Ground and hum.....???? Explain

[R.G.]

doesnt grounding stuff shield it from external interferences so of there is resistance on a grounding lug of a pot then there isn't enough ground on that pot and it won't be shielded as well.

Sorry, I couldn't translate that into my internal pictures very well. Can you explain some more?

So how would I hook up my Fuzz Face for twisted pair ins and outs?

You can't.

Well, OK, you can, but no connection of a shielded twisted pair carrying balanced signals does what you expect when you connect it directly to a FF without also losing the hum-immunity. It's not just the shielded twisted pair, it's also the  balanced signal on the STP.

A balanced STP requires a differential amplifier receiver to preserve its hum immunity. The FF is a no-immunity single ended input. So you have to put a differential to single ended converter in front of it to convert it back to single ended before the FF. And then you've lost the sensitivity to the pickup impedance that is part of the FF sound, and you have to go for pickup-faking circuits to get some of it back.

It's little "oh, yeah..." things like this that make the idea of simply getting rid of hum once and for all impractical.

[idlechatterbox]

very educational thread (if somewhat depressing). Thanks for taking the time guys.

I guess I now know why people walking around with acoustics and no shoes look so happy 

[petemoore]

  Heres' some ideas:
  Run the in house AC supply wires through grounded conduit.
  When in an area where conditions of abundant noise are prevalent, Use a grounded, shielded room.
  Some of this is impractical, conduit wouldn't be too bad in a new structure...for purposes of discussion...
  OT
  I read about steel cages installed to defeat microwave transmission caused by multiple large HV tube amps lining the walls of a studio, the grounded steel cages were built to separate the amp emissions causing the microwave, defeating the HF 'ring' noise. SRV could then begin recording again.
  Some CRT's have a shroud/shield encompassing much of the emission components.
  I had trouble with a CRT's noise injection to guitar when using the computer to record...I would put the cursor over 'Record', turn off the tube, carefully press the left click..I was later able to eliminate the CRT from the recording room.

[zpyder]

Great thread...

There is a hum eliminator that you could do. It's pretty simple, too. You take the AC power line voltage and half wave rectify it. Then you run it through four bandpass filters, one each at 60Hz, 120Hz, 180Hz and 240Hz. Each of those goes into its own two-stage phase shifter, and the outputs are all summed at variable volume with the audio you're listening to.  The result is the ability to fine tune out 60Hz hum by diddling the volume and phase of the 60Hz filter, and likewise, its first three harmonics. This will get the majority of the power in any power line hum.

This hum canceller is not a general solution, because you have to tune it to exactly the hum caracteristics you have RIGHT NOW, and it will de-tune if any of the hum changes. But it might get you past your immediate problem.

Hmmmmm... this sounds interesting.  You said it's not a general solution because you have to tune it to whatever house you're in - but to me this almost sounds like a good thing.  For instance, when I rock out at my own house, I do have some preatty unfortunate hum particularly with heavy distortions, but when we go play at the Downtown Lounge, my board goes nuts with hum.  Short of rewiring their stage for them, a tuneable hum canceller sounds like a great alternative!

But... is it SAFE?


Until then, I'll continue to use my simple, cheap, tried & true hum eliminator device: my passive volume/attenuator pedal.  For those silent moments between the huge hits, I dump all to ground - tedious for the first couple years, but you start to get used to it   And it works in 100% of situations.

cheers,
zpyder

[idlechatterbox]

I know this sounds stupid, but I was actually going to ask if there wasn't some way to flood my apt. with some kind of "reverse" 60hz frequencies to cancel out the wall outlet

[R.G.]

But... is it SAFE?

ACK - no, not as I stated it. I meant to say:

You take an isolated, lower voltage version of the AC power line voltage and half wave rectify it.

In practice, the thing is powered by the AC line through a small AC wall wart. The wall wart not only provides power, it provides the sample of the AC line to filter upon.

