active pups and making preamps

[black_labb]

hi guys, the bass player in the band i just got going mentioned that hed love to play a 6 string bass. im thinking if i do it it might be a good idea to use actuive pups due to the larger range of frequencies needed, so ive been thinking wether it is possible to make your own preamp easily? ive been recently getting interested in making pedals to get some experience to make a tube amp, and noticed that they basically use a chip and a battery to amplify the sound, would a preamp simply be one of these connected properly in the circuit?

anyone ever do this or seen something similar done? im not looking for the whole equalising things, more just using lower resistance coil to get a flatter response curve, to get a better response across the spectrum.

i dont really want to have to buy the preamps, as im really looking for a relatively cheap solution. i should also say i dont play bass much, but would like to make a 4 string for myself at some stage.

thanks for any help

[PerroGrande]

Hi Black_Labb,

You mentioned both changing coil resistance (lower) and a preamp in your message. I'm only going to address the preamp side of things. Changing the pickups is rather different than adding a preamp. Of course, changing to a higher output picked will boost the signal, in effect, when compared to the previous pickups... To me, a pickup change is more about changing the sound and character of the instrument. Adding a preamp is more about bringing out the existing character/sound of the instrument.

I'm not an expert at winding pickups, so I'll stick to preamps

One consideration right from the beginning -- are you considering an internal or an external preamp?

If you want to go internal, you'll need to have room inside the guitar for a booster circuit *and* a battery (or two). You'll also need to consider the impedance of the amp relative to the pickups. I like to go with a high input impedance preamp if the guitar will be plugged directly in (or is internal). MOSFET, FET, and FET-based Op Amp circuits are usually the best for this.

If you want a simple, good-sounding boost that could potentially be fitted inside an instrument, check out the AMZ MOSFET booster circuit. (http://www.muzique.com/schem/mosfet.htm). It is well known, successful, good sounding, and possible to shrink onto a board small enough to go into a guitar. Other circuits are out there using OP Amps, FETs, BJT's and various combinations thereof.

So -- to answer one of your questions -- is a preamp just a chip and a battery properly connected? Well, yes, it can be. Or it can be a few inexpensive components instead of a chip (both have their pros and cons).

For example: My bass contains a combination of a booster plus a tone circuit. It runs from two 9V batteries. It is really nothing but a buffered tone control and a boost. It is accomplished with a dual op-amp and a handful of components on a board. I'm in the process of adding a similar booster to one of my guitars.

[black_labb]

thats exactly what i was looking for, thanks alot

i was looking to keep it internal if possible, but if its not i could make myself a little box.

when i said i was going to change the coil resistance, that was to make a low impedance pickup to be boosted by the preamp to a high impedance signal to go into the amp, would one of those preamps giv out a high impedance signal from a low impedance pickup?

basically for the pups i would be winding it with larger size wire and probably weaker magnets than usual.

[PerroGrande]

In general terms, you want a low impedance output connected to a high impedance input. (Note -- this is not *always* the case. In RF, one wants to carefully match impedances, and there are cases where one intentionally loads down a circuit for a particular purpose).

A decent booster is going to have a high input impedance (on the order of 500K ohms or better) so it will not load down the output of the guitar. The booster should have a fairly low output impedance so it can drive the next stage without it (the booster) being loaded down.

FET and MOSFET boosters have moderately low impedances, and the output should be sufficient to drive an amp or a pedal(s) without issue. If you're concerned about it, you could always build your booster with an OPA 134PA op amp. That thing has enough oomph (okay -- current) to directly run headphones. (I am an advocate of running op amps on split supplies -- two batteries -- for maximum headroom. They can be run with one, but gain is more limited, naturally)

So, if you've got a pickup that has a good sound but low output, a little boost (such as the AMZ circuit I mentioned) is a great way bring out the subtlety.

A couple of other considerations to mull over:

1) You're going to want to have the battery (or batteries) in a reasonably accessible location. My bass has an access panel on the back that makes changing the batteries fairly easy.

2) You will need to have a way to shut the circuit off. The most common is to use a stereo jack to act as a switch.

3) You may want to consider making a true bypass switch (if you have room) in case you want to run without the booster or the booster stops working.

Of course, all these points are fairly moot if you go with an external stomp box, but I've always liked the elegance of the internal boosters.

[black_labb]

thanks for that, seems exactly what im after. i like the idea of running headphones directly from a guitar/bass as well...

might incorperate it into other guitars if i can make it small enough to not abstruct anything, or even as a separate box that can be used on regular pups, either way thats alot of help, and i think i may do it.

[David Schwab]

I make "active" pickups. They have a very wide frequency response, and are very flat.

