[hometheaterguy]

If I turn a QSC RMX-850 off, noise still remains in great amounts. It doesn't leave until I unplug the beast. Where is Bob from QSC when I need him? I could disable the fan. (BTW, all the other three power amps have no noise at all). I would rather get a value on a cap Parts Express sell to eliminate this nasty 60Hz hum. Any ideas? TIA

 

[DavidT]

What specifically are you talking about?


About the fan noise when the unit is on...


Is there a hum from your speakers when the amp is plugged in ONLY...


Your post is kinda vague

 

[hometheaterguy]

The noise is only present when the QSC is plugged in. Meaning, even if I turn the QSC off (the fan and amp) there is noise. It only goes away when I unplug it.

 

[Stereodude]

Quote:

Originally posted by hometheaterguy The noise is only present when the QSC is plugged in. Meaning, even if I turn the QSC off (the fan and amp) there is noise. It only goes away when I unplug it.

Ahh, the ground loop. Is it plugged into the same outlet as the rest of your gear?

 

[catapult]

You haven't told us what the noise is or anything about your other components. Sounds like it's a 60 Hz hum? If so, it's not the amps fault, you have a ground loop. Gotta fix that. It's pretty hard to trouble shoot that without being there but, in general, each component should only have one path from chassis to ground. Time to get out the old ohmmeter and figure out where to break the multiple paths -- most likely by using different wiring on the interconnects from the (unbalanced?) preamp to the balanced amp.

http://www.rane.com/note110.html


Using a cheater plug on the power cord of the QSC to break its ground is an easy option but it can cause a shock hazard so it's not the best choice.


Edit: just saw stereodude's reply and that can do it too.

 

[DavidT]

Just do a search on this forum for:


Ground Loop

 

[hometheaterguy]

Thanks for the replies. All my amps are unbalanced RCA interconnected. My QSC amp doesn't have XLR balanced connections. I had two power outlets (4 total receptacles) wired to the electronic closet. One goes to a Panamax power strip. There are 10 units requiring power plugged into these outlets. The QSC is not plugged into the strip.

It appears I must wire earth ground to the QSC amp chassis. I still wonder if it is a ground loop. The reason I wonder if it is a loop, or not is none of the other components have noise when on, just the QSC amp. (Why do none of the other components have any noise)?

 

[DavidT]

I had a 850,


I got rid of it mostly cause of the fan noise when turned on. I never had the hum you've described. But, if you only have it when the QSC is plugged in and running, either the QSC is bad or you have the ground loop problem. Can you run an long extension cord from another part of the house, plug in the QSC from that, and see if the problem still exists?

 

[hometheaterguy]

That's a good idea David. I will do it this afternoon and get back with you.

 

[DavidT]

Roger Doger


Taking my son to a soccer game and will check back later this afternoon

 

[catapult]

You still haven't described the noise. Is it a 60 Hz hum played through the speakers?
Quote:

All my amps are unbalanced RCA interconnected. My QSC amp doesn't have XLR balanced connections.

That makes no sense. QCS amps only have balanced connections. They do not have unbalanced RCA connectors.
Quote:

The QSC is not plugged into the strip.

It should be so all your components are using the same ground path.
Quote:

none of the other components have noise when on, just the QSC amp.

Now it sounds like you are talking about the fan noise of the amp. That's a separate (and fixable) problem from a 60Hz hum coming from the speakers.


Please be more specific about What the noise is (hum, hiss, fan), Where the noise is heard (speakers, amp itself), When the noise is heard (amp on, amp off, always). We can't help you if all you say is it's making noise.

 

[hometheaterguy]

I can't believe it. NO NOISE when by-passing the outlets in the electronics room! May have been a voltage flucuation, ground loop in the outlet, or both. I will get a multimeter on it and get some measurements and maybe make a filter for the QSC plug.

David, my hero

 

[hometheaterguy]

catapult, what I am wiring the inputs with the UN- Balanced 1/4 phono plugs. I am then using a sum adapter to get it to RCA. Running with clip indicator on, and parallel input to bridge mono, (this amp just drives the sub).

The noise seems to have disappeared after I ran the QSC to a outlet in another room. Makes no sense, but it worked to kill the noise. The noise sounded like a 60Hz hum, or a the sound of the fan amplified thru all 10 speakers in the theater. It did not increase in loudness, nor intensity with the turning of the VC.

