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Eliminating audio feedback from new desktop G4s System
Those really nice new G4s with the mirrored drives are great but ... there is often sound feedback from the sound out jack. I do not understand how it happens, but there is a tech article for service providers that provides the details and the fix.

Basically, go to Radio Shack and pick up a "ground loop isolator" and the problem will go away. Thank goodness the device only costs $15. However it only has RCA jacks so some converters or additional cables will be needed. But hey it is worth it! No more feedback!
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Authored by: pascalpp on Oct 22, '02 12:37:28PM

i'm using one of the new G4s as the center of an audio workstation using logic as the software and a fair amount of external hardware. the G4 sound out is running into my mackie mixer board and then out to my audio monitors (mackie HR824s). when i turn up the volume for the system sound on the mackie mixer, i hear a very strange staticy hum. is this the feedback sound you're talking about?

my setup is kind of complicated (the computer is actually in another room, with all the cables passed through a hole in the wall), but i didn't hear this static sound with my old G4 in the same setup. fortunately all my audio production is done using a MOTU 828 firewire I/O unit, so the static doesn't appear in my recordings, but it is annoying nonetheless when i'm listening to iTunes or playing quake or what have you.

could you provide a link to the tech article you mentioned?

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Authored by: DavidRavenMoon on Oct 22, '02 09:58:43PM

A scratchy sound is just interference from the radio producing parts inside the Mac. Or it can also be what's known as "DC offset," which is where there is a DC voltage along with the AC audio signal in the audio path. A ground loop, which is what Apple is referring to, sounds like a humming or buzzing noise.

You should never use the built in audio jacks for anything important. The Mac is filled with all sorts of interference inducing circuitry. I use an M-Audio Audiophile 2496 and it sounds great, nice and quiet.

Another way around ground loop problems is to make sure every piece of gear is plugged into the same outlets. If you have equipment with three pin grounded plugs, use adapters on the AC plug to "lift" the third pin, you know, the kind used to plug into the old 2 pin wall sockets. Only leave one power cord, the Mac, grounded.

Another strategy is to make new patch cords with the shield grounded only on one end, and plug that end into the Mac. This way you avoid the ground loop.

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Authored by: sly808 on Oct 24, '02 03:42:57AM

Just go to the Apple website and click on the support link. In the text window, type in "G4 Speaker Problem" and the article should come up. I just bought a new G4 last week and noticed this problem right away, even when using headphones connected to the jack up front. What bothers me is that Apple knows about this but to fix it you have to spend another $15 on top of the 2 or 3 grand that you spent for the machine in the first place. For that kind of money, this G4 should have zero problems.

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Authored by: eagle_eyes on Oct 23, '02 12:46:22AM

I do not know if the particular sound you are hearing is the symptom that I am talking about... For $15 it would be worth it to me to try it and see. When I read the tech article I did not think mine fit, but I guess it did.

As for the tech article it is in the service providers area only. ie even if I did link it it would not be readable unless you had access to service provider support. Sorry:( That is why I posted it here was so that all of you would at least be aware.

BTW I was interested in finding a better sound card (one that supports surround sound) is there such a critter for the mac. Preferably PCI and not with digital sound out, I just want to plug in my logitech z-560 system.

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Authored by: tekmo on Oct 24, '02 12:47:54PM

Motu's are not up to date on the OsX support. The 828 driver for osX is not out yet afaik. I have a delta 1010 (Also running Logic ...blah blah...)and it's rock solid for OsX. Audiowerk 2/8 would be what you're looking for in terms of surround sound compatibility/mixdown, and the drivers are on the way -- hey it's made by the newly acquired by Apple, Emagic Gmbh (o wait -- that might not be there any more).

Ando for the original question about the lowlevel hum in the Mackie -- might be your cables -- don't forget unless you're digital you are facing the ground/floor noise issue. Take your shoes off, stand barefeet on the ground and start touching the different connection points in the signal path -- from the out jack on the g4 to the actual ring on the in-jack on the mackie. Hehh what you described sound exaqctly like a ground loop -- you have to isolate it yourself, man, sorry. Also touch the different parts on the console that are adjacent to the g4 fader/knobs

Hoep this helps....


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Funky feedback definition
Authored by: WillyT on Oct 23, '02 07:55:17PM

Audio professionals usually define audio feedback as output signal getting back to the input signal and causing a runaway regenerative output ie. a howling higher pitched unpleasant tone.
What is being described here seems to be either a ground loop where some of the signal contains a difference in potential of the mains ground (earth) or rf leakage that gets rectified in the next input stage causing a dc offset.
If the hum seems to be mostly odd harmonics of 60Hz then it probably is a ground (earth) loop. If the hum is mostly even harmonics of 60Hz this would be from dc offset pulling the power supply down in the external audio device.
I admit that these could be described as feedback from a purely electronic perspective but should better be described as rf interference or ground loop.

As I have an older G4 I have not heard what is happening myself.

If this is actually rf a ferite bead on each signal line would be a more proper solution.(Less signal frequency degradation)


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