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Remove the thud/pop from my subbie ?


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Hi,

Every so often when I open or close my fridge door you can hear a rather large thud/pop noise from my subwoofer.

I have proven that this is electrical noise that is getting induced into the audio lead from the receiver to the subwoofer. That is the noise will only happen with the lead plugged into the subwoofer. The noise is not coming from the receiver as I can unplug at the receiver end and the problem still exists.

It is practically impossible to physically locate the audio lead away from all the mains power leads.

Any suggestions ??

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Hi AusZeus

I have a suggestion but I'm not sure if it would work.....

How long is the cable for the sub?

What cable are you using at present?

Anyway we will let the forum posters see what they think and correct me if it is not worth trying.

You say that it is interference from other cables?

Well have you tried replacing the cable with video cable?

This usually has superior shielding to audio cable.

Just a thought before you get into UPSs and power conditoning equipment.

J

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I would guess this is crappy 240v noise coming from your mains wiring - which happens a fair bit. Try powering the sub from a circuit other than the one the fridge is on. Microwave ovens and the likes can also cause this. Using an extension cord to another power point should prove this, or not :blink:

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If the Sub is self-powered which I assume it is,it most probably has a ' floating ' power supply,that is it is not earthed to the mains in any way.

This is to prevent earth loops inducing 'hum' into the audio circuits of your setup.

Unfortunately this leaves such power supplies open to mains transients,which are then amplified as the shielding of the input of the amplifier (as in the sub) is connected to the ground of the DC side of the power supply.

Though the ground of the DC side of the power supply is not directly connected to the mains earth, it is capacitively coupled to the mains active/neutral through interwinding capacitance and also by mutual inductance.

Shielding is virtually useless in this situation as the noise is injected via the mains connection (power point). Using the same power point connection as the other equipment may help though I doubt it. Try what was suggested by Nobby as this can prove successful in some cases,but usually a 'clean ' outlet is necessary to overcome the problem.

Another solution is to 'eat out a lot '

C.M

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But AusZeus has deduced it is caused by the cable.....

AusZeus, if you disconnect the audio lead at the subwoofer there is no popping noise?

However, if you disconnect it at the receiver the noise is there?

This is what I am inferring from your first post.

Hey Chickenman, if this is the case, surely shielding will be important?

J

Edited to fix dodgey spelling.

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AusZeus, if you disconnect the audio lead at the subwoofer there is no popping noise?

That is right. With the cable disconnected and the subwoofer powered up there is no noise at all.

However, if you disconnect it at the receiver the noise is there?

Correct. This was to show that the noise was not being generated in the receiver. Which only leads us to the conclusion that it is being inducted into the audio lead ?

I have lessened the problem to some extent by winding back the crossover control and also the amplifier gain on the sub. To compensate I have increased the sub output level on the receiver.

Powering the sub from a different power point also removes the problem - but this is currently an impractical solution as we don't want leads running from one room to another.

I also tried using a shielded rca-rca coaxial cable and this didn't help either - in some respects I think it was marginally worse.

Thanks for the suggestions so far.

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AusZeus,

You have found the ' fix ' to the problem and that is to use a different power point.

If you get an electrician to install an extra power point at the Sub, wired back from the power point that proved to be the ' fix ' then the problem is solved.

He could also swap the power cables over between the two rooms if that proved easier.

As I mentioned, shielding would prove useless as there is most likely a small potential difference between the two power points when the refrigerator door switch opens and closes. Poor contact resistance can cause arcing especially at the peak of the 240v mains sinewave (339.4volts).

The Sub amp is using its audio connection as a means of pickup for this pulse in potential difference. The braid of the audio cable may be terminated 10 or so ohms above the DC ground of the Sub's amplifier, this is by design to offset 'hum' through connecting to other equipment,it is common in amplifier design (Hum Bucking).

In other words, the braid(shielding) is not perfectly grounded at the Subs input.

Keeping that in mind, both the braid and centre conductor of your audio lead act as one, as if it were an unshielded conductor into the input of your Subs amplifier with a 10 ohm resistance to ground (of the amplifier).

So when the audio lead is unplugged at the Receiver but connected to the Subs amplifier, the ' thump ' is induced by capacitance.

Now this potential difference (pulse) will cause a current to flow through the audio lead when it is connected between the Sub amplifier and Receiver and so causing a voltage pulse across the 10 ohm (Hum Bucking) resistor and thus amplified as a ' thump'.

The only practical fix is the 'power point ' one suggested.

C.M

I hope this isn't too technical in explaination.

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Well done Chicken Man!

Thanks for your explanation.

Auszeus it could not be the cables if the problem is not there when you use another power point.

J

PS Does any one know where would you get an Australian product like the PS audio Power regenerator on the 'Secrets' site at the minute?

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