Two Pickups, Two Pots?

Uncle Krunkus

Hi everybody,
I feel like I should know this, but a friend asked me to re-wire his bass, and I'm kinda unsure about how to do it right.
The problem is there are two pickups, and two pots. No selection switch, no active circuitry. He wants it so each pickup has it's own volume control. The obvious, 2 volume style voltage dividers across the output, will short the other pickup out when when is turned down. I thought of reversing the pots, (wipers towards the pickups) but this puts a large impedance across the output. Doesn't it?
Any advice would be very appreciated.

Inventor · Posted: Sat Jan 31, 2009 5:09 pm Post subject:

That is a tough one. Perhaps try isolating the pots with series resistors on the wipers? It will be interesting to see what your solution is.

blue hell · Posted: Sat Jan 31, 2009 5:43 pm Post subject:

Hmm ... but then again ... a guitar always has high impedance ...

Uncle Krunkus · Posted: Sat Jan 31, 2009 5:58 pm Post subject:

I'm kind of happy that I saw putting a high impedance across the output as bad.
A typical volume pot on a guitar is about 500K, so with two of them turned around there would be a constant 250K (or less) across the output.
I still don't instinctively understand the input/output impedance thing, so I don't know if this is good, or why.
The other possibility is to go with 1 volume pot (500Klog) and a smaller passive balance pot (10Klin). This wouldn't allow completely muting either pickup though.
If it helps at all, a typical pickup has an impedance of about 5K to 15K.

blue hell · Posted: Sat Jan 31, 2009 6:11 pm Post subject:

Uncle Krunkus · Posted: Sat Jan 31, 2009 9:51 pm Post subject:

What about two parallel-pickup/pot combinations in series?
Using 500K log pots. When they were both wound out it would almost be like the pickups alone across the output. I hope. Smile

Looks like this,...

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Uncle Krunkus · Posted: Sat Jan 31, 2009 10:55 pm Post subject:

Filling in a broken wire, and inverting one of the pots, this is what I found out from the original wiring. I must admit that there doesn't seem to be alot wrong with this configuration. The only thing I don't like is that you can't get both on full. There's always a compromise between the two pickups. Is this a bad thing though?
What do you dudes think? Stick with this?

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Inventor · Posted: Sun Feb 01, 2009 1:52 am Post subject:

This article

http://buildyourguitar.com/resources/lemme/

has a table that lists pickup impedance values. Typical are 2.5 Henries of inductance in series with 5k of resistance followed by a shunt 100pF cap. There's your source model, it's a second order low-pass filter.

This is normally followed by the volume and tone pots and cable capacitance as you are aware. Whatever you come up with, you can do an AC simulation of it if you like. Good luck!

Uncle Krunkus · Posted: Sun Feb 01, 2009 3:47 am Post subject:

Inventor · Posted: Sun Feb 01, 2009 3:52 am Post subject:

Inventor · Posted: Sun Feb 01, 2009 3:59 am Post subject:

Here is one possible circuit, Uncle Krunkus.

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Uncle Krunkus · Posted: Mon Feb 02, 2009 3:44 pm Post subject:

Here's the same pic with more details.
I'm fairly sure the 10K Log pot needs to be a linear one.
What do you guys think?
Should I check the DC resistance of the pickups? Yeah, I think I will JIC.

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Uncle Krunkus · Posted: Mon Feb 02, 2009 8:16 pm Post subject:

Okay, I've changed my mind again. It should be a Log pot.
I've dicovered a bigger problem though. The main reason he wanted it re-wired is that the 10K pot "seems" back to front. Because,... he's got those gold hat type knobs which show "10" when the pot is fully counterclockwise.
So,... short of finding a 10K antilog pot (I don't like my chances there!) I think I should sub a linear 20K with a 20K resistor around one side to simulate an antilog pot. This actually could result in a far smoother log response than you can get from any "log" pot anyway.
Does that make sense to anyone else? I hope it does.

Danno Gee Ray · Joined: Sep 25, 2005 Posts: 1351 Location: Telford, PA USA

Why not just flip the wires on the pot?

Uncle Krunkus · Posted: Tue Feb 03, 2009 4:04 pm Post subject:

That's fine if it's a linear pot. But if it's a log pot it doesn't work, as the curve in the response is right for the higher volumes to be at the clockwise end.
That why we have (although they can be hard to find) antilog pots. They have a similar taper, but the other way around.
Log taper are designated "A"
Linear are designated "B"
And antilog are "C"

Danno Gee Ray · Joined: Sep 25, 2005 Posts: 1351 Location: Telford, PA USA

Roger that...DUH...Bummer.

Knobs without numbers?

...Bummer.

Uncle Krunkus · Posted: Tue Feb 03, 2009 4:53 pm Post subject:

Inventor · Posted: Tue Feb 03, 2009 6:13 pm Post subject:

Krunky Uncle, why are you using log pots? My thought is that linear pots would work best here. Also, no comment on my differential suggestion - it would have a balance pot and a volume pot which would be really easy to set, more intuitive I suppose.

Uncle Krunkus · Posted: Tue Feb 03, 2009 6:59 pm Post subject:

I only used the log pots 'cos they were there when I opened it up. I must admit that the differential idea is looking better all the time.

Inventor · Posted: Tue Feb 03, 2009 7:10 pm Post subject:

Uncle Krunkus · Posted: Tue Feb 03, 2009 7:50 pm Post subject:

I just found out (from a guitar site) that the reverse pot concept (pickup to wiper, full value across the output) is fine, and is the way jazz bass wiring is done. When the volume is up you have a much smaller impedance (the pickup) across the output, and you want the pot to be in the region of 250K-500K anyway so that you don't lose too much top end to ground.
So now I'm thinking of trying that with two 500K linear pots. A fixed resistor (about 680K) between the wiper and the top should logarise(?) the control.
Man,... I never thought getting this stuff right would be so fraught with hard decisions! Laughing

Inventor · Posted: Tue Feb 03, 2009 8:01 pm Post subject:

Uncle, I'm surprised that that's what works, but OK whatever. Let us know how it works out!