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RE: Tube amp output impedance and speaker impediance

All tube amps have a low damping factor primarily because they become unstable with lots of negative feedback, the main source of high damping factor in amps. And running a lower impedance output on a tube amp lowers the damping factor even more(DF = output-impedance of amp/load impedance).

The highest DF I ever heard of in a tube amp was about 25 in an English Radford amp. The Melos amps had a DF of about 20. But a DF around 10 or a bit more is most common in tube amps.

So all tube amps(and some solid state amps) react with speaker impedance to cause less flat frequency response. Look at the Stereophile graphs of the simulated speaker load frequency response.

It makes me think of the days before solid state amps when all amps were tubed with lowish DF causing more deviation from flat. Of course this doesn't show up in a conventional frequency graph for tube amps since those graphs are made with a constant, simple resistive load.

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  Kimber Kable  

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