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blend pot problems

A

Al Heeley

Dec 2, 2007
635
20
West Yorks., UK
#1
Blend pot for jazz bass
I have a problem with the blend pot wiring of my jazz bass (using the standard wiring schematic off Seymour duncan's site) - the centre detent position of the pot retains some residual resistance, which means that the combined blended pickup signals are significantly quieter than either the full bridge or full neck position.
In fact, the output drop - off is significant as soon as the pot moves below about 8 or above 2 position if you imagine a zero - to - ten range.
This volume drop means it is not a useable configuration for effectively mixing neck and bridge pickup signals together.
There must be available somewhere a true blend pot which does not give this residual resistance in the middle position, and allows for constant output level as the pot is turned from 0 thru 10. Any ideas? (I have heard some Stewmac blend pots also suffer from this)
 
#2
Normal enough for a blend and volume drops when maxing two volumes as well. There's been a good thread on this with numbers posted by Audere - try a search. Given as much I assume it's not a useable configuration for mixing while retaining the same volume without also adjusting the volume control.

Never looked into it cause it's not something I consider a problem but I've heard of active blends and I've read accounts of pre blends where the mix is smooth for whatever reason - but it's not the norm.
 
#3
The problem is not all blend pots are made the same. When it's done correctly, you get pretty much the full level at the center detent. The pots much be log/anti log taper, and even then, there are several tapers possible.

Some blend pots are also linear taper. So in the center, you have half of what ever the value of the pot is in your signal path, and to ground.

Where did you get the bled pot? I've seen some, like the one from Stew-mac, where both halves of the pot are audio taper, but one should be reverse taper, and it's not. But the pot in the center and take a reading on a ohm meter from the center lug to each half. You can see that the top half is reading the same as the bottom half, when it should be reversed.

So in essence, you never really get the full volume of both pickups when you are in the center detent.

I switched to two volumes on my basses for that reason... until I find a good source for blend pots.
 
#4
A lot of this stuff is covered in great detail here:
Link Removed
along with a link to the real truth behing log taper and audio taper pots - SHOCK HORROR! proper log pots are practically extinct for guitarists, they just use 2 different resistance ramps to very crudely approximate a log taper.
The blend pot I got was from AxesRus, UK - they are good people and very helpful. They state the pots are from the same source as Stewmac. There is only a tiny resistance left at half way but its enough to cause a signifcant output drop, and effectively render this wiring scheme useless in a gig environment.
 
#5
Al Heeley said:
A lot of this stuff is covered in great detail here:
Link Removed
along with a link to the real truth behing log taper and audio taper pots - SHOCK HORROR! proper log pots are practically extinct for guitarists, they just use 2 different resistance ramps to very crudely approximate a log taper.
Click to expand...

I can't get that link to work. Is that the same site as guitarnutz? If it is I wouldn't pay any mind to anything on there.

But you can indeed buy real log taper pots. http://www.potentiometers.com/ You can have any pot you need made there.

Al Heeley said:
The blend pot I got was from AxesRus, UK - they are good people and very helpful. They state the pots are from the same source as Stewmac. There is only a tiny resistance left at half way but its enough to cause a signifcant output drop, and effectively render this wiring scheme useless in a gig environment.
Click to expand...

OK, you have to get the pot wired up the right way. At the halfway point you will have more resistance on one side of the wiper than the other. This is because it is a log taper pot. You want the smaller resistance to be toward the output, and the larger resistance toward ground. The problem is the other section of the pot should be going the opposite way, but it doesn't. So on that half when you are at the halfway point, you have a small amount of resistance between the pickup and ground, and a larger amount between the pickup and the output.

The StewMac pot actually seems to work better than it should, but it's still not a real pan pot. It's a stereo volume control with a center detent.

You can try getting one from Duncan, theirs seem to be right (at least the last time I saw one, which was a while ago.) The EMG pot (which is the same make as the Duncan) also works correctly, but it's low resistance. My SoundGear bass had a got pan pot too, and the one I saw in a Padulla also worked the right way. So the good parts are out there.
 
