8 Ohm vs 4 Ohm Speakers - AVS Forum
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post #1 of 35 Old 11-24-2013, 04:11 AM - Thread Starter
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Can someone enlighten me what are the benefits /advantages for each figures?

What I learned from this forum and reviewers' sites are 4 Ohms speakers are harder to drive compared with 8 Ohms.

Links or comments are welcome.


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post #2 of 35 Old 11-24-2013, 07:30 AM
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You are correct. 8 ohm speakers definitely have the advantage of being easier to drive, by most AVR's. 4 ohm speakers can "tax" most AVR's at higher listening volumes, since the load is much more difficult. I currently have a Denon 4520, and that AVR drives my 4 ohm speakers just fine and that with a sensitivity of 88db. You have to keep into account the sensitivity rating of the speakers as well.
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post #3 of 35 Old 11-24-2013, 09:27 AM
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An advantage of 4 ohms is that with a quality amp you'll get anywhere from 50 - 100% more watts for the same price. smile.gif
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post #4 of 35 Old 11-24-2013, 10:33 AM
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Quote:
Originally Posted by abatsky View Post

Can someone enlighten me what are the benefits /advantages for each figures?
A 4 ohm speaker will reach the same output level with 0.7 times the voltage swing as an 8 ohm speaker. That's beneficial if your amp is underpowered, but otherwise, not really. The downside is that the current demand with a 4 ohm speaker is doubled compared to 8 ohms, and if your amp isn't totally comfortable with a 4 ohm load that can result in overheating and shutdown.
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post #5 of 35 Old 11-24-2013, 02:26 PM - Thread Starter
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Thank you for your shares. smile.gif

I keep on learning in this forum. Free AV education:D

My MC amp is NAD T975 (specs atttached) with a pair of B&W 683s (8 Ohms). Now, mild OT. I have a small Yamaha center spkr with 6 nom. Ohms but can not catch-up with the B&W in terms of loudness. I have a spare 8 Ohm center speaker, can I connect this together with the Yamaha and still be safe? Let's forget (for the moment) about sonic issues as I'm limited with budget and sources for a matching single unit center spkr. Any pieces of advice are welcome though.

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post #6 of 35 Old 11-24-2013, 03:26 PM
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Quote:
Originally Posted by abatsky View Post

My MC amp is NAD T975 (specs atttached) with a pair of B&W 683s (8 Ohms). Now, mild OT. I have a small Yamaha center spkr with 6 nom. Ohms but can not catch-up with the B&W in terms of loudness. I have a spare 8 Ohm center speaker, can I connect this together with the Yamaha and still be safe?
The combined load would be 3.4 ohms. If you can't get it right by adjusting the individual channel levels it means your small Yamaha center is probably too small.
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post #7 of 35 Old 11-24-2013, 03:34 PM
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I think it is important to understand that the problems with low impedance speakers occur when they are driven hard. As Bill mentioned, they draw more current to get to the same volume level as an 8 ohm speaker. Reality is that few of us ever come close to using full output power from our amps and amplifier impedance ratings are made at full output power. Most AVR's can drive 4 ohm speakers to normal listening levels without a problem whether they are rated for it or not. In my bedroom I use a bottom of the line AVR not rated below 6 ohms. I drive a pair of 4 ohm speakers loud enough that my wife will complain from downstairs about the volume. I'm only using about 1 watt per channel on average. 2 ohms speakers wouldn't even be a problem. While it is good be knowledgeable and aware of speaker impedance issues, it isn't something that most of us need to worry about. Some do, of course, but not most of us.
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post #8 of 35 Old 11-24-2013, 04:13 PM - Thread Starter
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Again, thank you all. You are more than helpful and generous in sharing your knowledge. smile.gif

A salute from an AVS fan in the Philippines!
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post #9 of 35 Old 12-06-2013, 01:28 PM
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Thank you FMW for such a useful viewpoint. I have been struggling to understand the impedence 'matching' issue ever since I acquired a couple of Totem Dreamcatcher speakers + center speaker (4 ohm ratings). I use them as front mains and center in a 3.1 AV setup with my Marantz SR5006 which has a rated output of 100W per channel. I think I understand that the primary issue is the amp overheating from too much current going through it with 4 ohm speakers attached. However, while Marantz isn't rating this amp to go into 4 ohm speakers, I am assuming from all I've read that since I don't listen at high volumes, and am only using three channels (FR< FL and Center) into 4 ohms out of the 7 available, that I am probably OK. What just occurred to me though. is that the speakers are rated for peak power handling of 100W, but since I'm running "100W" into 4 ohms, I'm actually running about 170 W into the speakers. Again, I assume that since I'm not going to max out volume, I'll be OK. But am I right in thinking about the max peak power this way?

