4 ohm speakers seem like they would raise your power bill - AVS Forum
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post #1 of 26 Old 06-13-2014, 09:19 AM - Thread Starter
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4 ohm speakers seem like they would raise your power bill

Say it's summer.

You hook up 4 ohm speakers rather than 8 ohm speakers. Now, it seems a typical amplifier will waste more power via the output transistors dissipating more power as heat.

It seems like unless you can drop the supply voltage, you have to supply the output transistors with the same voltage with 4 ohm speakers or 8 ohm speaker. I could be mistaken, but this means that even though the 4 ohm speakers might be able to produce the same SPL at half the voltage, the heat dissipation seems likely to go up.

You need half the voltage and twice the current at the speaker ( simplifying things a lot,) which would be the same power into 8 ohm or 4 ohms but the transistor is a different story.

Who cares in cold winters. You get a musical heater. But in the summer, it seems like you are paying twice - once for the wasted watts and again for the AC power to remove the waste watts.

So unless I am missing something, you save some money by using 8 ohm speakers.

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post #2 of 26 Old 06-13-2014, 10:20 AM
 
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The voltage halves and the current doubles - thus the power draw from the amp is the same.

Your amp's power draw doesn't care about impedance, it's just P=VI.

The case is different if the amplifier isn't designed for 4 ohm loads though.

And if your speakers can take the extra power, the amp will be able to output twice as much power to a 4 ohm speaker than an 8 ohm - which of course increases heat, but it's also increasing the power.

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post #3 of 26 Old 06-13-2014, 10:30 AM - Thread Starter
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Quote:
Originally Posted by JWagstaff View Post
The voltage halves and the current doubles - thus the power draw from the amp is the same.

Your amp's power draw doesn't care about impedance, it's just P=VI.
I agree that the speaker would draw the same power all things being equal. I am talking about the power dissipation at the transistor.

I am not 100% of my calcs, but using an example at ESP, for 4 vs 8 ohm speakers, I get 45 watts power dissipation vs 10 watts power dissipation. That's with a sine wave, and music is not a sine wave, so I am sure real world behavior is different.

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post #4 of 26 Old 06-13-2014, 10:47 AM
 
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The transistor power draw basically ends up being P=Vout*Iout, so the power shouldn't be different for different loads.

P=VBEIB+VCEIC

Where VCE is the voltage across the transistor (collector and emitter). VBE and IB are << than VCE*IC so you can assume it's 0.

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post #5 of 26 Old 06-13-2014, 11:53 AM
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Quote:
Originally Posted by MichaelJHuman View Post
Say it's summer.

You hook up 4 ohm speakers rather than 8 ohm speakers. Now, it seems a typical amplifier will waste more power via the output transistors dissipating more power as heat.

It seems like unless you can drop the supply voltage, you have to supply the output transistors with the same voltage with 4 ohm speakers or 8 ohm speaker. I could be mistaken, but this means that even though the 4 ohm speakers might be able to produce the same SPL at half the voltage, the heat dissipation seems likely to go up.

You need half the voltage and twice the current at the speaker ( simplifying things a lot,) which would be the same power into 8 ohm or 4 ohms but the transistor is a different story.

Who cares in cold winters. You get a musical heater. But in the summer, it seems like you are paying twice - once for the wasted watts and again for the AC power to remove the waste watts.

So unless I am missing something, you save some money by using 8 ohm speakers.
Hi,

IMHO, if you'd really like to save power why not use more efficient speakers. The higher the sensitivity the less amount of power is needed to reach the same sound pressure level. Am I also missing something? You save some money by using higher efficiency speakers, even if they are rated @ 4 Ohms.
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post #6 of 26 Old 06-13-2014, 12:45 PM
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Thats why you will see more adoption of SMPS power supplies and Class D amplifier solutions, going forward..
The global movement to being more green has momentum where Europe has more stringent power consumption standards and these are starting to be more visible in North America due to the subject global product platforms. Started about 5 years when Europe instituted standards for power consumption when a product was in Stand-By mode, now it has matured into broader standards...

