My bass onboard preamp project

[egasimus]

Hey everyone.

I'm contemplating the onboard preamp for my bass. The instrument is a Hohner B2B - a compact, headless model with little space in the control cavity. Not too tight, but tight enough for me to plan on this being my first SMD project. Man, I'm gonna suck at this.

Anyway, what bothers me most - besides the possibility of not being able to squeeze 2 switched and one double-tiered pot in there - is that, even for tamer control schemes, it would be hard to fit both a PCB and a 9V battery in the control cavity. Therefore, I'm thinking about using a rechargeable battery, and a voltage multiplier.

That's where I reach the limit of my skills, and humbly ask for your help. How hard is it to boost a 3.6V battery (cell phone or similar) to 9-18V in order to provide a good power supply for an onboard bass preamp? And, what would be a clever way of charging it save for an USB port on the back of the bass? Maybe there's a clever way for me to provide power via the signal cable?

[Seljer]

Theres plenty of ways to provide power over the signal cable Either go the full phantom power route with voltage on the signal lines like a microphone or DI box would use, or just use the ring section of a stereo jack to supply +9V.

[amptramp]

Check out the Tillman preamp on the first post in this thread:

http://www.diystompboxes.com/smfforum/index.php?topic=97593.0

Note that you could put the JFET and its source and gate resistor in the guitar and the drain resistor and power supply in an outboard box (no stompswitch required).  R1, R2 and a FET do not need to be surface mount devices (unless you want them to be).  R3, R4, C1 and C2 would fit in an outboard 1590A along with the battery or power jack, so there is no need for a battery in the guitar.  For bass, you could increase the capacitor values.

[egasimus]

I actually have no idea why onboard electronics are much more popular with basses than with guitars. Maybe it's about not having to lug around an extra box, maybe it's about the accessibility of the tone-shaping controls.

On the other hand, having a piezo pickup does necessitate having an onboard preamp, and I plan to add one along my single P-pickup. All in all, I still think I don't want to rely on constant external power, and would very much like to see how a preamp fares on a cell phone battery

[Mark Hammer]

I actually have no idea why onboard electronics are much more popular with basses than with guitars. Maybe it's about not having to lug around an extra box, maybe it's about the accessibility of the tone-shaping controls.

In some respects, it may be because guitar players are more likely to be using an array of pedals that will alter the tone moreso than anything situated on the instrument.  It is also fair to say that getting a pleasing clean tone is more important for bass players than for guitar players, so they will either lean towards low-impedance/low-output pickups and an on-board preamp, or onboard buffering to avoid tone suckage from that single long cable to the amp.

[amptramp]

Alembic started the ball rolling with basses that had an on-board preamp.  Their low-impedance low-output pickups needed some boost and some tone controls were added that used the internal preamp.  A lot of people were happy to pay three times the price of a Fender Bassman for a quality instrument and the internal electronics were part of the reason for the good sound.  A lot of bassists would like to have an Alembic or something just like it and some other brand with a preamp installed after the fact sounds like a cheap way to get Alembic sound out of a lesser bass.

[Gurner]

Anyway, what bothers me most - besides the possibility of not being able to squeeze 2 switched and one double-tiered pot in there - is that, even for tamer control schemes, it would be hard to fit both a PCB and a 9V battery in the control cavity. Therefore, I'm thinking about using a rechargeable battery, and a voltage multiplier.

I'd strongly recommend you find room for a battery (smaller 9V battery - http://www.gadgetbatteries.co.uk/scripts/prodView.asp?idproduct=959 ....if you can't fit that battery  in your guitar, then you're gonna struggle with any solution I reckon!).

Voltage multiplers can quite bad for oscillator spewage into the guitar signal (unless you use a multiplier IC that operates above hearing range)

[artifus]

^ and remember to carry a big bag of spare batteries around with you for every gig and session you intend to use it on, just in case, for peace of mind - and to check out your local environmentally friendly disposal services too.

[Gurner]

^ and remember to carry a big bag of spare batteries around with you for every gig and session you intend to use it on, just in case, for peace of mind - and to check out your local environmentally friendly disposal services too.

agreed, the current delivery is pants. I wasn't advocating that as the sole final solution, just feeding in alternatives....you could parallel them up, where space allows (egasimus never indicated where the dimension limitations were...if it's simply battery width, then a few of thiose small batteries in parallel will do. They're fairly cheap batteries too...

http://www.ebay.co.uk/itm/Trade-Lot-20x-Alkaline-batteries-9-volt-10a-L1022-MN10-/330429339541?pt=UK_ConsumerElectronics_Batteries_SM&hash=item4cef1f9795#ht_1753wt_1163

about 40p per battery (about 60c for those who don't understand UK currency.).

