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I heard that an active DI box doesn't amplify the signal at all in anyway so what is the benefit over a passive unit and is it always preferable?

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2 Answers

Well, a passive DI uses a transformer, and there will be some signal loss.

An active DI, which may not actually amplify the signal, shouldn't cause any signal loss, and theoretically should have better frequency response. And because you have a pre-amp, there's better isolation from the source.

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The DI box presents a large input impedance, so that the large output impedance of the guitar isn't loaded down, and then it sends the signal into the mixer from a small output impedance, so that it isn't loaded down by the input impedance of the mixer. If you connected the 100 kohm output impedance of the guitar directly to the 1 kohm input impedance of the mic pre, it would act as a voltage divider and drop the signal level by 1/100. This would also have bad effects on frequency response, interference pickup, etc.

http://en.wikipedia.org/wiki/DI_unit

http://en.wikipedia.org/wiki/Buffer_amplifier#Voltage_buffer

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