# Thread: Building a 3.5mm Battery Box (power source)

1. ## Building a 3.5mm Battery Box (power source)

Hi there,

I'm very, very new to this but i found an article on this site regarding microphones and how they work which has given me a little bit of an understanding.

What I'm trying to do is this - I've purchased a Sanken COS-11D which requires between 3-10 volts. I want to feed it as much as I can. It is terminated with a 3.5mm plug.

I've read online and found a number of diagrams and some products that provide this "plug in power" at about 9v for this exact purpose.

However, there seems to be a nagging variable, which is that it seems suggested that these devices alter the quality of the sound, and some have roll off filters etc.

I am not familiar with all of this. All I want is something that can power the microphone and then forward the sound from it to my recording device.

So basically, how does this work over a 3.5mm TRS wire and how would I build this without affecting the quality of the sound at all? (ie, make the power box as transparent as possible)

Thanks.

2. If you look at the diagram of any of these circuits, you will see more or less the following:

1) A source of DC power. The only requirements are that it be the right voltage range and clean (no hum or garbage on the DC.)
2) A resistor. This is required because you want to get the DC to the microphone, but if you connect them directly together, the power supply will short out the audio signal. So the resistor feeds the DC to the microphone through a high enough resistance to permit the voltage to vary as it must.
3) A capacitor. This is to pass the AC (audio) signal on to the input of the recording device, but block the DC. At high audio frequencies, a capacitor is a short circuit, like a piece of wire. At DC, it is an open circuit.

The issue here is that, at progressively lower frequencies, the capacitor becomes progressively less like a wire and more like an open circuit. If it isn't large enough, it will begin to block frequencies you want. Let's say the input impedance of the recording device is 5k ohms; for response flat to 30Hz, you need a capacitor that is 1 microfarad or greater.

It is necessary to know the input impedance of the recorder, in a general way. If you have a 150-ohm professional balanced microphone input, this isn't going to work well. A condenser mic connected to a 1/8in. plug is designed to work into a load of 5k ohms or greater. This will always be the case with a pink computer microphone input, but recording devices....well, all bets are off. Maybe you can give us the model number of the recording device? The Sanken mic will work into any load of 1k or higher, maybe less.

What are you recording? It's only necessary to provide the full 9V with very loud sounds.

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