The main reason I'm asking is that I'm thinking of getting another pair of Totems for another room where I have a four channel bridgeable Harmon/Kardon PA2000 amp. Right now I run it bridged into two 8 ohm speakers for which it is rated at 100W per channel. Unbridged, it is rated at 45 W per channel. My ultimate question here is: if hooked up to two 4 ohm speakers, would I be better off running the bridged 100W into them (which would really be 170W) or unbridging it and running the 45Watts, which is really more like 75W?
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post #10 of 35 Old 12-06-2013, 02:02 PM
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Most people have a grossly overblown concept of how much amplifier power they use. They get that primarily from the audio press whose purpose it is to support the audio manufacturers. The problem with low impedances is heat dissipation. As long as your amplifiers are running warm but not hot, then all is normal. If they get too hot to touch comfortably, then you are reducing the lifespan of the equipment. I use a little less than 1 watt per channel in my bedroom system with music peaks around 15 watts. Since the amps can handle up to 80 watts of power dissipation each, I don't stress the receiver at all even with my 4 ohm speakers. It runs quite cool even though it has very poor ventilation. It is mostly a common sense issue. You just don't want your equipment to run overly hot.
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post #11 of 35 Old 12-06-2013, 02:20 PM
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Soooo, I'm getting the sense that with my 4 ohm speakers, and the resulting higher 'watt rating', I should just unbridge my amp and run the rated 45 watts (75 into 4 ohm by calculation)...this would actually give me an extra pair of channels (at 45W@) to run another pair of speakers.
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post #12 of 35 Old 12-06-2013, 05:46 PM
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Quote:
Originally Posted by hippiedave View Post

Soooo, I'm getting the sense that with my 4 ohm speakers, and the resulting higher 'watt rating', I should just unbridge my amp and run the rated 45 watts (75 into 4 ohm by calculation)...this would actually give me an extra pair of channels (at 45W@) to run another pair of speakers.

If I remember right, bridging an amp cuts resistance in half. That 4 ohm load becomes a 2 ohm load as far as the amp is concerned. Is your amp rated for 2 ohms? If you do bridge your 4 ohm speakers, check your amps documentation to make sure it's safe to bridge it with 4 ohm speakers.

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post #13 of 35 Old 12-06-2013, 06:03 PM
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The documentation doesn't address this directly. The Power Output Spec just says:

"Normal Mode -- 4 x 45 watts @ 8 ohms, 20Hz – 20kHz, <0.07% THD, all channels driven
Bridged Mode -- 2 x 100 watts @ 8 ohms, 20Hz – 20kHz, <0.07% THD, all channels driven"

Can one tell anything useful on this question from this ? Seems like prudence is pointing me to run the 4 ohm speakers in "normal" hon-bridged mode?
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post #14 of 35 Old 12-06-2013, 06:39 PM
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Psycho is correct. A bridged stereo amp sees 1/2 the nominal impedance.
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post #15 of 35 Old 12-06-2013, 06:42 PM
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then how do I interpret the manual's spec that in bridged mode the amp pushes 100 wpc into 8 ohms? Wouldn't it be saying 100 wpc into 4?
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post #16 of 35 Old 12-06-2013, 07:43 PM
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The speakers are 8 ohms so the power ratings are still based off that, however, your amp sees half of that. If they don't mention bridging in to a 4 ohm load, I'm guessing the amp wasn't designed for that. The good news is you have a decent amp that should have protection from a low load (though I still wouldn't want to trip the protection circuit), but I have seen plastic parts on an amp melt off (literally put a big hole in it) from driving too low of a load.

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post #17 of 35 Old 12-06-2013, 07:46 PM
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So what do you think of the pros and cons of just unbridging the amp and running 4 ohm speakers off of a pair of 45W feeds?
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post #18 of 35 Old 12-06-2013, 08:41 PM
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That's 45 watts in to 8 ohm speakers. I think you said it was 75 watts in to 4 ohm speakers. There won't be much of a difference between 75 and 100 watts. All else being equal, you need to double your wattage to hear a noticeable difference in volume. I think you need 10 times the wattage to double the volume (i.e. 1000 watts is twice as loud as 100 watts). Since you will be using different speakers, however, you need to take the sensitivity of each set in to consideration. If your new set of speakers are more sensitive than your old set, they could be louder on 75 watts than your old set was at 100 watts, however, your B&W speakers (90dB) are more sensitive than the Dreamcatchers (87.5dB).

A few things to consider.

1) The Dreamcatchers are rated for 80 watts each. The 75 watt load (taking your word) from your amp in to a 4 ohm speaker will be a very good match for them. Based on the power handling of the speakers and not knowing how your amp would handle bridging in to a 4 ohm load, I would rule bridging out. If you did, you could damage both your amp and your speakers.

