If you want to identify studio equipment that’s susceptible to grounding problems, a simple DIY project based on a wall‑wart PSU can help.
My Ground Control article in SOS August 2023 (https://sosm.ag/ground-control) raised the topic of good and bad audio equipment design practices in respect of correctly managing interference and noise currents. In poor designs, unwanted noise currents from ground loops or RFI (radio‑frequency interference) introduced via the shields of cables connected to other equipment are allowed to intermingle with the audio signal currents within the amplifier circuitry, adding hums and buzzes to the wanted audio. In contrast, good designs ensure such noise currents are kept well away from the audio circuitry, leaving the device blissfully quiet and trouble‑free.
Unfortunately, there’s no way of telling whether a piece of equipment is designed well or badly in this respect just by looking from the outside — but there is a simple engineering test to find out, and the necessary equipment can be constructed with moderate electrical DIY skills and a spare wall‑wart power supply. This device, called a Hummer, was invented by John Windt and first described in the AES Journal (June 1995, Volume 43, No. 6). It was created in response to another article published in the same issue by Neil Muncy, who discussed at length equipment susceptibility to noise currents injected via external audio cables — something he christened the ‘pin‑1 problem’.
Conceptually, the Windt Hummer is nothing more than a reliable source of an audible noise current that can be injected (harmlessly) into the ground paths of any audio device. If a hum or buzz becomes audible (or the noise floor changes noticeably) at the output of the device under test, it clearly has a ‘pin‑1 problem’ and will be highly susceptible to ground‑loop or RFI issues when installed in an audio system. If all remains quiet under test, it is a well‑designed device that handles noise currents properly. It really is that simple!
So, the source of noise current in the Windt Hummer is a standard (double‑insulated) wall‑wart power supply with an AC output. Any AC PSU with an output current rating of more than 30mA and an AC output voltage of between 6 and 24 Volts is acceptable — my own tester was built using a redundant 1.9A 9V AC PSU from an old phone charger that I found in the back of a drawer. The wall‑wart transformer provides a safe, electrically isolated (ie. floating) and low‑AC signal at 50 or 60 Hz (depending on the local mains supply). The...