Wiring volume pot

[rjlight]

Maybe I'm missing something. I'm wiring the volume pots on the archtop I'm 'electrifying' according to the Gibson method used for 2 humbuckers with a volume & tone control for each. This involves the leads to the amp being connected to the two outer terminals of the pot, which means that when the volume is set to minimum there is a 500k resistance across the amp. This causes my Fender amp to buzz. When the leads to the amp are connected to the center and one outer terminal, at minimum volume the amp leads are effectively shorted out and there's no amp buzz. I've tested this on a hi-fi amp just to eliminate the amp as being the cause, but with the same result.

Has anyone else found the same?

It has to be wired this way to avoid disabling both sets of controls when just one is set to minimum.

Is there a way around this? I hope someone can help 'cos it's driving me mad!

Roger

[flateric]

If you look at the popular schematics around on various sites (Seymour Duncan, guitar nutz, Guitar wiring,ect.) there are a number of alternative methods for wiring up 2 hB's with separate vol & tone pots. I followed a basic Les Paul schematic from Seymour Duncan with a rewiring job on my Epi dot recently (separate independant vols) and it does not suffer from a buzzing problem with my marshall tube amp. Is this a screening or earthing issue?

[UnklMickey]

Rjlight,


first, welcome to GN2 !

i'm hoping some of the guys that have worked on mods with "backwards" volume pots will be able to give you more help on this one. but, 'til then, i'll offer what little advice i can.


an archtop is a poor choice to be experimenting with because of the difficulty threading things to where they need to go.

but you might find yourself doing some experimenting to get things right.

a cheap lp copy might be a good vehicle to work with if you can get your hands on one. that way, you can work out the details through experimentation, without all the assembly/disassembly headaches.

alternately, you might just try doing the wiring without installing it in the guitar for testing. you'll still be able to evaluate the amount of hum when the volume control is turned down.

with the controls backward, you do indeed have an antenna-like situation, when the volume for the selected pickup(s) is turned to minimum.

"shielding the beast" is obviously not an option for this guitar.

this is a case where careful wiring and attention to detail in the grounding might make the difference between acceptable levels of hum, and dismal failure.

you will no doubt want to make sure that ALL of the potentiometer cases are grounded. use shielded cable for ALL interconnections between pots, pickups, and the output jack.

double check to make sure you have the shield and hot going to the correct terminals on the output jack.

also, don't forget when the time comes to put this in the guitar, to provide a string ground.

if you can't reduce the hum to a manageable level, when the volume(s) are at minimum, you may need to come up with another volume control strategy.

you might be able to get (minimum volume) hum down to a reasonable level by turning the volume controls forward, and putting a 10k or 22k resistor in series with each volume wiper's connection to the selector.

this is where that lp copy will come in handy. you will be able to evaluate the control function and experiment.

i expect the resistors in series will cause about a 3dB drop in volume when switching from one pickup at max, to both pickups on, (when the other one is at zero).

since the pickup in use will now see a lower resistance loading it, it will also have the high frequencies attenuated.

well, that's all that comes to mind right now, but i'm sure that within a few days, lots more guys will add to this.



unk





EDIT: since this is one of those applications where every little bit counts, make sure that the end of the tone caps with the black band or bar, is connected to ground. the band indicates the outer foil.

[rjlight]

Blimy, I never expected such a quick response! Thanks for the welcome to the forum.

As far as I'm aware my earthing is pretty good, though I am working on the shielding, which on an archtop is a bit of a challenge! I've even used a small steel box to house two of my coil tap switches.

I think the 'aerial theory' is about right. There are 3 types of hum / buzz that I'm getting. The first is a buzz which is slowly getting less as I improve the shielding, the second is hum when I go out of phase or switch to sigle coil on the neck pickup, which is to be expected, and then third there's this 'amp buzz' when the volume goes to minimum on either pickup which is a completely different sound to the other two. Would this be aleviated if I improve earthing and shielding on the pots? I'm not convinced it would. Maybe I need a filter of some type.

