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previous: Kyle OT's and damping -- 4/19/1999 9:56 AM View Thread

Re: OT's and damping - long

4/19/1999 11:31 AM
Re: OT's and damping - long
A speaker has a resonant freq. at which its impedance is higher. At this freq. you need very little excitation to make the cone move. At this freq. the back EMF at the speaker terminals is maximum. If the amp has a low output impedance, it "looks" like a short circuit to the speaker. The back EMF will be shorted to ground acting like a "break" to the resonant oscillations of the cone. Try the following:  
1) gently tap the cone of a speaker with its terminals not connected to anything: it will give a "boom" sound that will die after some time.  
2) gently tap the cone of a speaker with its  
terminals shorted by a thick and short piece of wire. There will be less boom and it will die sooner.  
So a low output impedance amp will act like this short and make the bass tighter by damping the speaker natural boominess (resonance). You can't always improve the damping by reducing the amp output imp., as the speaker itself will impose a limit (it has internal electrical and mechanical impedances).  
If you are using feedback, you are subtracting from the input of the amp a fraction of the voltage at the speaker. This voltage is a combination of the output of the amp and of the back EMF of the speaker. If the back EMF goes up, the amp output will go down. If the back EMF of the speaker goes down, the output of the amp goes up. The amp's abillity to damp the speaker boominess is increased because a fraction of this "boominess" is subtracted from the input and so the output is "compensated" so that it acts in opposite direction to the EMF. The net effect is that the amp acts more like a constant voltage source - its effective output impedance is lower.

Kyle damping -- 4/19/1999 1:38 PM