For the more Audio savvy of you, I seem to be having a little issue here.

I just picked myself up a nice old Akai AA-1050 stereo receiver Amp to hook up to my MBP. Lovely warm sound, really pumps it out, but... I started to notice a rather annoying hum through my headphones, not so noticeable through the actual speakers. At first I thought it was just the connections in the back, so i changed the input from AUX to Phono as phono appeared to be nice and quiet, but the hum followed me there once I plugged the cable in. (Line out to RCA).

I also noticed that when I do something on the computer, like scroll through a finder window, I can hear the action taking place within the humming. so obviously there is some computer interference.

Also, the interference seems to diminish when I disconnect external devices like my external CRT monitor and my two Ext FW HD's, though it will not disappear completely.


I didn't think hooking it up this way would be a problem and I can't reproduce these symptoms on my old Powerbook. In fact my powerbook is nice and quiet.

How can I remedy this?

Cheers.

I don't have a lot of experience with audio on computers but I have done a bit of audio work and this sounds like a classic earth loop. The usual solution is to organise it so there is only one earth connection. You may be able to do this by using the MBP on battery - do this first to check if it solves the problem - and/or by using the two pin plug instead of the power lead on the power supply. The fact that it's worse with extra gear plugged in tends to support the prognosis but it may make eliminating it more difficult.

Some equipment can safely be "earth lifted," which means disconnecting the earth wire internally, but you should not attempt to do this yourself - seriously - because the risk of electrocution is high if the equipment is not designed to run with a floating earth.

Soulman's advice seems good. Some other notes:

Phono is phono... it's the input with a record player preamp, for use only with record players. This is a different impedance and therefor shouldn't be used for line level inputs (CD players, iPods, computers etc).

Computer outputs are computer outputs... computers are very noisy from an electrical interference standpoint. This means that sensitive analogue circuitry (like parts of analogue audio outputs) can be effected quite badly. The strange noises you've heard while scrolling etc would be due to the power requirements changing on the CPU and/or GPU or other things being used internally. That's normal (my G5 sent horrible noises out the audio output).

This is why anyone serious has an audio interface where the AD/DA converters are in a separate box.

Alright, first up, many thanks to both of you for your time and information.

But why didn't either of you ask me to try another cable! (joke)
Seems fine now. Problem solvered for the meantime.

Yep. Phono inputs have higher gain than line level inputs because they're designed specifically for turntables. They also have a heavy EQ on them called RIAA equalisation that produces a linear output from vinyl, which has the opposite EQ applied when it's cut. They are pretty much unusable for any other purpose.

No, that's a valid point to make. My advice didn't follow that useful rule of troubleshooting - try easy things first, even if they're not immediately considered the most likely. And cables should always be on one's list of things to check. Glad you got it sorted!

Err... yeah... check the cables first

From what I've found, the Macbook Pro's appear to use a floating ground by design.

Anyone with a macbook pro has noticed the buzzing feeling when running your fingers over the casing when plugged into the wall. changing to the 3 pin duckhead makes no difference.

When I have said interference coming from my audio-out on my PowerBook when going to a particular amp, I just switch to a Griffin iMic that I have which seems to eliminate it by using the USB out instead of and 3.5mm socket. Don't know why, but it works.

You know, i've been sitting here the last few days doing some video work with my headphones on, and I still notice these little blips and weee squeak sounds intermittantly. The Amp is still going fine as it's not the same issue as per my first post.

I never had such audio issues on my powerbook. I've found that when ichat is running the sounds are a lot more prevalent like ichat is doing a little audio hacking when idle.

Now the MacBook Pro have an optical digital out. Would the optical side of things be anything to with this? Any other ideas or should I just try an alternative like spargos imic.

What do you use marc? Or what would you recommend?

I use a Metric Halo Mobile I/O 2882: http://www.mhlabs.com/metric_halo/products/mio/2882/

Love it! It's rather full on though, with loads of inputs and outputs

I'd say anything that has the output in a separate box that's powered separately should do the trick. Maybe even something that's just USB would be ok. Grounding issues are very hard to chase down though, so a lot of trial and error is in order unfortunately.

That is some serious kit you are running there! I'm looking for something with maybe 8 inputs with 2 preamps. Any suggestions? (Not digidesign).