Hopefully someone has experienced exactly the same thing and has the perfect cure! I'm open to suggestions, and will certainly try the series resister you suggested Unk.

Rog

[sumgai]

Rog,

to the forums!

What unklmickey meant when he said

you might be able to get (minimum volume) hum down to a reasonable level by turning the volume controls forward, and putting a 10k or 22k resistor in series with each volume wiper's connection to the selector.

was that you should turn around the currently backwards wiring of your volume pots. The output jack's contact lug (the one that touchs the tip of the plug) should be connected to the center of the pot(s). As you face the pot from the shaft end (as if you were about to twiddle the knob), and with the three lugs turned upwards, the right-most lug should be grounded, and the remaining lug should be coming from the pickup selector switch (as appropriate for each pickup).

That's the industry standard way of wiring volume controls. The incoming side always presents the same resistance to the pickup. The outgoing wiper arm presents a varying amount of resistance to the amp's input - when the control is at it's lowest, the amp is effectively shut off because the input is grounded.

Doing this will still might cause problems, however. This is why unk said to insert those resistors. For more on the why's and how-to's of this, check out this page on the original GuitarNuts site: Typical Gibson wiring (with a mod). Look about half way down the page, at the second blockquote (inset paragraph) that's labeled "response". But to understand it, first read the query above it.

HTH


sumgai

[rjlight]

Thanks for that sumgai - I think the mists are slowly clearing, but there are still one or two whisps swirling about.

Forwards wiring does make most sense as it effectively means zero resistance across the amp at minimum volume, hence no amp hum. But is does also couple the two volume controls together, which means that if one volume is set to minimum, it kills the other pickup when the selector is in the central position - path of least resistance means both pickups are shorted out.

Am I right in saying that fitting the series resistors will alleviate this condition by not fully grounding the other pickup when one pickup is set to minimum? This does make sense to me, so I hope I've got thet right.

One thing worth noting is that I have tried forward wiring one of the pickups just to see the effect on the amp hum when the selector is set to just that pickup. At minimum volume - no hum as you'd expect. BUT, as you slowly increase the volume, the hum slowly kicks in, peaks at about half way and then reduces to nothing as the volume reaches maximum. Presumably this reflects the shift in balance of loading on the amp between the volume pot and the pickup - halfway, the effect from the volume pot's resistance is greater, full volume this is then dominated by the pickup.

Would the series resistors aleviate this issue? I think they might, but I'll just have to try it to see.

I do still wonder if it might be possible to just kill the hum with a parallel capacitor across the jack? It works with power supplies, so might it work with a low level passive input like a guitar? Again, I need to experiment!

Any further thoughts would be much appreciated.

Rog

[UnklMickey]

Rog,

i think the series resistors will give you a compromise, between interaction of the pots, and hum problems at minimum. the problem with the increased hum in the middle of the pot when wired "forward" will still be there. in fact, since the minimum volume will have 10k or 20k to ground, some hum will will be present there, and increase as you move to the center of the pot.



paralleling a capacitor across the output jack will eliminate the hum, if you have a large enough cap.

unfortunately, it will eliminate the signal, long before it eliminates the hum.




unk

[rjlight]

Funny you should say that, 'cos I tried it and that's exactly what happened! Oh well, another great idea down the pan.

I think you're right about the compromise - no easy answers for this one, just find the solution which works best.

Rog

[JohnH]

Heres my attempt!

First thing, just to check - how is your cable? when I bought a guitar and amp set-up for my son, it came with a bad cable that buzzed unless shorted out. It is now used to hang washing.

Setting that aside, it seems that your amp likes to be driven by a low impedance source, in order to ground out some noise in its input circuit.

The guitar is naturally of a variable impedance, zero with a forwards-wired volume pot at 0, fairly low at full volume and high at in between volumes, or with a reversed volume at zero. Theres no one arrangement which will always give what the amp would like.

Forwards volume controls are best for overall changes in volume, or setting a different volume for each pup separately. When I got my LP copy, it was wired that way, and indeed, if one volume was turned to zero with both on, the sound was lost. If I had had to, I could have lived with that, because if I want to turn a pup to zero, then I would just de-select it with the switch instead. The two pots worked OK together in the range 5 - 10, for doing mixes.

