The Pin 2,5, 8, 11,16,22 and 25 problem…Why We Must Solve the AES59 Grounding Trap
https://www.aes.org/standards/comments/cfc-draft-rev-aes48-xxxx-251124.cfm
The “Pin 1 Problem” Multiplied: Why We Must Solve the AES59 Grounding Trap
By Anthony P. Kuzub Chair, AES-X249 Task Group SC-05-05-A
In the world of professional audio, the transition from XLRs to high-density DB25 connectors was a matter of necessity. We needed more channels in smaller spaces. But in adopting the AES59 standard (often called the TASCAM pinout), the industry inadvertently created a trap—an 8-channel variation of a problem we thought we had solved decades ago.
We call it the “Pin 2, 5, 8… Problem.” And if you care about the noise floor of your equipment, you need to understand why it is happening.
The Ghost of Neil Muncy
Decades ago, the late Neil Muncy identified the “Pin 1 Problem.” He famously demonstrated that when a cable shield (Pin 1 on an XLR) connects directly to a device’s internal circuit board ground rather than the metal chassis, it turns the shield into an antenna. This dumps Radio Frequency (RF) noise and power-line leakage directly into the audio reference, muddying the signal.
The DB25 connector has made this problem exponentially easier to commit and harder to spot.
Because DB25 connectors are almost always mounted directly to a Printed Circuit Board (PCB), it is convenient for designers to route the ground pins (2, 5, 8, 11, etc.) straight into the internal ground plane of the PCB. This is a fatal flaw. It takes the RF noise harvested by eight different cable shields and injects it directly into the heart of the device’s sensitive audio circuitry.
It is the design flaw where the shield grounds of the DB25 connector (pins 2, 5, 8, etc.) are connected to the device’s sensitive audio circuit ground instead of the equipment chassis.
The Mechanism (Muncy’s Principle)
Neil Muncy’s “Pin 1 Problem” highlighted that cable shields act as antennas, picking up RF (Radio Frequency) noise and carrying power-line leakage currents.
- Correct Design: This noise should be dumped instantly into the metal chassis (the “Faraday cage”) to protect the insides.
- The Problem: If that noise is routed onto the circuit board’s “ground” traces, it modulates the audio reference voltage. The noise effectively becomes part of the audio signal.
Why it happens with DB25s
This problem is often worse or more common in DB25 implementations than XLRs for two reasons:
- PCB Mounting: DB25 connectors are almost always mounted directly to a Printed Circuit Board (PCB). It is very easy (and lazy) for a circuit designer to route Pins 2, 5, 8… directly into the PCB’s internal ground plane rather than routing them separately to the chassis screw or faceplate.
- High Density: Because there are 8 channels, there are 8 shield pins. If all 8 of these carry noise into the delicate internal ground tracks of a converter or console, the cumulative noise pollution can be significant.
The Solution: The Shield Plane
The work currently being finalized by AES task group SC-05-05-A aims to legislate this flaw out of existence by codifying the “Shield Plane.”
We are moving beyond viewing the DB25’s mechanical parts—the shroud flange and jack screws—as merely mounting hardware. In the revised standard, these components are defined as critical electrical conduits. They are the gateway to the “Shielding Enclosure” (the chassis).
By utilizing a Shield Plane—a dedicated conductive layer or path that segregates shield currents from audio signals—we ensure that noise flows from the cable shield, to the connector shroud, through the jack screws, and effectively “splashes” against the chassis (the Faraday cage), never touching the internal audio reference.
Why This Matters
This is not just about adhering to a standard; it is about protecting the integrity of modern audio. We live in an environment saturated with RF interference, from Wi-Fi to mobile signals. If we continue to allow “The Pin 2, 5, 8… Problem” to persist in equipment design, the high-resolution digital converters and pristine analog consoles we pay thousands of dollars for are being compromised by the very cables used to connect them.
The new guidelines for AES59 are simple but strict: No painted metal contacts. No routing shields to the PCB signal ground. No ambiguity.
By respecting the Shield Plane and ensuring a low-impedance path to the chassis, we finally honor the principles Neil Muncy taught us, ensuring that our high-density connectors deliver the high-fidelity performance they promised.
An Origin Story: The Value of Being “Wrong”
This initiative began fifteen years ago during a debate with a seasoned engineer regarding the routing of these very grounds. At the time, I was firmly told that my concerns were unfounded and, more critically, that “it didn’t matter.” I owe a sincere debt of gratitude to that engineer. That dismissal did not silence my hunch; it radicalized my curiosity. It forced me to dig deeper into the physics of interference and to understand why I felt the nuance mattered. That decade-and-a-half journey of proving a “wrong” hunch right has culminated in this work, ensuring that future engineers never have to guess whether the details matter—the standard will assure them that they do.
“curiosity killed the cat but satisfaction brought it back”