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jasonconanan 23rd December 2009 08:40 AM

bass onboard preamp circuit
does anyone know where i can get circuits for an onboard preamp circuit for an active 6 string bass? or any active bass for that matter?
im trying to redo mine.

the knobs would have to be
volume bridge pickup
volume neck pickup

or i really dont know. it has spaces for 4 knobs and i just really want to control the volume for both pickups and i dont know how to make a passive preamp circuit?

i saw someone make an active/passive circuit, with a switch controlling them?

this is my first time to try and do this so if anyone has any suggestions and tips i'd be very very welcome. :D thanks

Minion 23rd December 2009 05:38 PM

Well I suppose if you want to controll the Volume and tone for each pickup seperately then you would Wire the Tone and Volume of each pickup seperately with the output of each Volume pot comeing together to the 1/4in Jack , If you wanted to put in an active preamp then you could wire it between the Volume pots and the 1/4 in Jack .....

Ive built an active preamp in both my Guitar and bass and they work really well but my Guitars were allready wired up so I just installed it between the Volume and 1/4 in Jack ......

If your Bass isn"t wired up then wire up each pickup like this : Guitar Wiring Diagram 1 Humbucker/1 Volume/1 Tone

(I know it is for guitar but it"s the same wireing)

Then install the preamp between the Volume pots and the 1/4 in Jack ....

This is the Preamp I used :

If you decide to use it you might want to set the gain a bit lower , and increase the Value of the cap in the feedback loop , you will also need to get a TRS Jack so you can wire the battery so it gets disconnected when you remove the patch cord ....


jasonconanan 25th December 2009 06:22 AM

thanks. i have a question though, in ur preamp circuit,
would i have to change any of the values for a bass?

also, i found out that my pickups had a resistance of about 20kohm each.
this is also the impedance right?

so if i were to make a volume control for each pickup, i would have to use 20kohm potentiometer right? i plan to use a b245 FET

Nigel Goodwin 27th December 2009 02:42 PM


Originally Posted by jasonconanan (
also, i found out that my pickups had a resistance of about 20kohm each.
this is also the impedance right?

No, the impedance will be considerably higher than that - it's also VITALLY important that you feed it into a much higher impedance, otherwise you ruin the sound. High 100's kilo-ohms, or low mega-ohms are where you should be aiming.

Minion 27th December 2009 05:33 PM

Hi , I would change the 68pf cap that is in the feedback loop to say 220pf for bass use , you might also want to adjust the Value of the 100k resistor in the feedback loop , this controlls the gain , you might want to lower it a bit depending on the output of your pickups and how much gain you want ......

If the impedance of your pickups is 20k then that is a bit high for pickups but not a problem , the preamp has a input impedance of 470k which is plenty high enough for 20k pickups and with both pickups in paralell they will be 10k total .......

If you want Bass and treble controlls that will controll the Bass and trebble for both the pickups and a volume for each seperate pickup then I think you will need to wire the Volume pots first , then paralell the Volume outputs which then go into the bass and trebble controlls which then go into the Preamp section then to the 1/4in Jack ......

I warn you before hand that Passive bass and trebble controlls on a bass guitar suck , they only cut frequencies and don"t boost them so You will allmost never use them , I would suggest that you build an active tone controll set up or build an EQ pedal ......

You don"t need a Jfet to controll the volume, just a pot , and I would suggest useing 250k pots for Volume controlls .......


jasonconanan 27th December 2009 07:04 PM

oh ok. thanks for the help with the impedance thing. i thought that it was supposed to do that so that i could deliver maximum power to the preamp circuit. saw it in an article on impedance matching. thanks for the correction!
ive been studying electronics engineering but sometimes everything gets so theoretical that i don't really know how to apply some of the stuff.

i think im going to stick with the albert kreuzer design.

but i want to make it an active/passive circuit

so, can i just put an on/off switch that bypasses the JFET stages and goes directly to the tone? also i would have the same switch bypass the 2nd jfet stage in albert's design and just have it end with a 1meg ohm resistor parallel to the output?
this switch would also turn off the 9v supply. like a SPTT (single pole triple throw) if i can find one.

i'll post the design design in a few days. just downloading software.

i cant seem to find the j201 nor the bf245 in my local electronics store so i might have to change to the mpf-102 design by james hawes

FET Preamplifier, part 2

thanks in advance and happy holidays!

jasonconanan 29th December 2009 05:44 PM

this is my temporary design.
i have yet to put it on a breadboard so its all theoretical

i made a few edits to albert kreuzer's design
first, i changed the bf245 to j201, they draw less current

next, i removed the switches to the tone part of the design

also i added a passive/active switch, in the form of an SPDT.
the first part is between the unamplified outs of the potentiometers, and the amplified outs from the j201s, which goes to the tone part of the circuit.

next, from the tone circuit, it either goes to the next amplifier stage, or directly to the out.
for the passive, it doesnt go through either of the amplifiers, just to the 4.7uF cap then out to the 1Meg resistor to out.

so what the switch does is either
pots -> tone -> out
pots -> amp -> tone -> amp -> out

also had to add a 4th switch for the 9v battery. to turn it off when the switch does the first.

also, i moved the mute switch to the tone part out, so that i wouldnt have to turn the 2nd amplifier on if i wanted to mute the out in the passive mode.

please comment on the edits i made. thanks :D

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