Bill WhitlockBill WhitlockDisturbance signals, such as hum, buzz, or noise, can be extremely frustrating. And troubleshooting them is oftentimes a time‑intensive process of trial and error. Until I learned of Bill Whitlock and his education[1] on Audio System Grounding and Interfacing.

Whitlock recommends relatively simple means to — methodologically — identify causes for disturbance signals. And while his teachings have always appealed to me, I never got to the point of actually building some of the tools myself. Until recently.

Whitlock's Balanced Dummy and Windt's HummerFigure 1Figure 1 shows my first builds of Whitlock's Balanced "Dummy"[5] (for hunting cabling‑ and/or termination‑related problems) and John Windt's "Hummer"[6].

The Hummer is great for identifying Pin 1 problems which regrettably seem to also plague my favorite audio interface (for sound system measurements).

"When the unit is grounded, a slight hum may occur, depending on the particulars of your installation."

- Roland OCTA‑CAPTURE Owner's Manual -

Pin 1 Problem

The shielding (or screening) of audio equipment, cables, and microphones can be critical for electromagnetic compatibility (EMC). The Pin 1 Problem considers a self‑inflicted problem — within audio equipment — when audio‑signal circuitry and shielding share a common impedance. From XLR‑connector usage, where Pin 1 is standardized as the designated shield contact, this has been identified as the Pin 1 problem.

Despite the AES standardizing grounding and EMC practices for interconnections in 2005[7], my interface (which was released five years later in 2010) nevertheless appears to suffer from Pin 1 problems.

The Hummer Test

Using the Hummer to inject 80 to 100 milliamperes' worth of half‑rectified AC into the shielding — via Pin 1— one can listen as well as look for Pin 1 problems.

Notice when the black probe touches the interface's rear‑panel ground terminal, the green LED on the Hummer illuminates (signifying a return path), and hum is clearly audible as well as measurable.

In order for a ground loop to occur (and potentially cause hum), a shield current must make it into the device — and — have a return‑path to flow back to its source (typically via a safety ground). However, this audio interface features a power supply with a 2‑prong power cord without safety ground.

So — by design — there is no return‑path for any ingested shield currents. And as long as the device remains ungrounded, one "could" get away with Pin 1 problems. Untill the device ends up getting grounded unintentionally.

Unintentional Ways Of Grounding

Unintended ground loopsFigure 2Two ways can cause an audio device — without safety ground — to become grounded unintentionally.

  • Rack mounting

    Attaching — conducting — rack ears to the audio device‑chassis can cause the unit to become grounded. Nineteen‑inch racks must be grounded which typically includes the rack's rails as well, and subsequently the device.

  • Interconnections with one or more devices that feature
    3‑pronged power cords — with — safety grounds

    When the audio interface is connected to one or more devices that feature 3‑pronged power cords with safety grounds. Shielded interconnections (including, but not limited to, data communication to computers) can cause currents to flow through the device (Figure 2).

In this video, the return‑path is completed — indirectly — through the safety ground of my computer's 3‑prong power cord (white Shuko connector).


Solutions to problems like these are beyond the scope of this article. But the references linked below provide ample guidance.

In closing, please allow me to evangelize:

NEVER defeat safety grounding
to solve a noise problem!


[1] Standards Webinar: Analog System Interconnection Standards, 2021
[2] Bill Whitlock - Signal Interfaces Debunked - 4/27/2021, 2021
[3] AC Power, Grounding, and Shielding Super Session, 2019
[4] AC Power, Grounding, and Shielding, 2017
[5] An Overview of Audio System Grounding & Interfacing, 2012
[6] An Easily Implemented Procedure for Identifying Potential Electromagnetic Compatibility Problems in New Equipment and Existing Systems: The Hummer Test
[7] AES48‑2019: AES standard on interconnections - Grounding and EMC practices - Shields of connectors in audio equipment containing active circuitry