Just my $0.05...
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post #7 of 26 Old 06-13-2014, 12:55 PM - Thread Starter
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Quote:
Originally Posted by mogorf View Post
Hi,

IMHO, if you'd really like to save power why not use more efficient speakers. The higher the sensitivity the less amount of power is needed to reach the same sound pressure level. Am I also missing something? You save some money by using higher efficiency speakers, even if they are rated @ 4 Ohms.
Yes, if my main goal was to save on power, I would do what you say. More efficient speakers and more efficient amps.

My main thought though, was unless there is some inherent advantage in 4 ohm speakers I don't know about they seem like a worse choice than 8 speakers. They seem like they would use more power unless an amp was designed for 4 ohm speakers, and lowered the power supply voltage with the understanding they need less voltage.
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post #8 of 26 Old 06-13-2014, 01:10 PM
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Except for considering amp compatibility, impedance would be pretty much at the bottom of my list of things to check out when buying speakers... There are many design trades and lower impedance could be an advantage for some parameters. I would say say 4-ohm speakers are 'worse" than 8-ohm speakers or vice-versa based solely on impedance.

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post #9 of 26 Old 06-13-2014, 01:18 PM
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Quote:
Originally Posted by M Code View Post
Thats why you will see more adoption of SMPS power supplies and Class D amplifier solutions, going forward..
The global movement to being more green has momentum where Europe has more stringent power consumption standards and these are starting to be more visible in North America due to the subject global product platforms. Started about 5 years when Europe instituted standards for power consumption when a product was in Stand-By mode, now it has matured into broader standards...

Just my $0.05...
Even though I live in Europe I don't really care about so-called stand-by mode. What the heck can an un-plugged cell phone charger save compared to the fridge in the same household that consumes hundreds of watts on 24/7 basis. Let's see how Bruxelles can regulate fridges workin' in stand-by mode.

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post #10 of 26 Old 06-13-2014, 01:38 PM
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Originally Posted by MichaelJHuman View Post
Yes, if my main goal was to save on power, I would do what you say. More efficient speakers and more efficient amps.

My main thought though, was unless there is some inherent advantage in 4 ohm speakers I don't know about they seem like a worse choice than 8 speakers. They seem like they would use more power unless an amp was designed for 4 ohm speakers, and lowered the power supply voltage with the understanding they need less voltage.
Your point here is also my point. I purposely bought 8 Ohm (Dali) speakers that are placed all around me (7.1 with 2 height speakers). Denon amp is always lukewarm. Tell it a friend and call it a day.
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post #11 of 26 Old 06-13-2014, 01:48 PM
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Your point here is also my point. I purposely bought 8 Ohm (Dali) speakers that are placed all around me (7.1 with 2 height speakers). Denon amp is always lukewarm. Tell it a friend and call it a day.


My Denon doesn't seem to get hot with my Martin Logan 4ohm speakers. Anyway, my video projector produces far more heat than the audio system !

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post #12 of 26 Old 06-13-2014, 01:57 PM
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My Denon doesn't seem to get hot with my Martin Logan 4ohm speakers. Anyway, my video projector produces far more heat than the audio system !
I think with that projector contributing to global warming you would be arrested in Europe. Just kidding!
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post #13 of 26 Old 06-13-2014, 02:47 PM
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There does seem to be a strange fear of 4 ohm speakers in the audiophile community. I think people seem to forget that amplifier impedance ratings are made at full power, something we never use (or shouldn't use.)
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post #14 of 26 Old 06-13-2014, 05:55 PM - Thread Starter
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Originally Posted by FMW View Post
There does seem to be a strange fear of 4 ohm speakers in the audiophile community. I think people seem to forget that amplifier impedance ratings are made at full power, something we never use (or shouldn't use.)
I suspect it would be rare for speakers or receiver/amp to be damaged just because the speakers were 4 ohm. I mean, if you choose to abuse your system at 8 ohm or 4 ohm, that's your risk.

