Installing Apple's G4 Noise Mod

The Problem

Apple's "Mirror Drive Door" Power Mac G4 systems are quite a bit louder than previous Power Mac models. Basically, this is a result of Apple trying to put too much stuff in the case. The case is the same size as the original Power Mac G4 case (which I dearly love), but the MDD models have much more functionality:

The MDD power supply is rated for twice the output as the original Power Mac G4, and with all of the added features they should have just switched to a larger case. There's just not enough room in there to cool things right. (The second optical drive bay causes most of the problems; it's right up against the cpu heat sink, and it prevents the fan from being centered on the heat sink.) Apple used a high airflow Delta fan to overcome the poor geometry, and the result was plenty of noise. The Delta also injected noise into the G4's audio outputs.

You can read (lots) more about the problem at G4 Noise.

Apple's Solution--Does It Work?

After releasing the MDD's successor, the quieter G4 FW800, Apple made a retrofit noise kit available to MDD owners. It comprises a new Papst fan ("Multifan 4212H", 12 V, 5.3 W), a new power supply, and a good set of instructions. The power supply looks the same, but presumably has new fans. It has been derated to 360 W, down from the original 400 W. I should point out that this mod kit does NOT include a fan cowl such as is used on the FW 800 machines. This means the coupling of the fan to the heat sink is still not very efficient.

Below you can see the installation steps. Should you install the kit yourself or get an Apple Service Provider to do it? It depends how comfortable you are working inside your computer. It's much harder than putting in a new hard drive, but not as hard as, say, installing a water pump in a car. I'm pretty technically proficient but it still took me an hour. Mostly of the time was wasted trying to get the power supply in, which is a bit tricky.

By the way, if you are having trouble installing the new fan, you probably missed (as I did) the Correction for Page 17 sheet that Apple included in the directions. The Pabst fan really is a bit thicker than the Delta it replaces, and Apple had to have Pabst mill out a slot in the fan in order for it to fit in the Delta slots. In order to use the milled out slots, the fan must be installed as specified in the correction sheet, a 90 degree rotation from the original specification. (If you managed to jam it in the other way, just leave it alone; cooling-wise it doesn't make any difference.)

So, does Apple's solution help? The good news is yes! In my case, I also found that the low frequency beating that was formerly injected into the G4's audio output is now gone. This in itself is a huge improvement.

These were recorded using an old beige G3 (in my server closet, but still sound from it could leak through onto this recording) and an Apple Plaintalk microphone taped to a table about 8 feet from the MDD machine using Amadeus II. After recording I chopped the files down to about 6 seconds each and then used Amadeus to compute the RMS power of each sample. The results were:

Pre-mod Post-mod Change
-37.33 dB -44.95 dB -7.6 dB

The absolute values here are meaningless, but since they have the same basis we can subtract one from the other, yielding an improvement of about 7 dB. (Since I took this data the fan has started to run at a higher rpm and it's a little louder now. I've added a recording of the "warmed up" machine above. It measures 2 dB higher than when I first started it up, and there's an annoying beating at about 2 Hz.)

Apple's modifications don't make this machine into the "silent" or even "quiet" department. But it's definitely now in the "normal" range for a computer, which is a big improvement from its old status as "unreasonably loud". You can now hear the hard drive accessing files, which before was lost in the fan noise.

You can still hear some "beating" due to the two power supply fans spinning at slightly different rates, and the Papst fan does have a discernable tone to it. The coupling of fan to heat sink is still not very good. If you need a really quiet Mac, keep an eye out for the Verax fan mod. Their design is much better since the fans are tightly coupled to the cpu heatsink.

Still, I'm happy that Apple provided this kit. It's a huge improvement over what I had before, and at this point the machine is not actively irritating. An added bonus is that the annoying buzzing noise that used to be injected into the audio output is now gone! I think much of the electrical noise in the audio output was due to the Delta fan, and now that it's gone I can finally run iTunes again...

Installation Photos

G4 during disassembly.
New and old power supplies.
G4 power supplies.
Problem! (Actually, the problem was that
I didn't follow the corrected page 17.)
Problem 2.
New Papst CPU fan.
Completed modification, fan side.
Completed modification, optical drive side.

Future Hacks

My computer is quiet enough now, but I doubt it will be once summer comes and my non-air-conditioned house heats up. I'll probably try making my own fan cowl, and will also consider the Verax kit if they ever produce it.

27 March 2003 (updates and corrections on 18 April 2003)
Rob Calhoun
rob at-sign calhoun period net