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Introduction

I've recently entered the prosumer/pro audio market by buying JBL LSR305 studio monitors and noticed that the tweeters produce constant high-pitched noise (most people apparently call it ‘hiss’) that can be heard even from two metres away. See this spectrogram for a graphical representation. The noise is present with and without input and practically regardless of gain.

Disappointed, I've been researching the issue and it appears that the noise is intrinsic to all Class-D amplifiers and, more worryingly, to all Class-D based monitors. I've learned the basics of audio amplification and the differences between the classes here and found some very helpful information here and here.

Intriguingly, some people claim to either not hear the noise or hear it only when they put their ears right to the tweeters! Could be hearing sensitivity, could be something else.

Question

Naturally, my objective is to remove or at least attenuate the noise, but the question is more general and related to the amplifier itself, hence hopefully relevant.

  1. Since some people claim to not hear the noise, what variables aside from individual differences in hearing could cause it?

Andy aka wrote: “Class D amps are extremely susceptible to hum on the power feeds to the amplifier […] and any AC voltage superimposed on these rails (ripple aka hum) gets transferred to the speaker.”

  1. Could it be that my electrical supply is too noisy? (I live in a flat; 230V/50Hz.) If yes, can I buy/build something to filter it?

Some people claim that using a balanced connection solves the issue, others disagree. It's present even with no input, though.

  1. Can the nature of the input (balanced vs. unbalanced) be related to the issue?

Thank you!

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  • Suffice to say that this most likely not caused by your mains. Buy different monitors w/o class D built-in amp. Or better, buy un-amped "monitors" and build/buy a linear amp with a linear power supply. Unless you plan to demolish the walls of your flat (with SPL), chances are you won't notice the difference on your electricity bill. And I'm guessing you didn't make spectrogram yourself, or you wouldn't be asking this kind of question. – Fizz Oct 18 '15 at 22:39
  • As for balanced connection... if it makes the noise without any signal source/cable plugged into it, then that surely won't fix it. Otherwise, how good is your source in terms of SNR? Can you listen to it (your source) with anything else? – Fizz Oct 18 '15 at 22:46
  • @RespawnedFluff First off, I'm not sure what ‘kind of a question’ do you mean but I did make the spectrogram myself (not sure how it's relevant, though). Second, you provide no useful explanation that would help me understand the issue. The SNR of my source is 115 dB and yes, I can listen to it with something else… without the noise. Finally, I'm obviously trying to understand the origin of the noise and why some people hear it and others don't, not merely solve my issue by buying something else. Thanks. – Harold Cavendish Oct 18 '15 at 23:54
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    It's worth pointing out that Class-D amplifiers do not inherently have some sort of unavoidable hiss. While all amplifiers will have some sort of noise (see Johnson noise), well designed Class D amplifiers can have very low noise floors. There are many products with class D amplifiers that have almost imperceptible hiss at full gain. It's not so much that you need "non class D" amps, you just need better-designed monitors. – uint128_t Jan 29 '17 at 19:54
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    The best example I can give is comparing Mackie SRM450s with QSC K12s. I used to run the Mackies as tops supporting orchestra; during concerts, the hiss of the Mackies was audible at the back of the venue when no one was playing. Switched to K12s and the hiss was only audible when I was ~10ft away. Both are Class-D, the K12s simply have a much lower noise floor. – uint128_t Jan 29 '17 at 19:59

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