prevent loss of treble. The value of this will depend upon what pickups you have.

As far as component supply goes I cannot really advise you, I have a pretty large stock of components from buying them, and building stuff, since about 1966. I also worked in a large company that regularly threw development PCBs in the bin. I was always rescuing them (especially the analogue ones) and when my wife was watching the usual rubbish on TV I would sit and strip them down if I was not building anything. I now rarely have to buy anything.
Politics is the art of so plucking the goose as to obtain the most feathers with the least squawking. - R.G. 2011
Jeez, she's an ugly bastard, she makes my socks hurt. I hope it's no ones missus here. - Ice-9 2012

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Post by omni »

On a 2 pickup bass could this circuit be used on each pickup without the treb/bass portion so that it has it's own volume but be summed for a master tone (treb/bass)? Also would a buffer be needed on the input for high impedance pickups?

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Post by deltafred »

omni wrote:On a 2 pickup bass could this circuit be used on each pickup without the treb/bass portion so that it has it's own volume but be summed for a master tone (treb/bass)? Also would a buffer be needed on the input for high impedance pickups?
Not sure I am following you here.

A better/simpler solution might be

pups - mix pot - unity gain buffer - Baja preamp.

The unity gain buffer could be FET, BJT or opamp.
Politics is the art of so plucking the goose as to obtain the most feathers with the least squawking. - R.G. 2011
Jeez, she's an ugly bastard, she makes my socks hurt. I hope it's no ones missus here. - Ice-9 2012

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Post by wrnchbndr »

After nearly a year of putting this project on a backburner, I opened the envelope from uncle fluffy and assembled a few of these preamps. I only had two ICs so I've put some on order. I'm not hearing any effect from the treble control. I'm using a regular (not reversed) 1M audio pot but I'd assume it would work -- just not with a proper controllable rotation??? I'd think that I hear something at one end of the rotation. I've changed the 1.8nf cap to a few caps of larger value with no effect at all. The bass control works well as does the volume pot and the tone is clean with just a minor increase in gain over the passive pickups. I'm using a cheap budget 15W practice amp at home for my testing -- could it just be the amp -- I'm not hearing any effect at all. I checked and double checked the wiring of the pot from every direction. Would a regular audio taper pot just prevent treble control totally? I can't find a suitable reverse taper 1M pot. Would a linear pot be better than what I'm using?

This is my bass. Its a one-piece slab of swamp ash, sugar maple neck w/Madagascar rosewood fingerboard. The fingerboard is multiradiused with a smaller radius under the bass side going progressively flatter to the treble side with compounding radius as you get further from the nut. The fretless length is 2 octaves or the equivalent of 25 frets. The trussrod is dual adjustable. The headstock is an experiment but it works. The pickups are musicman knock-offs from Guitarjonesusa. The bridge is hand carved from aluminum billet. The pickguard is bakelite. At the moment, the body only has a sealing coat of shellac and I'll be using black grain filler and topping it off with a slightly ambered lacquer.
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Post by deltafred »

wrnchbndr wrote:...I'm using a regular (not reversed) 1M audio pot but I'd assume it would work -- just not with a proper controllable rotation???

This is my bass. Its a one-piece slab of swamp ash, sugar maple neck w/Madagascar rosewood fingerboard. The fingerboard is multiradiused with a smaller radius under the bass side going progressively flatter to the treble side with compounding radius as you get further from the nut. The fretless length is 2 octaves or the equivalent of 25 frets. The trussrod is dual adjustable. The headstock is an experiment but it works. The pickups are musicman knock-offs from Guitarjonesusa. The bridge is hand carved from aluminum billet. The pickguard is bakelite. At the moment, the body only has a sealing coat of shellac and I'll be using black grain filler and topping it off with a slightly ambered lacquer.
It should work with a standard audio taper but all the changes will be really cramped up at one end. A linear pot would be better but still cramped.

If you swap the outer ends of the pot, that will give you the correct taper only backwards (if you understand what I am am trying to say). This will at least allow you to check that the preamp is working correctly.

In mine I have a linear pot. It is still a bit cramped at one end but not too bad

Nice looking bass, I'd love to have a go at making another, made my first in about 1967 but have learned a lot since then. It was a total disaster.
Politics is the art of so plucking the goose as to obtain the most feathers with the least squawking. - R.G. 2011
Jeez, she's an ugly bastard, she makes my socks hurt. I hope it's no ones missus here. - Ice-9 2012

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wrnchbndr
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Location: Mid New Jersey soon to be eastern PA in the woods

Post by wrnchbndr »

Thanks for the confirmation. Knowing that the pot is not going to outright kill the treble boost lets me focus on other things. I need the preamp to function with the knobs rotating in the proper direction because I'll be installing them into guitars and basses that I build and eventually sell to the public. I'll keep looking for a 1M reverse audio. I did find one at Mouser but I think that the one they shipped me was not the physical spec that was indicated in the catalog. They're out there -- they're just not consumed enough for vendors to stock. I'm going to double check my component values. I may have made a mistake with a decimal point -- I don't like the use of nanofarads but I was very careful.

