Amplifier High Frequency Noise - AVS Forum | Home Theater Discussions And Reviews
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post #1 of 1 Old 11-13-2019, 01:55 PM - Thread Starter
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Amplifier High Frequency Noise

The 8 channel QSC CX168 amplifier I installed a few weeks ago has a weird issue with at least 2 of the channels. It may affect more, but I am using it to power active speakers and the noise produced may be filtered out through the MiniDSP crossover on other channels. The high frequency noise starts at perhaps 8000Hz (just a guess) and continues higher and higher until it is almost gone due to the frequency being so high that it is barely audible. After awhile, the noise comes back around and does it all over again. Initially the QSC had some ground loop noise, but I took care of that, and was still left with the high frequency noise.

The noise exists in at least channels 1 and 2 as those are powering the tweeters of my left and right speakers. Channel 3 is the center channel tweeter, and the noise is not present in it. Channel 4 is the center channel midrange, which is crossed over to the tweeter at 2500Hz using a MiniDSP 2x4 Balanced unit. Channels 5 and 6 are bridged for the left speaker midrange, and channels 7 and 8 are bridged for the right speaker midrange. L/R midranges are crossed over to the respective tweeters at 2500Hz as well. I have a separate amplifier for the woofer sections for all 3 speakers.

Just for clarification, the signal chain is setup as follows:


AVR Left Channel RCA out ------ MiniDSP balanced #1 ------ 2 channels to QSC CX168 (CH 1 Tweeter, CH 5/6 Midrange)
.................................................. .................................|
.................................................. .................................----- 1 channel to Crown XLS 1502 (Woofer)


AVR Right Channel RCA out ------ MiniDSP balanced #2 ------ 2 channels to QSC CX168 (CH 2 Tweeter, CH 7/8 Midrange)
.................................................. ...................................|
.................................................. ...................................----- 1 channel to Crown XLS 1502 (Woofer)


AVR Center Channel RCA out ------ MiniDSP balanced #3 ------ 2 channels to QSC CX168 (CH 3 Tweeter, CH 4 Midrange)
.................................................. .....................................|
.................................................. .....................................----- 1 channel to NHT A1 Monoblock (Woofer)


My thoughts are it has a capacitor going bad or something is off in the switching PS.

I decided to replace it with an 8 channel Crown amp. The Crown has neither the ground loop issues nor the high frequency buzzing, so I am happy for now, though I would like to repair the QSC eventually.

I just don't have enough time right now to sit down with a multimeter and trace everything on the main board, but maybe I will in the future. I thought perhaps someone on here might have some insight into what might be the problem?

A happy bonus with the 8 channel Crown is that the noise floor is lower even though the power output is greater. I'm really only using perhaps 25% of the available power to drive my active speakers to very loud levels, so the extra power isn't really utilized, but its there if needed.
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