Ground loop from cable coax

[jeffjmr]

I’ve been using an inline isolation transformer to eliminate hum on my sound system after months of unsuccessful troubleshooting.

I conpletely dismantled, rerouted and replaced some inferior cables this week so took a serious look for the culprit through sequential connection of all the many components; Preamp, sub crossover, two subs amps, power amp, TV, sound bar, Blu-ray, CD, FIOS set top box. Found the offending item.

The only device with 3-prong grounded power is the power amp. After plugging in all devices to mains and still no hum, I thought I inadvertently solved the issue until I plugged the FIOS coax into the set top box.

Aha! I thought. Lift the ground on the demark. Nope. 2 prong plug, no ground.

Demark feeds the FIOS router as well, but it’s DC. I’m out of ideas. Is there such a thing as a coax cable isolation device? Yes I can lift the ground on the power amp but it is an all-metal case that is grounded and I’m unsure of the wisdom of lifting it.

Help!

Jeff

[KE5FX]

You can lift the ground on your power amp with a device like one of these.  This is an expensive way to put a couple of antiparallel diodes (6A6 or similar) in series with the ground pin, so that the safety function is maintained while blocking low-level AC circulating currents that cause hum.

Googling will yield countless debates regarding the safety and/or wisdom of this solution, particularly in areas prone to lightning or other line disturbances that might fry the diodes without being noticed.  But it will most likely solve the problem for you.

[Kim Christensen]

Is there such a thing as a coax cable isolation device?

They do exist (Just an example). Apparently it's a common enough problem.

[jeffjmr]

Thanks, KE5FX.

Could you elaborate on the diode setup? If that is all the mains ground isolator is, $59 at Amazon sounds wasteful.

Would it be as simple as two 6A6 or 6A8 diodes cathode to cathode in series with the ground lead? And the one reverse biased in case of a ground fault at the amp would short? Seems the other risks could be mitigated with a surge protector.

And Kim, thanks for that link. I will look into that device. Having trouble figuring out the theory on that one but I don’t need to know how it works, just that it does.
Jeff

[KE5FX]

I'm not necessarily advocating altering your power amp's 3-wire ground connection, but the idea is that the diodes are in parallel but with one reversed, so that a voltage difference (of either polarity) above about 0.6V-0.7V is conducted while lower voltages are isolated. 

[Kim Christensen]

And Kim, thanks for that link. I will look into that device. Having trouble figuring out the theory on that one but I don’t need to know how it works, just that it does.

It works in the same way as your AC mains inline isolation transformer, except at RF and much lower power levels. Essentially, it is a 1:1  75 RF transformer.

[jeffjmr]

So the coax shield is just the second conductor?

[coppercone2]

Man I think you can do better then putting diodes on ground. Look at shafner filter its just a capacitor that's rated for such things and it still blows up.

whenever I hear about 'your area' and ground I think about like guide lines for planting tomatoes.... thats where that phrase belongs, not the power grid.

[KE5FX]

I don't think a Schaffner filter is going to do much for 60/120 Hz hum. 

[coppercone2]

I don't think a Schaffner filter is going to do much for 60/120 Hz hum.

What I am saying is that the part is meant for AC use and people have em blow up all the time, a diode gotta be worse

[jeffjmr]

I do indeed have a Schaffner filter mains power cable on the power amp.

I think I see how a diode pair might work, but wonder if a 10 amp or higher type might be better. It’s a 15 amp circuit and I assume we are trying to maintain protection in the event of a ground fault at the amp, to result in a tripped circuit breaker. That comes from not understanding the failure mode of diodes. In the event of a current exceedance do they fail shorted?

[Kim Christensen]

So the coax shield is just the second conductor?

Yes... And the coax shield will be grounded on the cable company side via their network. That's why you can also get a ground loop between it and your equipment/building/power ground system.

[coppercone2]

no actually diodes like to crack open quite ab it when you put mains on them. Like a hair line fracture. I have seen like 8 diodes go on a PCB where I can gently push them with tweezers and they open like chineese fortune cookies  without the paper

PTC also do this, slightly differently, but they def fail open too

its a linear ceramic structure with a junction in the middle.

[jeffjmr]

Success! Coax iso transformer worked like a charm. Thanks, Kim.