My point though, is why not just get 8 ohm speakers, unless there's some compelling reason to get 4 ohm speakers, as there seem to be some disadvantages.

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post #15 of 26 Old 06-13-2014, 06:20 PM
 
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Originally Posted by MichaelJHuman View Post
I suspect it would be rare for speakers or receiver/amp to be damaged just because the speakers were 4 ohm. I mean, if you choose to abuse your system at 8 ohm or 4 ohm, that's your risk.

My point though, is why not just get 8 ohm speakers, unless there's some compelling reason to get 4 ohm speakers, as there seem to be some disadvantages.
Driving 4 ohm speakers loud on a typical AVR is not good for it at all and will shut it down quite easily. But any decent external amplifier will easily handle 4 ohm loads, many even 2 ohm. There is no disadvantage going to 4 ohm, it's an advantage usually as the amp puts out double the power to 4 ohm so it's actually cheaper to drive them. AVR amplification just isn't designed for 4 ohms though.
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post #16 of 26 Old 06-13-2014, 07:07 PM - Thread Starter
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Yes, certainly if an amp is designed for it, 4 ohm speakers do have the advantage that the amp can push almost twice the power into them (vs 8 ohms.) I am sure that's a selling point. I would never need that much power, but I can understand that would be appreciated by many people

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post #17 of 26 Old 06-14-2014, 01:11 PM
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Do your own lab experiment. Using a Kill-a Watt or similar device that let's you check current draw on a 120 volt device, make an A/B comparison of 4 ohm / 8 ohm speakers with a mic showing identical SPL levels.

But now to the point that's a bigger factor than impedance. That would be speaker efficiency. That will have a bigger impact on current draw from the amp at a GIVEN VOLUME than impedance.

As to the statement that AVRs aren't designed for 4 ohms is wrong. My Onkyo TX-NR929 is rated for 4 ohms as are other AVRs. My 90 lb. Outlaw amp will work easier powering 4 ohm speakers but my 929 will do it all day and NO it doesn't run hot.

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post #18 of 26 Old 06-14-2014, 01:13 PM
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Quote:
Originally Posted by Patrick Collins View Post
Do your own lab experiment. Using a Kill-a Watt or similar device that let's you check current draw on a 120 volt device, make an A/B comparison of 4 ohm / 8 ohm speakers with a mic showing identical SPL levels.

But now to the point that's a bigger factor than impedance. That would be speaker efficiency. That will have a bigger impact on current draw from the amp at a given volume than impedance.

As to the statement that AVRs aren't designed for 4 ohms is wrong. My Onkyo TX-NR929 is rated for 4 ohms as are other AVRs. My 90 lb. Outlaw amp will work easier powering 4 ohm speakers but my 929 will do it all day and NO it doesn't run hot.
What speakers do you have? (Brand/model).
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post #19 of 26 Old 06-14-2014, 03:25 PM
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I realize this is a bit of an academic question as opposed to real world consideration. On what basis do most people select speakers? How good they sound. What does impedance, especially a nominal impedance rating, have to do with sound? Next to nothing.

Not trying to be critical of OP's question but it strikes me as being like asking whether a car is more expensive to operate if you have to change the oil every 12k miles instead of every 15k. It has no effect on the car's performance and as a % of cost of ownership or of all of the owner's expenses, it's negligible.

Like someone else said, you need to measure with a kill a watt meter to see what the actual practical difference in power draw is, as the 4/8 ohm ratings are nothing more than a generalization. Let's say it does draw twice the current from the wall outlet with Speaker A compared to Speaker B. Your amp may typically draw an average of 100 watts at moderate volumes with moderately efficient speakers and amplifier. So if you average 10 hours a week listening a week listening, the increase in cost is roughly 520 hours/year*100 watts*$0.10/kW-Hr = $5.20/year. You're pole vaulting over mouse droppings.
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post #20 of 26 Old 06-14-2014, 03:26 PM
 
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Quote:
Originally Posted by Patrick Collins View Post
Do your own lab experiment. Using a Kill-a Watt or similar device that let's you check current draw on a 120 volt device, make an A/B comparison of 4 ohm / 8 ohm speakers with a mic showing identical SPL levels.

