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Happy possessor of a powerbook and having wanted to have fun me with skype, I bought myself a microphone helmet of the market for about ten euros. Now when I connected the microphone on the jack input of the computer, it didn't work since this is an line-in input with higher levels than the levels of an microphone input.
A solution would have been to buy a USB helmet or buy an USB audio interface as the imic, but seen the surrounding prices about sixty euros for the one and an about thirty for the other I abstained and I keep my helmet microphone. I decide therefore to realize myself an interface for some euros (ten, maybe less).
The principal component of the mounting is a LM386 which will amplify tension input 200 times. It requires very few external components this that simplifies enormously the mounting.
Here the complete scheme of the mounting:
We will require the following components
- 1 LM386
- 1 resistor 10k
- 1 ajustable resistor 10kOhms
- 2 condensers 10µF (tension de 10V)
- 1 condenser 250µF (tension de 16V)
- 1 support for LM386 (optional)
- 2 tupip connectors
- 1 essay board
- 1 9V battery connector
- 1 jack 3,5mm male mono
- 1 jack 3,5mm female mono
It have next to connect the paws between they as on the scheme knowing that the paws of the LM386 are numbered as follows:
The Vs represents the + battery 9V (the red wire of the connector), the earth sign, the - (black wire of the connector). The two tulip connectors will be linked up between them if one uses a microphone helmet, one will disconnect them when one will want to preamplifier another signal that does not requires any supply.
Here a realization example, that is not the better arrangement but that works:
back view :
For the wiring of the jacks, the red wires of the photographs corresponds to the round full on the scheme
 
After essays, the mounting works perfectly. It allows therefore to use all the helmets of the commerce, including the less costly one to the opposed purchase of pricey interfaces or USB helmets.
If you want more precisions, ask your questions on the forum.
The mounting can easily be adapted to a small box, as did Shinbo (http://shinbo.free.fr).
Here are two photograph of the mounting :
 
I can sell to you the board for 3euros (france) ou 6 euros (outside france). A notice manual will be provided with the board. To order the board, please send yours euros with your adress at :
Paul HEROIN
4 place Edouard BRANLY
APPT C209-1
57070 METZ
You just have then to buy the parts in a serious electronic shop such as gotronic.fr or selectronic.fr.
Here is a preview of this board :
Paul HEROIN.
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