Ground Loop Humming in Classroom Audio Systems

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If you have ever listened to an amplified sound system you have probably heard the dreaded hum. It’s common. From presidential speeches to movie theaters to concerts to classroom audio systems the hum can be heard. What do you do? There are so many factors and so many things that can cause that horrible humming.

The typical culprit are mishandled high voltage grounds. When the facility has several electrical circuits (and they all do), some of the circuits have different grounds with slightly different voltages or resistance values. Many people say that this can cause DC to creep into the electrical system and cause humming issues. While this is typically correct, frequently that DC component has some AC characteristics, but that’s another discussion.

Once the DC creeps in, it can travel a long distance from one component to another. Let’s say that you have a classroom with a laptop that is plugged into your plenum rated amplifier in the ceiling via a 3.5mm audio cable. The devices now have a shared ground connection over the shield of the audio cable. If the laptop’s ground has a small bit of voltage on it, it will try to lift the ground level of the amplifier and will probably create a humming issue. At this point you have several options.

1. Disconnect the power supply from the laptop. (Not a good choice, since your battery will eventually run out).
2. Get a ground pin eliminator for the power cord. (Not a good choice since this is dangerous possibly illegal).

3. Cut the shield to the audio cable. (Not a good choice as this could cause other audio issues).
4. Get a ground-loop isolation transformer that maintains the signal integrity but, through magnetic decoupling does not allow ground differentials to transfer from one device to the other. (Bingo!)

Yes, number 4 is the correct answer. Roemtech’s HummBuster technology does exactly that. We put several ground-loop isolation transformers in each amplifier that has the HummBuster feature. So if the user ever hears a hum in the speakers, he/she can simply flick the switches until the problem goes away.

The PMA-350H+ takes this technology to the next level. The amplifier can actually search all of the inputs and deduct which one is causing the problem, once it is identified it engages the ground-loop filters until the problem is gone. This happens with the simple push of a button. Only Roemtech has this technology and it is so unique, we filed a patent on it.

To see how this feature works, refer to our “Videos” page for a demonstration.

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