Question about JBL PRX612M 12 Inch Two Way Self Powered Monitor Speaker 1000 Watt Portable and Flyable

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I have just bought a PRX612M. When I play my digital piano through it at low volume there is a buzz which is most noticable as the notes volume dies down. Loud it sounds great but i would love to know what is happening here. I know it has a 1000W amplifier inside so asking it to perform well at 10W is a big ask but it would be nice to have that capability too. The buzz sounds digital and is very similar to the speaker coil scraping. The piano is a yamaha P120 which sounds fine through my other amp.

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1000 Watts? Well, those are peak values. !000 watts RMS into a 12 inch speaker and it would smoke... this is some of our inflated advertising. But that is beside rhe point... now on to your problem: What you are having is a ground loop. You may have to do more than one thing to correct this here is the things to do in the order to do them to solve the problem:

1. Connect the piano and the speaker to the SAME power source or receptacle, even if you have to run an extension cord, three wire of course. Plug all of this ONLY into a grounded outlet, if your house has only two wire plugs, then you need to get an electrician to install a grounded receptacle.

2. Use Balanced lines. This means XLR or TRS balanced cables between the piano and the speaker. Use the 1/4 inch jacks output, not the RCA ones.

3. Since the AUX outputs do not support balanced lines, the next step require you use DI box. They are about $20 from Guitar Center and others. With very short 1/4 inch mono cable connect the piano to the DI box from the L/R mono piano to the high impedance input port of the DI and set the ground lift switch to "LIFT". Use an XLR to XLR cable from the DI box to the speaker. This last step if needed is the last thing in our bag of tricks to break the ground loop that can cause the buzz. Dirty power, if you have lamp dimmers can be a source of the noise.

Posted on Apr 06, 2011

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  • arling_tree Apr 06, 2011

    A big thank you. Your response arrived about an hour after i posted my problem and your first suggestion has now fixed the problem. All smiles again and i thought i was going to have to do a 2hr trip back to the shop with it.
    I'm in Australia and we have 3 pin power here. The wall outlets were about 6m apart. Does this mean that they're not earthed correctly?
    A big thank you again.
    steve, perth

  • Fred Yearian Apr 06, 2011

    In regard to your question about earthing... This is a little bit of engineering theory... EVERY piece of wire is an inductor and a resistor and noise can rattle the grounds AND the other wires relative to those at another receptacle. An even bigger problem is a receptacle that shares different feeds from the breaker box and has OTHER loads on that feed. It can bounce all over the place. Our audio equipment can tolerate about 15 volts before being damaged, but long before we get to that voltage we will likely notice hum and buzz in our equipment. Print my suggestions for future reference should you run into trouble again. Thank you for letting me know the outcome...

  • arling_tree Apr 11, 2011

    After having a bit more time to play with the prx612M I have found that the problem was from using a 20 ft unbalanced lead. With a 10 ft unbalanced lead all works perfectly but with a 20ft lead it was picking up noise from the fixed mains wiring under the floor-boards. I thought that plugging the keyboard and amp into the same outlet had fixed the problem but it hadn't - unless the signal cable was kept away from all mains cables. I have oredered a passive di box for longer cable runs. steve in perth.

  • Fred Yearian Apr 11, 2011

    Yes, use balanced line XLR to the speaker with a DI near the keyboard and a short (few feet at most cable from the keyboard to the DI box. Put the DI box on "Ground Lift" to avoid a power ground loop on the balanced cable. Use HIGHER gain OUT of the keyboard (say at 70% setting) and LESSER gain at the speaker to improve signal-to-noise ratio.



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