#6
Yes thats the guitarnutz forum. Why do you say "I wouldn't pay any mind to anything on there."?
I've always found their advice and info spot on.
Pot's wired the right way - I double check, dismantled and redid it.
I will have to continue my search for a proper blend pot ;)
 
#7
Al Heeley said:
Blend pot for jazz bass
I have a problem with the blend pot wiring of my jazz bass (using the standard wiring schematic off Seymour duncan's site) - the centre detent position of the pot retains some residual resistance, which means that the combined blended pickup signals are significantly quieter than either the full bridge or full neck position.
In fact, the output drop - off is significant as soon as the pot moves below about 8 or above 2 position if you imagine a zero - to - ten range.
This volume drop means it is not a useable configuration for effectively mixing neck and bridge pickup signals together.
There must be available somewhere a true blend pot which does not give this residual resistance in the middle position, and allows for constant output level as the pot is turned from 0 thru 10. Any ideas? (I have heard some Stewmac blend pots also suffer from this)
Click to expand...

The common pot for a passive blend is the type stamped M/N, often called a balance pot. With this type, when the the pot is set in the centre click, there is zero resistance to each pickup and so both are full level.

For each track, the resistive section only covers half the rotation, and from the centre to the other end, is a zero ohm section. The two sections are mirror images.

I'm certain the Bartolini blend, a Noble pot, is this type, and there are others too. There is a wiring diagram on the Bart site, for reference.
 
#9
Al Heeley said:
Yes thats the guitarnutz forum. Why do you say "I wouldn't pay any mind to anything on there."?
I've always found their advice and info spot on.
Click to expand...

A lot of the information is bogus. For example, they go on and on about star grounding and ground loops, but it's absolutely not needed in an electric instrument, like a bass. Nothing in the bass has enough of a difference in potential to create a ground loop. Ground loops occur when you have two pieces of equipment, like your amp and the house PA system, both with a ground connection to the mains, and they are patched together, like with a DI box or output on the amp. This is why they have a ground lift switch.

The other dumb stuff there is wrapping a bare copper wire around your finger and connecting it to your bridge to get rid of noise.

They also show how to shield pickups by wrapping foil around them, which is the quickest way to kill the tone of your pickup, because you are creating a closed loop of conductive material around the coil.

And as I said that "truth about pots" is incorrect. Maybe it's true of certain cheap brands, but I've never found a pot like that.

Al Heeley said:
Pot's wired the right way - I double check, dismantled and redid it.
I will have to continue my search for a proper blend pot ;)
Click to expand...

Unfortunately, with that pot, it will be right for one pickup, and not the other.
 
#10
DavidRavenMoon said:
A lot of the information is bogus. For example, they go on and on about star grounding and ground loops, but it's absolutely not needed in an electric instrument, like a bass. Nothing in the bass has enough of a difference in potential to create a ground loop.
Click to expand...

That's an opinion that is not shared by many, many professional luthiers. I agree with some of your other points, but that one is - to be highly charitable - HIGHLY debatable.
 
#11
Pilgrim said:
That's an opinion that is not shared by many, many professional luthiers. I agree with some of your other points, but that one is - to be highly charitable - HIGHLY debatable.
Click to expand...

Think of an effects unit. You have a metal enclosure, and all the pots and switches are making a connection to that chassis. No ground loop.

The pots and any shielding in a bass or guitar are all part of the same ground plane. having more than one ground wont do anything because nothing has a higher or lower ground potential.

many professional luthiers don't know much about electronics. And many many instrument are made, and work properly without star grounding.
 
#12
Interesting study done here by Chris on the Gnz forum about BLEND and PAN pots.
Link Removed
His work shows the difference between true blend and pan pots, as well as the inherant resistance of pan pots at the centre detent point, which causes the drop in output. Trouble is, these guitar parts outlets are selling pan pots (boo hiss) as blend pots (hurray) and there's little to tell one from the other until you've bought it and soldered it in place.
 
#13
Al Heeley said:
Trouble is, these guitar parts outlets are selling pan pots (boo hiss) as blend pots (hurray) and there's little to tell one from the other until you've bought it and soldered it in place.
Click to expand...

I agree that retailers should give better information on what kind of pots they sell, but the answer to telling one from another is a multimeter. What we want are pots that bring each side from 0 to 100% at the center, instead of crossing in the middle with 100% at the ends. A cheapo multimeter will tell you what you have in about 10 seconds by running each section (top and bottom in a stacked pot) through its complete rotation and watching the resistance reading. If the resistance keeps going up (or down depending on how you are measuring) when the pot passes the center detent, then your circuit will not work the way you want it to. You basically have partial volume of each pickup when the pot is in the center detent.

Passive blend setups have yet another thing working against them: pickups are reactive not resistive and don't behave in a linear fashion like a resistive load. You also have to be very aware of how each pickup is phased. You could have the right pot and still not have it work correctly based on the nature of the two loads and their relative phase. A well matched pair of pickups with a known phase relationship will have the best chance of working correctly.

