Can anyone help me determine what I have here?

[Kurt Veltman]

I have a wiring harness for a bass that I bought a few years ago, but I can't remember from where. It did not work out for the project it was intended for, and I want to use it now, but without a wiring diagram, I don't know what pot does what, or how to hook up the pickups and battery. I realize this might be a long shot, but I am hoping there is someone out there that will enjoy the challenge. I can do the soldering if someone tells me what goes where, but don't really have a clue about myself. I have enclosed a pic of the entire harness and the only identifying mark on the label. The only open/loose wires are a red one and a black one.

Thanks for any help you can offer

[Mark Swanson]

Two questions, do you know where the output jack is, and is the red wire a shielded wire, or just a single?

[Kurt Veltman]

Mark, the red wire appears to be just a single, unsheilded wire. As for the output jack, it did not come with one. I have my own that I have to work in.

[Mark Swanson]

The black and red wires are probably power wires- is the PC board marked with a + or a - for those wires? I'd want to just hook it up on the bench, and then you can see what each control does.

[Kurt Veltman]

There are no markings for the red or black wire. The red goes to the board, but is not marked. The black goes to the bottom of the stacked pot, to the frame, as if to ground it.

[Greg Robinson]

Ok, a few guesses here.
Red wire is positive, goes directly to the battery.
Black wire goes to sleeve of output jack.
Negative terminal of battery goes to ring of output jack.

This preamp is for one pickup only right?

The pot labels indicate:
1: 100k linear
2: 100k linear
3: 10k audio
4: 100k linear

The other parts of the code would indicate the detents, push-pull switches, etc.

I would guess that number three is the volume control, maybe the push-pull allows passive? Number 1 is a middle/frequency control. The other two would be treble and bass.

The pickup is probably supposed to attach to the tone controls somewhere, and the output will probably come from the volume control. Strange that there are only two unsoldered wires, makes figuring it out much more tricky!

[Kurt Veltman]

Thanks for your input Greg. Someone else (not on here) mentioned that the push/pull could be for passive/active, or possibly dual/single coil. They also said what you did about #3 being volume. I was hoping this would work for two pickups, since that's what I am using.

[Greg Robinson]

You could use it with two pickups and a switch, no problem. Just most bass players seem somewhat obsessed with the idea of individual volume controls for each pickup, which I don't think this offers. You could build a small jfet or opamp buffer if you really needed individual volume controls though.

[Kurt Veltman]

I'm not concerned with separate volume controls, but would like a pan/blend control to blend output between the two.

[Greg Robinson]

To do that, you would probably need to buffer the two pickups, or at least the blend control.
Unless there's some input on the board itself that I'm not seeing, then the maximum input impedance of it is 100k, which is not particularly high. Trying to insert a blend control between even low-impedance pickups and a 100k input would be very tricky without a buffer.

[Kurt Veltman]

I was keeping up with you pretty well until this last comment. I am very much a novice at electronics, so it doesn't take much to get over my head.

[Greg Robinson]

Ok, sorry! What I'm really saying is that you can't just throw a blend pot in there without losing a lot of high frequency response and probably a lot of output level without adding some active circuitry to buffer it. So probably won't be practical if you don't have a decent bit of experience with electronics.

[Kurt Veltman]

Not what I was hoping to hear. This is going to make my electronics ignorance glaringly obvious - is this thing not a preamp? If not, what is it? And why two little blue boxes (the name escapes me) if it is for only one pickup?

[Greg Robinson]

Yes, this is a preamp. The problem is trying to incorporate an extra control that wasn't designed into it.
It seems that there is only a single input to this preamp, and it seems like its a fairly low input impedance input at that. The problem is that when coupling between stages, you need the input impedance of the successive stage to be higher than the output impedance of the previous stage. The rule of thumb for audio is that the input impedance should be 10x the output impedance. This provides "voltage coupling", which means that the signal is kept at a decent level and not attenuated. Any frequency response of this interstage coupling is designed around this rule of thumb. If the output impedance is too high, then you lose treble response. If the output impedance is lower than expected, you get better voltage coupling, and extended frequency response, which is not normally a problem.

How all of this applies to your problem, is that there is one input, with an impedance of 100k. Most passive pickups are going to be around 10k. So, this preamp is well designed to accept a standard passive pickup, the input impedance is around 10 times the expected output impedance.
Now, if you had two pickups, and wanted to blend them, then you would need a blend control with around a 100k input impedance to accept the pickups. You could mix the two signals with a passive resistive mixer, which doesn't work very well, or you could use an active mixer.
If you use a resistive mixer, then the output impedance will be at minimum 100k. This will not couple well directly to the 100k input of the preamp. You could buffer this though, and still have it work.
Better would be to use an active mixer, which would have a low output impedance anyway, probably around 1k for a well designed discrete jfet mixer, or around 100ohms for an opamp mixer.

So you see, adding what seems like a simple control can get quite tricky when dealing with preamps.


Those two blue boxes are called trim pots. They are meant for adjusting the performance of the circuit, and are not intended to be used often. They are set and forget. Usually on a preamp they will set the gain, or maybe the q-factor of the tone controls or something like that. One will definately set the maximum gain of the circuit, not sure what the other one will be.

[Kurt Veltman]

Wow, that's a brain-full. Thanks for all the info. I'm getting the very strong sense that I would be better off saving this for a single pickup bass, and getting a different preamp for this one. Hopefully things start falling into place with this thing - I'm sick of it fighting me!

Thanks Greg!

[Greg Robinson]

No worries Kurt.

Remember though, I am making a few assumptions based on the info you've provided, so some of what I've said may not exactly apply to this preamp. But, unless you can find out or remember what it is and get some documentation, it's probably going to be best to go with something else for a two pickup instrument with blend control.
Also, if you're using a 3 way switch it will be no problem (neck, neck & bridge, bridge), it's just a blend control which would cause problems.

Good luck!

[Halgeir Wold]

Without a proper wiring and/or schematic diagram, this could really be a long shot!
There's a row of 5 transistors along the top long side of the PCB, and one opamp ( the 8-legged bug)....
This coud be a version of the Kreuzer pre-amp, but just by mentioning this, it could pull opinions i a faulty direction.
Are there any markings or components on the other side of the PCB?
Any recollection of type names, supplier, anything that might give a clue?
There's literally dozens or maybe hundreds of variations floating around on the net....

[Kurt Veltman]

Halgeir, sorry - I don't have anything but what is posted here. The best clue would be the numbers on the wrap around the wires. The back of the board has velcro stuck to it, but I will try to pull it off to see if there is something back there.

[Halgeir Wold]

The number on the cable looks more like a date code for the cable, but I may be wrong.
Are there any numbers on the 5 transistors ( the 5 3-pin bugs in a row along the top of your first picture)?
Type number on the op-amp? The 3 round little cans by the red wire are electrolytic capacitors. These have a + or - ( or both) marking on the side. Is the red wire routed to the + end of one or more of these capacitors?

[Kurt Veltman]

Woohoo! I peeled the Velcro off the back (good GOLLY that stuff is sticky!) and found: Seymour Duncan 412125 RO2. Now I have something to work with!