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Requirements for being tube friendly- not mentioned so far in this thread

Posted by Ralph (M) on January 15, 2015 at 09:31:07

In Reply to: tube friendly speakers posted by rws on January 14, 2015 at 11:32:48:

1) the designer of the speaker uses a tube amp. That's simple enough.

2) You don't need a flat impedance curve. Quite often an impedance peak in the bass is helpful- what is often at issue is distortion, and all amps (including transistors and class D amps) make less distortion when driving higher impedances. Music tends to have most of its energy concentrated in the bass. So if you make less distortion making bass the amp will be smoother and more transparent.

3) A low phase angle in the crossover is helpful. Simple crossovers often work better, but that is not a hard and fast rule.

4) If audio quality is your goal, avoid low impedances like 4 ohms (a dip to 4 ohms at a crossover point is perfectly fine). All amps, tube, solid state and class D make less distortion driving low impedances as mentioned earlier. A simple way for any speaker manufacturer to make their speaker seem smoother and more detailed is to simply design it for a higher impedance.

5) Putting woofers in series to obtain a higher impedance works a whole heck of a lot better than putting the same two woofers in parallel. Tube amps don't double power as you cut impedance in half- and output transformers (if there is one) will cut off as much as an octave higher on four ohms as opposed to eight or sixteen ohms! So dual woofers that pose a 4 ohm load are a bad move from a design perspective. Contrary to popular belief the woofers will act exactly the same in the box with the same efficiency if put in parallel. What is different is the sensitivity- which is unimportant to a tube amp. Efficiency is.

People often confuse efficiency and sensitivity. They are only the same if the impedance of the speaker is 8 ohms. Sensitivity is a spec that became popular with the advent of solid state- it makes less sense if you are talking about tubes. By reducing the impedance while the efficiency is kept the same, the sensitivity is increased. Here is the math:

Sensitivity is 2.83Volts at 1 meter. If the speaker is 8 ohms, that's 1 watt. If the speaker is 4 ohms, that's 2 watts, a 3db difference. But in reality the speaker's efficiency is unchanged because to get 2.83 volts into 4 ohms the amp is making 2 watts, not one (that 3 db difference). Its power that makes a speaker play at a certain volume; this fact has to be understood if one is going to have success with tubes.

• http://www.atma-sphere.com/Resources/Paradigms_in_Amplifier_   (Open in New Window)

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