[ZephyrApprentice]

Got a power hum and I was wondering if anybody had any suggestions for a rookie. If I turn the volume past 50% on my McIntosh there is a noticeable hum. I am bi-amping my Martin Logan SL3's with a Forte and a Threshold. When I turned the system up and heard the hum I turned off the Threshold, the hum died significantly. Any help would be appreciated.

[richidoo]

Isolate the hum between two components. If your MLs have active bass amp consider them a source too. Get the two devices to hum by themselves with nothing else attached. Hum travels over interconnects (actually that is usually the problem).

Once you have it isolated then you can start to think about a fix. Better shielded cables, twisted ground conductors, better ground connections, or less grounding altogether. Plug the offending components into the same outlet for a common safety gound, or try a temporary cheater plug to lift the safety ground TEMPORARILY only for diagnostics. You cannot will not must not leave the cheater installed, even if it cures 100%. Without a safety ground the circuit breaker cannot know to shut off the power when someone is being shocked or a short is burning. Shorts do happen, and people do touch hot wires. Without a ground death is possible. Insurance won't pay a dime when the fire marshall finds the cheater plug. Call me Mr. wet blanket!! Sorry, but I love you man...

If wiring/ground improvement strategies don't work, then you can get more drastic. The signal ground inside your components can be disconnected from the safety ground. This may not be legal where you live and I can't say it is perfectly safe either, but it is common practice in DIY stuff, never get UL approval though...  Here is another approach that is a little safer. Reconfiguring your ground is a purist approach. Usually the preamp is safety grounded to earth, and the rest of the system has no signal ground connected to safety, all components have their own safety ground (3 prong plug). That's less convenient and probably not as safe as:

Adding a humbuster. A device coming between the components that are looping which breaks the ground connection safely. Jensen makes the best dedicated humbusting parts. They have it for coax, RCA, balanced. You will not be able to hear the effect these have on the signal, other brands may not be so good. Jensen is expensive but worth every penny compared to the hassle of rewiring a ground and risking safety. Be sure the hum goes away when you disconnect the lead before you buy it. Don't try to figure it out mentally, you gotta prove it by disconnecting the loop creating lead. Finding the source of hum is the hardest part, especially with tuners whose circuits extend beyond the stereo system proper. Remember no individual component is responsible for the hum by itself. It takes two to tango. The best components in the world can have hum. It is an audiophile rite of passage - this is your big chance, your initiation!! haha   It's not that hard..

See if the whole stereo system is earthed on the same circuit. 3 dedicated power lines each running 120 feet back to the panel can create loops by themselves when linked with an interconnect cable connected to safety grounds. A simple power strip will allow you to test whether the ground connections are too long. This is rarely ever the problem because the ground wires in the wall are large ga solid copper so resistance is minimal over 50 feet. Higher resistances result in potential differences which allow current to flow which you hear as hum.

I once received this email from Ken Bernacky, owner of Stereo Surgeons in Hartford CT regarding hum I was getting through my tuner he had just overhauled. It really helped me understand what hum is and I was able to give up the purist approach of trying to fix it electrically and move forward and solve it (with a Jensen coax humbuster). My safety grounds are all in place so I can sleep when lightning storms come, and the signal is dead quiet, and transparent. With the isolator and Ken's mods, my tuner is now my best component.  Here's Ken:

"Assuming you actually do have a ground loop:  A ground loop exists in any installation because there is a difference in potential between the two, or more, "grounding points".  Being at ground potential is not an absolute, there is no such thing as 0 volts, there is more and more precision, ie: 0.1 v, 0.01 v, 0.001 v, etc., all can be considered to be at ground potential for certain applications.  The problem develops when we connect an audio amplifier chain with it's large associated gain, typically 100 dB. or greater.  What in one situation without such gain would be an adequate ground the large gain situation magnifies this seemingly small difference in potential, say 0.01 v to an audible level of 60 hz. hum.  What is clear is this pipe in the dirt ground is not at the same ground potential as the amplifier chain ground and thus your 60 hz. hum.  You never said it is 60 hz. hum, I'm assuming this.  I am also assuming that even with the tuner turned off you get the hum, as you specifically said it is not affected by the (tuner?) volume control.  I also suspect if you had headphones connected to the tuner output and had no interconnecting cables to the amplifier chain the currently perceived hum would not exist.  I suspect the difference in ground potential current is flowing down the coax, going over the interconnecting cable's shield from the tuner to the input of the amplifier, thru the amplifier to whatever the amplifier is using as it's "ground" point in your home.  This current flow is being coupled, possibly capacitively in cables, amplified, and you hear it as hum.

