Cavity shielding - always necessary ?

Mr Fingers

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2,492
Way back, I ran a Seymour Duncan Quarter Pounder (passive) and and EMG humbucker (active) in a Tele and it not only worked well, it worked really, really well. Probably just dumb luck as I knew nothing back then. As for shielding a cavity, I can think of no good reason not to do it, unless you think you're going to screw it up. We all have guitars that play ddead-quiet (so we think) until we end up someplace where we start getting interference. Why not shield?
 
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...a Seymour Duncan Quarter Pounder (passive) and and EMG humbucker (active)...
Ha, I did almost the exact opposite, a Seymour Duncan Metal Live Wire (Active) and an EMG Select humbucker (passive). Also worked really well. Quite a level difference, which would upset the "oh horror! my Strat pickups are not identical in output!" brigade around here. Pickup selector was a tone/distortion control.


...have you found a good skinny shielded cable for internal stuff?...
Another good source is old audio snake. The outer jacket is the protection, so the inner cables are really thin and flexible. A foot of snake is eight feet of skinny shielded wire (for example).
 

walterw

Gold Supporting Member
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37,524
Way back, I ran a Seymour Duncan Quarter Pounder (passive) and and EMG humbucker (active) in a Tele and it not only worked well, it worked really, really well.
Ha, I did almost the exact opposite, a Seymour Duncan Metal Live Wire (Active) and an EMG Select humbucker (passive). Also worked really well. Quite a level difference, which would upset the "oh horror! my Strat pickups are not identical in output!" brigade around here.
horrified, i'm just horrified


and yes, it can work fine as long as you aren't trying to run them both at the same time, because you'll just hear the active pickup.

also, ideally you'll want separate volumes for each one; i've put plenty of single active pickups in like gibsons, simple enough to have the "active side" with its own 25k volume and tone pots and the "passive side" with 500k pots. each will sound like it should by itself, they just don't mix.
 
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782
Love the cat!

...it can work fine as long as you aren't trying to run them both at the same time, because you'll just hear the active pickup...
The Seymour Duncans seemed OK in this respect. I never learned why EMG specified the 25k pots - an active output should be relatively insensitive to load - voltage division is just a ratio. SD active pickups came with 100k controls in the box, but instructions for using them also with 250k. I had a pickup selector, and single 500k volume, no tone - it worked well. The middle position between active and passive was genuinely a middle position - not dominated by the active pickup - I can't adequately explain why. I've never bought active EMGs, so no personal comparison/experience.

The pickup instructions were still around - they had more diagrams than this, but here are two examples of mixed install. (Phone has no idea what to do with the white balance, the paper is strikingly yellow - the HSS photo is more accurate.) There was also a 200k resistor, to add in series with the active pickup if mixing with passive pickups. I didn't use that, as I wanted ALL THE GAIN, and I thought I'd add it later if there was a problem (never did).

BTW - this pickup is the sound of Jim Martin on Angel Dust.



 

walterw

Gold Supporting Member
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37,524
SD active pickups came with 100k controls in the box, but instructions for using them also with 250k.
interesting; not so familiar with these versions, maybe they had a higher output impedance than modern actives? that might make them behave more like passive pickups
I never learned why EMG specified the 25k pots - an active output should be relatively insensitive to load - voltage division is just a ratio.
they are, which is the problem. with higher impedance pots the sweep is no good, they act more like on-off switches; the pickups are in fact so "impervious" to the load that you have to load them down hard (with only 25k between hot and ground) to get an even response out of the pots.
 
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782
...maybe they had a higher output impedance than modern actives?...
Maybe?


...with higher impedance pots the sweep is no good...
The voltage divider has to be the right way around, then it's purely ratio, it's not about load, and sweep is fine. The Fender style diagram above is correct - output connected to wiper. The Gibson style diagram is backwards - pickup connected to wiper - that's not going to play as well with an actively driven output - I wouldn't use a pot that way ever, not in a boat, not with a goat. Hadn't noticed that until you mentioned sweep.
 

walterw

Gold Supporting Member
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37,524
The Gibson style diagram is backwards - pickup connected to wiper - that's not going to play as well with an actively driven output - I wouldn't use a pot that way ever, not in a boat, not with a goat.
yeah, saw that too, seemed kinda weird
 
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782
...maybe they had a higher output impedance than modern actives?...
Could accurately measure it with an exciter coil to give constant input - could be a little project - exciter coil would be useful anyway.

The EMG actives surprised me by not being that low - 10k output impedance - basically set by R7 as shown in Electrosmash schematic - fairly similar to a passive.

The SD Live Wire was pretty brutal, adding a pad probably would be a good thing in the modern world. It was fine for hitting a valve amp really hard, but it clipped a few analogue input stages for A/D converters.


