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| How to adapt lower impedance speakers to a higher impedance? |
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| 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:
--- Quote from: Andreas S on April 01, 2019, 05:48:31 pm ---As i understand i cant measure impedance like simple resistance. --- End quote --- 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. |
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