Hello all,
so i'm trying to put a blend pot on this electric hurdy gurdy i'm working on, to blend the pickup under the drone strings with the pickup under the melody strings, but my electronic noob-ness is cathcing up with me once again!
Here's what i did, i wound two pickups (which by themselves work fine) and wired them to a 250k balance pot following this schematic i found on the forum.
at both extremes it works, you can hear one pickup and not the the other.
In between, however, there's just a hum and no output, like it's not grounded correctly or something...
Does anybody have any ideas what i'm doing wrong??
Thanks!
Replies
Ok well I said I'd be back with a graphic...
................
If you were to graph out the response of a regular everyday typeB (linear) potentiometer it might turn out like this:
BUT..
If we measure across B->common we get the yellow triangle, which tapers in the opposite direction.
So if you were to implement a typical volume circuit like half of your diagram above then the only difference if you were to swap the ground and output connections (A & B) would be that the volume knob would work in the opposite direction, right ?
.......
ok so lets say that we do the same type of graph for a typeA (log) pot...
hmm... ok now we can see that the 'response curve' actually does curve. So swapping A&B over does more than just reverse the direction of the knob right? because the yellow and blue triangles (triangle-like-shapes) are not equal in shape or area. The typeA pot is designed to compensate for the logarithmic response in your ears/brain, so that between the pair of them they achieve a response (or the illusion of a response) more like the first graph. Its like spectacles for your ears. With a typeA pot we need to be sure we wire it around the right way, using the wrong triangle here would be like putting your spectacles on backwards, the corrective lenses actually bend light in exactly the wrong direction, doubling your problem. This especially comes up when you make a left handed electric guitar...
........
Now a typical 'balance pot' is two typeA pots stacked but with one of the wipers reversed, and both of them offset so that their tapers do not overlap (thereby enabling both to be at 100% or '10' in the centre where you feel the detent there and putting each taper on opposite sides of that detent.
Like this.
Do you see ?
you're in the blue. Swap A1<->A2 and B1<->B2 to get in the yellow but keep each pickup at the extreme of the rotary action that they are now, or swap A1<->B1 and A2<->B2 to get in the yellow and switch that as well.
You really need a multimeter to test out all pots and switches but especially more complex components with more solder lugs (balance pots, switches with many throws/poles) to visualize how your circuits work when building them.
the reason for lifting the grounds is because signal can escape back across the other wiper to ground when they are summed at common1/common2 with no rectifier, remember current will always follow the path of least resistance to ground..... (edit sorry, you were summing A1/B2 and A2/B1, one for output and one for grounding. the same applies, current intended for hot can escape across the other wiper to ground..)
allright, after an evening of family (they get grumpy when you want to work on your instruments too much) i'm back on my project.
thanks for your explication, it makes sense. Are you a teacher or something? You really have a knack for clear and sufficiently detailed explanations.
But alas, between theory and practice there is a big gap,
I rewired the pot the way i understood your last instructions and now it looks like this:
However this yields exactly the same result as before, it still works at both extremes and buzzes like crazy in between...
I'm actually surprised as to how many different ways i can wire this thing to get the exact same result!
Could it be something else then the pot?
As i said before, connect the pick-ups to a female jack directly and they both work fine.
I did use some quite thick gauge copper salvaged from an old microwave, could it be they have too much resistance (i'm talking here without knowing...)?
Or maybe the pot is broken?
Curious to see what's next!
aw man, I'm sorry to hear that.
i think the next best step for you is to nip down the auto-parts superstore for a $10 multimeter. Set it to resistance (Ω, ㏀, ㏁ ) and you should pretty easily work out which side of which pot is yellow and which blue by reading from the centre lug to each of the side ones as you turn the knob. This really is an essential skill to learn as far as I'm concerned, once you start to get your head around it you'll be free of internet circuit diagrams forever. You might also find a cheap can of contact cleaner, this pot might benefit from a squirt and its always handy anyways..
good luck, I'm sure a multimeter will enable you to either work it out or establish that the pot is faulty. Sorry i couldn't help further :(
allright, back to the drawingboard so to say.
i actually own a multimeter allready.
thanks for your time in any case...
Nope it's not a phase issue. (Flipping the phase on one of the pups may lead to a more pleasing muxed signal, but that's not what this is)
You've just used the wrong side of the taper on each of the pots. So you know how you had a diagonal connection for ground (which we clipped) and you still have one there for your hot..? Well we're using the wrong side. So clip the remaining diagonal connection (that's important).. Then replace it with the opposite (the one which was ground). But don't send it to ground, send it to hot...
This is because of the special taper on balance pots.. When I have a minute I'll make a graph in illustrator to help you understand it.
Have you managed to get the pickups to somehow cancel each other out ? They both work in isolation through the pot but if one is wound clockwise and the other anticlockwise, could the signals clash when they're mixed ?
I'm no electrical guru so I don't know but whether this could happen, but it may be worth switching the wires on one of the pickups to see whether there's a difference.
I've used blend pots a few times with the ground in place and it worked well ........except when I tried to blend mag and piezo. That failed completely :-\
Good luck
the other way you could do it..
although i do just what i said above...
is to put a diode (or other cheap rectifier) between each of the lugs you've used for ground and the wire, so that current can only travel across the connection in one direction....
(or alternately, between the two 'output' lugs and their connecting wire.)
The reason I posted these alternates is not because i think you should bother, but because if you think about this it might give you some insight into the flaw in this circuit (and why you can't ground both volume pots in a Les Paul if you want it to work right..)
Okay, so i wired it up according to my understanding of what you told me, removed the grounds from the pot and sent them straight to the output jack ring and removed the wire connecting the two grounds on the pot.
Bit i'm still having the same result, output at the two extremes and just buzzing in between.
I've added a picture of my rig,
what am i doing wrong?
thanks!
thanks for the lightning fast reply!
i will let you know if it works!
it does, I've probably done a hundred of em :)
-
1
-
2
of 2 Next