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How to choose a tube friend speaker? Two thumb rules: impedance and efficiency!
1) Because of tube amp uses output transformer to match the speaker's impedance with the output tube's load to max the output power. Some of speakers designed with impedance changing as frequency changes. Those speakers are NOT tube friendly. Say for 8-Ohm speaker, if it varies from 3 Ohm to 20 Ohm in the entire audio frequency range then a tube amp cannot output uniform power cross entire audio frequency spectrum. Especially for those speakers of which the impedance swings considerably in the low frequency, you will feel definitely the amp losses the bass unless you use a separate self-powered subwoofer. Make sure pick up a speaker with very static impedance curve that varies within 30% in the entire audio frequency range.
2) Today most high-end speakers are highly efficient that can produce very detailed and accurate sound with relatively low power. In fact, a lot of audiophiles try to use as low power (amp) as possible and as high efficiency (speaker) as possible to preserve neutrality and dynamics of the music reproduced. The industry standard measurement of efficiency on a speaker refers to measure the sound level at 1 meter away from the speaker with 1 watt input power for 1 kHz pure tune. A good tube friendly speaker with very flat impedance response shall give a uniform efficiency distribution cross whole audio spectrum. In average, if one sits in the middle of live concert, his comfortable listen levels shall be within a dynamic range of 65 - 96 dB (of course, this is depending on who and what type of concert). From a mean sound level of this dynamics, the table below suggests the output power required for given efficiency of speaker assuming the listen room size is 18'x25' and the distance between the listener and the speaker is about 15':
|
Efficiency of speaker |
Output power required by amp (watt per channel - wpc) |
|
87 dB |
> 10 wpc |
|
90 dB |
> 5 wpc |
|
93 dB |
> 4 wpc |
|
96 dB |
> 2 wpc |
|
99 dB |
> 0.5 wpc |
Following example shows you how to select a speaker for your amp. If your listen-room size is about 20 X 15 ft you usually do not want your sound level goes beyond 96 dB. Otherwise, it may be too loud for you. If you have a pair of speakers with 88dB efficiency (at 1 Watt/m), say you sit 3 ft away from the speakers (1 meter) so you need an amp to gain at least 8 dB in order to reach 96 dB level. The amp powered the speakers shall have at least 6.5 Watts (because of 10log(6.5/1)=8 dB). For speaker with 92 dB, you need 2.5 watts power to get 96 dB sound level. I listened 92 dB speakers with VP-300B using about 3 - 4 watts most of time at my living room since I sit about 6 ft away from the speakers. They sound great! However, if you have a pair of speakers with 96 dB (at 1 W/m) you may need about 1 -2 watts most time, depending on how far away from the speakers.
Why are the low power/high efficient speakers better than the high power/low efficient speakers? The loudness of sound to human ears is not linearly proportional to the power of signal input to the speaker. It is proportional to the sound pressure level (SPL) that is a logarithmic measure of wattage in dB unit: SPL = 10 log (W). The dynamics of SPL is expressed as the slope of the curve (SPL verses W). Now, if you do the derivative (slope of the curve) of SPL respect to W, d(SPL)/d(W), you can see the lower W the higher value of derivative (slope). This is why the lower power and higher efficiency speaker has better dynamics. In two channel recording, the music source recorded are in two “points” scheme (left and right channel), that are supposed to be reproduced through your ears (two points) from the speakers. Since the recording through microphones uses very low power, the music recorded on the medium shall have the low power nature to preserve the original dynamics. The "best amplifier" is no amplification (zero wattage) at all (just pure silver wires from RCAs to the speaker binding posts -- 100% transparent). But, even for most efficient speaker, one still needs at least few of wattage to reach the SPL. So "passive power amplification" is not practical.
Why not high power amp, say 300 wpc? As stated above, the higher power on a lower inefficient speaker will have less dynamics. Indeed, you do not need high power for most home audio. You do not listen to music at 110 dB level at home (well, unless you have a big party and try to shack down your house). Also, a higher power amp has more complicate circuit design and more nonlinear electronic components, which may produce more “Grain” and worse imaging/soundstage**. Can I use a high power amp for a purpose of low power usage? Not a good idea. A high power amp is design to optimize an operation regime at high power output. For example, the linearity of 1 watt output from a 300-watt amp is much worse than that of 1 watt output from a low-power tube(300B, 6V6GT, 6L6, etc) amp.
Given following two choices, which one shall I choose if my usual listen level is less than 98 dB and assuming price and other facts are the same?
a) a 20 wpc amp with 92 dB speakers, or
b) a 200 wpc amp with 85 dB speakers.
The answer is a) of course. What if I want to listen to a loud bass, say around 100 dB bass? You shall then get a good 300-watt self-powered sub to take care of bass below 40 Hz (most sub-woofer or woofer drivers are < 80 dB under 50 Hz at 1 watt and 1 meter). Thus, you get both best worlds.
**The audiophile press has varied colloquialisms to describe sound coloration. “Grain” is a common term referring to a the interpretation of low level non-harmonic noise added to the signal.
“Imaging” refers to the clarity of the perceived stereo sound field. Commonly the better the “imaging” the easier the listener can resolve the spatial locations of individual instruments.
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