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Thread: Home pre-amplifier for balanced XLR output/input on a Crown pro power amplifier

  1. #1
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    Home pre-amplifier for balanced XLR output/input on a Crown pro power amplifier

    I own 3 Crown professional power amplifiers that were purchased new a few years ago. These amplifiers have ONLY balanced XLR input jacks. Each Crown power amp pumps out 500-watts per side into 8 ohms and they can really crank on JBL speakers. All my sources (1) CD player, and (2) Bluetooth radio have standard home RCA outputs which are currently plugged into a Denon home receiver with pre-amp output jacks going to the Crown power amps.

    Unfortunately, they all HUM (ground loop) and it drives me nuts.....

    I have tried hooking these power amplifiers up to my Denon receiver using many different RCA to XLR cables with no successful elimination of the HUM (ground loop). The connections I'm using include 3-pin adapter jacks that go from XLR to RCA. These XLS-to-RCA conversion jacks have ONLY two wires (+, -), however.

    A few weeks ago, I researched "HUM" and "ground loop" on the Internet and found a few interesting threads that discuss ground loop problems. The first suggestion was to insure that all electronic pieces are properly grounded and attached to the same electrical circuit on my circuit box (fuse box) but this exercise did not solve the HUM (ground loop).

    The second suggestion was to use an Artcessories Clean Box Pro This unit is advertised to convert unbalanced output to balanced output. This did not work either but I am connecting this unit with the same XLR-to-RCA conversion jacks and the RCA cable wires only have two wires (+, -) which I am thinking defeats the purpose of the Clean Box Pro unit. I'm going to order two XLR cables (Male-to-female) that will have 3 wires to see if that solves the HUM when I connect them to the Clean Box Pro unit.

    At this point, if this does not work, I'm thinking that I have only three choices, (1) dump the Crown power amplifiers in favor of some other large home power amplifiers with RCA inputs like the Adcom 555, or (2) use a cable that connects a 1/4" stereo headphone jack to an XLR jack, (3) buy a professional disco (??) pre-amplifier that will accept RCA inputs like a CD player and which will funnel its output into an XLS cable that will match the Crown professional power amplifier input jacks.


    Any ideas on this is much appreciated.

  2. #2
    Senior Member richluvsound's Avatar
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    Older Amps ...

    Hi Robert ,

    I had an old Bryston pre .... the XLR connections may need to be reversed on the crowns also ...

    Rich

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    Senior Member 1audiohack's Avatar
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    Robert how old is your house? Old houses often have two wire power with the neutral and ground bonded at the outlet when the old two wire recepticals are replaced. This can float the "ground" well above zero when the circut is under load and depending on the ground scheme of the components in use can generate this issue. A true three wire circuit with a proper saftey ground bonded to neutral ONLY in the service panel/breaker box will hold the ground at zero V since there should be zero current in that wire unless there is a fault at which point the breaker should disconnect the load.

    When you wire a consumer (single ended / unbalanced) component output to a pro (differential drive / balanced) inout, connecting the RCA shell (ground) to pin 1 of the XLR and RCA pin to pin 2 of the XLR, this should work if the ground and chassis voltage truly held at zero. I get that it is not working for some reason.

    I assume that with the pre disconnected from the amps the hum is gone yes?

    If so, I suggest you try a quality audio isolation transformer like a Jensen or Radial. This will take the + - (where - is what ever voltage the chassis is) from the pre and inductively drives a secondary coil to deliver to the amp / amps a + - that is only connected to pin 2 and 3. Connecting nothing to pin one at all. This completely breaks the chassis ground connection through the audio connections.

    I like the Jensen DM2-2XX. These have no cable length restrictions and are completely user configurable, and will set you back +$300.

    I am curious to know the answers to the questions above.

    Barry.
    If we knew what the hell we were doing, we wouldn't call it research would we.

  4. #4
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    Have you tried removing the "ground strap" on the Crown? Sometimes that's all it takes.

    In fact, IIRC, I had to remove my ground strap on my PSA-2XH to remove the hum and I don't use my balanced inputs. (My PSA-2XH has both balanced and unbalanced inputs.)


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    ...does disconnecting the ground strap on the Crowm amp mean using a plug that allows only two of the amps electric power cable wires to pick up electric power from the wall outlet? I tried that also with no change.

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    Had. Similar issue when I first got my Ashly crossover. There were hums in some amps.

    Since I was making my own cables, it took a bit of trial and error with shielding etc to get things to settle. Since then I've moved my problematic connections to balanced as the Ashly can mix balanced and unbalanced.

    What type of cables were you using? For cheap cables to experiment with, Monoprice makes RCA-XLR cables that may be worth a shot. Not sure how they are shielded but know several who have had luck with them.

    Be sure to rule out any issues with those older amps too, maybe they cause the hum regardless of other components... thats what I gotta do today too.

  7. #7
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    Quote Originally Posted by robertbartsch View Post
    ...does disconnecting the ground strap on the Crowm amp mean using a plug that allows only two of the amps electric power cable wires to pick up electric power from the wall outlet? I tried that also with no change.

    No, not the same. On the picture of the Crown you see a jumper between the chassis ground and the electrical ground. On mine, I simply removed that jumper to kill the hum. That's what it's there for.

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