Amplifier Damping – Another Wet Blanket.

By John Strohbeen • Ohm Speakers • Tuesday, September 1, 2015


How does amplifier damping factor (DF) impact the sound of loudspeakers and, thus, the sound of music in your room? Tube amps usually have DFs of about 20 or less while many solid state amplifiers have DFs of 2000 or more. Quite a difference! You would expect to hear a major change.

What is damping factor? For consumers, it is usually defined at the ability of the amplifier to control (stop) the speaker’s cone with no input to the amplifier. A higher number is better as it does a better job of preventing boominess at the system resonances. For an electrical engineer, it is defined as the ratio of the load impedance divided by the amplifier’s output impedance. Both of these impedances vary with frequency, so the DF specified is often stated as the minimum in the audible range. An example: a 6 ohm load with an amplifier having a 0.3 ohm output impedance will have a DF or 6/0.3 = 20. Another example: a 6 ohm load with an amplifier having 0.003 ohm output impedance will have a DF of 2000. This is like the difference in DF between tube and solid state amps.

Why are they so different? Solid state amplifiers usually use feedback coming from their own output to correct any distortion (or difference) from the input. This effectively reduces the output impedance to the resistance of the leads inside the amplifier. Tubes control much higher voltages and much lower current than transistors. Tube amplifiers use output transformers to match the tube’s voltage to the speaker’s impedance. The DC resistance of the output windings of the transformer is the minimum output impedance of the amplifier. It takes lots of copper and iron (with lots of cost) to keep this resistance as low as possible. The classic Marantz 8B tube amplifier is still considered one of the best sounding amplifiers ever built has a DF of 20 and the McIntosh 275 has a DF of “greater than 22.” These amps cost a great deal in their day and are worth even more today. This increase in value runs counter to the usual trend in audio (see our post on collecting gear http://ohmspeaker.com/news/is-collecting-audio-gear-a-good-investment/). PS Audio’s new BHK Signature 300 Monoblock has a DF of 100 and Jeff Rowland’s Model 925 Mono has a DF of >1000.

How does this difference in DF affect the sound? Not much! Even less when you consider the speaker wire. When the amplifier manufacturers are writing their specifications, the speaker wire is part of the amplifier load. The speaker wire is effectively part of the amplifier’s output when calculating the DF that you hear. In practice with just ten feet of AWG 18 speaker wire, the DF drops to 14 in the first example and 50 (yes, 2000 drops to 50) in the second example. As you can see, the speaker wire has a dominant influence in the amplifier-wire-speaker setup. For more information this, see our post devoted to speaker wire (http://ohmspeaker.com/news/does-speaker-wire-affect-the-sound/) Changing the DF from 5000 to 50 will not make much difference in the sound.

How much is not much? Reducing the DF will change the balance slightly. With a DF of 13, the deepest bass and highest treble will be increased slightly (less than 1 dB) compared to a DF of 1000. With a DF of 5000 moving to 50, the change is so slight, it is certainly inaudible. So, unless you have very thin wire or very long wires or are using a resistive loading speaker switchbox to ‘balance’ speakers, you can forget about DF.

The wet blanket. Moving your speaker a foot will probably make a bigger sonic difference than the difference in DF factor between any two amplifiers. Big, easily audible improvements are available if you change speakers or room layout. For the best sounding system, put your effort where you can hear it.

Enjoy & Good Listening!

John

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