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I'm a helicopter pilot. I'm thinking of buying a headset Bose A20. My problem is that the Bose headset comes with a high impedance microphone, but the helicopter I'm operating is using a low impedance 5 ohm system. The most common systems use high impedance.

My question is...

Is there any chance to build an adaptor to convert the signal of the headset to a low impedance microphone signal?

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  • \$\begingroup\$ The first step is to find out what radio the helicopter uses, then query Bose to find out whether their headset is known to work well with that radio, and/or whether they can recommend an adapter and/or a different headset. (For communications headsets, I personally am more partial to David Clark.) \$\endgroup\$ – John R. Strohm Apr 2 '13 at 19:57
  • \$\begingroup\$ Tony's comments will be deleted soon enough, for good reason, but he did make one valid point. Basically you shouldn't be screwing with avionics. If you are a commercial pilot, don't go anywhere near a home brew solution. Talk to Bose and see what they suggest. Don't try rigging something yourself. If something bad happens, whether its the fault of your jig or not, you could get into serious trouble. \$\endgroup\$ – Olin Lathrop Apr 2 '13 at 20:31
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To answer your question directly, yes. That device, most generally, is called an amplifier.

The effect, in your situation, of just connecting the microphone/radio anyway (without an amplifier) will be that your microphone will appear to be much less sensitive than you expect.

However, you can't hurt anything, so you should just try it. You might be surprised with how well it works (e.g. good enough). There are a few things working in your favor:

  • Good radio transceivers (especially in aerospace) use an auto-gain/leveling function on their mic inputs, which may compensate the lower input level.

  • Professional (expensive) microphones use larger elements and are typically much more sensitive than "standard" elements that the inputs are designed to handle.

  • The output impedance may be specified based on maximum compatibility rather than actual hardware impedance.

...so, yeah, you should just try it first.

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  • \$\begingroup\$ It's a bit of a shame to buy an expensive microphone and then don't use it at its best. Then why not buy a cheaper one and build a simple amplifier? :) \$\endgroup\$ – user17592 Apr 2 '13 at 6:32
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    \$\begingroup\$ @Camil -- It's a bit like saying it's a shame to buy an SUV (off-road vehicle) and drive it on a paved road. It's a shame to wear a bicycle helmet and not crash into things... There are many great things about a pro-headset (comfort, isolation, clarity, etc) and it may have the range to handle a wide-variety of inputs. There is such a thing as "good enough" -- as in not noticeably underperforming. Many hobbyist "audiophiles" make this mistake and get suckered into buying things that improve stuff in theory, but not observably in practice. \$\endgroup\$ – DrFriedParts Apr 2 '13 at 6:39
  • \$\begingroup\$ Yeah, well, that's a valid point you make, haha. You're right. \$\endgroup\$ – user17592 Apr 2 '13 at 6:58
  • \$\begingroup\$ what is surprising me it is just the Bose doesn't have this kind of amplifier, so there is no officially a original tool. What is the best suggestion to try get this goal? The headset is using a U174 connector. \$\endgroup\$ – Davide Apr 2 '13 at 8:35
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The impedance match may be achievable passively and with decent efficiency by using an impedance-matching transformer.

First of all, I suspect that the 5 ohm impedance refers to the source impedance of the microphones, not the input impedance of the mic preamps in your helicopter's instrumentation panel. If I were to guess, I would say that this impedance is probably in the hundreds of ohms, or even a thousand something.

Let's just go with 1000 as an example.

You should also find out the impedance that the high impedance microphone expects from its preamplifier. Let's say for the sake of the example that the mic works well if it faces a preamp with a 100,000 ohms impedance.

So, in this example, what we need is a transformer which makes 1000 ohms look like 100,000. It will be a step-down transformer, and the turns ratio is the square root of the impedance ratio. 100,000 / 1000 = 100, and the root of that is 10. So 10:1 turns ratio would do the job.

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  • \$\begingroup\$ how can I find out the impedance of the preamps with the headset? is it a big circuit or is it possible to fit it along the cable in a small extension? have you got an idea of some type or name that I may look for on the net? \$\endgroup\$ – Davide Apr 2 '13 at 8:57

protected by W5VO Apr 2 '13 at 20:25

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