loud hum when using RMX 2450 - AVS Forum
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post #1 of 10 Old 02-05-2012, 02:06 PM - Thread Starter
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Hi,

I'm hoping the expertise here can help me debug a problem I'm having
The amplifier on my Def Tech SuperCube I subwoofer died, so I'm trying to replace it using an external amp (QSC RMX 2450). This is how envisioned everything to be connected and just work (it doesn't):

Denon 3311ci subwoofer out on pre-amp

connected to (via RCA cable):

TRS input (1/4 microphone connector) on QSC RMX 2450

connected to (via speaker wire):

Definitive Technology SuperCube I subwoofer (w/o amp; just connected straight to the driver)

This setup causes a very large hum to the subwoofer and barely audible music. So I think haven't consider input impedance or speaker impedance properly or something. Any ideas?

And some more info:

- If I connect the 2450 amp to an iPod with a small bookshelf speaker, it seems to work fine, but I was a bit surprised that the speaker wasn't super loud at full volume. There is no hum and it is loud (actually quite loud) at 100% volume (on the iPod), but I expected to have to keep the iPod at 5% volume maximum because the amp is so powerful for a small bookshelf speaker (probably 50W max input). But maybe this is normal behavior ?

- Any of the 3311 pre-amp outputs (subwoofer, FL, FR, whatever) cause a very loud hum and barely audible music even if I use the bookshelf speaker instead of the subwoofer. This doesn't seem right to me. Maybe I have a problem with the 3311 as well? Or?

Any tips?

Thanks,
Steve
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post #2 of 10 Old 02-05-2012, 02:33 PM
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You may have an incorrectly wired rca to trs adapter. If it came from monoprice the chance is high.

To do it right, you really need a Dbox or really a Directbox. The transformer type should be OK because it is driven by your avr.

Lot's of low rent stuff stacked up into a medium rent pile.
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post #3 of 10 Old 02-05-2012, 02:38 PM
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Try a RCA>T/S instead of an RCA>TRS adapter.

"You take the blue pill - the story ends, you wake up in your bed and believe whatever you want to believe. You take the red pill - you stay in Wonderland and I show you how deep the rabbit-hole goes."
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post #4 of 10 Old 02-05-2012, 02:47 PM - Thread Starter
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> To do it right, you really need a Dbox or really a Directbox.

Hmmm. That's a new term to me, but it looks like it converts high-level input (speaker level) to low level (RCA). I shouldn't need that if I'm using the RCA out of the Denon AVR 3311, right?

Or are you suggesting high-level outputs from the AVR would be better?


> Try a RCA>T/S instead of an RCA>TRS adapter

What's T/S? Attached is from the manually for what I have to work with...


Thanks for the tips...

-Steve
LL
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post #5 of 10 Old 02-05-2012, 02:48 PM
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http://www.ebay.com/itm/DIRECT-BOX-U...item256c2b4fb5
You are trying to adapt form consumer unbalanced output to pro-type gear which requires a balanced input. Your amp does not have an unbalanced input. you can cheat by getting an adapter, but really, the sure fire way is to give the amp what it needs, a balanced input.

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post #6 of 10 Old 02-05-2012, 02:50 PM
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Quote:
Originally Posted by siverson View Post



> Try a RCA>T/S instead of an RCA>TRS adapter

What's T/S? Attached is from the manually for what I have to work with...


Thanks for the tips...

-Steve

Tip / Sleeve...... not Tip / Ring / Sleeve.

Scroll down and see the difference in the connectors.

Even though the page is for TRS, they show TS connectors as well.

http://en.wikipedia.org/wiki/TRS_connector

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post #7 of 10 Old 02-05-2012, 03:09 PM
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Ground loop. This link should be of some help: http://www.rane.com/note110.html
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post #8 of 10 Old 02-05-2012, 03:56 PM - Thread Starter
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Got it thanks. I see the error of my ways now. I've ordered some parts, and I'll update when I get this fixed.
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post #9 of 10 Old 02-05-2012, 04:08 PM
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"- If I connect the 2450 amp to an iPod with a small bookshelf speaker, it seems to work fine, but I was a bit surprised that the speaker wasn’t super loud at full volume. There is no hum and it is loud (actually quite loud) at 100% volume (on the iPod), but I expected to have to keep the iPod at 5% volume maximum because the amp is so powerful for a small bookshelf speaker (probably 50W max input). But maybe this is normal behavior… ?"

your ipod isn't outputting enough voltage to drive your amp to full power. commercial gear requires higher voltages than does consumer gear in order to hit full power.

Listen. It's All Good.
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post #10 of 10 Old 02-06-2012, 10:16 AM
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Quote:
Originally Posted by Oklahoma Wolf View Post

Ground loop. This link should be of some help: http://www.rane.com/note110.html

Try building it like #17 with XLR pin 1 not connected. Worked for me and others I know.
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