On board Bass preamp

[Skywalker83]

I'm looking for a schematic for a D.I.Y onboard preamp for a passive bass guitar. My bass currently has 2 vol (2 x soapbars) and 1 tone control. I'd like a preamp that has an active/passive switch, and a 3 band EQ, with a mid sweep pot if possible.

Does anyone know any sources for finding such a schematic?

[Ben N]

If you look around, you should be able to find schematics for Bartolini, Musicman or Fender onboard preamps. But this should not be that difficult to design on your own. The simplest thing would be a jfet-input, low current opamp, like a TL062, with the first stage configured as a non-inverting buffer or low gain stage, and the second stage as an inverting active Baxandahl. For the second stage, look at the eq section of this preamp: http://www.albertkreuzer.com/pics/el...eamp_sch21.gif. (The first stage of that Kreuzer design is inverting, and therefore has too low an input impedance for this purpose without another buffer stage ahead of it.) For bypass, all you need is a DPDT switch somewhere (there is plenty of info on the web abouit how to bypass effects--same thing just with a toggle or push-pull switch.) Also keep your existing volume control ahead of the preamp--if you give up the bypass option, you can use a 25k audio taper volume control after the preamp for better performance.

Alternatively, you could use the Kreuzer eq section as is, and add the jfet buffer in front of it from http://www.muzique.com/lab/buffers.htm.

Either way, I would suggest you breadboard your design and test it with your bass's tone control disconnected and vol control maxed so you can tweak the gain for minimum distortion.

[Skywalker83]

The set up i would want would be

Tone - for both Passive and Active
Treble - Active
Mid/Mid Sweep - Stacked pot- Active
Bass - Active
Pickup Blend - Passive and Active
Volume - Passive and Active
Passive/Active Switch


There's preamps made by John East which is almost exactly like what i want , but i reckon if i can figure out how to make it (with my modifications) it would be alot cheaper. My design is a hybrid of the U-Retro 01 Deluxe and the U-Retro 5 Knob deluxe.

John East Preamps

[Ben N]

I don't know how to do the mid/sweep stacked pot--you would probably need a different mid circuit than the modified Bax, perhaps something with an inductor. As for the rest, just leave you passive vol & tone controls in there, ahead of the preamp, and substitute a J-bass style blend pot or a balance control (available from Stew-Mac) for your p/u switch. This would work better in an all-active bass.

Do you really have room for that many knobs?

[Skywalker83]

Yeah i reckon i have enough space, i might need to use some slightly slimmer knobs for a couple of the pots, but it should all fit. I was going to use a blend pot for the pick up selection anyway.
i reckon it's going take some time to design the circuitboard as this is a pretty much unique set up. Unfortunatley i cant find any schematics for any preamps which are similar in nature to this one, such as Fodera, or the John East. I'm going to have to start piecing together combinations from various circuits.