Using EarBuds with an aviation handheld?

[Mark Horak]

I have a Sporty's SP-400 handheld aviation radio, and I would like to use it sometimes with a standard 3.5 mm stereo headset (with mic) that comes with just about every smartphone (other than current iPhones).

Sporty's can't help. They tell me they only support aviation headsets via the adapter cables they sell, and that they are not aware of any way to do what I am requesting. They also tell me that one pilot who called them with a similar request had mentioned that he would be seeking some kind of impedence adapter. They have not heard back from that pilot so they had no further information to share.

The SP-400 has separate mic and audio plugs (the audio is 2.5 mm mono). I have found the right connectors and a splitter to create a single combo 3.5 mm jack into which I can plug the little headset. The mono audio signal is being delivered to both ears, but I'm hearing some whining and I'm not sure yet if the mic even works.

Clearly there is more to this than just plugging in. Has anyone here done what I mentioned in the first sentence? If so, any hints on how I might be able to get there myself? A commercial adapter? -meh

[Terry Carraway]

Speakers and microphones have an impedance. Amplifiers are designed to work with specific impedance speakers and mics.

And a whine can be a symptom of impedance mismatch.

Basically, you use a transformer to impedance match. A ferrite donut, with two wires wrapped around it, with a difference in number of turns. Each coil will have the desired impedance, and the transformer transfers the signal between them

I have done this for ham radio gear.

[Mark Horak]

Thanks. I have no doubt the solution is in this direction, but I am not sufficiently knowledgeable to work it out, even though I sheepishly acknowledge I have a general class license. But I ain't quit breathin' so I ain't quit learnin' (an expression I first heard in Avsig long ago). I've found a fellow ham who has done a lot of work in this area and sells various adapters, w2eny.com. I suspect I may be speaking with him soon.

[Jeff Hartmann]

I have an Icom IC-A22, I just plugged in my I-Phone headset, got audio on one side, used the Icom built in mic to transmit. Listened on the scanner.
I'm sure the impedance is mismatched, but it worked for listening and side tone was clear. Same result with a cheap, free airline headset.
I don't see a way to use the I-Phone mic to transmit.

[Mark Horak]

Sporty's audio port is 2.5 mm mono, but I found an adapter to convert it to 3.5 mm stereo, and it almost works but for that whine. I could live with that for my limited purposes, but I'm kind of invested now in understanding the full picture.

In that light, I found a couple of columns that provide a nice explanation of how aviation com ended up where it is right now. For anyone else interested: https://www.aviationtoday.com/2001/1...-audio-part-i/, https://www.aviationtoday.com/2001/1...audio-part-ii/.

[Terry Carraway]

Transformers[edit]

Transformers are sometimes used to match the impedances of circuits. A transformer converts alternating current at one voltage to the same waveform at another voltage. The power input to the transformer and output from the transformer is the same (except for conversion losses). The side with the lower voltage is at low impedance (because this has the lower number of turns), and the side with the higher voltage is at a higher impedance (as it has more turns in its coil).

One example of this method involves a television balun transformer. This transformer converts a balanced signal from the antenna (via 300-ohm twin-lead) into an unbalanced signal (75-ohm coaxial cable such as RG-6). To match the impedances of both devices, both cables must be connected to a matching transformer with a turns ratio of 2 (such as a 2:1 transformer). In this example, the 75-ohm cable is connected to the transformer side with fewer turns; the 300-ohm line is connected to the transformer side with more turns. The formula for calculating the transformer turns ratio for this example is: {\displaystyle {\text{turns ratio}}={\sqrt {\frac {\text{source resistance}}{\text{load resistance}}}}}

Also - https://www.electronics-tutorials.ws...ansformer.html

[Robert Dubner]

Okay. I am going to commit some heresy, here.

Impedance matching in the modern environment is overblown.