Reversed volume controls don't have that shorting-out issue. Hence they are regarded as better for mixing a little of one pup with the other. But I think they are not so good for reducing overall volume, since they put a large resistance in series with the output. That causes loss of treble due to the capacitance of the cable acting like a tone control. And it doesn't help your amp hum.

Unkls idea of a resistors after the forward wired pots helps with the loss of volume from the other pup when one is at zero. As he points out though, there are some losses and the value of the resistors is a compromise between too high, and it causes a loss of treble due to the cable capacitance, and too low and it also causes a loss of treble and volume on the other pup. Whatever value you pick, my belief is that you don't quite get all your signal - you lose some high-end edge. Personally, I have no spare edges, I need all mine.

My recent answer to all these issues is a small active buffer built into the guitar. It always gives out a consistent low impedance output, no matter what is fed into it. With this, it can drive past long cables, into dodgy input circuits. It may help your amp noise (? to Unkl and Sumgai - what do you think?).

My buffer is built into the guitar, and gives a high impedance input for the guitar to feed. Hence, I could do the forward volume controls, with the extra resistors. Now however, instead of being limited to a low value for those resistors of 10k to 22k, I could use a much higher value of 56k. It all works fine, and you can read about it here (the LP maximiser - just look at the volume and buffer wiring):

guitarnuts2.proboards45.com/index.cgi?board=schem&action=display&thread=1138768962

The buffer is very simple and about $2 in parts.

My buffer is built into the guitar and that is the best place, but there is a chance that a similar device, as a separate unit between guitar and amp might give it the low impedance source that it wants, without building it into the guitar.

John

[UnklMickey]

johnh said:

...it came with a bad cable that buzzed unless shorted out. It is now used to hang washing....

LSH...



John,

i knew eventually you would ring in on this one.

i like where this is headed.

there are some small challenges, though.

if the buffer is built into a tiny box, with a plug on one end, and a jack for the cable on the other, that keeps things simple in the guitar. but it adds length to the connection at the output jack. some might find this undesirable.

inside an archtop, there is tons of room. so making a custom jack-plate on which the preamp and battery are mounted would be a possibility. even though battery changes would be infrequent, it would require removal of the jack-plate. so each time, the screws would have to come out.

another important issue would be vibration concerns. mechanically, the construction of the jack-plate / preamp and battery frame, would have to be quite solid. if there were any sympathetic vibrations, it would sound ugly, when played acoustically.

all in all, i don't see the challenges as being "show stoppers".

just considerations to be addressed in implementing the idea.



unk

[rjlight]

Thanks for all the input on this one. For now I'm going with the forward wired volume pots with no additional mods. I tries the series resistor mod and it did exactly as you said Unk, but I wasn't happy with the loss in overall volume as I do need everything I can get with this guitar. The archtop I have is ok but certainly not the best and lacks a bit of sustain, which is why I need every ounce of volume, harmonics and edge from the pickups. The volume cut off with the selector in the middle position is not ideal, but then again not something which is really going to cause me any problems - I can live with it as the other benefits to forward wiring are greater. I've still got the amp hum at the mid-volume range which is certainly not ideal, but hey, something I'll have to live with for now.

I'm definately going to tinker with the LP Maximiser solution. One question I have and one I'm not too clear on is whether it is necessary to do the full mod or if it is possible to just have the main active circuit between the selector and output jack. This would be the perfect solution if possible.

Rog

[JohnH]

Rog - the buffer circuit works fine as a stand alone addition. It will work well with your forward wired volume controls and standard wiring.

cheers

John

[sumgai]

Rog (and gang),

Argh! I go away for one measly night, and look what happens! Anyway.....

Rog, your analysis was correct, and you've proven it beyond a doubt. For that, this thread is destined to be linked into the upcoming FAQ and Reference section, coming soon to a GuitarNuts2 forum near you! ;D

I have to second John's suggestion, you can always do good things with active electronics. The big fly in the ointment, of course, is supplying battery power. Not to mention being able to replace dying/dead batteries as needed.

