[reezekeys]

The title says it, when I use my MBP with the power adapter I hear a buzz, accompanied by a slightly louder buzz that cycles around 1x/second. The curious thing is that it does not happen all the time. So I'm thinking that it might be a noisy AC circuit that the power adapter is plugged into. Or, it may be related to the battery charging status -- if the battery is fully charged the noise is absent or lessened. I'll have to take more notice of this.

Interestingly the noise is present both on the built-in audio, and when I use the analog output from my Emagic 26 USB audio interface. So the idea of digital hash going into the analog sound circuit inside the computer due to being on the same motherboard may not apply here - this looks like something else. One thing is certain: as soon as I disconnect the power adapter and let the mbp run on its battery, the noise is completely gone. I have tried both the standard Apple adapter (a brand new one AAMOF) and a cheapo Ebay replacement. I hear the noise with both of them -- although it's different (the Ebay is more steady-state, the Apple "pulses").

I am using my mbp for live gigs, streaming samples from a bus-powered FW drive and running two USB peripherals (the Emagic audio interface and an Edirol midi keyboard). As tempted as I am to just play the whole gig on the mbp battery, I'm a little nervous trying this since I am draining it faster than typical usage would.

I'm interested if anyone has been down this road and knows where this buzz comes from. Is it a characteristic of these cheap switched power supplies, or perhaps induced by the PMU in the laptop? I was thinking of trying an AC line filter since they're not that expensive but something is telling me it would be a waste. Any tips appreciated, TIA!