...The Gibson style diagram is backwards - pickup connected to wiper...
Ah OK, this is something Gibson have varied over the years? I hadn't paid attention in class. I suppose EMG were a little bit stuck then - if people were blindly swapping EMG pickups wire-for-wire into a pickup-to-wiper Gibson, EMG might have some unhappy customers. Enforcing a pot swap would fix that, but still...
 

vortexxxx

Supporting Member
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10,076
horrified, i'm just horrified


and yes, it can work fine as long as you aren't trying to run them both at the same time, because you'll just hear the active pickup.

also, ideally you'll want separate volumes for each one; i've put plenty of single active pickups in like gibsons, simple enough to have the "active side" with its own 25k volume and tone pots and the "passive side" with 500k pots. each will sound like it should by itself, they just don't mix.
For some reason mixing active and passive never crossed my mind until it was mentioned here. Interesting though.
 

vhollund

Member
Messages
3,519
Hello!
Apropos shielding i just gave a double graphite coating to the cavities of my tele and my les paul 54 RI, today.

Well... half the les paul, because i run a p90 in the neck together with a virtual vintage p90 humbucker bridge.
;) I don't know if it is a no-no but the blend seems to work without any wierd phase issues. No active though, god forbid.

Anyway while halfway through 2nd coat, i realise that the screws that hold the P90 in place go right through the middle of the pickup, and will screw into the cavity shield paint.
Isn't that a potential issue that might change the sound of the pick up?

I know vaguely of the term eddy courant and also
If the screws connect to ground in the pick up via the cavity shield AND through the black lead/PU to ground , might it become a potential ground loop /antenna for amplifying the noise?
 

Dave Weir

Gold Supporting Member
Messages
1,220
Hello!
Apropos shielding i just gave a double graphite coating to the cavities of my tele and my les paul 54 RI, today.

Well... half the les paul, because i run a p90 in the neck together with a virtual vintage p90 humbucker bridge.
;) I don't know if it is a no-no but the blend seems to work without any wierd phase issues. No active though, god forbid.

Anyway while halfway through 2nd coat, i realise that the screws that hold the P90 in place go right through the middle of the pickup, and will screw into the cavity shield paint.
Isn't that a potential issue that might change the sound of the pick up?

I know vaguely of the term eddy courant and also
If the screws connect to ground in the pick up via the cavity shield AND through the black lead/PU to ground , might it become a potential ground loop /antenna for amplifying the noise?
I don't think a ground loop is a possibility. Also, I would not be concerned with eddy currents in the screws. I can't say I completely understand it, but there really doesn't seem to be anywhere for the currents to flow in that screw. Having the screw grounded would maybe microscopically lower the noise level, but probably not noticeable with a P90. I don't see any problem.
 

vhollund

Member
Messages
3,519
You're probably right
They did not mesure any connection to the pickups ground
My shield doesn't seem to make that much of a difference though
Still noisy as hell
It's hard to say if it is more or less noisy but it is VERY noisy
I don't know maybe i did something stupid
I was taught not to create loops in the shielding because they act as an antenna but the bridge in a tele has 2 ways to ground
One via the shield and one via the ground wire

I don't think a ground loop is a possibility. Also, I would not be concerned with eddy currents in the screws. I can't say I completely understand it, but there really doesn't seem to be anywhere for the currents to flow in that screw. Having the screw grounded would maybe microscopically lower the noise level, but probably not noticeable with a P90. I don't see any problem.
 

Dave Weir

Gold Supporting Member
Messages
1,220
At best, shielding is going to reduce noise the same as touching the jack body or grounded strings. It won't do anything for P90 60 cycle hum. P90s are why humbuckers were invented.
Ground loops in a guitar aren't a real thing.
 
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vhollund

Member
Messages
3,519
The term ground loop was badly chosen yes
And i agree with you on what to expect

What John Suhr told me 10 years ago was that a shielding should only have one connection to star ground
That a loop in the shield path is a potential antenna
I just found ou my tele pick up connects to the PU plate which connects to ground via a cable.
Which means that there is one lead to ground too many, if I try to follow the advice.
Unfortunately thats my only approach because I have too little competence in the matter of electronics

Anyway, what CAN be done is a noise canceling coil that connects between PU and ground. I doesn't suck tone, it's passive and a cheap DIY solution
I'm trying out on I made winding around an old maxi box of Twinnings
It seems to be working.
Not completely but some, probably due to lack of sufficient windings.
It was A LOT of winding to do manually ;)
 
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Husky

Member
Messages
11,733
Old post!

