input in guitar combo

[teemuk]

Originally Posted by wakibaki

The connection between the guitar and the amplifier is not a 'transmission line' in the radio sense, it's too broadband and it's 'electrically short' but the most efficient transfer of power still takes place when the sending and receiving resistances are the same.

In audio applications you actually care more about efficient transfer of voltage than power.

There's little sense of transferring power of few milliwatts efficiently but much more sense in transferring as much of the signal voltage as possible.

Regaining power is cheap for an amp, sacrificed SNR is something you can't really fix afterwards.

Hence Impedance bridging (Impedance bridging - Wikipedia, the free encyclopedia) where you - by rough rule of thumb - try to couple the source to an input impedance that's at least 10x higher than the source/output impedance.

Despite the common terminology (which is erroneous) you don't really "impedance match" anything in the preamp. That's just an old saying that still confuses people. When people use this term they most often really refer to impedance bridging instead.


Where did you get the idea that a guitar's output impedance is 250k?
The source impedance of a typical pickup is in the few kilo-ohms range (ca. 5K - 20K). Even less if it's an active pickup. The volume and tone controls are shunts in parallel to pickup's coil and the only thing raising the Zout to 250K range is actually rolling the volume way down, which increases the series resistance. The 250K is more like a figure for a volume control set to zero, at which point you'd practically have no output at all because the signal is attenuated by the voltage divider to begin with.

[Enzo]

I don't know the amp in question offhand, but another reason to roll off the bottom end is so they can use a cheaper speaker that wouldn;t be happy with fuller range signal.

[padamiecki]

Originally Posted by Enzo

I don't know the amp in question offhand, but another reason to roll off the bottom end is so they can use a cheaper speaker that wouldn;t be happy with fuller range signal.

Enzo
I would say that too much low end gives too fat sound for the guitar and causes then mud in overdrive section.