Would the 500 pf cap have an effect on the treble boost? I have two completed PCBs. The one I'm using has a 680pf and my other one has a 471pf installed -- I just didn't have any 500pf caps on hand.
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Post by deltafred »

wrnchbndr wrote:
Would the 500 pf cap have an effect on the treble boost? I have two completed PCBs. The one I'm using has a 680pf and my other one has a 471pf installed -- I just didn't have any 500pf caps on hand.

I don't think that 500pf to 470 or even 680 is going to make much difference, probably just shift the range slightly. It certainly shouldn't stop it functioning. If it really is having no effect I would suspect the pot first, look for open circuit wiper connection, but it could be the pot, a dud 1n8 or 500pF capacitor, a dry (cold) solder joint, or even a broken track.

I know what you mean about cap markings, when I was an apprentice caps were marked in their value so you knew what you had. Now I pick up one marked 470 and never know for sure if it is 47pF (47 x zero tens) or 470pF actual value, so I have to break out the cap meter to make sure as some of my caps are so old they pre-date mojo.
Politics is the art of so plucking the goose as to obtain the most feathers with the least squawking. - R.G. 2011
Jeez, she's an ugly bastard, she makes my socks hurt. I hope it's no ones missus here. - Ice-9 2012

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wrnchbndr
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my favorite amplifier: Fender Twin
Completed builds: A series of modified 5E3s, 5E6 and 5F8s in exotic hardwood cabinets. Too many handcrafted guitars to list. I play around with stompboxes now and then. I've built a fair number of one off specialized instruments and a few for people with special needs. Everything is custom and I never do the same thing twice.
Location: Mid New Jersey soon to be eastern PA in the woods

Post by wrnchbndr »

The second PCB works. ...and!!! I found my stash of 1M Rev Audio pots that I bought last year. I wish I knew where I got them from because I've put in hours of time looking for them. They look like Alphas and maybe these are the proper ones from Mouser page 766. I think you may be right about the pot simply being a dud -- it happens but usually not a total dud --- but then, I've seen that once before too. The treble boost is spectacular. I keep my real gear at work and I can't wait to plug this into something substancial. I'll getr back to the first PCB later and give a definative verdict.

I need to make a long overdue email of thanks to Uncle Fluffy for the PCBs.
The otters threw me out because I was too messy.

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Post by deltafred »

Pleased to hear you got one working right.

I've had a couple of dud pots recently, both had lost contact where a lug was riveted on. One was due to being overheated by bad soldering (not mine) the other was brand new.

Those Uncle Fluffy boards do look superb.
Politics is the art of so plucking the goose as to obtain the most feathers with the least squawking. - R.G. 2011
Jeez, she's an ugly bastard, she makes my socks hurt. I hope it's no ones missus here. - Ice-9 2012

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Post by Victor Nery »

Firstly I would like to thank you for the support. I tried the same circuit with another bass and got the same problem. I tried the same steps. First I wired them in parallel both ways phase and out of phase. Second I wired them in series; again phase and out of phase. The hum was there in all the ways I tried. I did not try to flip the magnets because I was advised I could break the wire so I did not check this (yet). But this time I could listen to some radio interference and it lead me to the point that I probably misunderstood a loud hum (louder than any single pickup in the world can produce) with the radio interference. What do you think? Is there a way to fix it and have you experienced something like this? Thanks to you all.

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Post by deltafred »

Victor Nery wrote:But this time I could listen to some radio interference and it lead me to the point that I probably misunderstood a loud hum (louder than any single pickup in the world can produce) with the radio interference.
A few random thoughts.

Picking up radio signals points to a bad solder joint or connection, a faulty component, grounding issue, or possibly a problem with your amp.

Have you checked the bridge earth, guitar lead (chord) and amp mains lead and tried your amp in a different mains socket.

Did you use the LM4250 or 'improve' the circuit by substituting a different opamp as this will increase the gain way beyond the audio spectrum, well for humans anyway, increasing the possibility of noise by amplifying signals and noise outside the pickup output range.

Edit as I re-read you prev post.
It looks like it is the board as you have tried it on a different bass and it is doubtful if that has an identical fault.

If you have built it onto a control plate (as MM do) is the control plate being earthed?
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Jeez, she's an ugly bastard, she makes my socks hurt. I hope it's no ones missus here. - Ice-9 2012

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Post by Victor Nery »

I‘m sorry I didn‘t mean the very same circuit. I built another one with an equal layout and equal component values. I isolated the pickups from the circuit and in this bass the noise is much more accentuated. I thought of adding a resistor from IN to ground let‘s say 120k, considering that there‘s a resistance when the sinal goes through the pots (when no active circuit was added). Does it make sense? Again thanks a lot for your help.

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Post by rafammbass »

Hey Guys!

Baja, thanks for this project, this is really useful and I have to express my respect for your work. Incredible!

Can you give me an update on what is the current version for this Stingray preamp? I have seen so many discussions on improvements/other values that I cannot recall what is the real deal.

Thank you very much!

Best!

Rafa

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