Mirrored Drive Doors Audio out Buzz

[cmchamp]

I'm one of those official "Helpers" at the Apple Discussion groups. There have been frequent questions/concerns regarding the 'buzz' eminating from the headphone/line out on the new MDD G4's. I susspect that this 'buzz' is an impedance issue. Does anyone have one of these machines and can verify the output impedance for me so I can make recommendations to those in 'agony' on the Apple Discussion boards?

Thanks

[DAS]

I do not know the impedance (I will try to find out when I have a little time), but I'm not sure why you would think this would cause a buzz. What is leading you to this hypothesis?

[cmchamp]

Well, again, I'm not sure. I don't have one of these babies to check it out.

http://docs.info.apple.com/article.html?artnum=75408&SaveKCWindowURL=http%3A%2F%2Fkbase.info.apple.com% 2Fcgi-bin%2FWebObjects%2Fkbase.woa%2Fwa%2FSaveKCToHomePa ge&searchMode=Expert&kbhost=kbase.info.apple.com&showButton=false&randomValue=100&showSurvey=false&sessionID=anonymous|155361610

The article linked above is the Apple Knowledge Base Article speaking about the difference between balanced and unbalanced lines. I know the difference and know that the Mac and 90% of home audio equipment doesn't have balanced ins/outs.

Any solution won't make much difference to me personally because I wouldn't be using the built in audio ayway, but if a solution could be found, I'd gladly pass it along on the Apple forums.

Presently, I've recommended making sure there aren't any florescent lights, motors, etc floating around anywhere as well as recommending ferrit RF Chokes.

Since I've not experienced the symptoms first hand, I can't say it's a ground loop or an impedance issue.

Thanks for the reply though.

Cory

[Scott Gould]

From the discussions & reports I've heard (and experience with the built-in audio on my 7 Macs) the solution is to use a pair of passive direct boxes with ground lift switches. It is almost certainly a ground problem, but the impedence can be a factor when connection to pro audio gear. Star grounding sometimes works, but sometimes not (in my experience the audio ground isn't always at chassis ground potential). Peavey, ProCo, DOD and several other companies make passive direct boxes with ground lifts in the $40-50 range. Not a negligible amount, but it works well and shouldn't break the bank.

Scott

[cmchamp]

Thanks Scott:

The question, however, isn't with connecting to balanced gear. It's the 'buzz' when connecting to unballanced gear, such as a home stereo receiver.

Additionally, besides the issues mentioned above, ground loop, impedance, is the question of the architecture of the motherboard and where the audio components lay in relation to all the fans, drives, processors, etc. I received my November MacWorld last night, and after looking at the review, looks as if the audio components are just too close to the processor, ATA buss and hard drive. There may be nothing that can be done with this issue, but engineering improvement on Apple's part.

I thank all who've put forth thought to this topic even though it really doesn't apply to "Pro" audio or studio issues.

[DAS]

I have a G4/867 (plus several other older models) and I have it hooked up to a hi-fi preamp, which is then driving a pro audio power amp and a pair of JBL studio monitors. This is my Final Cut Pro machine and I monitor FCP audio that way all the time without problem. The key for me is that my preamp is only about five feet from the G4.


Two things are very possible with these kinds of setups:


1) Ground Loops - there's a definition for this in our WFTD Archives:

http://www.sweetwater.com/insync/word.tpl?find=GroundLoop

A great solution for this is a product made by Ebtech, which uses transformers to break the loop (similar to the DI solution mentioned, but without changing levels or impedance significantly).

http://www.sweetwater.com/store/detail/Hum/?action=nolog


2) Long, unbalanced cable runs - try to run an unbalanced signal too far and you're going to end up with noise, especially if it's a high impedance run. One possible solution for this is to put a passive direct box at each end. The first one (at the computer) balances the signal, which can then be fed to the second one some distance away. That one unbalances it and basically returns it (from a level and impedance point of view) to roughly the same type of signal that left the computer. Of course, for stereo this will require a total of four direct boxes so the cost is not insignificant.

[cmchamp]

Thanks everyone. I think there's enough here to answer the Apple Discussion querries. If there are no objections from the moderators, I'll poste at Apple with a copy of the link to this page. I will wait 24hrs prior to posting at Apple as to wait for permission.

