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Re: Damping Factors

Posted by Ralph (M) on September 19, 2000 at 16:23:23

In Reply to: Re: Damping Factors posted by garland on September 18, 2000 at 13:46:26:

There has been controversy about the output impedance of the M-60 in the past. This is due to a common mistake wherein the measured output voltage is taken to represent the output of the amp.

What is important about measuring the output impedance of an amplifier is that power has to be considered and any voltage measurements should be viewed as a means to calculating the power present, using P = R x (I squared).

On to the M-60: the output impedance is roughly 3.5 ohms, depending on tube condition: maximum power is obtained at about 14 to 16 ohms, decreasing at impedances more or less then that value (although above that impedance power decreases *very* slowly). The amp is producing 1/2 that power into about 3.5 ohms, thus that is its output impedance. I don't know why the voltage has been so often used to arrive at a higher (incorrect) value, but it is one of the more common mistakes that even engineers will make.

As to the matching issue: For best match, a speaker (in particular a woofer) should ideally be an impedance at or above the impedance of maximum power transfer for the given tube amp. In the case of the M-60, anything above 14 ohms or so is ideal. Very practical results can obviously be had with 8 ohm speakers. As with any amp, when the load impedance approaches the value of the internal impedance, distortion increases and power of course decreases. This is why it is important to make a careful decision about the speaker to be used *WITH ANY TUBE AMPLIFIER*. In most cases, all other things being equal, a tube amp will always perform better on an eight ohm speaker then a four ohm speaker (an deven better yet on 16 ohms) *regardless of the tube amplifier technology*.

Sorry for such a long-winded answer.

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