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Forums » General Discussion » Home Theater » 4 ohms, 6 ohms &, 8 ohms / M80/M60/VP150/QS8


#86561  03/22/05 06:58 PM 4 ohms, 6 ohms &, 8 ohms / M80/M60/VP150/QS8  
old hand Registered: 03/17/05 Posts: 88 Loc: Manitoba, Canada 
I have a question in regards to Receiver/amplifier output in comparison to varied ohm ratings of different rated speakers and differant speaker locations in a 7.1 or 5.1 system.
How does it work? Let's say you have a 7x 100 Watt receiver. How is the output ratings of the M60(8 ohms)compared to the M80 (4 ohms)output, with the VP150 and QS8's(6 ohms). What do you hear and /or is the receiver smart enough to recognize this and distribute the power as required. I am trying to figure out what size of amp is really required to get good use of each setup, with the ohm ratings as stated.
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M80 VP150 4QS8 EP600 Monitor Audio S8 NAD T 773 Anthem MCA3 II Hitachi 57" projection 
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#86562  03/22/05 09:27 PM Re: 4 ohms, 6 ohms &, 8 ohms / M80/M60/VP150/QS8  
axiomite Registered: 08/25/04 Posts: 6024 Loc: Bowmanville, Ontario, Canada 
Regrettably, the answer is "depends on the receiver".
For any given input signal and level setting, amplifiers try to drive the speaker at a constant voltage  constant in the sense that with two slightly different speakers the voltage would be the same with each speaker even if the speaker had a slightly different impedence. Note that because we're talking about AC signals and music the voltage isn't really "constant", just "predictable and repeatable". We're also talking about AC impedence not DC resistance... in other words I'm simplifying all over the place, but it doesn't affect the outcome. If the receiver is rated at 100W RMS per channel then it can put out enough voltage to drive 100 watts into 8 ohms. Since power = voltage times current and current = voltage over resistance (impedence) power = V squared over R. 100 = V squared over 8, V squared = 800, V = about 28 volts RMS (say 40 volts peak, 80 volts peaktopeak). In case you're interested, the current required here would be 28/8 or a bit under 4 amps RMS. Note that a good amplifier will actually be able to put out quite a bit more voltage AND quite a bit more current in order to properly handle the peaks and transients... but the distortion will probably be higher and the amplifier will overheat if you draw higher voltage or current continuously. Replace the 8 ohm speaker with a 4 ohm speaker, what happens ? Amplifiers put out a constant voltage, so instead of (28 squared / 8) or 100 you get (28 squared / 4) or 200 watts, ASSUMING THE RECEIVER CAN PUT OUT ENOUGH CURRENT TO MAINTAIN THAT CONSTANT VOLTAGE. Here's where the fun starts. Most receivers can't put out enough current to drive the same voltage into 4 ohms as they did into 8 ohms... and the ones that can will overheat if you try to run at 200W output instead of 100W. Part of the problem is that the energy lost in a wire (or an amplifier) is a function of current not voltage  this is why long distance power lines step the voltage up by a factor of 1000 or more, reducing the current and lost energy by the same factor. In other words, 100 W into 8 ohms is 28 volts and 4 amps; 100 W into 4 ohms is roughly 20 volts and 5 amps, so the amplifier will waste about 25% more heat in the output stages of the power amp and run hotter as a result. Receivers that have enough current and power reserves will typically put out 50% more power into a 4 ohm load than their 8 ohm rating, but will run a bit hotter as a result. Cheaper receivers will overheat and crap out, sometimes at pretty low power levels. It's not that 4 ohms is "bad", it just happens that 8 ohms is the most common standard for low cost home audio. Note that most bigass power amps are designed to drive happily into a 4 ohm load all day, and are usually factory rated for something like 50% more power into 4 ohm than 8 ohm. Denon and HK receivers also seem to have no problems driving into 4 ohms, do NOT have factory ratings for 4 ohms, but anecdotal evidence suggests they can drive 3050% more power into a 4 ohm load.
