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RE: Why does the same speaker sound so different with two different amps

Posted by tomservo (M) on December 4, 2014 at 07:13:29

In Reply to: Why does the same speaker sound so different with two different amps posted by airtime on December 4, 2014 at 06:40:50:

At the point the industry adopted the Thiele & Small design approach to low frequency drivers, the assumption was that the woofer was driven by a Voltage source signal, in amplifier terms, that would be one where the driving impedance was VERY VERY small compared to the load impedance.

If you have a woofer not connected to an amplifier and tap the cone, you hear it “boing” or resonate. If you short the woofer terminals and then tap the cone, you hear it is COMPLETELY different and “dead” sounding. This is the effect of “electromagnetic damping”, the closer your amplifier resembles a short across the terminals with no signal, the more damping effect it has.

You have probably heard the term “damping factor”, this is roughly the ratio of the load impedance to the source impedance and in real life a Damping factor of 20-30 or more and one has reached the point of diminishing returns and has the assumed “Voltage source” that the T&S loudspeaker parameters assume.

Tube amps often had a significantly higher output impedance and so exert less damping. This means the bass will be lifted compared to the Voltage driven alignment. Maybe a better way to describe the effect of a higher source impedance is to say that if you look at the speakers impedance curve, anywhere (frequency) that impedance is low, the speaker will have a reduced sound output while anywhere the impedance is high, the speaker has more output all relative to the response driven by a Voltage source.
Hope that helps
Tom Danley

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