It's not that easy
If the coil has more than about 400 ohms of DCR you are going to start to loose high end and it's not easy to make a coil that produces the same amount of hum as the pickup with 400 ohm or less. Ideally that coil should not be under the strings.
Shielding will protect you from hi freq trash and RF but won't help with induced low frequency hum. I wouldn't worry about the shield so much creating a double ground. Just don't physically wire up a second ground. Shielding paint is not 100% conductive but works fine for RF.

for instance back in the early 80's I connected all 4 Les Paul pots with a ground loop by connecting the back of the pots together with a wire, they were already connected via the braided wire and partially with shielding paint. I was getting RF like a radio bad. I cut the wire between two of the pots creating a U instead of O and it went away instantly.
The term ground loop was badly chosen yes
And i agree with you on what to expect

What John Suhr told me 10 years ago was that a shielding should only have one connection to star ground
That a loop in the shield path is a potential antenna
I just found ou my tele pick up connects to the PU plate which connects to ground via a cable.
Which means that there is one lead to ground too many, if I try to follow the advice.
Unfortunately thats my only approach because I have too little competence in the matter of electronics

Anyway, what CAN be done is a noise canceling coil that connects between PU and ground. I doesn't suck tone, it's passive and a cheap DIY solution
I'm trying out on I made winding around an old maxi box of Twinnings
It seems to be working.
Not completely but some, probably due to lack of sufficient windings.
It was A LOT of winding to do manually ;)
 
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Husky

Member
Messages
11,733
shielding will protect against high end noise and trash but won't do anything much for hum which is induced in to the coil.
Hello. I need some technical advice on the electronics. Not my strong side, especially the theory:confused: !

I am building a Les Paul Junior style guitar, and have finished doing the control cavity rout.
I will be installing a Gibson humbucker at the neck position and a Schaller piezo tune-o-matic style bridge, the latter requiring a 9 volt battery powered pre-amp fitted into the body. I bought the matching Schaller "Flagship" pre-amp for this purpose.

http://guitar-parts.biz/hp425601/Flagship.htm

I have asked Schaller in Germany if putting everything (pots, battery, preamp) into the one cavity created any issues by doing so, and their answer was 'no'.

But just to be sure to be sure, with using the passive and active pickups, and the 9volt power supply etc., is there any electronically related reason for me to shield the cavity with shielding tape or paint, or to position components differently ?


Thank you ,

Don
 

Husky

Member
Messages
11,733
EMG are sensitive to the fact that distributed cable capacitance is different than a simple capacitor. That cable can be part of the tone.
interesting; not so familiar with these versions, maybe they had a higher output impedance than modern actives? that might make them behave more like passive pickups

they are, which is the problem. with higher impedance pots the sweep is no good, they act more like on-off switches; the pickups are in fact so "impervious" to the load that you have to load them down hard (with only 25k between hot and ground) to get an even response out of the pots.
 

vhollund

Member
Messages
3,519
Hello John,
Yes old post, but the advice you gave is still solid.
Good to see you still find time to lurk around and shine some light.

Actually finally I had to dismiss my homemade coil as it brought more noise at certain angles than with regular ground. I don't know if it was polarity or number of windings, but it proved you right. Not that simple. I did use a 10k pot too
I'll probably try it with a P90 before disgarding it. Damn.

I still have the Suhr backplate i bought from you back then and it works great with the V60LPs in the strat. No noise and no alteration. If Illitch could take down the ambitious price tag just a bit, I think it would be in most guitars in no time.
I think you still put in your guitars right ?

Old post!

It's not that easy
If the coil has more than about 400 ohms of DCR you are going to start to loose high end and it's not easy to make a coil that produces the same amount of hum as the pickup with 400 ohm or less. Ideally that coil should not be under the strings.
Shielding will protect you from hi freq trash and RF but won't help with induced low frequency hum. I wouldn't worry about the shield so much creating a double ground. Just don't physically wire up a second ground. Shielding paint is not 100% conductive but works fine for RF.

for instance back in the early 80's I connected all 4 Les Paul pots with a ground loop by connecting the back of the pots together with a wire, they were already connected via the braided wire and partially with shielding paint. I was getting RF like a radio bad. I cut the wire between two of the pots creating a U instead of O and it went away instantly.
 
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Husky

Member
Messages
11,733
We developed our own system which has some differences and works better for our purposes. But his system works as well. We also now do an active system which leaves the pickups passive which we use for the real bad noise makers like a P90. I love that system and it's adjustable. It does require a battery but there is 0 tone change and when the battery dies the hum simply comes back.
Hello John,
Yes old post, but the advice you gave is still solid.
Good to see you still find time to lurk around and shine some light.

Actually finally I had to dismiss my homemade coil as it brought more noise at certain angles than with regular ground. I don't know if it was polarity or number of windings, but it proved you right. Not that simple. I did use a 10k pot too
I'll probably try it with a P90 before disgarding it. Damn.

I still have the Suhr backplate i bought from you back then and it works great with the V60LPs in the strat. No noise and no alteration. If Illitch could take down the ambitious price tag just a bit, I think it would be in most guitars in no time.
I think you still put in your guitars right ?
 




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