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[Peter Kowalczyk]

Hey Folks:

The situation:

One of two installed Turbosound Athens TCS122/94-AN has a pronounced buzz.  It is powered by a different circuit than the DSP (Ashly PEMA Amp DSP out) that drives it.  When powered off the same circuit, the buzz disappears.

The two AC circuits appear to be on different phases - 0V Ground-to-Ground, 0V Neutral-to-Neutral, 208V Line-to-Line.

My Question:

Is the phase difference a red flag or a red herring?  Should I ask the electricians to re-wire the speaker circuit to be on the same phase as the DSP?  Or is there something else I should look into?

[John Roberts {JR}]

more likely a pin 1 problem (product input stage design).

JR

[Peter Kowalczyk]

more likely a pin 1 problem (product input stage design).

JR

Thanks JR - so would you recommend disconnecting the signal Pin 1, or running thru a transformer?

[Tim McCulloch]

Phase has nothing to do with hums/buzzes.  Each phase comes from a common source if from the same service. 

What is more likely - a wiring error (neutral/ground swap or hot/neutral swap) between the circuit breaker panel and the outlet on one of the circuits.

Any Pin 1 issues will be revealed by electrical shenanigans.

[Peter Kowalczyk]

Phase has nothing to do with hums/buzzes.  Each phase comes from a common source if from the same service. 

What is more likely - a wiring error (neutral/ground swap or hot/neutral swap) between the circuit breaker panel and the outlet on one of the circuits.

Any Pin 1 issues will be revealed by electrical shenanigans.

Hmm, thanks.   I just disconnected the shield connection at the DSP end, and the buzz disappeared.  Perhaps worth noting, I've had buzz problems with other self powered Turbosound products (TFX112 monitors to be specific) which presumably use similar or identical input/dsp/amp circuits.

So sounds like it'd be worthwhile to go through each outlet on each circuit with a cube checker and look for errors?  The two circuits in question appear to be correct relative to each other, but sounds like a single miswired outlet could wrench up the works?

many thanks...

[John Roberts {JR}]

Thanks JR - so would you recommend disconnecting the signal Pin 1, or running thru a transformer?

Modern product should not have pin one problem.

I do not advocate lifting pin 1... While wiring shields do not make robust safety ground they are better than no safety ground (safety ground can telescope through shields protecting ungrounded chassis.)

The cheap 3 lamp outlet testers IMO are pretty much worthless...

Perhaps with a VOM, meter across the shield you opened up and see how much AC voltage is present. Too much is too much. If not a lot of voltage you can restore the shield effectiveness at RF with a small cap...

If you see too much voltage fix that.

JR

[Chris Hindle]

Hmm, thanks.   I just disconnected the shield connection at the DSP end, and the buzz disappeared.  Perhaps worth noting, I've had buzz problems with other self powered Turbosound products (TFX112 monitors to be specific) which presumably use similar or identical input/dsp/amp circuits.

So sounds like it'd be worthwhile to go through each outlet on each circuit with a cube checker and look for errors?  The two circuits in question appear to be correct relative to each other, but sounds like a single miswired outlet could wrench up the works?

many thanks...

Disconnecting a shield (or ground) should be considered a troubleshooting step. IT'S NOT A SOLUTION to a problem. FIX the problem.
Lives matter.
Chris.

[Frank DeWitt]

Disconnecting a shield (or ground) should be considered a troubleshooting step. IT'S NOT A SOLUTION to a problem. FIX the problem.
Lives matter.
Chris.

I disagree.  Disconnecting the shield (pin 1) at one end of a audio cable is a good fix, as is a 1:1 isolation transformer.  It does no harm, it is not a safety issue, and it works. 

Disconnecting a AC safety ground is dangerous and should never be done except for trouble shooting. 

[Kevin Graf]

I disagree.  Disconnecting the shield (pin 1) at one end of a audio cable is a good fix, as is a 1:1 isolation transformer.  It does no harm, it is not a safety issue, and it works.

True, some experts recommend disconnecting the shield at the receive end of line level balanced interconnects (this won't work for phantom powered mic cables, which means that you have to have two sets of cables). They suggest a hybrid connection at the receive end consisting of a small RF capacitor.
But other experts recommend NEVER disconnecting the shield.

Disconnecting a AC safety ground is dangerous and should never be done except for trouble shooting.

Exactly!

[John Roberts {JR}]

I disagree.  Disconnecting the shield (pin 1) at one end of a audio cable is a good fix, as is a 1:1 isolation transformer.  It does no harm, it is not a safety issue, and it works. 

Disconnecting a AC safety ground is dangerous and should never be done except for trouble shooting.

opinions vary.

Indeed shields are not an official safety ground but for lots of gear it is the only path between chassis and safety ground. Gear with 2 wire line cords require double insulation for mains wiring, but we've all seen 3 wire gear with ground lift adapters.

In live sound we have a higher probability of encountering rogue mains wiring (like mis-wired outlets) and some squirrelly back line gear. If an isolation transformer is concealing a hot chassis situation that is not good for human safety.

For live sound an audio isolation transformer can be a quick fix problem solver but for a permanent install I'd like to correct the fault at its source. Sometimes the transformer is the right choice if your' VOM determines that the ground potential is not hazardous, and it can't be addressed any cheaper. 

Further if the shield wire turns red hot and catches the cable jacket on fire you have an early warning system. 

JR

[Kevin Graf]

A component with a double insulated enclosure (square within a square safety symbol) does not have a connection to the Safety Ground. While there may be continuity from it's shield to the Safety Ground of other components, it must not be modified to add a Safety Ground.

[Stephen Swaffer]

And, yet, installing it in a rack with 3 wire chassis grounded gear will do just that...

[John Roberts {JR}]

A component with a double insulated enclosure (square within a square safety symbol) does not have a connection to the Safety Ground. While there may be continuity from it's shield to the Safety Ground of other components, it must not be modified to add a Safety Ground.

Unintended safety ground paths can easily occur via interface wiring, and often do. The hum being discussed may be caused by one such unintended path flowing current between grounds.

The scenario where such an unintended safety ground path could cause harm is insignificant (IMO) compared to the opportunities where it could do some good. While it is clearly not a valid safety ground bond***.

I do not advise people to modify anything (like adding or especially breaking a ground path, shield or otherwise).

But I am curious why the "must not" and what are you referring to?

JR

*** back when I was designing gear for the install market (last century) UL insisted that any external connection (including audio) labelled ground "must" be bonded to safety ground and tested to carry tens of amps without significant voltage rise long enough to trip a fuse/breaker. 

[Kevin Graf]

And, yet, installing it in a rack with 3 wire chassis grounded gear will do just that...

Yes, and it's expected to happen in that situation and is very acceptable. But opening up a double insulated cabinet and adding a Safety Ground wire to an internal chassis is not. 

[John Roberts {JR}]

Yes, and it's expected to happen in that situation and is very acceptable. But opening up a double insulated cabinet and adding a Safety Ground wire to an internal chassis is not.

again who (two words, second one dirty) suggested that?

JR

[Stephen Swaffer]

the lawyer

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