hum loop solution

[Reffc]

I think that was the problem Nick; By lifting the earth, the RCD was rendered useless. I wasn't there when it happened but I was shown the aftermath. A mass of burnt plastic inside the amp where wiring had obviously burnt off the insulation and a fried mains transformer.

[Naughty Nigel]

It can be a can of worms Richard. I know of one person who lifted an earth on a (valve) power amp claiming that the RCD was protection enough. A fault later developed and the power supply insulation melted, with the amp catching fire and the chassis also remaining live long enough to fry whoever would have touched it. The RCD on that occasion didn't trip for whatever reason and the mains fuse protection didn't pop until enough current had been drawn to set fire to the thing. Not worth shortcuts, especially when other family members might be using the hifi. It never ceases to amaze me just how many in the gene pool are willing to put their lives and those of their families at risk for the sake of a problem which can be otherwise safely addressed. Natural selection eventually comes to bear

This is a fascinating thread which raises a number of important issues.

Firstly I would like to explain that RCD devices (earth leakage circuit breakers) will only trip if the device detects a leak to earth of about 10 milliamps or more. (Most are rated at 30 mA, but will usually trip at a lower value). RCD devices work by comparing current in to a circuit with current out, and tripping the supply if there is an imbalance (suggesting a leak to earth). RCD devices do NOT protect against fire or overload unless the fault current happens to flow to earth. The test button on RCD devices allows a small current to flow to earth via a resistor to check its operation.

RCD devices protect against electrocution as long as fault current through the body flows to earth. They do not prevent electrocution caused by touching live and neutral conductors at the same time! (Believe me, I have tested this myself more than once! )

Fuses and overcurrent circuit breakers will only protect against excessive currents. They do not protect against electrocution, and do not necessarily protect against fire in fault conditions. As an example, a one amp fuse can provide 240 watts of power at mains voltage. If a fault situation arises where 240 watts is dissipated across a half watt resistor or other small component the outcome will be obvious!

None of these devices is infallible under all fault conditions. Cleary then, audio circuits must be designed so that dangerous fault conditions are very unlikely to occur. Anyone modifying a circuit or changing its components must understand these parameters if safety is to be preserved.

So much for the safety lecture. Hum loops are created when magnetic fields generate alternating currents in earth circuits between audio equipment. Usual practice is to avoid hum loops by only having one device earthed (usually the amplifier), although as discussed this can be problematic if two or more devices are fitted with three core cables. (This includes computers of course.)

In my view it is dangerous to 'lift' the earth connection on an earthed appliance under any circumstances, as the circuit is no longer fail-safe. (An isolation transformer may be acceptable, but this usually is an expensive and impracticable solution.) Fitting external earth cables is NOT an acceptable substitute as earth continuity is not guaranteed.

(There are numerous design features intended to maintain the safety and integrity of earth connections, such as the longer and larger earth pin on 13A plugs, and the longer earth cable inside the plug, so that the earth connection should always be the last to be broken. These things are all there for a good reason.)

So how can hum loops be dealt with? Prevention is always better than cure, and the first step is to minimise the size of any loop, thereby minimising the effects of magnetic fields. Wherever possible, mains leads to different earthed HiFi separates should run together from the same source, such as an extension socket, rather than forming a loop. Plugging separates into different 13A sockets is asking for trouble! Interconnects should be as short as possible and run close to one another, and close to (but not alongside) mains cables (again minimising the physical size of any loops).

I should mention that separates fitted with two-pin 'figure of 8' chassis connectors can also be problematic owing to capacitive coupling of internal mains-voltage circuitry to the metal chassis, especially when switched off. Unplugging these connectors and reversing the connections often reduces mains hum significantly. (Likewise two pin European style mains plugs.)

If hum loops remain, and earth connections really do need to be 'lifted', the safest solution is to fit a pair of blocking diodes into the earth circuit of the offending appliance. These are simply a pair of heavy duty rectifier diodes connected in reverse parallel formation. The diodes allow a fault current to flow safely to earth, but the forward voltage (Vf) of the diodes (typically 0.6 volts) prevents unwanted current flow that might cause hum.

The voltages involved in hum loops are very low (usually just a few millivolts), whilst induced currents are similarly low (milliamps or micro amps), so the 0.6 volt Vf of the blocking diodes will be like a brick wall!

As a matter of interest, such diodes are commonly fitted into the earth circuits of yachts where shore lines are used. This is not to prevent mains hum, but to prevent electrolytic corrosion of metal fittings caused by earth leakage faults in marinas, and sometimes in other boats nearby.