It's actually pretty easy. I'll give the Alembic recipe first. Rick Turner made the original Alembic basses and pickups. He would 1,000 to 1,500 turns of 40 gauge wire around a ceramic magnet. He picked such a low number or turns because we was really winding by hand... no winder! And he didn't want to wind 8,000 turns! I'd imagine the 40 gauge wire was either easier to come by, or just easier to work with. Larger gauge wire has lower resistance for the same number of turns.

Larger gauge wire gives you more top end. If you use thinner wire, you will get more mids and lows. You can even use something like 36 gauge (like a Charlie Christian pickups).

Or use something like 42 gauge and wind like 2,000-4,000 turns.

For a preamp/buffer, build one of these FET preamps (also of Alembic origins):

[black_labb]

thats pretymuch what i was thinking, im just not sure where to get to 40 awg or similar. might see if i can get some from a local electrical repairs place that rewinds electrical motors, to avoid getting too much (the one i have now is 42.5 (0.06 mm, an odd size but they had that in a 1.6kg spool, as opposed to a 3-4kg spool for the others, and seem sabout right for pups)

for that diagram, where does the -ve attach to? is it the ground? sorry im quite new to electronics that actually use a power source... (everything but passive guitars)

[Satamax]

Black labb, if you're registered on the mimf forum there's a big preamp schemo called mimf preamp in the library, worth a check.

[CustomTabs]

David,

can I use this preamp for any active PU? I'm trying to fix an old Ibanez Musician bass (2 active P-style PU's called Super J60). 1 PU goes dead after ca. 1 minute of operation. I guess it's the electronics, the coils are OK - already took it apart.

Thanks,

-Alois

[David Schwab]

David,

can I use this preamp for any active PU?

You probably wouldn't want to use it after an active pickup, as it would amplify the thermal noise (hiss). Since you took the pickup apart, I'm assuming you removed the built in preamp. So just substitute this FET preamp.

The circuit works real well on high Z passive pickups, which is what it was designed for. With lower Z pickups just boost the gain up a bit.

[CustomTabs]

Right, I want to replace the built in preamp (whatever) as I think it's causing the failure. I didn't find replacement PUs (dimension-wise) so I'll have to use the original housing. I'll also use the coils (they work) but might need a new or different preamp. It's a little tricky as it's all epoxied but I can disclose things little by little. It's fun. I also bought a used S.Duncan p-stype active PU on ebay, so if all goes wrong I'll just glue that SD into the original housing - as a last resort. However, I'd like to keep it mostly original, so I'd restore the original preamp - or take that Alembic like circuit.

Where could I find information about the original circuit, the components that have been used? Probably it's just 1 part not working correctly - so if I replace them or copy the layout to a new board 1:1, things would work again, right?

Any ideas anyone?

Thanks for input,

-Alois

[Mark Hammer]

The one caveat I will add to this otherwise sensible and informative thread is that one should add gain to such a preamp judiciously. Some bass outputs can be very hot, and adding even a bit of gain can make other things misbehave.

For instance, did you plan on running the bass into a compressor or phaser? Both are predicated on seeing a particular signal level. In the case of a compressor it may result in too much squish, while in the case of a phaser it may result in nasty clipping.

At the amp level, some amps handle hotter signals easily and some not so much. For instance, it there is one or more gain stages at the preamp end of the amp before any attenuation control, a hotter input signal may result in distortion unless you turn the bass down. If there is some attenuation early in the front end of the amp that may be less of a problem and you'll be able to leave the bass volume pot up full.

Personally, I find that unless you have deliberately wound some very low output pickups, anything more than a gain of 4 or 5 can be more of a P.I.T.A. than an improvement in sound. Whatever you make, incorporate some means of adjusting the gain to something the bassist can live with.

And make sure that it is LOW CURRENT so that the battery lasts and you don't have to keep changing it and stripping the threads wherever you have to unscrew things.

[CustomTabs]

Thanks for the heads up, Mark! I will keep the "means of adjusting the gain" in mind.

However, my goal is to repair the broken preamp in the bridge PU. The neck PU is still working and so I will have a good role model how the (new) preamp in the bridge PU should behave. To me, the bass doesn't sound louder or hotter that a regular passive bass would, so I guess the gain isn't very high. A single coil has about R = 3 kOhm. I will post pics as soon as the preamp circuit board has been disclosed fully.

Interesting detail in fabrication: There's no metallic housing, but the inside of the plastic cap had been painted with graphite and there's a kind of "ground wire" to it.

Thanks,

-Alois

[David Schwab]

The circuit I posted has a gain control that goes from unity to about 10. Just adjust it to match the other pickup.

[black_labb]

thanks for the help, i have been planning to do a low output coil anyway. the drain on the battery is related to the watt level of the chip used?