In July 2025, I picked up a pair of HIFIMAN HE-400 planar magnetic headphones at a flea market for $20. At that price, it was worth the gamble—but what I didn’t expect was just how good they sounded. The HE-400 delivers that classic early planar sound—solid bass, smooth presentation, and an overall musical signature that still holds up today. But like most older HiFiMAN gear, it came with a couple of issues that needed attention.
The Challenges
1. Aging Ear Pads
The ear pad mounting rings were dry-rotted and falling apart—pretty typical for headphones of this age. That was an easy fix with replacement pads.
2. SMC Connectors
The bigger challenge was the SMC connectors.
These are not commonly used in modern headphone cables, which makes finding replacements or alternatives inconvenient. Even though my pair came with a nice aftermarket cable, I wanted the flexibility to use standard dual 3.5mm cables.
The Goal
Create a custom adapter that allows:
SMC (headphone side) → 3.5mm female (cable side)
So I could use widely available headphone cables instead of being locked into SMC.
The Parts
- SMC extension cable (cut in half for donor connectors)
View on eBay - 3.5mm female panel-mount connectors
View on Amazon
The Build Process
Step 1: Harvesting the SMC Connectors
I cut the SMC cable in half to create two leads—one for each ear cup.
This gave me a clean starting point for wiring into the adapters.
Step 2: Wiring the 3.5mm Female Jack
Each SMC lead was soldered to a 3.5mm female connector. (I chose mountable jacks to use instead of some cable end models as the spring retention was much stronger than any of the cable end models I could find.)
Here’s the important detail:
- The connector is TRS (Tip, Ring, Sleeve)
- Each ear cup only needs TS (Tip and Sleeve)
The Fix:
I bridged the Ring and Sleeve connections to make the TRS jack behave like TS. This ensures compatibility with standard stereo headphone cables without signal issues.
Step 3: Final Assembly
After soldering, I used heat shrink to secure and protect the connections.
I also marked left and right channels for clarity.
Installing the Adapters
The adapters connect directly into the original SMC ports on the headphones.
Using Standard Cables
With the adapters in place, I can now use a typical dual 3.5mm cable.
Final Result
The finished setup looks clean and works perfectly.
The Outcome
Pros
- Standard cable compatibility
- Easy cable replacement
- Preserves original headphone connectors
- Fully reversible mod
Cons
- Slight added bulk at the connectors
- Requires careful soldering
- TRS-to-TS workaround needed
Learn More About the HE-400
If you’re interested in the original headphone, this Drop page is a good reference:
To hear the latest from HIFIMAN’s Dr. Fang Bian, check out Lachlan’s recent interview on his Passion for Sound YouTube channel:
Final Thoughts
This turned out to be one of those projects that checks all the boxes:
- Cheap entry point
- Fun DIY challenge
- Real usability improvement
For a $20 flea market find, the HE-400 has earned a permanent place in my collection—and now it plays nicely with modern cables.
