November 2017
I bought a 110V AC line isolation transformer to use with some old two-wire radios to see if I could reduce the AC hum in the audio. The audio is great, but now I’m bothered by the hum of the isolation transformer.
I read that it’s probably due to the transformer interacting with the metal enclosure. Should I remove the transformer from the metal enclosure and put it in a plastic one?
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The transformer hum is due to loose steel laminations in the transformer core. The alternating magnetic field (60 Hz) sets up a vibration within the core that can also be considerably amplified by the enclosure, if the enclosure and transformer mounting happen to resonate.
Your enclosure material is not the actual problem. Try placing the transformer on rubber isolation cushions to isolate it from the chassis/enclosure. Rubber grommets work well for this. This may reduce the hum, however, this will not address the loose laminations within the transformer and it will still make some hum sound. I don’t think much can be done about that except replacing the transformer.
It could be the case, but it also could be the laminations in the transformer core itself rattling after warm-up. Remove it from the case and make a trial run with the bare transformer sitting on a insulated surface (plywood) to find out. Or wear headphones and ignore the noise.
Line-frequency transformers always hum because their core is made of laminated steel. Hum is more pronounced in inexpensive transformers where lower-grade steel is used and it is driven further into saturation on each alternating-current half-cycle. The hum that you’re experiencing is most likely acoustically coupled to the transformer enclosure into or onto which it is mounted.
Try to isolate the mechanical coupling — e.g., use rubber grommets around the mounting hardware.
