Kurt, I salute you on asking your question instead of making an assumption. First, a minor clarification. Your Gem's have an efficiency of 87 dB for one watt input at one meter. They have a "nominal" impedance of 6 ohms, and a minimum impedance of 4 ohms. In other words, they are of pretty much average efficiency and a little more difficult load than average, but no sweat for an amp in the league of Levinson or Ayre or Pass. Amplifier power for stereo amplifiers is customarily per channel, unless otherwise specified. Thus, your "125 watt" Levinson amp is 125 watts per channel, and so on. The question of how much power is enough depends more on your listening tastes and room. For example, say you like orchestral music at the kind of volume you'd get halfway back in a concert hall, maybe 95 dB peaks. And, let's say you sit 12 feet from your speakers. We're going to ignore some factors to make this simple. Okay, one of your speakers will give you 87 dB at 3 feet (one meter, give or take). Two of them, each getting one watt input, will give you 93 dB at three feet. Back up to 12 feet, and the sound pressure level will have fallen off to 87 dB. This is fairly loud, but remember we want peaks of 95 dB. Now here comes the important part. When we increase the wattage input by a factor of ten, we increase the sound pressure level by ten decibels. So at 10 watts per channel input we get 97 dB at our listening chair, at 100 watts input we get 107 dB, and at 1000 watts input we get 117 dB. So you can see that a 15 watt tube amp would give us our 95 dB. I'd like a bit more reserve power for occasional forays into hard rock, but I'd rather have 15 real good watts than 1500 crummy ones. Of the amplifier brands you mention, the Pass Aleph series would be my first choice. Hope this helps.
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Jazzzman, South43, Thanks for your response. Jazzman, your explanation is wonderfully precise! I will cut/paste it into magnetic immortality for future reference. To be sure I understand, let me pose a follow-up. When you say that a ten-fold increase in wattage offers a 10 decibel rise in sound pressure, is this a doubling of sound pressure? That is, twice as loud? Just how loud is "loud"? I've heard the reference made to "good vs crummy" watts before and would like to make sure I understand the distinction between "good" watts and the amount of them. That is, the influence of quality AND quantity of watts. I suspect a "good" watt is "clean", consistent, distortion free watt. "Good" watts make for accurate, pleasing music, so a few "good" watts are preferable to a legion of "crummy" watts. Quality trumps quantity. But what if all the watts are the same? Is there some sonic virtue in having more "good" watts, that lies beyond the volume equation? I have heard people say so! Let me put this another way. If I understand your example correctly, If I NEVER want to hear music above 95 dB -- a 15 watt tube amp will produce that sound level. A 30 watt amp of EQUAL "goodness" will not produce "better" 95 dB music by virtue of its higher QUANTITY or "good" watts alone -- yes? I don't yet fully understand this, but I think it is a very important concept. I've heard "experts" say that more "good" watts are always better, even at low listening levels. They seem to claim that 95 dB produced by our 30 watt amp will sound, fuller, rounder, snappier, than the 95 dB out of the 15 watt equivalent. I don't understand this. Their argument seems to be that those 15 "good" watts can get tired running back and forth to the speaker. If you have 30 "good" watts, they will take turns carrying the load and produce a faster, fuller, richer, sound. Is there any merit in this argument? On our system, we listen to jazz and symphonic music, usually at conversational levels; with the family sitting on the floor about 8 to 10 ft back from the speakers. I really like the intimacy of this "near-field" listening position and very rarely play anything loud. (I've joined an amateur band again, so at home I like the music -- soft!) Thanks again for your very informative post. We did choose the Pass amplifier, but still think we may want to have some fun with tubes. I've read about and wanted to try some of the low power tube amps, but the lack of power has made me hesitant. Now, I may try to pick up something used to have fun with. Thanks again, Kurt
"Loud" is herby defined as 85 dB at the listening position. "Twice as Loud" is 95 dB. "D*@# Loud" is 105 dB. And "TOO [email protected]#& Loud" is 115 dB. Okay you nailed me on the "good watt/crummy watt" thing. I'll take a shot at it, but I can't back up anything I'm going to say with math. Some amplifiers make you tired of listening, make the music sound grainy or harsh, give you a headache, have no life. You want to turn the volume up so you can hear what's happening in the music. And other amplifiers (often with much higher distortion figures) sound liquid, musical, full of life and energy, like this is easy and they're having fun doing it. You enjoy the music immensely at all volume levels. It's hard to pin down just what the difference is. It's more of a feeling than a tonal difference. But one thing is for sure - we really don't have a consensus on which measurements are of sonic significance. So this is where trusting your ears (and feelings) comes in. Yes, amplifiers do sound different. The difference may be subtle, but it's profound. In my experience a really beefy power supply is very important. I personally lean toward output transformerless (OTL) and single-ended triode (SET) tube amps, but I've heard excellent amps of many different types. Amps that run Class A, or that use little or no negative feedback, often sound (or feel) more musical to me. Okay there's another, more empirical way to predict sound quality - how many pounds an amplifier weighs per watt of output. The heavier, the better. And sadly often the more dollars per watt the better. Yes all things being equal having twice the power will give you a greater sense of ease. The thing is, all these generalities are nice until you get down to figuring out what you can buy with $1500 (or whatever your budget is). Then the more experience you have (not the more cheap [ahem!] advice you listen to), the better choice you will make. I hope you can borrow and listen to a bunch of different amps in your system and come to very different conclusions from the ones I have, because my conclusions have cost me dearly.