Thanks

Cory

[DAS]

Permission granted - Hope this helps.

Let us know the location where this conversation is taking place. I'm sure some of us would also be willing to pop in over there and help out. I ask questions in the FCP forum quite a bit and I'm usually met with really helpful input. I like reciprocating wherever possible.

[cmchamp]

Link posted at the Apple Discussion Groups.

http://discussions.info.apple.com/

Then follow these links:

Power Mac
Power Mac G4 (Mirrored Drive Doors)
Usage
Any Audio Experts Out There?

I don't think you have to register to view, but you do have to register to post.

[Justin]

I hope we don't suffer "the Slashdot Effect" :) Well, I suppose it would be the "Apple Forum Effect" in this case.

Yay /.

:bounce:

[BluesRascal]

With over 30 years of recording studio exprience troubleshooting these types of problems, I can tell you the 'buzz' eminating from the headphone/line out on the new MDD G4's is NOT caused by external ground loops or impedance mismatch.

This is an EM/RFI noise problem due to faulty or inadequate shielding/grounding on the motherboard. This noise seems to be generated by the fans (it changes pitch every time the big fan changes speed) and by the video display card (a sputtering sound when anything on the display changes). It appears on the audio line out, the headphone out, and (from other posts I've seen) it looks like this noise can get into PCI audio cards that have D/A on the card as well. There are dozens of angry posts on the Apple Discussion Forum and the MOTU-Mac Forum re this problem. Thank god the A/D stuff on my PCI-324 2408 is well isolated from the computer!

Apple's lame document titled "Power Mac G4 (Mirrored Drive Doors): How to Eliminate Audio Buzzing or Humming Noise" claims it is caused by external ground loop:

http://docs.info.apple.com/article.html?artnum=75408

Their solution is to "connect the line output of the Power Mac G4 to the input of the stereo isolation transformer, and connect the output of the transformer to the audio equipment."

Well, I can tell you from my tests, isolating the ground on the Mac line out with a transformer does NOT solve the problem! This is not typical 60 Hz ground loop noise, it's EM/RFI digital hash.

I think the only real solution is a properly shielded and grounded system. Call Apple support and get a case number so you can take in to get a proper fix, if and when a fix is available.

Michael Darooge

[cmchamp]

Michael:
That's kinda what I was thinking. Though I don't have one of these new MDDs (for several reasons, mostly Apple's engineering - - or lack there of) from the descriptions of the sound, I had thought they were experiencing the same thing I used to experience with my Centris 610. RF & EFI from the ATA controllers, Fan controllers, etc.

Thanksf or the post.

[trafalgar]

Sorry folks, not trying to turn this into an Apple board, I just wanted to verify BluesRascal's post.
I have the G4 dual 867 (mirrored drive doors), and experience exactly the described symptoms. To quote my post at the Apple site:
"Someone said it sounds like a zipper when he rolls the mouse across the dock...that's exactly it. Pulling down a menu emits a squeal that raises and lowers in pitch as you go up and down. Launching a program sounds a bit like pulling a bad tooth. There's a buzz whenever the mouse is moved, and on and on. Even with volume set to a minimum, it's incredibly annoying."
Does anyone here have any ideas on modifications we can make? I'm perfectly willing to roll up my sleeves and open this baby up if it will help!
Thanks to everyone here for having addressed this issue, and to cmchamp for coming over here and asking.

[BluesRascal]

While isolating the ground on the audio out does not eliminate the noise, it does bring it down to a tolerable level. At least I can listen to iTunes without being distracted by the zipper noise every time I move the mouse around.

I picked up an inexpensive ground loop isolator from RadioShack ($15) that works as well as my expensive direct boxes. The part number is 270-054. I had to call a couple of local RadioShack stores before I found one in stock.

[audiomatt]

If this is not a ground loop problem then why does the interference only exist when the computer audio outs are connected to a powered amp or other device? In other words, the problem goes away when just headphones are connected to the outputs.

Matt
Romper Room Music

[trafalgar]

Originally posted by audiomatt
If this is not a ground loop problem then why does the interference only exist when the computer audio outs are connected to a powered amp or other device? In other words, the problem goes away when just headphones are connected to the outputs.

