AC line loss causes hum

[Scott Helmke]

Here's an interesting thing I ran across a couple weeks ago. I did a service call to a big church with the praise band, to troubleshoot hum problems in their monitor wedges. We (me and the house tech) went through all the usual things. Finally found that the hum occurred between the monitor position and the amps, and could be mostly removed with isolation transformers.

However, it also turned out that the 20 amp circuit to monitor world had a rather high neutral to ground voltage, a bit more than 3 volts. None of the other dedicated audio power had anything that bad. Now here's the "aha!" moment. They had a pretty full load on that circuit, big old Yamaha M3000 console plus a bunch of UHF-R, some analog outboard, and power amp for cue wedge. So there was measurable line loss, even though they weren't overloaded on the circuit. We borrowed a big heavy extension cord from the maintenance guy and used it to power the monitor console from a random outlet backstage... and the hum was drastically reduced. We started turning off wireless and the cue amp, and the hum dropped even further.

Now here's the interesting part. Electricity travels in a loop, Kirchhoff's Law, symmetry, etc. So line loss happens not only on the hot wire, but equally on the neutral wire. As the hot wire drops, the neutral *rises*. And we all know that a high neutral to ground voltage is usually found right alongside system hum and noise. Which means that we really want to send thick wire and extra circuits on that long FOH run, to minimize line loss.

 

[Scott Helmke]

Re: AC line loss causes hum

The followup question here is why a high neutral to ground voltage causes hum. That leads to a bit of headscratching and thinking about power supply topology. But I think what happens is that you end up with some common-mode 60Hz voltage on the power supply input. That's no problem, any decent PSU can get rid of hum. It does that by shunting it onto the ground. And if the problem comes from long skinny wires, all that crap that the PSU wanted to get rid of now has to go over a long skinny wire as well. So our system now has localized dirty ground(s).

 

[Greg Cameron]

Re: AC line loss causes hum

Hum is caused by ground coupled noise that infiltrates a poorly designed audio interface. Even with higher voltages between neutral and ground, a well designed audio interface will not allow ground loop noise into the audio system. The voltage created between the ground and neutral is created by the magnet field generated around current carrying conductors. Like a transformer, they induce potential in the ground wire relative to the neutral line. The longer the wire and the higher the current, the more voltage you'll get. Check out Bill Whitlock's Grounding Tutorial.

 

[Scott Helmke]

Re: AC line loss causes hum

Thanks for the link, that's a very interesting presentation.

However, it doesn't seem to cover what I'm talking about here.

 

[John Roberts]

Re: AC line loss causes hum

The followup question here is why a high neutral to ground voltage causes hum. That leads to a bit of headscratching and thinking about power supply topology. But I think what happens is that you end up with some common-mode 60Hz voltage on the power supply input. That's no problem, any decent PSU can get rid of hum. It does that by shunting it onto the ground. And if the problem comes from long skinny wires, all that crap that the PSU wanted to get rid of now has to go over a long skinny wire as well. So our system now has localized dirty ground(s).

A high Neutral to Ground voltage doesn't or shouldn't cause hum. Neutral and Hot mains cord connections inside the product are both very isolated from chassis ground or anything that could shock humans (HiPot tested for leakage with thousands of volts.)

The elevated neutral voltage suggest high current is flowing in the associated wiring. High current means more magnetic field to induce hum in nearby wiring. Using heavier gauge wires to reduce that voltage drop will not alter current of magnetic field.

This is data, but not significant wrt your hum. If a transformer can make it clean up, a well designed active balanced input should also.

JR

 

[Scott Helmke]

Re: AC line loss causes hum

We did try a transformer, but it was not any more effective than adding a second circuit (that big extension cable). Did not really have the opportunity to try a different input topology, though. I forgot the make/model, but it was a relatively recent professional rack DSP unit. Changing outputs at the monitor console (direct from the console, or through outboard graphic EQ) did not make a difference.

Now as it happens that was a big enough church to have their own electrician, so adding a couple more circuits was no big deal. In this situation it seems to me that adding stuff in the audio chain would be considered a band-aid, versus clearing up the problem by improving the electrical supply.

 

[Greg Cameron]

Re: AC line loss causes hum

Thanks for the link, that's a very interesting presentation.

However, it doesn't seem to cover what I'm talking about here.

Sure it does. Starting at page 21. You're question was, "The followup question here is why a high neutral to ground voltage causes hum. (?)"

