After wasting countless hours with Apple Tech Support and getting no resolution to audio and fan noise problems, I decided to do what I had to do to make my Mac happy again. This is the infamous MDD dual 1.25 (dual boot version).

The issues of noise fans cycling up and down in speed, expensive audio cards cooking in a box that was poorly designed for the components it contained, and an irritating tk,tk,tk noise that permeated anything that you interconnected the mac with via the line-out led to an extreme makeover.

Problem

Issue# 1 Audio noise and cooling

This was a problem. I discovered that my irritating tk,tk,tk audio noise was coming from the power supply circuit that was supplying power to the fans. This circuit is controlled via temp sensors at the cpu and power supply. When temp of the cpu or power supply reaches a predetermined threshold, the fan power circuit ramps up the voltage to the fans. Also I noted that when the voltage from the circuit was below 5 volts, the noise was not present. My first thought then was to figure out a way to keep things cool enough to keep the fan control circuit from exceeding that 5 volt threshold. I tried many ideas to achieve this, with some improvement but eventually things would heat up and with that the fan cycling and audio noise.

Issue # 2. Excessive fan noise.

Even though I had the improved version of the MDD with pabst 120mm fan and revised power supply, still it was to noisy for a studio environment. The power supply fans produced a very irritating whine that was unbearable to me.

In my experiments I discovered just opening the case helped dramatically in cooling things down, thus keeping fan noise down, so I rigged up a shroud of sorts with 2-80 mm fans to keep the cpu cool and ran the computer that way for a month. I disconnected the main 120 mm fan and monitored the cpu temp and did effectively drop it to 43-44C measured at the heat sink base. This was an improvement over the stock configuration and the 2-80mm fans running at 12 volts were less noisy than the pabst 120mm. Overall better

Of course I couldnt keep the computer like that permantly and I couldn't close it up because the 80mm fans just were to big. Also, even though things were much cooler, and quieter the temp sensor would still send the control circuit over 5 volts and that tk,tk,tk noise would be there.

I was against hacking my case to death like many who came up with there own solutions to the issues and none of them really dealt with the audio noise. In trying to trace down the source of the audio noise, I discovered if I disconnected all the fans the noise was totally gone regardless of the voltage the circuit was sending. My guess on this is that the circuit that controls the fan voltage is just poorly designed (poor filtering) and that any load on the circuit then produces the noise. Some have suggested that the fans themselves produce the noise but I disagree with that. If that were the case, I would expect to here the noise even when the fans were running at lower voltages (ie below 5 volts). Even the small cd bay fan alone on the circuit is enough to trigger the noise. Also, the nature of the noise doesnt change as the voltages rises and does not correspond to fan speed. If it were some kind of back emi as some have suggested I would expect a correlation between the noise and fan speed. Once all the fans are disconnected ...noise gone. Of course you can't run the computer like that. I realized I would have to design a custom case and run the fans on a different power supply to eliminate the audio noise. I also needed to improve overall cooling and at lower noise levels. the final result was the "rackmac" My apologies to anyone with a product called that, its just my nickname for the case I created and not a product name.

Solution

Build a custom case, rack mountable for studio envirnment. Design the case with better air flow and improved cooling. Devise a better way to cool the power supply unit and do away with noisey and inefficient 60 mm whinny little fans.

One thing I ran into with working with various fans is that the mounting is crucial. The fans must be totally isolated from the case or the case will amplify the sound of the fans significantly. Verax does this via there rubberhead mounts and other isolation components. I did try the 60 mm replacement for the power supply and while they are quiet with a superior mounting system that aids in overall noise reduction, I couldnt use them becuase my case design required reversing the air flow of the power supply and I was unable to mount them in a reverse configuration. If I were able , I would have still used them over the originals. In my earlier experiments I made fan mounts out of balsa wood mainly because it was easy to work with and cheap for prototyping. However it would resonate and amplify the fan. I finally ended up using a damping material that is used to silence heating and cooling ductwork. It is a dense limp rubber like material that when applied to my basla wood mounts, dampened the fan noise very effectivly and also added strength to the structure.

Results

#1 Audio noise is gone once and for all!

#2 My cpu temp right now measured with temp monitor program running OSX is 47.8 C with the ambient room temp of 76 F. I don't have my temp sensor at the heatsink anymore but typically it runs 8-10 C less than the cpu sensor. The highest my cpu has reached was 51C while my house was 80F. This is a MAJOR improvement. I swapped the aluminum heat sink for a copper heatsink and that dropped overall temps about 3-4 C. I am using 2-92mm fans for the cpu heatsink

#3 I now am cooling the power supply with 2-80mm fans and the exhaust temp is typically 28-30C

I completely removed the 60mm fans from the power supply, built a housing that fits over that end and am pulling the air with the 80 mm fans through the power supply. The ps is mounted under the cd bay shelf and draws air from the bottom of the case which is an open mesh cover then through the ps and out the top of the cd bay shelf where the 80mm fans are mounted above exausting out the top of the case.

#4 Cooling of the pci audio cards and video card is supplemented by 2 more 80mm fans Also the fan shroud for the heat sink is opened up where the video card sits so air catches the cards processer heat sink. I added a fan controller to enable me to adjust the voltage of the fans. There are 4 channels so that I can set the fan speeds for all my fans independently. Presently I am running them all at 12 volts with the exception of the small cd bay fan that is running at 5 volts. Even though it isn't cooling the cd/dvd housing anymore, it is moving air underneath the cpu board so I chose to keep it in the mix of things. When winter sets in and my house is cooler I am comfortable in ramping those fans down. Even with them all at 12 volts, it is quieter than the "original" computer was when it was at its lowest "just turned on" state. And it just stays at that level...no more irratic cycling, no more audio noise, no more wind tunnel when things heat up a little. Total cost was around 300.00 Yeah that is more than most would want to spend on something that should have worked from the get go, but looking at the options, I believe it was worth it for a nice rack mount unit. I found a company that actually rack mounts a G4 but they wanted about a 1000.00 for it so that was out of the question for me.

Browsing the pics below, you will note that I originaly had the 120mm fan cooling the pci card slots from the rear. I did away with that and opted for the overhead 80mm pair because it was more effective and quieter.