[zpyder]

So let me see if I can conceptualize this:

Take the existing hypothetical wall-wart I have, and splice a second plug onto it.  Route the first to my pedalboard as usual.  Route the second to my new filtering circuit, which I will place in the last position on my pedalboard (closest to amp).  The internal workings of the filtering circuit are as such: We take the +9v in (from the newly spliced second plug on our wall-wart) and half rectify it using diodes.  Then we split that rectified wave 4-ways using 4 bandpass filters (caps+resistors) at 60Hz, 120Hz, 180Hz, 240Hz (60Hz+Harmonics ... seemingling we could do as many bandpass filters for harmonics as we felt warranted).  We now have 4 "hums" isolated.  We then phase shift all 4 of these hum signals independantly and variably via 4 variable capacitors.  I'm guessing we're going to end up wanting around 90 degrees of shift??  The end result is that we should be able to cancel out that hum by sampling it, isolating and phase shifting it, and then superimposing this new "inverted hum" over our original signal (much like a balanced-XLR reciever).

Seems this could be done in a pedal with one switch (not necessarily a footswitch), 4 pots, 1 9v jack, and in & out 1/4" jacks.  My guess is that it would be most appropriate at the end of a signal chain, right before the amp...

yea?

zpyder

[R.G.]

Close.

The wall wart has to put out AC, not DC. If that's what your pedalboard eats now, OK. Otherwise you have to have a second wall wart that make AC for the filter.

The filter does two things with the AC it gets. It makes its own DC power supply, and it makes a half wave rectified signal from the incoming AC. This is a replica of the AC line, exact in frequency and phase because it's derived from whatever the AC line is doing around you at the moment.

You split that half wave signal four ways into four bandpass filters - resistors, cap and OPAMPs - to filter out a signal that's essentially pure fundamental, second, third, and fourth harmonics. Because of the way filters work, these will in general not be in the right phase to cancel anything. So the output of each BP filter goes into a 0-180 degree variable phase shift stage, two opamps for each filter output. The result is four signals that are then mixed all together with the incoming audio signal and passed to the amp. There is a volume control on each section and a phase control.

You use it by setting up your rig and the "filter" with all the filter volumes at 0. This should pass your signal unaffected. You will hear the same hum as if the filter was not there.

Now you turn up the 60 (or 50) Hz output a bit. The hum will increase or decrease. Diddle the 60hz phase a bit. At some phase, the hum will decrease. Mess with the level and phase until you get the best you can on 60Hz. Now do the same on 120Hz, 180Hz, 240Hz, etc.

You are creating a custom anti-hum. The hum components are selected by you turning the knobs and dialing the hum as low as you can get it. The anti-hum cancels any hum in your whole rig, either before or after the filter. It doesn't even have to be in your instrument line if your amp has a second instrument input. In fact, not hooking up your guitar when you do the adjustment is an advantage.

Now for the gotchas. The filter only compensates for the specific conditions right there and then. It can not compensate for any changes. So whatever your guitar causes by picking up hum and that changing as you move around will not be cancelled. It also creates hum where there was none. If you had a perfectly quiet amp, any anti-hum you add will still come through. If someone touches something and makes hum, it will not be cancelled until you go null it out.

It's a nice trick, but not as generally applicable as it sounds. I made myself one, but I don't use it much. Frankly, the amount of work it is would better be put into fixing the hum sources in your rig.

[David]

I've had good luck with the "100R resistor and the monster electrolytic capacitor to ground" filter trick.  Is this a valid attempt at a solution, or am I delusional (always possible)?

[Paul Marossy]

I've had good luck with the "100R resistor and the monster electrolytic capacitor to ground" filter trick.

+1

[R.G.]

I've had good luck with the "100R resistor and the monster electrolytic capacitor to ground" filter trick.  Is this a valid attempt at a solution, or am I delusional (always possible)?

It is a GREAT solution - if your only hum problem comes from incomplete filtering on the incoming power line. It is no help with capacitive hum, may make it worse, or inductively coupled hum.

[Meanderthal]

 For those of you who induct their hum through interference(power lines, vaccuum cleaners, etc) you can always build a Farraday cage if you're really serious about eliminating this problem. Basically you just shield an entire room with screening or chicken wire... radio signals won't even get through!
I may have somehow missed it, but I'm surprised no one mentioned ground loops(not that it applies to the situations in the origional post).

[Paul Marossy]

It is a GREAT solution - if your only hum problem comes from incomplete filtering on the incoming power line. It is no help with capacitive hum, may make it worse, or inductively coupled hum.

Yeah, that's a good point.