Page 21: "MYTH: Most Noise is Caused by “Improper” AC Power Wiring"
- Small voltages between outlet safety grounds is NORMAL in proper wiring
- Parasitic transformer effects in wiring
- Lowest between nearby outlets on the same branch circuit
- Highest (up to a few volts) between distant outlets on different branch circuits
- INTERFACE problems cause the NOISE!"

Then he goes on to explain how the interface allows the noise to occur.

 

[John Roberts]

Re: AC line loss causes hum

We approach such issues from a different perspective.

It is easier for the OP to change the mains wiring than the product input stage design.

As a product designer it is easier for me, to make the product design right than change everybody's mains wiring.

It's all relative. Some day all new gear will get it right. Until then there is still a mix of legacy gear in circulation with marginal/faulty input design.

JR

 

[Scott Helmke]

Re: AC line loss causes hum

Thanks, John - that sums it up nicely.

I started this thread as a "here's something else to try before tearing out the rest of your hair" thing.

 

[Geoff Doane]

Re: AC line loss causes hum

We approach such issues from a different perspective.

It is easier for the OP to change the mains wiring than the product input stage design.

As a product designer it is easier for me, to make the product design right than change everybody's mains wiring.

JR

I ran into what might be a similar problem with some Focusrite compressors a number of years ago. It seems that these rather high end processors would hum, seemingly at random. Testing with a Variac revealed the problem. Everything was fine at normal (115V) line voltage, but when it dropped down below 110V, the hum appeared on the outputs, even with no input connected. It turned out that the power supply rails were falling out of regulation, and ripple would appear on them at lower line voltages. My preferred solution would be to replace the the power transformer with one with a higher secondary voltage, but Focusrite's answer was to simply lower the power supply rails from 15V to 12, giving 3V of regulation headroom.

Lower the rails affects the maximum output capability of the device, but no one has ever noticed that there is less headroom, and hum hasn't been a problem since. Something similar might be going on in the OP's situation.

GTD

 

[John Roberts]

Re: AC line loss causes hum

I ran into what might be a similar problem with some Focusrite compressors a number of years ago. It seems that these rather high end processors would hum, seemingly at random. Testing with a Variac revealed the problem. Everything was fine at normal (115V) line voltage, but when it dropped down below 110V, the hum appeared on the outputs, even with no input connected. It turned out that the power supply rails were falling out of regulation, and ripple would appear on them at lower line voltages. My preferred solution would be to replace the the power transformer with one with a higher secondary voltage, but Focusrite's answer was to simply lower the power supply rails from 15V to 12, giving 3V of regulation headroom.

Lower the rails affects the maximum output capability of the device, but no one has ever noticed that there is less headroom, and hum hasn't been a problem since. Something similar might be going on in the OP's situation.

GTD

Anything is possible but live SR is notorious for lousy mains power, so major manufacturers typically accommodate realistic low-line voltages. 110v is not an acceptable low-line voltage, but some high end designers live in a bubble, so don't have the experience gained from dealing with world-wide mains voltage variation. It isn't just low voltage that bites you. The outback is notorious for capacitor blowing high-line voltages too.

But these are just two sides of the same coin, inexperience wrt mains voltage design tolerance occurs along side inexperienced input stage design for pin 1 rejection.

JR

 

[Othmane Alaoui]

Re: AC line loss causes hum

When you have an unbalanced 3 phase network, it does happen that you get zero sequence issues, this imply the presence of the third harmonic at a certain level, 180Hz. The hum is not only at 60Hz but go very often higher. Even when we connect our stuff directly to 120Vac, the line (120ac) is part of either 240V LL or 208, 3 phases. its very important to always make sure your loads ( amps, lights, smoke machines ... ) are equally spread on you distribution network.

an overvoltage of less than 10% normally should not affect our equipment, they are and should withstand 10% of overvoltage. Ground loops are an other common issue, people sometime lift the ground so we do not loop ...

 

[Greg Cameron]

Re: AC line loss causes hum

When you have an unbalanced 3 phase network, it does happen that you get zero sequence issues, this imply the presence of the third harmonic at a certain level, 180Hz. The hum is not only at 60Hz but go very often higher. Even when we connect our stuff directly to 120Vac, the line (120ac) is part of either 240V LL or 208, 3 phases. its very important to always make sure your loads ( amps, lights, smoke machines ... ) are equally spread on you distribution network.

The one type of neutral current you can't get away from with 3 phase systems is triplen currents cause by switching PSUs used by a ton of gear these days. No matter how balanced your current draw is between legs, there's no getting away from them.

Ground loops are an other common issue, people sometime lift the ground so we do not loop ...

Which ground? Signal only I hope...