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Old 8th December 2010, 12:34 AM   #31
Sheldon is offline Sheldon  United States
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Join Date: Dec 2003
Location: San Diego
Originally Posted by artosalo View Post
I see clipping has something to do with NFB. Especially with difficult load such as a real loudspeaker is. Imagine a non-NFB pentode output stage with relatively high output impedance, say 100 ohms, is connected to the speaker having the impedance curve like this:
Click the image to open in full size.

At the bass resonant frequency the impedance of the speaker is some 70 ohms while the nominal impedance is 10 ohms.

Now at bass resonant frequency the output voltage of the amplifier will rise some 12 dB compared to 1 kHz due to the high load impedance of the speaker. This big voltage rise can obviously cause clipping even at relatively small output levels.

However, if this amplifier was be equipped with such amount of NFB that the output impedance will drop to 10 ohms, the voltage rise will be only some 4,5 dB and the amplifier is not that "sensitive" to clipping.
The horrible sound you might hear when that speaker hit the resonant frequency is not the amp clipping, it's the driver beating itself to death.

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