Making Fan Noise Bearable

In Uncategorized by tfwm

Power amplifiers inevitably generate heat. That’s because they are less than 100% efficient; they take in somewhat more electrical power from the AC line than they send out to the speaker. The remaining energy turns to waste heat: every watt of power that comes in on the AC cord that doesn’t go out on the speaker cables turns into heat at the rate of 3.4 BTUs (0.86 kilocalorie) per hour.

Audio power amps thus are designed with some means of cooling by convection-that is, transferring heat from metal heat sinks to the air, lest the temperature buildup damages the electronic components. Some amps use passive convection, with finned heat sinks exposed to the ambient air, while others use active convection, with fan-forced airflow through the heat sinks.

Fan cooling allows engineers to design more powerful amplifiers in smaller and lighter packages, but with a trade-off: noise. Passive-convection amps are silent in comparison, while fans add noise by both their mechanical action and the turbulence of the airflow.

The airflow noise, a general rushing sound, is usually much less noticeable then the mechanical noise, which arises from the fan’s vibrations as it spins, resulting in a whine or hum. Much of the energy in these vibrations tends to conduct to the chassis, which acts as a large surface for radiating sound, like the soundboard in a piano.

Particularly in small worship spaces, the fan noise might be objectionably loud, but there are ways of reducing it. Here are some tips.

1. Change the location of the amp. Is it necessary to keep the amp in the sanctuary, or can it be put into a nearby utility room?

2. Mount the amp securely. Mounting it in a sturdy equipment rack, with rigid support at the back corners as well as the front, adds more mass to absorb the vibrations, especially if you mount other equipment immediately above and below it.

3. Choose your power wisely. It’s far better to have a little extra amplifier power than not quite enough, but don’t go overboard. A more powerful amp may tend to produce more waste heat (but not always; that’s covered next), requiring the fan to run harder.

4. Choose an efficient amp. Techniques for improving amplifier efficiency, such as Class G and Class H output circuitry, are mature and well-developed enough that they don’t audibly degrade audio performance; however, they tend to be implemented on the more powerful amp models, where the power savings better justify the additional cost. This introduces the irony that within a series of similar amplifiers, a higher-power model may use less electricity in normal operation than one of less power.

5. Choose an amp with a variable-speed fan. Many models now have this feature; when the amp is just idling, as during quiet moments, the heat sinks are cooler than when it’s really cranking out power, so the fan runs at a slower speed, producing less noise as well. When the amp pumps out more power, it’ll need more cooling, so the fan will speed up but the sound from the speakers will be louder, too, which will help mask the noise.

6. Control the environment. If the temperature of the ambient air is relatively low, it’ll cool the amp more effectively, and if the amp has a variable-speed fan, it won’t have to run as fast. If your church is air conditioned, the amp will like it as much as the congregation will.

7. Practice good fan hygiene. One thing that makes fans get noisier over time is dust buildup on the vanes; the dust may accumulate unevenly, causing an imbalance that makes the fan wobble severely as it spins. Every once in a while, it’s good to blow the dust off the vanes with compressed air or a canned dust blower. Don’t use a filter on the fan unless you’ll remember to replace or clean the element regularly; it’s better to risk some dust buildup within the amp than overheating and thermal shutdown due to a clogged filter. A similar type of noise can arise if the fan’s bearings are going bad; the fan will make a rattling or whirring noise even if the vanes are clean. If this happens, have it replaced by a qualified technician.

8. Baffle the noise. If the amp has to be in a rack in the sanctuary, put partial front and/or back covers on the rack, over the amp, that will not impede airflow, but will instead deflect it upward and/or downward. They should be at least a couple inches away from the front or rear panels of the amp so they block noise from the amp but still allow the cooling air to flow freely.

Sometimes people ask me if it’s all right to disable the fan because the amp will be used only for light duty. Never! Doing so will almost certainly kill the amp and your manufacturer’s warranty.

Taking all these steps into consideration should lessen the annoyances caused by fan noise, and also extend the life of your amplifier. If you have any questions about this article, please email them to me at bob_lee@qscaudio.com