Tgi: Passive mode simply turns the GFP-750 into a "switchbox" for what source ( CD, Tuner, Cassette, etc...) you want to listen to. There is no signal processing in terms of potential for gain ( added volume to what already exists ) or tone controls, etc.... You can alter the balance of the signal and you can attenuate ( turn down ) the level of signal that the original source device is feeding to the amp. Since most line level sources ( tuner, cd, dvd, cassette, etc... ) put out enough voltage to directly drive an amp, some people go this route as they think that adding additional circuitry only "clouds" the signal.
Turning the GFP 750 into active mode, additional circuitry is added to the signal path while retaining all of the original switching capacity that the unit previously allowed. The added circuitry adds gain ( increases volume ) and helps stabilize the impedances of the system. This allows one to run longer runs of cabling and / or potentially reduce issues with impedance matching.
There are benefits to using both methods and units such as the GFP 750 and a handful of others allow you to select which one you want to use at the flip of a switch. Sean
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PS... Line level typically refers to any type of component that is not phono related. Phono systems put out very little signal and require an added gain ( amplification ) stage. Once the very weak signal is amplified, that signal is then fed into the line stage like any other "line level" device. As such, you can't run a phono system in passive mode because there just isn't enough signal to directly drive the amplifier.