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HT: M60ti/VP180/QS8, SVS PC+ 2039, EP500 Music: M5HP, Sierra1, M40ti Idle: M2ti, M3v4, VP100 John 
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#86563  03/22/05 09:31 PM Re: 4 ohms, 6 ohms &, 8 ohms / M80/M60/VP150/QS8  
axiomite Registered: 08/25/04 Posts: 6024 Loc: Bowmanville, Ontario, Canada 
One last point  seems like pretty much every receiver and amp can drive 6 ohms without any problem. You will get a bit more power than the factory rating; potentially you could get up to 30% more power but I would be thinking about 10% or so more at the most.
I believe M80s are 4 ohm so that they can make the most of big honkin' power amps with heat sinks the size of a small dog. For a normal receiver I wouldn't plan to take any more than the rated power into 8 ohms, and even then the receiver will be running a bit hotter than normal.
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HT: M60ti/VP180/QS8, SVS PC+ 2039, EP500 Music: M5HP, Sierra1, M40ti Idle: M2ti, M3v4, VP100 John 
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#86564  03/22/05 10:46 PM Re: 4 ohms, 6 ohms &, 8 ohms / M80/M60/VP150/QS8  
shareholder in the making Registered: 05/11/02 Posts: 10621 
Peg, a speaker, regardless of its impedance, draws the amount of power from the amplifier that it needs to put out a certain sound level. So, the receiver doesn't have to be "smart", it's the speaker which decides how much power it needs. Assuming that the receiver won't shut down if some of the speakers have a 4 ohm impedance(generally it'll be okay)the fact that the speakers differ in impedance is of no significance.
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 Enjoy the music, not the equipment. 
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#86565  03/22/05 11:27 PM Re: 4 ohms, 6 ohms &, 8 ohms / M80/M60/VP150/QS8  
old hand Registered: 03/17/05 Posts: 88 Loc: Manitoba, Canada 
So, simply it comes down to having a strong enough amp to run a 4 or 6 ohm speaker. More the 4ohm M80ti.
The sound level of the speakers take care of themselves and balance according to receiver sound selected(5.1, dolby, 2 channel, etc) On this note a 100 watt @ 8 ohm Denon or Onkyo may stumble if powering the M80ti at louder volumes. What would the minimum power be suggested for the M80ti as mains with VP150 and 2pr SQ8's. Sub to be determined.
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M80 VP150 4QS8 EP600 Monitor Audio S8 NAD T 773 Anthem MCA3 II Hitachi 57" projection 
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#86566  03/23/05 12:01 AM Re: 4 ohms, 6 ohms &, 8 ohms / M80/M60/VP150/QS8  
axiomite Registered: 08/25/04 Posts: 6024 Loc: Bowmanville, Ontario, Canada 
Here we get into the usual "how big is your room ?" and "how loud do you need ?" questions. Axiom guidelines are roughly :
 under 2400 ft3 is fine for bookshelves  over 2400 ft3 should be towers  over 4000 ft3 use M80s (HK or Denon receiver should be OK at normal volumes)  over 6000 ft3 use M80s with a big power amp Running with M80s in a smaller room doesn't seem to be a problem as long as you don't end up sticking the M80s too close to the wall. EDIT  also note that power ratings on receivers tend to only have a vague relationship to what the receiver can do. HK receivers seem to be rated with just over half the power of comparably priced models from other brands, but the actual measured power is very close. I think that's why most people here talk in terms of "you need a 1905" or "go with a 2805 buddy" Edited by bridgman (03/23/05 12:08 AM)
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HT: M60ti/VP180/QS8, SVS PC+ 2039, EP500 Music: M5HP, Sierra1, M40ti Idle: M2ti, M3v4, VP100 John 
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#86567  03/23/05 05:29 PM Re: 4 ohms, 6 ohms &, 8 ohms / M80/M60/VP150/QS8  
old hand Registered: 03/17/05 Posts: 88 Loc: Manitoba, Canada 
I am currently looking to set up for 4000 cu/ft room. The M60 seems to fill most forum member requirements. The M80's might be more then I need. I just want to cover all options before purchasing.
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M80 VP150 4QS8 EP600 Monitor Audio S8 NAD T 773 Anthem MCA3 II Hitachi 57" projection 
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#86568  03/24/05 01:16 PM Re: 4 ohms, 6 ohms &, 8 ohms / M80/M60/VP150/QS8  
devotee Registered: 02/22/04 Posts: 346 Loc: Wisconsin, USA 
You have the room size, and if you have a receiver/amp that can drive them...I'd get the M80s if I were you.
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