 

[catapult]

Okay, so you are doing diagram #19 on http://www.rane.com/note110.html . The mono phone plug is shorting the (-) and ground connections of the balanced input at the amp. That is a common way to do it with off the shelf wires but you are finding out why it is the incorrect way to connect unbalanced and balanced gear.


You should be using either a transformer (best) or diagrams 17 or 18 (second best). 17 and 18 make the signal wires independent of the shield and float one end of the shield so there is no extra path to ground for your other gear. It is your other gear that's making the hum.


The easiest way to make a cable is buy a standard XLR cable, cut the female XLR end off and solder the twisted inner wires to an RCA as shown in diagram 17. Don't let the shield touch anything at the RCA end.

 

[Vic C]

Im a pro audio guy and that doesent sound right. Why is that clip indicator on?


That shouldnt be. You might be shorting out an input on that amp. The 1/4" inputs should be TRS balanced inputs (Tip Ring Sleave) Tipically TRS plugs are Tip + ring - Sleve ground. An unbalanced 1/4" mono plug could be used in those jacks beacuse the - and ground become the same lead. So if you just want RCA inputs on that amp you should just use 1/4" to RCA adaptors. (rat shack 1.99)

Im was sure that amp had XLR connections as my USA 900 does in which that is a way better means of connection and I would personally use them instead.


Im glad to see other people using pro amps in there theaters I personally cant see using anything less then pro audio grade amps. Im using a CREST pro series 6002 power amp a wopping 430 watts per channel @ 8 ohm stereo for my subwoofers (custom 10" pro grade subs we build with pro drivwers) a QSC USA900 for the fronts and Im looking to buy another 900 to replace a ROTEL amp. I feel the ROTEL is not up to spec with the rest of my gear and would liek to have all pro amps and dump this "audiophile" amp. Ill take "PRO AUDIO" any day over "AUDIOPHILE" grade.


On another note your only getting noise with that amp because its a high current amplifier and it is amplifing the ground noise of your system. By goignto a nother circut you bypass the noise.

 

[hometheaterguy]

David, Stereodude, Catapult, and Vic thanks for the words of wisdom. I did want to edit what I said earlier. I DO have balanced XLR inputs on the QSC, I meant to say I do not use them. My current Lexicon prepro uses RCA un-balanced inputs only. I plan to get a Lexicon MC-12B in February which allows balanced XLR inputs.

Vic, it is the clipping limiter. The clip indicator LED is on the front and fixed. Bob Lee of QSC said in a Parts Express forum to someone to keep the clip limiter on. But the RMX-850 manual suggest when to use it: "When driving subwoofers, some users let the amplifier clip without limiting because it gives extra "punch" to kick drums and similar sounds". So I guess it is like anything else in audio, the key is experimentation.

 

[DavidT]

VR Audio Systems,


I was using a QSC RMX850, but got a killer deal on my new baby, a Crest amp to power my SVS subs, its a Crest CPX2600 pushing 750 w per channel @ 4 ohms. Of course I dont need that much power....yet!


hometheaterguy,


I am glad you've got this almost figured out. Kinda what I was figuring the problem was.


I was also considering going pro audio on all my amps soon as more funds become available, only drawback I can see is the space requirement for all those amps. Also, how would you adjust the volume on them...through the receiver? I am just starting in the spearates world and dont really have a clue

 

[hometheaterguy]

 www.dolby.com has some diagrams that show systems both commercial and home that use separate power amps. You can run the amp inputs via XLR, or RCA thru the prepro. The prepro control the volume. The good thing about power amps is: no tone controls, better damping factor and slew rate, much more headroom than even the meanest receiver, clipping indicators. I have all 10 components in this PC room. The volume, sound formats and the DVD functions are controlled remotely by the Xantech IR remote repeater system I installed. Under our 6' fixed screen is a little red LED, (the main sensor). Just aim the remote at that and you control anything. I like this setup because all the electronics are out of the way. The LEDs and LCDs are not distracting to the viewers. Some audiophiles abhor pro audio amps because they think the power is crude and un-controllable. If you get the QSC, Crown, or Hafler amps that have the THX label then you have the confidence they meet headroom and sonic quality that the strict THX program demands of amps. www.partsexpress.com has QSC THX amps that generate over 4000watts. Talk about putting the THEATER in home theater! "Got 220v?"

 

[DavidT]

WOW,


Now that is alot of power


So my receiver can operate the volume controls of the pro amps, I should just set the gain on the amp and all should be well with the world!

 

[hometheaterguy]

You could use your receiver as a master volume control, yes.