HI,
 
I have a strange HUM problem.
Im SURE there is more going on here, but ive made a dent in the problem,
and I would like to know if I should implement these changes to all units in the rack.
 
Ill try to keep it simple.
 
 
Guitar has *active pickups*
Problem ONLY there when Distortion used.
 
 
When I would use Mono 1/4" cords to go from my LINE 6 Pro XT
Into my Behringer EX2200 (Its an exciter Think BBE sonic max)
I got "hum from hell"
my noise floor W/ distortion turned on  be something like -12dB
 
 
Yet, when I used XLR cords, plugged into 1/4" Impedance  transformers
To go from the POD XT pro (XLR OUT)  into the Behringer EX2200 (1/4" IN)
the noise floor dropped to abt -28db
 
Crazy part:
Pod XT pro XLR Ohm out is BELOW 50K ohm
The Impedance Transformer "range" is 250K-50K Ohm.
Yet, it still made an impact.
 
Ive used XLR to 1/4" cords, and they had no effect.
But the Impedance Transformer on the end of the XLR cable DID!
 
 
Hum still there, but...
A -28db noise floor is WAY better then a -12db Noise floor.
 
 
Ive got a few other units in this rack,
some units have different Ohm levels on their in/outs
Most are 30K-40K but none of them above 50K ohms.
 
 
Im pondering yanking ALL 1/4" cords
plugging in XLR cords W/ Ohm transformers on *BOTH* ends
as not all units have XLR ins/outs.
 
 
Total bill for doing this is in the $380+ range, so its a real hoot.
 
 
My biggest questions:
If the XLR cables going from the POD XT PRO XLR OUT
into the EX2200 have XLR to 1/4" Ohm Transformers....
 
Would that then mean that ALL cords connected to the NEXT units in the signal chain
carry the same Ohm current to each unit below it?
 
 
Or is each ohm level, effected by the IN/OUT specs of each unit,
regardless of the ohm levels of the units BEFORE them in the signal chain?
 
 
And IF each unit,  DOES effect all units that are NEXT in the signal chain,
Does it seam at least mildly sane to TRY and spend the $
to see if this impacts those other units too?
 
 
ANY other ideas welcome,
but if you can answer the above I'll worship you.
Thanx!

If you have a rack with several units, any one of the units can be causing the ground loop hum problem. You need to isolate all the units from the rack rails (or at least start isolating them one at a time until you find the one causing the problem). "Humfrees" isolating tabs are sold just for this purpose - to insulate the racks from the rack rails. If its just the connection between two units an Ebtech Hum Eliminator will take care of it (and a lot less than the $380). http://www.musiciansfrien...channel-hum-eliminator I used to do amp repair (in Nashville) and customers would come in with ground loop hum problems and the fix many times was not the obvious or suspected problem. BTW, I use a POD X3 (bean type).

So... start at the beginning...

When you turn on the studio and the gear and NO GUITAR is connected... do you have hum or is the system quiet?

If quiet..... hum is a result of the guitar or the amp/sim/distortion level.  

If hum.... you would need to look to find the source of the hum...process of elimination. 

However, in the third paragraph in post #1, you said it was the guitar and only when the distortion is used.


This is quite common. I have this issue too. I have 2 guitars..... a Gibson with factory pups on it and a new Fender (china) with less then original fender pups.  Both are quiet as one would want in a studio situation when they are running clean.  Throw some gain on either and the noise floor rises depending on how much gain is cranked in and which model is used on the POD. Same deal on my Boogie... touching the guitar lessens that noise/hum to some degree. 

When you crank any amp into the distortion range, the amplification will raise the noise floor since all the inter-circuit noise is amplified along with signal levels.  This is one reason some guitarists use a noise gate. 

Using the balanced lines tends to help since they are designed in such a way as to not pick up the interference like the unbalanced lines do. That's why we use XLR balanced on mics and other things rather than 1/4" unbalanced. Yup... the guitar cord is unbalanced and so it picks up noise. It acts like an antenna. Most the the internals in a guitar are also unbalanced. Hence the noise. 

I also noticed that the guitar you speak of has "ACTIVE PICKUPS".... translated, that means it has a built in amplifier in the guitar to boost the output.... correct?  So you are adding yet one more level of amplification to the signal chain..... wow.....  Being a guitarist, I can understand the desire to have the hottest pickups, since I had a guitar at one time that came with some "hot" pickups as compared with the Gibson with factory pups. Nothing is ever free..... what you gain in gain and tone.... you trade off in other areas like noise and hum..... even with humbuckers.... crank them sufficiently and the noise canceling aspect of their design can not keep up. 

You can spend the money to try to eliminate it.....or do a few less expensive work around solutions.

1. use a footswitch on the amp to go to the hi-gain channel on demand. My Boogie has this and it works well. So does the POD. 
2. try a noise gate
3. fix it in the mix.... use envelopes to pull the faders to the bottom in the silent parts.
4. reduce the gain enough to keep the noise under control. 

It sounds like you've already concluded that the problem is the result of an impedance mismatch, but that seems unlikely, especially given that your guitar has a built-in preamp (which should have a low enough output impedance to drive just about any load).

What's more likely is a wiring problem within the guitar, perhaps a ground wire solder connection that's come loose or was poorly soldered by the manufacturer. If the preamp was added rather than coming with the guitar originally, the technician that installed it may have made a mistake.

You hear the defect when distortion is used simply because you're boosting the gain to achieve the distortion effect. I'd suggest taking the guitar in to a knowledgeable technician and have them check for hum and examine the internal grounding for the pickup enclosures, preamp and tone control wiring. They may be able to add shielding around the electronics, assuming it's not the pickups themselves that are picking up hum.