I worked with a passive blend setup on two different jazz bass setups and it never worked right. I had a lot of desoldered parts and build notes for my trouble. After that experience I feel that an active blend is a much better way to get the desired results.
 
#14
Check Duncan's schematic against the one on the Fender website, available in the Mr. Gearhead section for your particular Jazz bass....You will also find the Fender part number for the original pot used in the guitar.....Hope this helps
 
#15
Al Heeley said:
Interesting study done here by Chris on the Gnz forum about BLEND and PAN pots.
Link Removed
His work shows the difference between true blend and pan pots, as well as the inherant resistance of pan pots at the centre detent point, which causes the drop in output. Trouble is, these guitar parts outlets are selling pan pots (boo hiss) as blend pots (hurray) and there's little to tell one from the other until you've bought it and soldered it in place.
Click to expand...

Yeah, as I said originally in post #3, the pots being sold from places like Stew-Mac aren't the correct pot. They do exist however and when you get a pan pot from people like Bartolini, EMG and Duncan, etc., it's the right part.

As John East said:

John East said:
The common pot for a passive blend is the type stamped M/N, often called a balance pot. With this type, when the the pot is set in the centre click, there is zero resistance to each pickup and so both are full level.
Click to expand...

That pot they show in the middle photo, the 250K Fender pot, is the one marked M/N that Bartolini also uses.

The M/N is signifying the taper.

Here's an example of some tapers available on pots.
 
#17
Well, the first thing to do is to make sure your pickups are wired in phase through the blend pot. Anytime there's a significant volume loss with both PUPs on full compared to any other combination, I suspect that the PUPs are wired out of phase.

A real blend pot will have resistance over only half the travel of each gang. If they're wired correctly (which means you need to make sure you've got the PUPs wired to the correct terminals of each gang) then starting with the pot fully counter-clockwise, as you roll towards the center, it's not turning DOWN the bridge PUP, but it's turning UP the neck PUP. So the bridge PUP is on full from fully CCW to center, and the neck PUP gradually (or not- taper is another factor) is being turned up. At center both PUPs are of full, just like having a typical V/V/T set up with both PUPs wide open. From center as you continue clockwise, it's already got the neck PUP on full, and it's rolling the bridge PUP OUT of the circuit.

It's easy to see with a proper schematic (NOT a simple wiring diagram, but a schematic that shows what's going on electronicallY) but I don't have a way to draw a legible one and post it...

But the OP's problem sounds like it works OK until they get to center where there's a big drop off. That sounds like simple out of phase to me. Check the resisance track with a decent VOM then make sure you've got the PUPs wired correctly. That means to the correct gang of the pot to get the movement you want, and that the hots from the PUPs are connected correclty.

jte

Ch
 
#19
OK here's a link to my quick 'n' dirty schematic that shows how to wire a real blend pot and compares it to the volume controls on a typical V/V/T Jazz bass set up. I eliminated the master volume and the tone controls for clarity.

Yeah, I understand that the extra volume control used for a master volume does load passive PUPs some, but that's got nothing to do with volume loss in the center.

http://farm4.static.flickr.com/3499/3284625829_ba5bc22451.jpg?v=0

3284625829_ba5bc22451.jpg


If the wiper is all the way to the right, the N PUP's hot is grounded while the B PUP's hot is connected directly to the jack's hot. There's the resistance track between the N PUPs hot and the output jack so some signal will leak through, but it's the same as the V/V above it with the neck PUP rolled all the way off. As the wiper moves to the left, there's no difference to the B PUP's connection to the hot of the jack. But the N PUP's seeing more resistance between it and ground, and less between it and the hot of the output.

At center, both PUPs are wide open

As it rolls to the left, the N PUP is still wide open, and the B PUP is seeing more resistance between it and the output jack, while seeing less resistance between the PUP and ground. All the way to the left and the B PUP's hot is grounded. It's exactly like the way the circuit at the top of the drawing works.

jte
 
#20
hi John, nice to see you here. Maybe i am your first j-retro deluxe customer in mainland China. i bought it at bestgear.com 2002, at that time i wanna made myself the best jazzbass, with jretro and lindy fralin, badass........but 7years past i only made a jazz body.........
Hope you could help me find a right blend pot.
my friend who is a luthier(www.kuanchi7.com basskuanchi7.spaces.msn.com) is your fan but he never used your preamps just by watch the pics,funny.
 

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