Curing Hum

Hum can be an annoying problem with any low power AM transmitter. Work through the causes below and you should be able to find and eliminate it.

Audio source level too low

Increase the signal level (volume) from your audio source.

Ground loop in the audio cables

A ground loop happens when the audio source is connected to a different ground from the transmitter, and it puts hum on the audio cables.

To check, unplug the audio source from the transmitter. If the hum drops a lot or goes away, you probably have a ground loop.

• Try disconnecting the transmitter earth, so it is earthed through the audio cable instead.
• Or fit a ground loop isolator in the audio cable, so it is earthed through the transmitter.

Hum induced by the mains wiring

The most common source of hum indoors is mains hum modulated onto the RF signal by the building’s wiring. The transmitted signal is picked up by the mains cables and re-radiated with mains modulation on it.

To check, disconnect the audio cables from the transmitter. If the hum is still there, it is coming from the wiring. Often a radio close to the transmitter has no hum, while one in a distant room does.

• Connect the transmitter ground to a grounded metal point such as a copper water pipe. This bypasses much of the signal that would otherwise couple into the wiring.
• Find the source. Anything with a switch-mode power supply can cause hum: low-energy lights, lamp dimmers, phone chargers, Wi-Fi routers, PCs. Turn lamp dimmers fully off.
• If your radio has an internal directional aerial, try rotating it. There is often a position where the signal stays strong but the hum drops sharply.
• Change the operating frequency. It can make a big difference, and the higher the frequency the better.