But now to the point that's a bigger factor than impedance. That would be speaker efficiency. That will have a bigger impact on current draw from the amp at a GIVEN VOLUME than impedance.

As to the statement that AVRs aren't designed for 4 ohms is wrong. My Onkyo TX-NR929 is rated for 4 ohms as are other AVRs. My 90 lb. Outlaw amp will work easier powering 4 ohm speakers but my 929 will do it all day and NO it doesn't run hot.
I don't think anyone said that all AVR's aren't designed for 4 ohms. Most of the high end AVR's will do 4 ohms fine, most low end ones won't.
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post #21 of 26 Old 06-14-2014, 09:31 PM - Thread Starter
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You are right, Patrick. The two speakers would have to have identical sensitivites, but as they may not measure sensitivity the same way, and room acoustics may have an effect, that might be a test which is hard to do right.

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post #22 of 26 Old 06-15-2014, 08:58 AM
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A few points:
Actually JWagstaff, it was you that said "AVR amplification just isn't designed for 4 ohms though".

Mogorf, not sure if it's pertinent about my speakers as I've been collecting them since the sixties. My comments were in general about speaker ohm ratings and efficiency. But in my HT my LCR are GoldenEar Triton 7s and Super Center X, NHT Super Ones and Sunfire True Sub for 9.1.

MJH, today's speaker efficiency ratings conform to industry standards so should be comparable.
You should conduct speaker comparisons at home so that room acoustics don't play a part.

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post #23 of 26 Old 06-15-2014, 09:05 AM
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I noticed several comments above about comparisons without including the qualifier, at the same SPL level. That's what makes the comparison meaningful.

Patrick
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post #24 of 26 Old 06-15-2014, 09:49 AM
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Keep in mind that while you may have some beefy 100w/ch amps, the long term average power, even at high volume, is about 2w to 4w.

A 4 ohms speaker relative to an 8 ohm speaker does get the same voltage, but for a given voltage it draw twice as much current. So, referring to above, the average power is 4w to 8w, instead of 2w to 4w. Though 4w to 8 ohm would be substantially loud.

Power = Vots²/Resistance

So, if you apply 12 volts to 4 ohms and 12v to 8 ohms you have this -

P = V²/r = 12²/8 = 144/8 = 18 watts

P = V²/R = 12²/4 = 144/4 = 36 watts

If you have a Volt Meter that has a needle rather than a read out, you can put it on your speaker terminals and get an idea of the average voltage, then using the formula above, you can calculate the average power. It is not going to be has high as you think it is. If fact, I suspect it will be closer to 1w to 2w.

Remember speaker Sensitivity ratings are with 1w (or 2.83v). That will drive a speaker to 85dB to 95dB depending on the speaker. That is pretty loud. So, despite needing 100w/ch for music peaks, your average power is very low, far lower than you might think.

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post #25 of 26 Old 06-15-2014, 09:55 AM
 
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Patrick, before I said it wasn't designed for 4 ohm I said "Driving 4 ohm speakers loud on a typical AVR is not good for it at all and will shut it down quite easily." I know there are quite a few AVR's designed for 4 ohm, but I'm talking about the typical AVR that the average person has.
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post #26 of 26 Old 06-15-2014, 11:19 AM - Thread Starter
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Quote:
Originally Posted by Patrick Collins View Post
A few points:
Actually JWagstaff, it was you that said "AVR amplification just isn't designed for 4 ohms though".

Mogorf, not sure if it's pertinent about my speakers as I've been collecting them since the sixties. My comments were in general about speaker ohm ratings and efficiency. But in my HT my LCR are GoldenEar Triton 7s and Super Center X, NHT Super Ones and Sunfire True Sub for 9.1.

MJH, today's speaker efficiency ratings conform to industry standards so should be comparable.
You should conduct speaker comparisons at home so that room acoustics don't play a part.
I always assumed you needed an anechoic chamber for such things to be accurate

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