John, your description of an external box was actually the very first preamp to hit the market, the Electro-Harmonix LPB-1 (Linear Power Booster). A battery, four resistors, a capacitor and a transistor (a 2N2904, IIRC), and that was the humble beginning of history. The battery was not quick to replace (and you could lose the little screws); it could break off at the plug if you got wild and hairy on stage; and it was just a bit too big to fit into a Strat-style jack, but it added almost 20dB to the signal!!!

Remind me to write up a treatise on matching impedances between devices, eh? Right now I'm being paged to the dinner table, and that's an event that I don't wanna miss!

Ciao (pun intended!)


sumgai

[JohnH]

Sumgai - one nice feature of the buffer that I have been using is its measly demand for power, just 50 microamps. It also works well with dpleted batteries down to about 3 volts with normal pups - 4 1/2 v to be safe. 9V batteriies are rated for about 550 mA-hours to half voltage. So once the battery is in, it needent come out for a loooong time!

John

[sumgai]

johnh said:

Sumgai - one nice feature of the buffer that I have been using is its measly demand for power, just 50 microamps. It also works well with dpleted batteries down to about 3 volts with normal pups - 4 1/2 v to be safe. 9V batteriies are rated for about 550 mA-hours to half voltage. So once the battery is in, it needent come out for a loooong time!

John

Sounds intriguing! Can you post a copy of said circuit, here or elsewhere? Please?


sumgai

[UnklMickey]

sumgai said:

...Sounds intriguing! Can you post a copy of said circuit, here or elsewhere? Please? ...

one thing i've learned: read everything John posts.

that buffer is at the output end of his "LP Maximiser":

guitarnuts2.proboards45.com/index.cgi?board=schem&action=display&thread=1138768962


the battery life isn't too bad. if you played 24/7, it would only last about a year and 3 months.

[sumgai]

unk,

Well, toot my horn and call me a cab!

I first saw that as "two humbuckers" and left it at that. I use SSS, and don't ordinarily spend time on what I don't use. But yes, it's beginning to look like I'm gonna have to start being completely neutral about how I regard topic titles, doesn't it?

And as for the time I'm gonna need to read everything ever posted here, I'll have to give up sleeping.... ahh, who needs it anyway, right? ;D

Thanks.


sumgai

[rjlight]

I have a techie question for John - I've looked through my bag of transistors and have found that I have a 2N5459, but no 2N5457. As far as I can tell it's a similar beast in that it is a general purpose JFET, N channel but has a 2V threshold rather than a 0.5V threshold. Do you know if this would work, or do I need to get hold of the one you specify? Or can I use this but would need to change the value of some other component in the circuit?

[rjlight]

Sorry - I've just read this last post of mine and it doesn't exactly flow with the rest of the thread!

Just to clarify that this is regarding the buffer circuit which forms part of the LP Maximiser referred to in previous posts. I'm looking into this as a way of sorting out the amplifier hum problems I'm experiencing with my 2 humbucker archtop project .

Rog

[JohnH]

Rog
I would expect that the 2N5459 would work, but probably with a higher value for the 3.3M resistor, or possibly no resistor there at all. That type of circuit (a source follower) is very tolerant of different values. The ideal is to get a voltage of half the supply at the source, but it is not very critical because we dont need the full voltage output swing available for a guitar signal. So if you have a meter and some different resistors, give it a go.

John

[rjlight]

Thanks for that John. One question though - across where would I need to measure the voltage to check that I'm getting about half the supply at source?

Rog

[Runewalker]

JHs' buffer is trey cool, and really clarifies the dynamics of your instrument in constructive ways ---- then it require us to unlearn all the adjustments we made as guitarists to fussy sound.

I have discussed with JH some issues I have on cavity space and that honkin' 9 volt brick, and he has I believe tested it down to 6 volts, finding that power level acceptable. Which opens up some smaller volume watch battery options, assuming we can find specialized clips.

