Using EarBuds with an aviation handheld?

[Mark Horak]

You're right, it's not the splitter. I still get the whine when plugging the 4 pole Apple ear buds directly into the 3 pole adapter. I had not realized this before because doing so cuts out all audio, as if I was transmitting. But I have since learned that holding down the center button on Apple's mic restores the audio for some reason. So I need a miniature vise grips for that button. :-) In any case, I'm getting closer, and will follow up.

[Russell Holton]

But I have since learned that holding down the center button on Apple's mic restores the audio for some reason.

Maybe this helps explain it: 3.5 mm Headset: Accessory Specification. Granted, this is Android and you're Apple, but I'm sure it's close enough.

I think what's happening (the headset works when the button is pressed) is the 3-wire ground is getting applied to the "mic" section of the 4-wire earbud. Holding down the switch puts a resistor between the radio's ground and the headset common. (If that button is a mute button, then it may short the mic out.)

As for the whine, I'm leaning toward the idea of adding a resistor between the audio and ground to give a lower impedance load to the radio. If you make your own adapter, it would be easy to add a 1/4W resistor inside the plug.

[Mark Horak]

Thank you for that reference. I have a set of Samsung 3.5mm 4 pole ear buds, and they so not require holding down a button to hear the audio (and the whine). But seeing the spec provides ideas on what Apple is doing, and I think you're onto it. In any case, I have another adapter that I believe already incorporates some additional "input resistance" and I am going to experiment with that this weekend.

[Mark Horak]

The "another adapter" I have is Sporty's own adapter that permits a standard aviation headset to connect to the SP-400 with a 1/4" plug. Since no whine is heard in my aviation headsets through that adapter I figured that maybe some resistance had been built into it. However, after I found another adapter to convert the 1/4" female plug of Sporty's adapter to 3.5mm, the whine was still there in the ear buds. So it seems clear to me that Sporty's adapter is just a pass through (and mono at that), and that the whine is suppressed by the normal impedance of aviation headsets. It looks like I need to do what you all have been suggesting (or ignore the whine).

[Mark Horak]

FYI, I arrived at a decent solution using a combination of two adapters: 1) a 2.5mm mono to 3.5mm stereo adapter, and 2) a 3.5mm passthru adapter that adds 220 ohms resistance (er, impedence). The two adapters are only $5 and $8 respectively.

With this setup, the whine is gone, and the mic/controls on 4-pole earbuds from Apple and Samsung are successfully ignored. You would be using the SP-400's builtin mic to transmit.

The 2.5mm adapter: https://www.amazon.com/gp/product/B000HM3PUS/

The 3.5mm adapter: https://www.amazon.com/gp/product/B07FM8GJL4/

There is an 80 ohm version of the 3.5mm adapter available at the latter link, but I could still (barely) hear the whine with that one.

[Russell Holton]

Based on your description, I'm guessing the resistor is in series with the headset to lessen the sensitivity. Otherwise, I'd think the 80 ohm would work better.

[Mark Horak]

Would you mind explaining the scenario you are envisioning where 80 ohms might work better? I still have a long way to go in understanding audio technology.

[Russell Holton]

Would you mind explaining the scenario you are envisioning where 80 ohms might work better? I still have a long way to go in understanding audio technology.

There's two ways a single resistor might be added to the circuit: In parallel with the earphone (to add loading to the phone's output), or in series with the earphone (to reduce sensitivity of the earphone). If in parallel, a lower resistance would increase loading. In series, a lower resistor would be decrease the amount of sensitivity reduction.