Hum

A little mechanical hum from a power transformer is normal. A lot of it is a warning sign.

I get emails every week from people unhappy about the buzzing sound coming from their amplifier or preamplifier. Sometimes they describe a gentle hum that has been there since day one. Other times it is a new noise that appeared out of nowhere and keeps getting louder. The cause is almost always the same thing, magnetostriction in the transformer core, but what triggers it and how much you should worry about it varies widely.

Transformers work by passing alternating current through coils of wire wrapped around a laminated steel core. As the magnetic field cycles back and forth fifty or sixty times per second, the steel laminations physically expand and contract by microscopic amounts. That tiny mechanical movement is what you hear as hum. Every transformer does this to some degree. The question is how much is acceptable and when it means something is wrong.

In a well-designed power supply with a high-quality toroidal or R-core transformer, the hum should be barely audible. If you can hear it from your listening position, something is off. The most common culprit is DC offset on your AC power line. When the positive and negative halves of the AC sine wave are not perfectly symmetrical, the transformer core saturates on one half of the cycle and the hum gets dramatically louder. This asymmetry can come from devices on the same circuit like dimmer switches, LED driver circuits, large motors, or even solar inverters feeding back into the grid. Might be from you, your neighbor, or industries in your area.

A power regenerator solves this problem completely because it generates a fresh, symmetrical sine wave from scratch. That is one of the reasons our Power Plant regenerators make such an obvious difference in systems plagued by transformer hum. If a regenerator is not in your budget, a DC blocker is a simple device that removes the offset and quiets the transformer. Some companies sell them as standalone boxes for under a hundred dollars, and they work remarkably well.

If your transformer hum is new and getting progressively worse, it could also mean the laminations are loosening due to age or thermal cycling. That is a mechanical problem that typically requires service. But in my experience, nine times out of ten the fix is addressing what is on the AC line rather than what is inside the chassis.