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JBL PRX-515 low output

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  • JBL PRX-515 low output

    This is one I wish I'd passed up....

    Getting lower than expected output (242w RMS into 2 ohms dummy load) on the bass output at the point the overload LED comes on. The factory spec shows 400w. Sweeping the input frequency from 30hz to 1khz maintains a nice output waveform, but 242w is the most it will give.

    Similarly, the High output only gives 72w at best into 4 ohms (should be 100w) with a sweep of 1khz to 10khz.

    Does anyone have any service notes or has come across a similar problem with this model?

  • #2
    We replace entire modules on those. It's the best bet, and the most reliable repair.
    John R. Frondelli
    dBm Pro Audio Services, New York, NY

    "Mediocre is the new 'Good' "


    • #3
      I do not have a schematic for that since I am not a JBL service center anymore but if all looks good otherwise, is it possible that the output is not grounded on one side while your scope or meter is referencing ground? Is it a balanced output, or does it use a ground return.
      I have a PRX612m that came in yesterday, also no schematic that has a power supply problem with crispy PWM controller and a couple SMD transistors plus some shorted output devices. John is right, board replacement is the best solution if you have access to them.
      The power and modulator board is pretty impressive with layout, construction and design, plus cast aluminium shield box.
      I just found a board that is available in the US for $198 plus shipping so I am going to recommend to the pa company which has a dozen of these to replace the board and I' will try to repair this one as a spare, if I can find a schematic.


      • #4
        To their credit, JBL provides a lot of technical info, parts lists and schematics here: EON-G2 Series (EON-G2 Series - I don't know why the URL presents as "EON-G2")

        I notice the language on the technical cutsheet says "Class-D, 400W (continuous) at driver impedance". They may be playing a bit with the ratings since speaker impedance varies greatly with frequency & other factors, and they didn't say "nominal driver impedance".

        Can you get your hands on a known-working unit to compare via measurement and/or listening test?


        • #5
          Mark, thank you very much for the link. It has a very good set of schematics and layout details. It is just what I needed to repair the PRX612m I have on my bench.


          • #6
            Tremendous information - I never found this on a search for "PRX-515 schematic".

            The outputs I measured are floating - not ground referenced. I've requested that the customer brings in the other module for comparison. Further measurements show that the module can be pushed into delivering 450w into 2 ohms with a large enough input signal (3.5v RMS) and ignoring the overload LED. Possibly there's some attenutaion of the signal before it gets to the output stage, or the dynamic circuitry is limiting the signal too much. I'll study the schematics.


            It could be entirely normal for this amp.

            I've only had one of these in before and it was cutting out and an easy fix.


            • #7
              If memory serves we (at the JBL dealer where I used to work) thought the PRX series needed an abnormal amount of input signal compared to EON and Mackie powered speakers in order to get an equivalent output.

              On a subjective note I didn't really care for the sound of them either.

              I bet it's working as designed...


              • #8
                Well I got my hands on the working unit and at the point of the overload LED coming on it is producing 450w into a 2 Ohm resistive load with a 30hz signal.

                The only real difference between the two units was the faulty one needed a higher input signal and the level control turning right up to produce the same output.

                Gentle tapping/pressure on one end of the main PCB closest to the input gave an improvement in output with a lower input signal level. There looked like a few shrinkage tears in that location so I've reflowed some of the joints and now setting both amps up together and checking between them only shows a small difference between the settings of the level pots for the same output. The 'repaired' unit will actually produce more power (480w) than its counterpart.

                The customer tried them out at the weekend and mailed me to say they're now working fine. So I hope they stay fixed. The only thing is that I never identified what precisely was causing the problem. Just happy that it's working.