First my compliments to Jazzzman for a very good explanation. Then to the second question: are 30 'good' watts better then 15 'good' watts? In my opinion: no, unless.... The presumption here is made that we want to listen to a soundlevel of 95 db on the listeners seat. In 99% of the time this is loud enough, but there are days (or nights) when you want to listen LOUD (well, at least I like to every once in a while...). Then it's a good thing to have the power on tap, and that doesn't mean an enormous stack of Krell mono-amps. But the most important thing, as Jazzzman points out, is to listen before you buy. Especially, if possible, in your own home with your own equipment. If one amp sounds ok, and doesn't distort if you turn up the volume, buy it (that is, if it fits yor budget as well). Then forget about watts, db's, distortion etc. and put in another cd....
All good comments. I'd just like to throw in one more consideration. Current limiting is a consideration for speakers that have impedence values, even over a limited region (frequency) of less then 4 ohms. Amps that can't put out power to low impedences can be very harsh even at moderate listening levels. If your amp is rated at 100 wpc at 8 ohms but 150 watts pc into 4 ohms, you are looking at current limited operation. The 4 ohm wpc should be double the 8 ohm ie 200 wpc, not 150. The closer you get in the published specs to a doubling of the power from 8 to 4 ohms the better off you are. My Thiel's run down to about 3.4 ohms and I had to buy a Krell that doubles again into 2 ohms to make those beasts behave.
Keis hit on another good point; however, his explanation negates the use of a good tube amp. Most tube amps will not double power as impedance is halved. Many only maintain power as impedance drops. In fact, some VERY good tube amps (mostly OTL's) will drop in power as the impedance drops, making them very sensitive to the speakers they drive. I think the most important aspect is "recovery", followed closely by "graceful clipping". Does the amp recover quickly from an instantaneous overload and does it clip (or distort) gracefully. Recovery: Let's not fool ourselves. Measuring a 95db peak at the listening position with a radioshack SPL meter does not even remotely represent the kind of load the amp is "seeing" at that instant. The peak is madeup of a variety of frequencys, each with a different impedance (on most real world speakers) and each with a different phase lag (worse on speakers with complex crossovers). Each factor requires the amp to provide more current than a simple calculation using impedance, voltage, and power would imply. How the power supply recovers from this instantaneous overdraw of current is a large factor in how the amp/speaker sounds before, during and after the peak. Even low level peaks on real music can put a big amp in "recovery". If this were all, however, Keis' Krells would be the way to go. Giant power supplies and the ability to provide rivers of current. Graceful clipping: This sounds like audiophile nonsense, but it really is not. Recovery and the harmonic content of the distortion are really closely tied, but not the same. For modern high end gear, how the output devices distort the waveform during these peaks determines the "sound" of the amp more than how much the output devices distort. Most solid state amps with FETs, MOSFETS, JFETS, Bipolar Transistors... tend to have a higher ratio of odd harmonic content of that distortion to even order harmonic content compared with that of a good tube amp. A low THD with more 5th, 7th, 9th order harmonics will usually sound harder and grainier than a higher THD comprised of a higher ratio of 2nd, 4th, 6th order harmonics. The Krells and Levinsons are excellent amps. I owned a 27.5 (also able to double power into halving of impedance down to 2 ohms) for 5 years and was very happy with it. However, I am getting far more realistic sound with my Avalons (in my system/room) with a pair of Manley Ref 250 mono tube amps than I ever got with my Levinson. They just behave better with the Avalon's more complex crossover and, IMHO I think distort more gracefully and recover faster. Power ratings and amp/speaker matching are NOT simple subjects. Excellent question Kvolk.