2) It takes an increase of 3dB to hear a noticeable difference in volume (just like doubling the wattage). Combine the slight increase in wattage you had going to the B&Ws and the increased sensitivity of those speakers, the B&W setup will be noticeably louder, though not near twice as loud.

As a disclaimer, sensitivity ratings for speakers are generally taken with 1 watt being fed to them. I do not know if sensitivity can change as they receive more power.

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post #19 of 35 Old 12-06-2013, 08:44 PM
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Bridging typically is used to double the voltage swing into a high impedance load. It is typically not used to double current delivery into a low impedance load. If the spec for bridged output says 8 ohms don't run it into 4.

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post #20 of 35 Old 12-06-2013, 08:57 PM
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Thank you ...that is helpful. My 75 W number is based on a calculation using a formula I ran across that X Watts into 8 ohms is roughly 1.7X into 4 ohms. I've seen others say that it should be more like 1.4X....regardless, I think its clear from the input I'm getting that unbridging the amp is the way to go and that I should have a decent amount of power to run them. In my youth my Dad (an electrical engineer) always advised me that 50 quality watts was better than 100 bad watts every day...so for years I ran a Luxman 50 amp set up--I still can't believe I gave that unit away...or what I was thinking.
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post #21 of 35 Old 12-06-2013, 09:33 PM
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You know, I was confusing you with the topic starter. My comment on the B&W must have seemed strait out of left field.

Also, I believe different amps behave differently with different loads (as far as increase in wattage as ohms drop) but I could be mistaken.

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post #22 of 35 Old 12-06-2013, 09:40 PM
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's'okay---It didn't matter as we were talking 8 ohm speakers in comparison anyway. (I'm currently using some old Pyramid Metronome 7 speakers, which carry a impedence rating of 4-8 ohms.)
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post #23 of 35 Old 12-07-2013, 02:17 AM
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Quote:
Originally Posted by hippiedave View Post

So what do you think of the pros and cons of just unbridging the amp and running 4 ohm speakers off of a pair of 45W feeds?

That is the right thing to do.
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post #24 of 35 Old 12-07-2013, 05:05 AM
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Quote:
Originally Posted by hippiedave View Post

's'okay---It didn't matter as we were talking 8 ohm speakers in comparison anyway. (I'm currently using some old Pyramid Metronome 7 speakers, which carry a impedence rating of 4-8 ohms.)
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I'm curious what stimulated the line of questioning?

What's wrong?
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post #25 of 35 Old 12-07-2013, 07:43 AM
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Originally Posted by parkdarily View Post

I drive a pair of 4 ohm speakers loud enough that my wife will complain from downstairs about the volume. I'm only using about 1 watt per channel on average. 2 ohms speakers wouldn't even be a problem.
Perhaps. Some amps have thermal protection circuitry, which shut the amp down if the output devices get too hot. Some have current sensing circuits, which shut it down if the current draw is too high. Some have active sensors that can detect a load that's too low and will shut the amp down even with no signal present. And some don't have any protection circuits other than a fuse, which may or may not blow before output devices and/or power supplies are damaged. Prudence dictates that one should not run a lower impedance load than recommended. Besides, the wife who doesn't complain about the volume is a wife who's not home. rolleyes.gif
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post #26 of 35 Old 12-07-2013, 12:14 PM
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What stimulated it is demonstrated by post #26...there seems to be a fundamental disagreement or uncertainty in the audio world over whether you need to have an amp that is "rated" for a particular ohm rating for speakers.
I have two amps --both of which have 'power ratings' expressed in terms of going into 8 ohm speakers. I run Totem speakers which are 4 ohm. Some people think this is fine if the amp isn't noticeably over heating and say if you don't try to drive the speakers to max volume, you're probably OK. Others express the view that this a) not prudent or b) will eventually wreck the amp, even if it seems ok now.

So, I've been looking for some reasonably clear way of deciding what to do. My concerns are 1) not wrecking my amps and 2) not exceeding the rated peak wattage of the speakers, which I might do since the rated output of the amps into 4 ohms would be significantly higher than the rated output into 8 ohms.

The confusion seems exacerbated by the manufacturers practices. Some high end outfits, like Outlaw, for example, just give two power ratings...one into 8 ohms and one into 4, clearly implying that its OK to use their amp either way. Others, such as some Onkyos appear to rate their amps for 4 ohms, because they provide an impedence selector switch. However, as I understand it, that 'selector' is just a limiter and most experts recommend just leaving it at 8 ohms to provide max power into the speakers....which sort of begs the question and gets us back to square one.

