Inside a 3.5 mm plug from old Bose headphones

28.08.2021 10:16

Some time ago my dad told me about a problem he was having with a new ham radio transceiver. The transceiver has a RS-232 serial interface in a 3.5 mm stereo socket in the back panel. Connections that would normally be used for left and right audio channels are used for data RX and TX instead. Since the transceiver didn't come with a suitable cable my dad made his own: he found an old cable with a molded 3.5 mm plug on one end and soldered the other end to a DB-9 connector he could plug into his PC. However no matter what he tried, he could not get the digital interface working.

Surprisingly, we traced the problem to his adapter cable. Even though the multimeter showed that the cable had continuity on all cores and that no lines were shorted, the digital signals that passed through it came out distorted on the oscilloscope. I was curious what was going on, so I took the cable home with me for some more investigation. I commonly re-use the 3.5 mm plugs from old headphones as well and have never came across a problem like this before.

This is the 3.5 mm plug molded onto the end of the cable in question. It has nice gold plating on the contacts and a no-expense-spared molding with a combination of hard plastic and a soft rubber-like overmold. It probably once served some higher-end Bose headphones.

The 3.5 mm stereo plug on the end of the suspect adapter cable.

First thing I did was to measure the frequency response of the cable in the audio frequency range. I connected the stimulus signal to the 3.5 mm connector and measured the gain and phase of the signal coming out of the other end of the cable. This is the resulting Bode plot:

Measured frequency response of the adapter cable.

Obviously there is something more going on in this cable than just normal copper connections. At these frequencies I would expect to see practically flat, 0 dB gain and next to no phase shift across the cable. Instead, there seem to be a -10 dB band stop filter with a center frequency of around 2 kHz somewhere inside.

I found it unlikely that the actual cable was doing the filtering, so I focused on the molded handle around the actual 3.5 mm connector. After some application of unreasonable force it still refused to fully come apart. It did reveal what looked like tops of two MLCCs molded into the plastic. However it seemed that if there is some more electronics inside removing the rest of the plastic by force would also destroy the circuit.

3.5 mm plug with the partially removed molding.

Since I heard that acetone sometimes dissolves potting compounds I put the connector into a glass of acetone-base nail polish remover. I then forgot about it, so it soaked in the acetone for about 2 months. Still, this didn't have as much effect on the plastic as I thought it would. It did make it brittle enough so that I could chip it away until I revealed a small double-sided printed circuit board with a few passive SMD components:

Circuit board inside the 3.5 mm plug handle, bottom side.

Circuit board inside the 3.5 mm plug handle, top side.

The circuit board is marked "YODA" Ver A. Apparently someone working at Bose was a Star Wars fan. If I read the date code correctly this board was produced in 2005. The circuit is symmetrical and has two identical parts for the left and right channel. Each half consists of 2 multi-layer ceramic capacitors and two chip resistors. Tracing the circuit revealed this schematic:

Schematic of the filter circuit embedded in the 3.5 mm plug.

Note that the resistances in the circuit are low enough that a typical multimeter on continuity setting will see them as a short. This is what made debugging the initial problem so frustrating.

I couldn't get a reliable measurement of the capacitors, so 10 μF was a bit of a guess here. However simulating the response of this circuit in SPICE shows that it behaves similarly to what I measured before I destroyed the connector:

Simulated frequency response of the filter circuit compared to the measurement.

Why did Bose go into the trouble of embedding this circuit into the connector? I'm guessing they wanted to improve the frequency response of their headphones. Maybe the speaker has a mechanical resonance at 2 kHz and the filter circuit damps that electrically? I don't know much about hi-end audio engineering. This article mentions passive correction filters that are placed in-line with a headphone cable to equalize the speaker response. While the circuit mentioned is a bit more complicated than the one I found, the one Bode plot they show is very similar to what I measured.

One quick test I found that is capable of detecting this specific circuit without any elaborate setup is setting the multimeter for a capacitance measurement and connecting the probes between one of the channels and ground. On a normal audio cable the multimeter reads some low capacitance, but in the case of the embedded filter it shows around 20 μF due to the two capacitors in the circuit.

In conclusion, it seems that not all connectors are merely connectors. When all other options fail it's worth doubting the most basic assumptions, like that a cable you're using is not actually behaving like a cable.

Posted by Tomaž | Categories: Analog

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