It is true that to get maximum power transfer from an amplifier output to the next stage input, the output and input impedances should be matched. The power loss curve, however, when the impedances are not matched, is pretty darned flat. The mismatch has to be pretty spectacular in order to make a difference that matters. This is an issue when you are trying to move a lot of elections around, like when you are trying to get the output of a radio transmitter into an antenna at a radio station.

That said, it's not what we are talking about, here. Power transfer impedance is an issue at radio frequencies and up. At audio frequencies, which are basically DC, "impedance" has a different essential meaning. Keep in mind that at 3,400 hertz, which is the upper limit of the voice band, the wavelength is about 55 miles. Transmission line effects are *not* an issue.

(Impedance matching was more of a consideration when there were more transformers in audio circuitry. But modern circuitry -- by "modern", I mean "since the widespread adoption of the transistor" -- doesn't use transformers in audio circuits.)

The thing to keep in mind is that at audio frequencies, you are not worried about power transfer. What you do want is fidelity, which improves when the output impedance is less than the input impedance of the next stage. But you want them to sorta kinda be in the same ballpark; say in the range of one to ten, if for no other reason than you don't want the output stage to burn itself up when you accidentally short the output to ground.)

Modern cell phones have an interesting problem to deal with. They come with a single 3.5mm jack, which the user might use to plug in a headset, with an input impedance (which, frankly, in my opinion ought to be called an input resistance, since I have a prejudice that says impedance should be used when talking about something that might require a Smith chart to analyze) of, these days, around 16 to 300 ohms. The user might also use that same jack to connect to the "Line In" of an amplifier or recorder. The input impedance of such a gadget might be about 10K.

So, a really tricksy cell phone might try to detect the impedance of the next stage; if it sees 32 ohms, it might lower the output impedance and change the output levels to match what it assumes is a headset. Otherwise it might switch to 600 or so ohm output impedance and change the output levels to match the specs for a line input. I'd have to look up what the industry regards as "standard" voltage levels; I'm pretty sure they are different for a passive headset and an active line input.

So, that all said: If you have plugged in a stereo headset using a 2.5mm to 3.5mm mono to stereo adapter, and it works, it's probably working. The whining is possibly because the manufacturer of the stereo headset expects you to be using it not just for voice, but for listening to music. So, it has a significantly higher frequency response than the aviation band supplies by design. Modern frequency allocation has the bands spaced at as little as 8.33 kiloHertz, which means the maximum payload bandwidth is 4.16 kHz, and the radios probably restrict that even further to simplify the design. Which means that the speakers and headsets that come with the radio almost certainly don't have much frequency response above 4kHz, again by design. I am, therefore, suggesting that the whine is actually there, and you just couldn't hear it until you attached a headset with a higher frequency response.

If I am right, what you need is a low-pass filter to put between the radio and the headset. It could be done passively, but now audio frequencies are not your friend. I haven't done work like this in twenty years, but for a first cut, just to see what would happen, I'd put a 32-ohm resistor in series with the mono output, and shunt a 47 microfarad 6.3 volt ceramic capacitor to the return, and see if that cut down on the whining. But that's a biggish capacitor, which I don't have (they are available, and cost about buck or so, but I don't have any around), so I'd actually try two 100 uF electrolytics in series, with the polarities reversed relative to each other. But that just ought to work, and probably wouldn't.

This is a variant of one of the very first lessons that every electrical engineer learns when they actually start building circuits they've designed: Circuits that are supposed to oscillate, don't oscillate. Circuits that are not supposed to oscillate, do oscillate. Nobody knows why this is true, but it is.

All in all, if you can live with the whine, you have half a solution. If you can't live with the whine, maybe you should try a headset designed for a portable phone (again, optimized for voice), rather than one designed for an MP3 player.

As for the other half, I don't know what's going to happen with the microphone. Microphones come with all kinds of impedances, and the question there is: will the mic preamp of the radio be able to handle the output level of the mic in your headset? From a standing start, there is no way of knowing. Tests and measurements have to be made in the absence of a specification.