I too thought multipliers would be a great solution...but wrestled with them for more nights than I care to declare....

[artifus]

[amptramp]

This is all you need:



Only the JFET, R1 and R2 need to be in the guitar.  The rest can be in an outboard box and even with a battery, you might be able to pack it all into a 1590A.  The guitar output will be the JFET drain and the output impedance will essentially be R3 in parallel with R4.  The impedance is sufficiently low to drive cables on a stadium-width stage.  There is no need for a battery in the guitar.

[Earthscum]

Only the JFET, R1 and R2 need to be in the guitar.

I have been considering this on my guitar and bass and technically only R2 and the fet need to be in the body. R1 is pulling the gate bias to ground which  is already accomplished by each pickup and pot.

[Earthscum]

Thought of something brilliant to go along with Ron's idea. My bass has a stereo jack already. Run Drain to middle and use a stereo cable. Just set up your ext box to run signal from tip to passive out and middle to active out. If you use a mono cable with a different rig, the Drain just gets pulled to ground. Only thing I can see being a problem is if the fet acts like a diode and excess signal drops from Gate to Drain.

[Mark Hammer]

Only the JFET, R1 and R2 need to be in the guitar.

I have been considering this on my guitar and bass and technically only R2 and the fet need to be in the body. R1 is pulling the gate bias to ground which  is already accomplished by each pickup and pot.

Correct.  I've told this before, but the very first onboard FET preamp I ever made was in 1976, and it was built into the head of a rat, and attached to the skull.  The purpose was to provide a suitably buffered-and-boosted output of an even lower output and higher-impedance source - a brain electrode - that we could run 50ft from a freely-moving animal to en EEG recorder in another room.  And, just as amptramp notes, the FET and the R2 equivalent needed to be on the animal (i.e., as close to the signal source as possible), but all the other components were situated in a separate box en route between the rat and the EEG amplifier.

A large part of what I know about signal processing - filtering, gain structure, noise risks, etc. - came from learning to record electrophysiological signals, be they muscles, EEG, EKG, or other.

[Gurner]

This is all you need:



Only the JFET, R1 and R2 need to be in the guitar.  The rest can be in an outboard box and even with a battery, you might be able to pack it all into a 1590A.  The guitar output will be the JFET drain and the output impedance will essentially be R3 in parallel with R4.  The impedance is sufficiently low to drive cables on a stadium-width stage.  There is no need for a battery in the guitar.

That's a cool idea, but the guitar is only gonna be playable with that external box to hand (ie it's not really  "I'll just take my bass to rehearsal & use whatever amps are there" type solution)

Another option is to build the tilman into a guitar cable, which at least then the bass remains playable in the absence of all the add ons we're talking about here.

[amptramp]

This is all you need:



Only the JFET, R1 and R2 need to be in the guitar.  The rest can be in an outboard box and even with a battery, you might be able to pack it all into a 1590A.  The guitar output will be the JFET drain and the output impedance will essentially be R3 in parallel with R4.  The impedance is sufficiently low to drive cables on a stadium-width stage.  There is no need for a battery in the guitar.

That's a cool idea, but the guitar is only gonna be playable with that external box to hand (ie it's not really  "I'll just take my bass to rehearsal & use whatever amps are there" type solution)

Another option is to build the tilman into a guitar cable, which at least then the bass remains playable in the absence of all the add ons we're talking about here.

You have some options here.  You can make a small box that has a male plug for your guitar and a female plug for the cable.  You just attach it when you want amplification, but it has to have all the components in it including the battery.  There are small key fob 12 volt batteries that are ideal for this.  A 1590A would be plenty big enough.

Another idea: You could gut an old cellphone and put the components inside.  You run a short cable to the guitar and have a normal jack for the output.  The cellphone case can then be put in a belt clip and you can wear it on your belt.  Older cellphones may have enough thickness for a standard jack.  Old transistor radios can also give up their cases for this use and they are more likely to be able to take a standard jack.  Use this style of jack to plug a box or cellphone or radio case directly into a guitar, remove the top and bolt the bottom section directly to the case:

[Earthscum]

This is what I was thinking as far as being able to use stereo for pre and mono for passive for other rigs...