How to adapt lower impedance speakers to a higher impedance?

[Andreas S]

Dear Forum,

i would like your input on a little project i am working on. I am a civillian pilot and sadly with getting older starting to develop some hearing problems. To save what i have left i want to use custom in ear monitors under my normal headset to further dampen ouside noise while still having clear sound from the aircrafts radio.

The challenge is that an airplane intercom in mono configuration expects to see a load of the headset speakers of 2x 300 ohms in parallel (=150 Ohms Impedance?). According to a Datasheet everything from 150-600 ohms is fine. But every headset that i own is rated at the 300 nominal per speaker.
Available in ear moniors (either for military aviation use or civilian music use) have an impedance between 20 to 50 ohms per ear.

How could i adapt such a lower impedance speaker for my use? As i understand i cant measure impedance like simple resistance. My usual clueless tinkering around to find a solution does help me in this as frying an expensive aircraft intercom system by overloading it would be somewhat less than optimal.

Is it a simple as a resistive divider?
Or would i need a transformer like this: https://www.mouser.de/ProductDetail/Tamura/MET-44?qs=sGAEpiMZZMv0IfuNuy2LUS0duD88xK2J5pwq1%252BF%252B6lo%3D
Problem here again, there are no transformers with the exact secondary impedance that i would need. Whithout a way to measure i have no idea what the load impedance would look like when looking from the audio source side. Example: Using a transformer rated a close numer of eg 300/12 ohms primary/secondary and the actual speaker is say 20 ohms.

I would be thankful for any input from the audio magicians in the foum ;-)

Best Regards
Andreas

[Gyro]

Welcome to the forum Andreas.

You may need to experiment a bit. A resistive divider - actually a simple series resistor might work. Even a 150R series resistor would prevent you from going below the lower impedance limit under any circumstances. It would reduce the damping factor and change the frequency response of your ear monitor but that probably would not matter too much for speech.

Resistive dividers are always lossy, so you might not get sufficient power to drive your monitor to high enough volume. A transformer is the most energy efficient way of adapting impedance. A 300/12R transformer wouldn't be a problem with 20R load, it would just reflect as a higher (than 300R) apparent impedance on the primary. Note that the transformer you linked is only rated for 40mW, you should check the power rating of your monitor, you may need a physically larger one to transfer sufficient power without distortion. A larger transformer will also have better low frequency response, the one you linked has a flat range of 300Hz to 100kHz, you probably want lower than 300Hz even for speech.

[magic]

As i understand i cant measure impedance like simple resistance.

Most of the time they are similar and usually impedance is higher than resistance, although some IEMs are actually exceptions to that rule.
This may be useful if you go the consumer headphone route: https://www.innerfidelity.com/headphone-measurements
Frequency response deviation with a series resistor is minimized by using headphones with reasonably flat impedance vs frequency.

Watch out for "multiple balanced armature" IEMs.They contain multiple drivers connected to the input through a crossover network, which means crazy impedance variation with frequency and possibly less impedance at high frequencies than the DC resistance you could measure. For example, Logitech UE900 at 6kHz.

[bson]

Get an inline volume control for the IEMs.  This is nothing more than a stereo pot to adjust the load resistance.

[richard.cs]

As you're going to in-ear you probably want significant attenuation anyway to avoid damaging your hearing, I would definitely start with a series resistor of 150 Ohms and see how it sounds. You'll present a load of between 150 and 200ish Ohms. If it's too quiet then a transformer would be best, if you can't find one with the impedance you want pick one with the next highest impedance but the right ratio.

[Andreas S]

Thank you Gyro. It is good to know that i can replace the impedance load by a largely resistive load. And a series resistor is delightfully simple and easy to place into a connector housing.

I was able to talk a maintenance guy at the airport in letting me borrow a hedset tester which is just a fancy amplifier for the headsets own microphone as well as a 300 hz tone generator.
Using some cheap 18 ohm (measured dc resistance) smartphone earbuds i tested a simple series resistor and the 330 ohms i had on hand turned out to be a good value for the most quiet setting. Playing some music from my phone to the microphone of the aviation headset (using the 18 ohms earbuds to listen) i noticed no difference in the frequency response. This is looking promising  .

magic: Thanks for making me aware of the multiple balanced armature topic. Checking some available options the single armature ones seem to be the cheapest which fits me just fine . Right now i am leaning to a custom fit silicone IEM which would cost me about 250€. They are rated to provide 26dB of noise attenuation which is about as much as some fully closed silicone earplugs that i got. Sound quality of the armarture really should not be an issue as aircraft raidos always sound like you live in a tin can.

bson: Absolutely, now knoing that a series resistor will do just fine i will use a regular resistor to set a fixed value followed by a maybe 100 ohm pot to adjust the volume per ear. As the manufacturers of the IEMs never put the values of their volume controls into the datasheet id rather make it myself.

richard.cs: Looks like being too quiet will not be the problem so for now i will pass on using transformers. You raise a valid point with the significant attenuation:

Checking around on the internet it seems old school ham radio guys used to built something like a limiter for the audio signal in their headphones by using 2 diodes in reverse configuration to short the speaker if the voltage is to high. This seems easy enough to put in my adapter box too. Has anyone of you done something like that before? I dont quite understand how to adjust the point at which the diodes would cut the signal off (doubling the diodes for twice the forward voltage seems to be a much too coarse adjustment).

What kind of resitors should i use? Metal film, Carbon film? Or is that irrelevant?

Thank you all so far for your great help.

To get everybody on the same page i attatched a generalized schematic.

[Gyro]

That's really good news - decent transformers get expensive very quickly.

With regard to the resistors, it doesn't really matter but as metal film are becoming pretty much universal these days, you might as well use them.

Diode clipping would provide some protection against excessive sound level but the audible distortion would be horrible when they clip. I think that's one to experiment with as it will depend on the actual sensitivity of your earpieces. Just try some 1N4148 or equivalent - get four (ok, get a cheap pack) so you can try two pairs of two series diodes if the sound level is too low at clipping with just one pair. Whether you actually need them is debatable though.