To do this properly this difference in ground potential between all ground points must be reduced further, you will never get it to absolute 0, but below the level at which an audible hum occurs.  This can be done by running the ground point for the amplifier chain to the ground you are using for the antenna with the largest sized wire possible.  The larger wire has lower resistance and this whole problem is occurring because of a voltage being developed because of current flow thru the resistance of the wire.  Since ohm's law states E=IR, stated another way: Voltage = (Current)(Resistance) one can see that reducing the resistance of the wire lowers the offending voltage, BUT IN THIS CASE THE VOLTAGE MUST BE REDUCED TO AN EXTREMELY LOW VALUE DUE TO THE LARGE GAIN OF THE AMPLIFIER CHAIN.  This may turn into a ridiculous extreme and is not what I would do.  First of all I don't know why you "grounded" the coax.  I would not do that.  Assuming you wanted lightning protection I would get the type of lightning arrestor which uses a small gap between the coax and the line run to "ground".  This would provide lightning protection and not cause a ground loop.  Also your tuner uses a 3.3 mohm resistor from one side of the AC line to the chassis, reversing the ac plug in the wall may affect the hum situation.

Ground loops can be frustrating because the neophyte thinks ground is ground, but once one experiences the epiphany that nothing is at the same ground potential when connected thru wiring (thru resistance), and this difference in ground potential causes a current flow, then it all begins to make sense.  Usually in cases like this the fewer ground points the less problems one has.  As an aside...on circuit boards many times foil patterns are designed to specifically not have currents flow thru certain "ground" areas because this seemingly infinitesimally small current gets amplified and adversely affects circuit operation.  To prevent this the circuit board designer uses a technique called star grounding, actually what you are currently doing is a crude form of star grounding, just not good enough due to the extreme gain of the amplifier chain.

Sincerely,

Ken Bernacky
President, Stereo Surgeons, Inc.
http://www.stereosurgeons.com"

Good luck
Rich

[bpape]

Are the ML's, the Forte, and the Threshold all plugged into the same 110v circuit?

Bryan

[ZephyrApprentice]

yes. Unfortunatly, all of the equipment is sharing one outlet. A couple of splitters and power strips and I was in business.

[bpape]

Then go down the path Rich is suggesting.  If they're on 2 different circuits or on a daisy chained set of outlets on the same circuit, the ground loop could be there.

That's an awful lot of stuff and a couple BIG amps on 1 outlet.

Bryan

[richidoo]

Bryan nailed it. The ground connections have to be solid to reduce resistance across the plug contacts which will cause hum, even in a very short loop. Thin dirty brass contacts in a $8 strip or cheapo splitter will add a lot of resistance to the ground circuit, causing current to flow and make hum. Get rid of cheap parts in the power line and invest in a good power strip, it should be HARD to plug into it..

Tripplite Isobar got a good audio oriented review online 2 years ago, but I can't find it now. I tried it and it is not suitable for tube amps which are finicky about strip and AC filters, but otherwise it is very well built. Mapleshade makes a plastic one that they say is the bomb in their own inimitable way, but I've never tried it. If you wanna spend some money, check out Acoustic Revive's newest. 

See if the hum improves by plugging as much as you can into only one strip, or better yet, the two worst hummers into the outlet. Good luck

[electricbear]

Give me a call at the store sometime and we'll arrange a good time for me to come over and help you diagnose and resolve this issue.

[jedi35]

Hey, there is a lot of great information in this thread!!

Adrian

[Wayner]

I have MartinLogan reQuests. You have to have the speakers electro-static power cord and the power amp on the same circuit. Richidoo is correct. The ground loop gets completed thru the speaker wires.

Here  is the train: outlet to power cord of amp, speakers wires from amp to speakers, power cord from speakers to outlet.....around and around and around........hummmmmmmmmmmmmmmmmmm.

Try same ground, I think you will like it.

Wayner