Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Posting-Version: version B 2.10.1 6/24/83; site fluke.UUCP Path: utzoo!linus!security!genrad!decvax!microsoft!fluke!rzdz From: rzdz@fluke.UUCP (Rick Chinn) Newsgroups: net.audio Subject: how much amplifier power... Message-ID: <75@tpvax.fluke.UUCP> Date: Mon, 14-Nov-83 16:20:05 EST Article-I.D.: tpvax.75 Posted: Mon Nov 14 16:20:05 1983 Date-Received: Tue, 15-Nov-83 22:15:54 EST Organization: John Fluke Mfg. Co., Everett, Wash Lines: 104 re: power amp advice wanted rre: tom@rlgvax Tom Beres, in his reply of nov 10, makes a fundamental error in his thinking. His statement: On the other side of the coin, I have never had the volume control over 1/2 way, either, so there is still plenty of power with which to shake the walls should I ever get the mind to. This statement contains the classical gain/power error made by many people. The position of the volume control relates to the voltage gain of the amplifier/preamplifier system, and has nothing at all to do with how much power is either available or left over. Since the volume control is merely a voltage divider, it only affects the overall voltage gain of the amp/preamp system. It doesn't matter if you have 10 watts or 100,000,000 watts, the position of the volume control is only relative to how much is being shoved into it, and what the gain of the system following it is. example: amplifier needs 1 volt for full output. volume control is set at -20 dB from full rotation. The input signal to the volume control is 10V. Since 10V is 20 dB higher than 1V, the amplifier puts out full power, even though the volume control is set way below maximum. Furthermore, there isn't any more power available, even though the volume control is miles from wide open, since the power amplifier is already at the threshold of clipping. Now, I will grant you this: There is one and only one signal level where the position of the volume control will actually relate to the %power level. That signal level is the voltage level that causes the onset of clipping to occur with the volume control set to maximum. If this is the condition (which is usually *NOT* the case), then Tom's comment is true and accurate. Now onward... What actually constitutes sufficient volume is certainly in the ears of the beholder, and most certainly in musical taste and personal taste. For *my* ears, 70 to 80 dB SPL = background level 80 to 90 dB SPL = loud 90 to 100 dB SPL = very loud > 105 dB = really loud I typically listen between 70 to 90 dB SPL. When I'm alone, or if I really need/want to experience something, then I might resort to much higher levels (100 dB +). My own system has sufficient headroom that when the volume control is wide open, the amps are just beginning to clip (true for phono source). A scientific way of deciding how much power you need is to find the speaker sensitivity spec (dB SPL @ distance), then determine how loud you really want to run (average SPL), then compute the power required. It's probably a good idea to add 10 dB to the average SPL to allow for peaks before computing power. Although your listening room's furnishings and construction enter into the calculations, a good conservative approximation is to assume no reflections, therefore the attenuation of sound in your listening room follows a square law relationship. example: a speaker is spec'd at 85 dB @ 1 watt, 1 meter. How much amp power is required to produce 95 dB average SPL @ 3 meters. solution: first add 10 dB to the average to get peak (probably a good idea to add 20 dB if you have a CD player). Now subtract to find how many dB over the reference the new SPL is: 105 - 85 = 20 dB. Now calculate how many dB loss the new distance represents: 20 log (3/1) = 9.54 dB loss. Therefore the level @ the reference distance needs to be 9.54 dB higher. So...20 + 9.54 = 29.54 dB above the reference. Now find 29.54 dB above 1 watt.... 10 ** (29.54/10) = 899 watts. Since most of us listen in stereo, (2 speakers, 2 amp channels) you can halve this number to get the per channel power requirements. I will admit that I picked speakers that were pretty greedy. It does point out how far 900 watts will go once you have defined the distance, speaker sensitivity, and level requirements. From my own experience, bi-amplifying was one of the biggest aids towards improving the headroom and reliability (freedom from blown drivers) of my system. Getting a 200 watt/channel amplifier was the other. I suggest that you resort to science and physics and leave the empirical evaluations to the salesperson in the showroom. disclaimer: these facts/figures/opinions are my own and reflect my own tastes and prejudices. Please direct all flames to /dev/null. caution: When you operate a *big* power amplifier into a speaker with a passive crossover, be sure that the crossover components, particularly any inductors, will withstand the power levels (peak). Inspect capacitors within to be sure they will withstand the voltage levels involved. Inductors are very suspect, since saturation usually means a cessation of their inductive characteristics, leading to changes in the crossover characteristics and (usually) blown drivers. Rick Chinn @ fluke !fluke:rzdz 206 356 5232