[Terry Carraway]

Interesting.

In my ham HF setup I use a ham audio headset, that does not match the impedance (or resistance for you), and I get a whine. When I was researching this when I was active, I found references that this was due to the impedance mismatch.

I started building a matching box for the headset, but never finished it to see if would work.

BTW, the same radio, audio output to its matching speaker did not have the whine.

[Randy Sohn]

........All in all, if you can live with the whine, ,,,,,

NOW you've dun gone and made me wonder, which is gonna be cheaper, a spouse or that?

best, randy

[Jeff Hartmann]

Mark wrote:

I'm not sure yet if the mic even works.

My question is, have you got a push to talk on the adapter? Are you thinking the call answer switch on the earbuds would work... ? (doubtful)

[Robert Dubner]

Terry, like I said, one of the fundamental observations of electronics is that things that aren't supposed to oscillate, do. So, given that a lot of output circuits use feedback to try to make the actual output match the expected output, I can't be surprised that having the wrong impedance -- that is to say, a load that the designers didn't expect -- on the output could lead to an oscillation.

At this point in time, I am wishing I could see a schematic for Mark's radio; it would answer a lot of questions. I tried looking on-line; I couldn't even find specs, much less a schematic.

Randy? I'm pretty sure that there has never been a radio as expensive as a spouse, with or without whining. Just sayin'.

[Randy Sohn]

pretty sure that there has never been a radio as expensive as a spouse, with or without whining. Just sayin'.

Chuckle chuckle and concur!

best, randy

[Russell Holton]

I think the whine might be one of two things. First, the ear buds might be sensitive enough that you're picking up normal background noise that was never noticed before. The giveaway here is if you have to really turn down the volume control knob.

Second, back in the 70's, I had a transistorized multi-band radio that would complain like that if it didn't have an adequate load on the output. Try tacking a 8 -10 ohm resistor across the output and see what happens. If it works, then you can try other values.

[Mark Horak]

I am going to forget about the mic side of this question for now. It might fun to play with later just to learn a thing or nine, but practically speaking, the SP-400's built-in mic is perfectly adequate. So the only real issue here is the whine that can be heard through the stereo ear buds with mic. Here are some additional factoids.

1. The back-lit display on the SP-400 operates normally at what I will call half-bright. After turning on the radio or touching any button, the display goes to full-bright for 5 seconds. The whine only happens when the display is at full-bright.

2. The adapter I found for the SP-400's 2.5 mm audio port is 2-pole mono on the male side, and 3-pole 3.5 mm stereo on the female side.

3. The stereo ear buds with mic are 4 pole. I have a splitter which has a 4 pole 3.5 mm female plug that is split out to two 3-pole 3.5 mm male plugs, one for the mic and one for the audio. So the ear buds go into the female plug, and then the audio plug of the splitter goes into the aforementioned adapter.

4. The whine persists across 3 different sets of stereo ear buds with mic, 2 from Apple and 1 from Samsung.

5. I found a pair of mic-less stereo ear buds with a 3-pole 3.5 mm plug. I plugged those directly into the adapter, sans splitter, and there was no whine.

After reading Bob's message, I suppose the whine could be related to the higher frequency response of the stereo ear buds with mic. Or maybe there is some problem with the splitter. The latter should be easy to verify by picking up another splitter.

[Russell Holton]

I highly doubt if the problem is the splitter. I don't see how it would affect things. I still think that the problem is either the earbuds are too sensitive and make a background sound that's always been there more noticeable, or they are too high of an impedance. Either way, the addition of some resistors to the circuit should take care of things.

Since you're not using the microphone, you may not need the second splitter (4 phone to twin 3 pole adapter). The first 3 bands are identical between headphones with mics and headphones without mics. Depending on where that first splitter tries to connect to ground, it may work just fine.