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| Loudspeaker impedance curve - Zmax and tube amps [message #52308 is a reply to message #52307] |
Thu, 10 July 2008 17:40   |
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Wayne Parham
Messages: 18787
Registered: January 2001
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Illuminati (33rd Degree) |
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If the amplifier has high damping factor (low output impedance), the impedance curve won't matter much. But amps with higher output impedance will interact with the load impedance and may cause some amplitude response fluctuation. Single-ended tube amps with no feedback usually have a few ohms output impedance, so the speaker's impedance curve is important.
The reason for the fluctuation is the source impedance and the load impedance form a voltage divider, and if the source is nearly zero then the signal is developed almost entirely on the load, even when impedance fluctuates. But in the case where the source impedance is higher, less of the signal is applied across the load when load impedance dips, and since the dips are usually closer to average impedance than the peaks, this makes the peaks seem more pronounced.
Add to that, the source impedance, cables and any crossover coil resistance are all in series with the woofer's voice coil, making it act like it has higher Re and Qes. This pushes the speaker towards an underdamped condition, which tends to increase bass output near resonance.
Say the amp has 0.1Ω output impedance and the speaker fluctuates between 8Ω and 50Ω. At 8Ω, the speaker receives 99.88% of the amplifier's open-circuit voltage and at 50Ω it receives 99.98% of the signal. That's only 0.01dB difference in the signals presented to the speaker at 8Ω and 50Ω, almost nil.
Now let's look at the same speaker, one with 8Ω average impedance and 50Ω peaks, on an amplifier with 4Ω output impedance. This is pretty common for SET amplifiers. At 8Ω, the speaker receives 83.33% of the amplifier's open-circuit voltage and at 50Ω, it gets 92.59%. This is a 0.92dB difference. That's still not bad - less than 1dB - but you can clearly see the increase of fluctuation in the transfer function.
A speaker with 50Ω Zmax is really not too bad. An amplifier with 4Ω output impedance isn't eaither, really. Pretty common for a SET amp. But once you go much past that, the fluctuations start becoming audible. A speaker with 125Ω Zmax, for example, has 1.3dB fluctuation with the same amp. The increased series impedance shifts the speaker circuit Q too, so it may sound muddy as a result.
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