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Discussion in 'Archived Threads 2001-2004' started by Bobby T, Jun 8, 2001.
How do you figure out how much amp power you need for different room sizes?
Are you asking how many amps you need for your dedicated electrical service for your equipment, Or are you referring to wattage?
Champagne tastes with a beer budget
Well, I think it all depends on what you are running in those rooms. Most rooms you can get by on 15 amp curcuits, if your just using them for lights, vaccuum cleaner, tv, etc. If you're using them for other things like stoves, fridges, washer/dryer you need a 20amp curcuit. If you can, run a separate line for your a/v equipment. I would do a 20 amp line if you have serious equipment. Otherwise 15 amp would do.
You determine amplifier power by knowing the enclosed volume of your room, HxWxD, the efficiency of your speakers, how far back you're sitting, and how much sound absorbing material you have on the walls. Let me know how big your room is, and how efficient your speaker are, and I'll get you in the ballpark. Amplifier power is kind of like horsepower and sex, its hard to have too much!
I'm sorry, I should have specified amplification power not electric. Thanks Tom, I'm helping 2 friends decide what to buy. That's why I was wondering if ther was a basic formula, like 3-5' times screen size to figure out T.V. size.
Figure out how loud you want the peaks to be. Example: if you want Dolby reference level, you need 105dB peaks at the seats from the 5 full range channels, 115dB from the LFE channel.
Subtract the speaker sensitivity. Example: my speakers are rated at 90dB given 1 Watt of power measured 1 Meter away. So, I need 15dB more than 1 Watt.
You loose 6dB of volume for every doubling of distance. So,
add 6dB * log2 distance in meters (log distance / log 2 will be the same thing for arbitrary log bases). I'm 8 feet from my main speakers, which is 2.4 meters away, so I add 7.6dB for 22.6dB.
Divide by 10. 2.26. Raise 10 to this power (the INV LOG combination on your calculator). So, I need 181 Watts to hit 105dB.
If I had less efficient speakers good for only 86dB @ 1W/1 meter, I'd need 457 Watts to reach the same output.
Note, however that amplifier power ratings are given for some distortion level at an average signal level; and that there's usually headroom for higher peaks. If you want to factor this into your equations, you can subtract headroom from the value you obtained after adjusting for your speakers. For example, if I have 2dB of headroom, I need 20.6dB beyond 1W, which is 10^2.06 or 114W with 90dB/1W/1M speakers (or 288W for 86dB/1W/1meter).
Room gain will make the required power less in practice, but is only really significant for sub-woofers (where you may have 10dB at 20Hz).
Using one subwoofer for both the LFE channel and the other speakers also complicates things a bit.
Also note that in practice, bigger class AB amplifiers often have a higher idle bias, which will mean that both positive and negative output devices will remain both on at higher volume levels and in their linear regions when the half of the waveform with opposite polarity is being reproduced (thus eliminating cross-over distortion at higher volumes). IOW, while you may not need the power, they're still desireable.
Thanks, all this info helps alot.
Drew... Now THAT'S what I call a response! All those formulas! You must be an Engineer or an audio installer!
Wonderful to have folks like you on the forum.