阻尼係數 - 喇叭音響

阻尼因素的高低跟聽感關聯性沒有這麼絕對
真空管機沒有高的 但是管機低頻控制就都很差嗎 相信市場上反應並非這樣
以下轉貼 Sterophile 訪問Tim de Paravicini 的片段（Jun 5, 1984)
談他的EAR 509
這機器的低頻控制力速度感都十分厲害 但DF只有20
請看看他如何解釋

Stereophile: We're looking at a damping factor of only 20, right?
De Paravicini: Yes, well, it may only appear to have a damping factor of 20
but that needs to be clarified. People have latched on to the idea that
damping factor is the only important criterion for tight bass.

Now a conventional dynamic speaker is essentially an electric motor in series
with an 8 ohm resistor inside that motor—if you measure the voice-coil
terminal it measures 8 ohms DC [(footnote 1)]. Now if you put a screwdriver
across those two terminals, you have put an infinite damping factor on that
loudspeaker, right?

Stereophile: Right.

De Paravicini: But that electric motor has still got 8 ohms in series with it
to damp its motion. It hasn't really got a short circuit. Now, if instead of
the screwdriver I put an 8 ohm resistor across those terminals, that electric
motor now sees 16 ohms in a series, so in fact it's got half the damping that
it had before, which is expressed as a DF of 2. If we lower that resistor
from 8 ohms to 1, for example, we now have a damping factor of 8. That
electric motor is now seeing 9 ohms, which is only 10% larger than 8 ohms.
We're closing the gap towards that perfect 8 ohm resistor, so if we go
downwards from below 1 ohm to ½ and then ¼ ohm, we're getting ever closer
to that 8 ohm minimum. It just doesn't buy you anything, because you can't
get below 8. So if you increase the amplifier's damping factor beyond 10, the
actual damping on the speaker remains for all intents and purposes unchanged.
Whether you have a damping factor of 1000 or 100,000, the speaker really
doesn't know the difference.

Stereophile: Then why is it that amplifiers with very high damping factor
usually give the tightest bass?

De Paravicini: The reason amplifiers sound apparently tighter or looser is
not their damping factor but the amount and the quality of their feedback and
their low-frequency stability under dynamic conditions. The bandwidth, the
frequency response, the way it clips, the way it recovers from clipping are
all problems that are really of great concern. They are all signatures of the
amp's sound characteristics.

The ideal amplifier should recover from an overload infinitely fast and not
have any subsonic misbehavior. What I am trying to do is to come as close to
that as possible in a transformer-output tube amplifier. That is why my
amplifiers may sound as if the bass is thin—simply because there isn't this
overhang.

Most tube amplifiers cannot deliver current. They have a rated power into one
rated impedance and the minute the impedance deviates significantly on either
side of that, their power output goes to pot. I designed the 509 so it would
still maintain its rated power down to 3 ohms. It has a peak current capacity
—something I don't publish in the specs but that should be mentioned—of 12
amps. That is one of the reasons for the exceptional tightness of the bass.
It can keep the speaker under control, and that's why a cheap 10W receiver
will often bottom its woofers. The amplifier loses control totally, and you
end up with a damping factor of less than 1, momentarily. From a
superficially high figure it disappears, then the speaker lets go and it goes
back, crash!

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