For those of you who induct their hum through interference(power lines, vaccuum cleaners, etc) you can always build a Farraday cage if you're really serious about eliminating this problem. Basically you just shield an entire room with screening or chicken wire... radio signals won't even get through!
I may have somehow missed it, but I'm surprised no one mentioned ground loops(not that it applies to the situations in the origional post).

Interesting idea. So the chicken wire or screen would be connected to the ground system in the building?

[idlechatterbox]

QUOTE:
" Basically you just shield an entire room with screening or chicken wire... radio signals won't even get through!"


I always knew there'd be a use for that Cold War era "Camouflage Netting, Radar-Scattering" that I saved! 

[Meanderthal]

 

So the chicken wire or screen would be connected to the ground system in the building?

Yes, just like shielding a guitar, but on a larger scale. Ever tried to use a radio inside one of those steel buildings? It's an ineficient farraday cage, but is a good example of how well this can work. To REALLY do it right the floor, doors, windows, everything would be included.

[aron]

Faraday cage anyone?

http://en.wikipedia.org/wiki/Faraday_cage

[R.G.]

Faraday cages work GREAT in all cases except the fine print.

That being:
- for all frequencies where the largest opening in the netting is smaller than 1/4 wavelength
- for netting conductivities and frequencies where the skin depth of the conductor does not exceed the actual conductor thickness
- if you don't then bring your hum in with you on that AC power supply cord

It's entirely possible to have hum inside a Faraday cage by running an AC power cord in which radiates capacitively or magnetically inside the cage, or by not rectifying and filtering that AC power properly and making your own personal hum.

The Faraday cages we used for RF experiments at school had their powered equipment powered by DC brought in through the screen.

[Paul Marossy]

It's entirely possible to have hum inside a Faraday cage by running an AC power cord in which radiates capacitively or magnetically inside the cage, or by not rectifying and filtering that AC power properly and making your own personal hum.

So, I have a question for RG:

Why does an inductor based wah pedal pick up hum even though it's in a metal enclosure? How does the inductor pick up hum from fans, wall warts, etc. when it's essentially in a Farady Cage with no actual AC inside the enclosure? Is it magnetically coupled?

[R.G.]

Why does an inductor based wah pedal pick up hum even though it's in a metal enclosure? How does the inductor pick up hum from fans, wall warts, etc. when it's essentially in a Farady Cage with no actual AC inside the enclosure? Is it magnetically coupled?

It is magnetically coupled.

Most people, even most EEs, even most analog design EEs (and those are rare!) do not understand how hard it is to shield out a magnetic field. Electricity and radio are fairly easy to shield against. There are insulators that will not conduct electricity. High frequency radio bounces off conductors. Low frequency E-fields are short circuited by conductors and are converted to eddy currents inside the conductor. An M-field stops at substantially nothing.

There are no magnetic insulators. There are just better and worse conductors. Some very few materials are weakly diamagnetic, meaning that they conduct M-field less well than free space, but it's nothing like an insulator. You can shield against a time varying magnetic field by using thick, highly conductive materials like sheets of 1/8" or more thick copper. This converts the M-field to an electric voltage, which is spent in eddy currents and opposing M-fields, but you have to have a LOT of it, and it's never a complete shield. You can use ferromagnetic materials to "short circuit" M-fields, both time varying and static. This will shunt the field away from sensitive parts, but some still gets through because the magnetic equivalent of conductivity for M-fields is only on the order of thousands of times better than free space, not billions of times better like electrical conductors. And that only works up to a certain field density, where it saturates and from there on it's just like free space. This low difference in "conductivity" between iron and free space is why transformers always leak a little - it's like trying to make electrical circuits in an atmosphere of weakly conductive gas where the entire environment is weakly conductive.

About the best anyone has done is for some of the high precision mike input transformers. These are put inside as many as eight alternating layers of drawn copper and iron or mu-metal boxes. Each succeeding box ate a bit more of the impinging M-field. It worked better than nothing.

So a pickup coil like a many-hundreds of turns inductor will be an effective pickup for hum, even after it is kept inside a metal box.

[Paul Marossy]

An M-field stops at substantially nothing.

This explains a lot of things!

So a pickup coil like a many-hundreds of turns inductor will be an effective pickup for hum, even after it is kept inside a metal box.

Yeah, I did see that parallel. Inductors are kind of like little guitar pickups in a way.