As for your final question, impedances are not transferred from one unit to another. The guitar sees one load impedance only. The device it's plugged into sees only one load impedance - that of the next device in the chain - and so on. The effects of an impedance mismatch, however (added noise, altered frequency response), will be propagated down the chain.

Adding the transformer isolated the circuit. That's why the hum died down...??? 
All the above is what you need to do. 


Test the guitar -  is there a hum when plugged into a loud amp with gain cranked up to stunn?? 
Test the Exciter - Nothing plugged in-- crank the output
Test the Line 6 with a different guitar

Agree on using a few guitars and amps to test the offending guitar.... you must isolate the problem before you can solve it. 

Impedance mismatches generally mean poor signal transfer with possibly a substantial signal loss..... compensating for the signal loss requires you to turn things up and that will amplify hum that is in the circuit. 

Both, the Line 6 and the Behringer have IEC power cords with safety ground.
If the safety ground is not the same in both units (long or broken cables), you will have a ground current going through the shield (braid) of the XLR cable. This current may induce a common mode voltage into the symmetric connection that the inputs of the Behringer cannot handle.
A transformer completely eliminates the common mode voltage (the same voltage added to both signals). I don't think you are seeing a problem that is related to impedance.

So, you should first investigate if you have a (potentially dangerous) problem with AC power: Are your IEC power cords and the outlets okay? (I assume that the Behringer and your audio interface alone do not show any problem.)
If everything is okay, you could make an XLR cable, where you disconnect ground (braid) on one side (ground lift). Or, you use a transformer between the units (DI box). It is a perfectly valid solution.

Wilko

I too was thinking that the transformers might be isolating stuff and creating the impression that they were a good solution. Those inexpensive in-line transformers aren't the best sounding audio transformers out there, so a more basic solution, as suggested above by shadow may be a good first thing to try.

Here's an illustration of the idea.

Running everything from basically one AC power source is not the answer.  Many times two devices are being powered from the same AC source and there is a ground loop hum.   For example a very common problem, two guitar amps are being fed from one Guitar (or effect) and both amps are plugged into the same AC outlet (or power strip).  There can be, and usually is, a ground loop problem unless one amp uses an AC ground lifter adapter.  

I really need to write this up<G>...

First - ground loops, and ground potential differences are NOT a problem, they exist everywhere, and you can't make them go away.

Second - single-ended interfaces, especially inputs, make the problem of noise pickup worse, but once again, they are not the problem, well, I think the world would be a better place without them, but that's cause I'm lazy<G>!

Third - you have to differentiate between different mechanisms for noise couping. And there are a bunch - it can be capacitively or indutively coupled, it can be coupled to the shield or to a signal conductor, etc. And the #1 problem is poorly designed equipment where the noise is allowed inside the case because the manufacturer violated the Pin-1 rule!

Fourth - Ground is not common, and common does not have to be ground. Yeah, makes my head spin too!

Fifth, probably should be first - Safety Ground is there for safety, not noise reduction. NEVER defeat it, not even temporarilly. It's marginal as a trouble-shooting tool, and it's dangerous. If you must break galvinic connections break them on the signal paths. And be wary of any device that claims to do so, the ONLY device that works is a transformer. If you are using it for troubleshooting you can get away with a cheap one, usually, If you are using it as a solution you'll need to spend the money on a good one, or suffer the audible penalty.

Trouble-shooting noise problems is, or can be, complex. So it's generally better to build for noise immunity.

The first step to a low noise system is a clean common reference, and power distribution. Note I say common, not ground. Ground is the final reference point, for safety reasons. Make every path to your reference as short and low impedance as possible.

You'll also need to think through the grounding scheme. There are two camps - single point grounding and mesh grounding. They both work, and they can even be combined, but you have to think it through. Single Point grounding is easier and less expensive, and has become the most common practice, at least in the US.

Power should be fed from a single breaker, or at least one leg. And the leg shouldn't have a lot of noise makers on it.

Balanced output to Balanced input is far and away the best interconnection... assuming the designers didn't do anything dumb - and you can't assume that I'm afraid.

Single-ended output to Balanced input is the second best solution. Wire it with a twisted pair, and connect the shield at the send only for starters. You may need to terminate the shield at the recieve end too in high RF environments.

Note that it is the twist of the signal pair that provides protection from magnetic fields. Shields have no effect at low frequencies.

While it's a bit more work, terminate the shields to the chassis, not the connector. You can, of course, do this only for devices that have a Pin-1 problem, but it's just as easy to start here.

If you are in a high noise environement you might want to modify single-ended sources to impedance balanced sources by adding an impedance between the low conductor and signal common.

Speaking of which - a balanced connection is balanced because the impedance to ground from either conductor is equal. It has nothing to do with the signal, and there is no requirment that both conductors carry a signal, only that each one has the same impedance to ground at the input. This allows the input stage to maximize Common Mode Rejection.

Single-ended source to Single-ended input, or Balanced source to single-ended input should be a last resort!

The worst is Balanced source because now you need to figure out if you need to ground or float the unused pin.

Avoid them and save me some typing<G>!

Other thoughts... impedance matching is not required, and in fact can make matters worse. Modern equipment (except guitars and guitar amplifiers) is designed for voltate transfer, not power transfer, and contrary to popular belief, characteristic impedance does not play a role in typical studio designs.

This is a huge topic - and an interesting one. I only skimmed it, so feel free to ask questions...