Blend Pot Wiring - it's more than meets the ear

Turnaround

Turnaround

Commercial User
May 6, 2004
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#1
There are two basic ways to wire a blend pot, and a variation to one of them.

First, the conventional way that you will find on most production basses.

upload_2024-1-6_13-12-31.png

Here one pickup is connected to the centre lug of one gang of the pot, and the other pickup is connected to the centre lug of the other gang. The leftmost lug of the upper gang is connected to ground and to the right lug on the lower gang. This type of wiring uses the potentiometer as a variable voltage divider, sending more or less signal to the output and the other part of the division to ground.

The complaint most often expressed about this kind of wiring is that it doesn't take much rotation from the centre detent to throw the balance largely to one pickup. That is to say when you rotate the pot to favor the neck pickup, very little rotation is needed before the bridge pickup is almost eliminated from the audible blend.

To remedy this situation the wiring is changed from a voltage divider to a variable resistor in series with the pickup. So instead of splitting the signal between the output and ground it simply provides variable resistance between the pickup and the output stream.

upload_2024-1-6_13-14-11.png


There is no ground connection in this configuration, and thus no need to connect the black diagonal shown in the first pic. However, you should still ground the case of the pot.

The complaint about this scheme is that you cannot take one pickup out of the circuit entirely - roll the blend to the extreme neck pickup position and there will still be a bit of signal coming from the bridge pickup, and vice versa. For many it's not enough to be concerned about, while others feel the need to be able to completely eliminate one pickup from the signal.

Which brings us to the third scheme - the no-load blend. Our friend Walter has posted about this in another thread, and he will chime in with his solution shortly.

But I should mention another scheme I ran across - the single gang quasi-blend solution. It uses a regular linear pot wired like this:

upload_2024-1-6_13-25-50.png


TB member @sissy kathy commented so well on this configuration that I will just quote here:

"That single pot is using a mid line tap so there is always resistance to each pickup. The output of the pickups is never at 100%.You only ever get a continously variable split between the pickups. that equals 50% of the potential total output of the two pickups; that is 100% of either pickup at the extreme positions but the other pickup is near 0%. It'll make your amp work harder for a given volume."
 
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#2
right, the ungrounded trick "slows down" the sweep and also eliminates the extra loading loss on the signal; otherwise, a 500k volume and a 500k dual blend/balance pot is like having three 500k pots on the signal, each trimming off a little bit of volume and treble.

without that ground connection you can use a lower value blend, like 250k or even 100k, and it won't load down the treble; the lower value gives you even smoother shading between the two pickups. the problem there becomes that with the lower value blend pot the "off" pickup starts to become less "off", it might start influencing the sound more.

to my ear a 250k blend is plenty of series resistance to "dim" the pickup you want shut off; yes you can hear something when you tap on it with a screwdriver but when playing i only hear the tonal character of the "on" pickup.

to fix that tradeoff you can cut the traces of the blend pot such that when rolled all the way over the "off" pickup is disconnected entirely. that will give you the smoother shading of a lower-value blend and 100% pickup switching like a straight up 3-way toggle switch.

there's an entire thread about how to do that trace-cutting trick here (wow, i started that one ten years ago this year!)
 
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#3
as for the types of blend pot to use, i recommend the "M/N" type; they're like home stereo blend/balance pots in that at the center detent they're 100% volume for both sides, only turning one side or the other down when you turn it. one of those with the traces cut will sound exactly like a 3-way switch in the middle and at the extremes, no loss at all, while still allowing for shading in between

the other common type found in basses is "A/C", which leaves a little series resistance in even at the center detent; there's an argument that the resistance "isolates" the two pickups from each other a little and that this is a good thing, making for a smoother sweep at the cost of a little output. this type might actually sound more like the old-school method of running a vintage jazz bass where both volumes are backed off a little, again for the goal of smoother shading between the two pickups at the cost of some output

i'm not big on the single pot idea, that just loses too much signal in the middle setting
 
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#5
sissy kathy said:
Elucidate us; would the single pot idea give you a more consistent volume across the sweep of the pot?
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i really don't think so

a standard M/N blend pot has both pickups at 100% in the center detent and that setting is no louder than the individual pickups by themselves (maybe even slightly quieter due to the inherent loss of parallel coils)

a single pot blend would put significant series resistance across both pickups at the center setting, reducing the output
 
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