The majority of seemingly knowledgeable folks seem to think this is not an issue if the amp ain't over heating and that you're fine if you don't constantly drive the system to its limits. A vocal minority (in this entirely unscientific survey of mine through a variety of sources) believe you are doing damage in the long run.

So what's the answer?
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post #27 of 35 Old 12-07-2013, 01:08 PM
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Originally Posted by hippiedave View Post

What stimulated it is demonstrated by post #26...there seems to be a fundamental disagreement or uncertainty in the audio world over whether you need to have an amp that is "rated" for a particular ohm rating for speakers.
I have two amps --both of which have 'power ratings' expressed in terms of going into 8 ohm speakers. I run Totem speakers which are 4 ohm. Some people think this is fine if the amp isn't noticeably over heating and say if you don't try to drive the speakers to max volume, you're probably OK. Others express the view that this a) not prudent or b) will eventually wreck the amp, even if it seems ok now.
So what's the answer?
Any quality amp should have specs for rated power into various loads. If it doesn't that in and of itself is worrysome.

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post #28 of 35 Old 12-07-2013, 01:16 PM
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Originally Posted by Bill Fitzmaurice View Post

Any quality amp should have specs for rated power into various loads. If it doesn't that in and of itself is worrysome.

And yet others disagree...and unless you're talking very high end amps...in my limited experience many quality brand amps such as Marantz and HamonKardon are completely silent on the issue. No rating other than 8 ohms, but no warnings either.
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If the amp does not get extremely hot or overheat with a 4 ohm load, it was designed to handle it. Your Marrantz was obviously not designed to handle it and if you keep tripping the protection circuitry you can end up damaging it. It is usually not good for the amp to run it to the point where protection circuitry kicks in. Your H/K amp is bridgeable in to an 8 ohm load. This means it should probably be OK with a 4 ohm load as long as it's not bridged, but I'm certainly not willing to guarantee that. Most home audio speakers are 8 ohms. Some are 6 ohms. Very few are 4 ohms. Listing 4 ohm specs could confuse a lot of people, depending on the demographic the amp was made for, and the specs are usually not needed anyway.

The bottom line is if your amp does not indicate it's OK to drive a 4 ohm load and you do so anyway, you are at the mercy of the protection circuitry which may or may not exist and which may or may not be adequate. If it does not exist and is not adequate, your amp is toast. If it exists but is not adequate, you can end up damaging your amp if you keep pushing it. Protection circuitry is not full proof and is less so with less expensive products. It is also designed to run within certain parameters. When run outside of those parameters, the protection circuitry can fail. I would absolutely not bridge your H/K amp in to a 4 ohm speaker. I would probably try running it in to the 4 ohm load unbridged, but I would be cautious.

Again, I've seen an amp melt from running a load it was not designed for. It did not take long to overheat and as soon as I realized how hot it was getting shut it off. i thought it was fine because it was still playing when I turned it off, but when I went back to it a few minutes later plastic components on it had melted off an it never ran again (I think it kept drawing power even after I shut it down because circuits were damaged). So yes, as the saying goes it's not an issue if the amp does not get hot to the touch or overheat. The problem with that is if it does overheat it could be too late.

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post #30 of 35 Old 12-07-2013, 03:02 PM
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Quote:
Originally Posted by hippiedave View Post

And yet others disagree...and unless you're talking very high end amps...in my limited experience many quality brand amps such as Marantz and HamonKardon are completely silent on the issue. No rating other than 8 ohms, but no warnings either.
Maybe consumer grade amps can get away with that. This is typical of the specs that should be provided:

QSC RMX 850a

Stereo Mode (both channels driven) 185 W

8Ω/ FTC 20 Hz – 20 kHz / 0.1% THD 200 W

8Ω/ EIA 1 kHz / 0.1% THD 280 W

4Ω/ FTC 20 Hz – 20 kHz / 0.1% THD 300 W

2Ω/ FTC 20 Hz – 20 kHz / 1% THD 430 W

2Ω/ EIA 1 kHz / 1% THD
Bridge-Mono Mode 530 W

8Ω/ FTC 20 Hz – 20 kHz / 0.1% THD 600 W

8Ω/ EIA 1 kHz / 0.1% THD 830 W

4Ω/ EIA 1 kHz / 1% THD
Signal to Noise (20 Hz – 20 kHz) 8Ω > -100 dB
Distortion (SMPTE-IM) < 0.01%
Input Sensitivity at 8Ω 1.15V (+3.4dBu)
Voltage Gain 31.6x (30 dB)
Output Circuitry Class AB Class
Power Requirements 4.5 A
1/8 power pink noise at 4Ω* 6.6 A

The RMX850a is a $399 amp.

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