Differential Amplifier

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Here's a good starting article about using op amps to make a differential amplifier:

http://www.electronics-tutorials.ws/opamp/opamp_5.html

This type of circuit is used by musicians in the form of an audio mixer. When used to compare voltages, hv's will have to be reduced by a pair of precision voltage divider resistors.  These are a couple of bucks each:

http://www.goldmine-elec-products.com/prodinfo.asp?number=G17753

I'll do some checking and see if I can find a specific circuit.  But it's also possible to buy a basic, upgradable differential amplifier for $15:

http://www.robotmesh.com/grove-differential-amplifier?gclid=Cj0KEQjwx_WuBRDJ7tSK2-W0pJkBEiQAEWgR8NXyiWgwd5jv8D7n-B_bDJNZ-9XjFhrLEYQrqeitnHkaAlh08P8HAQ

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I Will have a good look at it man thanks a lot! hope i can find this pieces... seems better to use as voltage dividers

normaly to buy things from abroad here it take too much time...