Matt
Romper Room Music
The problem I referred to above is present with no other equipment attached. It comes through headphones, USB audio out, speakers, anything.
I have never attached a powered amp or other device.

[BluesRascal]

It's true that using just headphones (with nothing connected to the audio outs) does reduce the noise to an almost imperceptable level. But it's still there, just very hard to detect.

This is not a typical ground loop problem in that it is not 60 Hz noise generated from external power line current induced in the ground side of the audio cable. The noise is digital hash generated inside the computer which appears at the ground side of the audio out jack (and perhaps on the ground of the PCI bus as well) as potential energy. When a cable is connected to the audio out, the resulting ground loop gives this potential energy (voltage) a path to flow as current through the audio cable. The cable acts as an antenna inducing the hash noise into the hot side of the audio cable and reflecting the noise back into the gain stages of the audio circuitry inside the computer.

So the external ground loop is not the cause of the problem, but rather the path that gives the already existing internal noise voltage a place to flow. All electronic devices generate EM/RFI noise, especially digital circuits, fan motors, power supplys, etc. With proper internal shielding and grounding this noise should not appear anywhere in the audio path, and certainly not as induced current in the external cable grounds.

Michael Darooge

[DAS]

I think different users are experiencing sligtly different problems, or causes. In your case (audiomatt) it may well be a ground loop.

[tingkumajique]

i have the same problem!!
just discovered it on sunday (today is wednesday). called tech support and none of them have ever heard of the problem. funny... so i'm begining to wonder if it's just certain units with defects or it happens to all powermac G4s...

another thing i'm experiencing is

1. plug in audio out to speakers
2. plug in headphones to headphone jack.

and you get no audio output from the built-in rite?

rite... then i unplug the headphones...
result: a slight pop and sound comes out from the built-in speakers!! even though the jack in the audio-out is still in!!

remove the jack, built-in speakers sound as normal... only slightly softer. rejack, and the built-in speakers don't sound...

is this suppose to be normal?

i'm having so much trouble with my PMG4 i'm begining to wonder why what did i do wrong to deserve this...:(

[tingkumajique]

took my powerMac G4 to a local service centre and sat with the tech support guy as he went troubleshooting the noise issue. this is what we did.

disconnected the 2 fans (optical fan and the main fan) and boot up. no buzzing... strange but that's what happened. reconnected both fans, one by one and came to the conclusion that both fans are causing the problem.

i have ordered the power supply replacement unit last week and it should be arriving sometime soon. hopefully that will resolve the isue, though... honestly i doubt it.

regarding the connection between the audio out, headphone out and built-in speaker, still a mystery

gabe

[tingkumajique]

not sure about you guys, but i find this problem very very unnerving...:mad:

have been bugging Apple Malaysia about this problem for nearly a month plus now...

according to them, they have contacted Apple Australia and directed the issue to them... bearing no results as they too were dumbfounded.

they suggested that i replace the motherboard, and that's what they did... am waiting for my unit to return... will keep you all informed.

did anybody notice that the noise level is higher when the mac boots up in OS 9?

[GJetson]

I experienced the same problem with my G4 867 DP. I had the audio out connected to my studio monitors, with noise everytime you moved the mouse, plus constant background noise. I especially noticed the "dock noises."

I purchased an Audiophile 2496 sound card, which has audio and MIDI ins and outs (available through the Apple Store). I needed the card for MIDI and audio anyway... Connect your amp or powered speakers to the audio outs on the PCI card, and no noise! Turn down the volume on iTunes before you hit play, it will be MUCH louder. Changing the volume on the keyboard will have no effect.

I would recommend that you go into the sound settings in your System Preferences, and allow the card to serve as your output, but use the internal Mac speaker for your system sounds and alerts. Also, turn OFF the "Play feedback when volume keys are pressed," or you will get a VERY LOUD click through your speakers (under Sound Effects in sound system prefs). I don't know why the volume key effect still goes through the card, even though sound effects are set for the Mac "built-in audio controller."

Hope this helps. I would guess that any good quality PCI sound card would work.