#881498 - 02/28/00 08:05 PM
Double Check Your Connections!
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Bart
Senior Member
Registered: 12/07/99
Posts: 44
Loc: St.Paul, MN USA
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I had a "eye opening" experience this past weekend that may make you re-check all of your input/out connections on your PA system.
I own a QSC MX3000a that I have had for about 3 years. For the inputs, I have been using a balanced TRS (tip ring sleeve) from the crossover into the amp. I thought that TRS connections were standard, where tip is + ring is - and sleeve is ground. Well, I have wrong for 3 years, the MX3000a inputs are tip -, ring + and sleeve ground as stated in the manual and back of the amp, but I never bothered to look, or think about it. The result, I have been running my low end in reverse phase, another words, the speakers have been sucking in, instead of pushing out! I was floored! A fellow soundman that used my FOH system enlightened me on this fact, and what a difference it made (I always thought I should be getting more low end).
I never knew that TRS inputs/outputs could be different. I would like to know if there are any other products like this or how common this is.
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#881499 - 02/29/00 12:14 PM
Re: Double Check Your Connections!
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Mark Amundson
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Registered: 10/30/01
Posts: 5
Loc: Cambridge,MN,UNITED STATES
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Yeah, you got really watch polarity, especially with amplifier inputs. TRS or XLR, some amps (my Carver PM-1.5A's included)have flipped polarity inputs.
Not only do sub-woofers suffer on this polarity issue, but stage monitors as well.
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#881500 - 03/21/00 10:45 AM
Re: Double Check Your Connections!
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karen
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Registered: 12/02/99
Posts: 7
Loc: Spotsylvania, VA
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Mark and Bart:
Just want to send you a huge "thanks" from my husband in regards to the above remarks. He took your advice and sure enough, he had the same problem. He's been pulling his hair out trying to figure why he wasn't getting the power and sound out of the p.a. he's saved and sacrificed for. He is very grateful for the information.
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#881501 - 03/21/00 12:57 PM
Re: Double Check Your Connections!
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Bart
Senior Member
Registered: 12/07/99
Posts: 44
Loc: St.Paul, MN USA
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Glad to be of help. I have a little follow-up on this topic. First off, reversing the +- on the speaker output will solve this issue as stated in the following from QSC website FAQ's . Here is the official explanation:
"Why does QSC use tip negative and ring positive 1/4" TRS connections on most
amplifiers?
Since input circuits respond to the difference between the plus and minus signals, if only an unbalanced
(single-ended) signal is available, the unused input terminal need only be grounded for normal operation, without loss of gain. The ability to reject cable induced hum and noise is lost, but this may not be needed
in well shielded environments with short distances between audio components.
Normally, an amplifier is expected to reproduce input signals in the same polarity, which is called the non-inverting mode, so that a drum beat, say, pushes the speaker out instead of in. However, if there is any
leakage from high-power (speaker) circuits to the amplifier inputs, the amplifier is much more stable in the iverting mode, since the leakage then tends to add to the negative or stabilizing feedback. Using an inverting mode power amp is the opposite of normal practice, but we have observed many cases where
"mystery problems" occurring in wide band amplifiers used in the non inverting mode are solved by switching to the inverting mode. Balanced line operation corrects this problem without regard to polarity, but is not always available.
You can always reverse the red to black polarity to all the speakers to restore correct polarity even when using the "more stable" inverting mode. In any case, be sure to use the same polarity for all of the speakers so they work together."
Whew!!
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