GA to 8ohm Headphone Adapter Completed

[Gary G]

Having posted to this group before, I though I would just provide
the follow up.

I completed a transformer design - thanks for people who
posted in the group with information.

Instead of paying $75 - $150 for an adapter, I constructed
what is the equivalent of 500 ohm source impedance to
4 ohm (actually, 2 8-ohm earbuds in parallel) adapter.
I've tested the unit and am quite happy with it.

I used a stereo 1/4" male adapter cable into a 3-piece
PVC tube assembly (2 end caps and a 3" piece - stock off
the shelf from Home Depot).
I used a Radio Shack $2.99 audio transformer and using
my old engineering days book, insured that I had the right
impedance transformation ratio.

Not including my labor (hee hee), I spent a grand total
of approximately $7 in parts.

In general, most aviation radios expect too see 200-ohm to 600-ohm
loads. Higher loads are typically fine, but lower isn't a good idea.
If one uses 16-ohm headphones or 32-ohm headphones (not uncommon
at all), the load impedance seen by the radio is higher.
This prevent excessive current loading, but higher will affect
"optimum" power transfer. It is safe, nonetheless.

The reason I did this: As an engineer and musician for a music company,
I have some very high-end custom molded, in-ear headphones that
I found are very comfortable and work very well in the plane.

Now - I have to obtain a boom mic to use . . .that hasn't been
as easy as it sounds for my "low budget" approach . . .

I thought some of you might be interested to hear how I finished that up.

Good flying!

Gary

[Charlie]

Gary G wrote:
> Having posted to this group before, I though I would just provide
> the follow up.
>
> I completed a transformer design - thanks for people who
> posted in the group with information.
>
> Instead of paying $75 - $150 for an adapter, I constructed
> what is the equivalent of 500 ohm source impedance to
> 4 ohm (actually, 2 8-ohm earbuds in parallel) adapter.
> I've tested the unit and am quite happy with it.
>
> I used a stereo 1/4" male adapter cable into a 3-piece
> PVC tube assembly (2 end caps and a 3" piece - stock off
> the shelf from Home Depot).
> I used a Radio Shack $2.99 audio transformer and using
> my old engineering days book, insured that I had the right
> impedance transformation ratio.
>
> Not including my labor (hee hee), I spent a grand total
> of approximately $7 in parts.
>
> In general, most aviation radios expect too see 200-ohm to 600-ohm
> loads. Higher loads are typically fine, but lower isn't a good idea.
> If one uses 16-ohm headphones or 32-ohm headphones (not uncommon
> at all), the load impedance seen by the radio is higher.
> This prevent excessive current loading, but higher will affect
> "optimum" power transfer. It is safe, nonetheless.
>
> The reason I did this: As an engineer and musician for a music company,
> I have some very high-end custom molded, in-ear headphones that
> I found are very comfortable and work very well in the plane.
>
> Now - I have to obtain a boom mic to use . . .that hasn't been
> as easy as it sounds for my "low budget" approach . . .
>
> I thought some of you might be interested to hear how I finished that up.
>
> Good flying!
>
> Gary
Is your intercom/radio capable of enough signal level (voltage, not
current) to compensate for the ~100-1 stepdown in voltage?

I took a shot at your idea several years ago but the local musician
supply couldn't even quote me a price for the in-ear phones.

If the custom earbuds meet your 'low budget', why not just get the mic
designed to work with the phones?

Charlie

[GeorgeB]

On Fri, 18 Nov 2005 20:45:27 -0600, Charlie >
wrote:

>> Instead of paying $75 - $150 for an adapter, I constructed
>> what is the equivalent of 500 ohm source impedance to
>> 4 ohm (actually, 2 8-ohm earbuds in parallel) adapter.

>Is your intercom/radio capable of enough signal level (voltage, not
>current) to compensate for the ~100-1 stepdown in voltage?

he has ~100:1 impedence ratio ... that will be about a 10:1 turns
ratio ... impedence in an n^2 relationship.

Question is still valid, though. The POWER, less efficiency, will be
the same, so 1mW at (say 400 ohm) will be 1mW at 4 ohm. So if the
sensitivity of the transducer elements is the same, same SPL.

[Gary G]

The turns ratio is about what you mentioned.
The issue is impedance and total power delivery.
You get an offset of voltage in one direction, and
current in the other. The radio itself won't really
need to drive any more current because it sees the impedance
it wants to see.

The answer is yes - it works fine.

The "low cost" approach to the flight is a good question.
The in-ear headphones are priecy - they ran me about $150
and are quite good.

BUT! That's part of my profession in the music industry.
Throwing another amount out wasn't really in the cards.

I am going to purchase a mic. I actually have one, but
can't get the schematics - and since it is powered, I have
to be careful.
I bought an old StarSet series, and can't for the life
of my se the schematics - it ha a dual plug for the headphone.
I'm going to remove that and just use the mic.
The headphone is more for quite-cabin use.

My headphones are very comfortable and have maximum isolation
for "non active noise reduction" type headphones.

Thanks for the follow up!

Gary