I have been turning some notions around in my head though about adapting a chord to the buffer. Maybe something with a short pigtail - 12 to 14" - so the guitarist could loop the chord through the strap interface as is commonly done, then the buffer would be a small in-line device next to that strap interface. That way you free yourself from having to always build it into the guitar, and the device is available for your entire harem.

I'm sure there a number of variants of this idea. I am having some definite space constraints, especially on the Strat builds, that could use an external solution. Or at least some smaller form-factor power units. The buffer circuit itself is very compact.

Remember, that JH prototyped this on a Hondo with enough cavity to put the guts of a cruise missile in.

[UnklMickey]

runewalker said:

...I have been turning some notions around in my head though about adapting a chord to the buffer. Maybe something with a short pigtail - 12 to 14" - so the guitarist could loop the chord through the strap interface as is commonly done, then the buffer would be a small in-line device next to that strap interface. That way you free yourself from having to always build it into the guitar, and the device is available for your entire harem....

well i s'pose one could build a micro-mini version that would have the preamp in one end of the cable and a battery or power supply at the other. obviously the box containing the batttery or ps at the amp end would be much less of a problem there.



unk

[JohnH]

Morning chaps:

To measure the correct output voltage for checking bias, test volatage across the 100k resistor, which goes from source to ground.

The buffer does work fine at 6V. in fact it was just about OK at about 3V, except for by hottest GN2 style series pup combos which have so much voltage swing that 3V is not enough.

As for preamp cables - theres a very neat idea by Tillman, using a slightly different configuration of FET circuit. a common source rather than a source follower - no more complex. With that a FET and resistor can be mounted in a jack plug and the rest connected, at the other end of a standard 2-core cable. Gotta catch a train right now - try googling 'tillmann' and 'pre-amp cable'

JH

[UnklMickey]

we hope your travels are pleasant John.

for anyone interested the preamp cable John referred to is here:

www.till.com/articles/PreampCable/index.html

the really slick part about using a common source configuration in the preamp cable, is only one conductor (in addition to ground) is needed for the cable.

if you were to use a source follower (like John's) it would require a 2 (inner) conductor cable.



also on that page is a stereo version for a RIC.

it is THE project for those of you who like the challenge of seeing just how much stuff you can fit into a small space.


unk

[rjlight]

And another question..........I notice that the active circuit in John's LP Maximiser does not appear to be shielded and uses unshielded signal cables. Is this because it's active and so doesn't have the issue with low level passive signals which are more susceptible to interference?

Rog

[JohnH]

Rog - Im gonna have to watch out and shape up!, with people like you reading my posts! - it is a good point that you raise.

In my case, the Cort LP copy that I used for the LPmax was fully shielded internally by black conductive paint. Inside the shielding, I'm not too worried about screened cables on a circuit like that with no voltage gain.

I had another circuit, on a different guitar, which had a built in high-gain overdrive circuit. I found I needed to use all screened cable for that, to stop large signals radiating out of it and feeding back into the pickups.

For the LPmax, if the buffer had had to connect through long uncreened routes, or a non-screened body, then screened cable would have been better.

John

[fobits]

an important link for this forum.

www.till.com/articles/PreampCable/

Or how about...

www.till.com/articles/index.html

He has some other good stuff there, too.

[finetuned]

I am experiencing the very same problem. I first used the wiring as shown on the Gibson website, now switched to the wiring at the guitarnuts ´stock LP´ page. Experiencing the very problems described here. My guitar is a solid body Les Paul copy. How are the volume pots wired on a real LP, and do they also suffer from hum when the volume pot is at minimum, and if not, how do the people over at Gibson control this (seemingly known) problem?

Thanks
Stan.

[rjlight]

As far as I can tell, and from what I've learnt through this thread, the hum due to the amp having a 500k resistance across it at minimum volume will always be there on any guitar wired in this way - just one of those trade-offs when balancing the pros and cons of different wiring options. There are ways to mitigate it, but it sounds like the buffer circuit mentioned previously would be the best solution as this ensures that a low impedance is always presented to the amp irrespective of how the guitar is wired.

Rog