But that is an entirely different subject for another day.

Nigel.

[Reffc]

Very good post Nigel. What you describe is exactly what I would (and have) done between pre/power amps in the past where neither had an earth lift and hum occurred (in that case, I fitted the diodes to the preamp section). It cures it completely and is intrinsically safe. The interesting part, and one which people often fail to grasp is that just because a circuit is fitted with an RCD, that in itself is not enough to guarantee protection against electrocution. The other thing worth mentioning is that at mains voltages, it only takes as little as 20mA or more for a severe (potentially lethal) shock partially because it's the threshold for sustained muscular contraction. It's a worthwhile thread (sorry for the slight thread drift to the op) because people need to be aware that messing around with mains unless they know what they're doing is plain dangerous and as you say, lifting a mains earth on a component designed to have one is dangerous. Finally, valve amplifiers hold some pretty lethal voltages so unless fully qualified to do so, no-one should go poking about in the inside of one especially as the power smoothing caps etc can hold some serious charge even after unplugging. As you say, lecture over but all important and worthwhile stuff to highlight.

[Naughty Nigel]

Very good post Nigel. What you describe is exactly what I would (and have) done between pre/power amps in the past where neither had an earth lift and hum occurred (in that case, I fitted the diodes to the preamp section). It cures it completely and is intrinsically safe. The interesting part, and one which people often fail to grasp is that just because a circuit is fitted with an RCD, that in itself is not enough to guarantee protection against electrocution. The other thing worth mentioning is that at mains voltages, it only takes as little as 20mA or more for a severe (potentially lethal) shock partially because it's the threshold for sustained muscular contraction. It's a worthwhile thread (sorry for the slight thread drift to the op) because people need to be aware that messing around with mains unless they know what they're doing is plain dangerous and as you say, lifting a mains earth on a component designed to have one is dangerous. Finally, valve amplifiers hold some pretty lethal voltages so unless fully qualified to do so, no-one should go poking about in the inside of one especially as the power smoothing caps etc can hold some serious charge even after unplugging. As you say, lecture over but all important and worthwhile stuff to highlight.

Thank you Reffc.

What I failed to mention is that earth bonding between separates components can also help to reduce hum; although any bonding cables should be as a short as possible (for the reasons already stated, and to minimise resistance), and with cable of at least square millimetre cross section. Offcuts of loudspeaker cable are ideal.

On your point about HT voltages, the voltage across terminals of a valve rectifier can easily exceed 700 volts, so I agree this is not something for anyone to play with unless they know exactly what they are doing. Even then, mistakes can happen.

If you do ever poke around inside a valve amplifier make sure you are insulated from the ground, (rubber soled shoes, etc.), and keep one hand behind your back so that current doesn't flow through your body.

Nigel.

[Barry]

Good post Nigel.

An alternative to the back-to-back diode arrangement (incidently, the current rating of the diodes ought to be as high as possible) is to 'lift' the ground with a 10 Ohm (5W) resistor. This will provide a current path sufficient to blow the fuse under a fault condition.

[Reffc]

Good post Nigel.

An alternative to the back-to-back diode arrangement (incidently, the current rating of the diodes ought to be as high as possible) is to 'lift' the ground with a 10 Ohm (5W) resistor. This will provide a current path sufficient to blow the fuse under a fault condition.

...or do both ;-)

Here's an over-kill earth lift circuit I installed into my last croft preamp to cure hum:



The white wire to the right is the link from the diodes to the chassis earth and the one coming in from the left is from the signal star-earth point which was floated. Bridge rectifier provides safe path to ground.

[Barry]

...or do both

Here's an over-kill earth lift circuit I installed into my last croft preamp to cure hum:



The white wire to the right is the link from the diodes to the chassis earth and the one coming in from the left is from the signal star-earth point which was floated. Bridge rectifier provides safe path to ground.

Yes, a 10A or 25A bridge rectifier would be ideal. I assume the capacitor in parallel is there to provide a low impedance path to RFI. What value did you choose? 100nF 400V?

[Reffc]

Yes, a 10A or 25A bridge rectifier would be ideal. I assume the capacitor in parallel is there to provide a low impedance path to RFI. What value did you choose? 100nF 400V?

That's right Barry. I think it was a 100nF ( probably still have a box of the things somewhere) so may check and put the circuit up for anyone interested in adopting something similar.