Why 4 ohm if you know you will use an AVR? - AVS Forum
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post #1 of 21 Old 10-08-2010, 12:37 PM - Thread Starter
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Some time ago, I asked in the speaker forum, why people would design 4 ohm speakers. I never felt I got a great answer. But I believe someone suggested a speaker with nominal 4 ohm impedance could be more efficient.

I wanted to ask people why they would consider/buy 4 ohm speakers when their plan was to drive it with an AVR that might not be designed for 4 ohm speakers?

Arguably, many receivers will drive 4 ohm speakers just fine. But some receivers may never have been designed for 4 ohm loads. They may have insufficient heat sinking. Is it possible their lifespans will be shorter?

For movies, maybe it's ok. They are so dynamic, I could see where the receiver's output transistors have time to cool down.

For music, I am not sure it's a good idea to hook seven 4 ohm speakers up to a mid range receiver and then crank up the volume. Seems risky?

"But this one goes up to 11"
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post #2 of 21 Old 10-08-2010, 12:48 PM
 
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I used to do it. And I sent all of my previous receivers into standby numerous times when I was using 4 ohm speakers. And all of my receivers were very powerful as far as receivers go. Nowadays, I would not trust any of today's receivers to run any 4 ohm speakers effectively.
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post #3 of 21 Old 10-08-2010, 12:51 PM
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Well then there's the whole issue of accurate ratings on speakers. There are plenty of examples of 8u rated speakers that rarely run even close to that. And there may well be 4u rated speakers that often run higher than that.

My opinion would be that cranking ANY AVR for sustained periods on multi-channel music is a risk to the speakers regardless of 4u or 8u rating.

I'm still trying to understand why anyone who's concerned with getting adequate power would ever choose an AVR in the first place - period.
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post #4 of 21 Old 10-08-2010, 12:55 PM - Thread Starter
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The obvious answers would that -
a) They fell in love with some particular 4 ohm speaker
and
b) They think this worrying about driving 4 ohm loads with mid range or lower receivers is misplaced
or
c) They never plan on pushing their system hard

I assume flagship, near flagship, or Ultra THX receivers should be fine with 4 ohm speakers. I am questioning people spending $600 on a receiver and hoping it will drive 4 ohm speakers.

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post #5 of 21 Old 10-08-2010, 01:02 PM
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This will surely ruffle some feathers, but what the hell.
http://forum.ecoustics.com/bbs/messa...79/556241.html

Sec 42-30
It is unlawful to make any loud music, in a manner that the same is heard on the public streets or sidewalks to the annoyance of persons or cause a disturbance of the peace. Pfft- whatever!
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post #6 of 21 Old 10-08-2010, 01:26 PM
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Quote:
Originally Posted by MichaelJHuman View Post

Some time ago, I asked in the speaker forum, why people would design 4 ohm speakers. I never felt I got a great answer. But I believe someone suggested a speaker with nominal 4 ohm impedance could be more efficient.

With a typical class A/B solid state amp (whether its in an AVR or a behemoth of a seperate,) you'll give back at least half of whatever sensitivity/efficiency you will gain by going to a 4u speaker design (vs 8u).

In short, it's not worth it on sensitivity grounds alone, mostly you just make the amp work harder (twice as hard in fact) to produce the same results. While at the same time, you run into current limitations and wind up *reducing* the available dynamic headroom of the amp.

It amazes me how many folks just don't understand this.
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post #7 of 21 Old 10-08-2010, 01:34 PM - Thread Starter
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For reference, I should quote Paul Scarpelli, who posted his answer to why people design four ohm speakers -

Quote:


If you look at specifications of raw drivers from the driver manufacturers, you'll sometimes notice 4 ohm and 8 ohm versions of the same units. The 4 ohm versions are usually more sensitive, and not by a little. A speaker system that's 4 ohms will usually cause a good amplifier to deliver more output than it would into an 8 ohm load, too. The net pickup from these two observations can be 3-5 dB more output. More output means less distortion, more headroom, and the capability of playing substantially louder.

I will try to explain what I think he means, but I could be wrong. A 4 ohm speaker will require less voltage to drive it. If you were voltage limited, that would make sense.

Amps designs trade off voltage and current in their design. You can raise rail voltage which you might think would help reduce the chance of clipping. But you can't do this with no tradeoff. With a higher voltage, the power supply (assuming unregulated) will likely collapse under load. Of course as you started with a higher voltage, you have some room to work with.

On the other hand, you can do what I think Harmon Kardon does. You start with a lower supply voltage to the amp. In return, you should get better performance into lower impedance loads. But you start with a lower supply voltage, so even though it should be more stable, the clipping point is lower.

If you knew for a fact someone was going to hook up a well behaved 8 ohm speaker to your amp/receiver, you would simply design for that. If you knew someone was going to hook up a well behaved 4 ohm speaker to your amp/receiver, you would use a lower rail voltage than our theoretical 8 ohm scenario, and your current capability should be better.

(This is probably a simplification, and maybe has some inaccuraries, as I am not an engineer.)

But anyway, in reality, you don't know exactly what people will hook up. So you probably design for a compromise. And you put in limiter circuits and thermal shut off circuits to protect your product from crazy people

Not sure what my point was, now that I think about it. Sorry. Been a long week.

"But this one goes up to 11"
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post #8 of 21 Old 10-08-2010, 02:10 PM
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MichaelJHuman:

I'm not sure who Paul Scarpelli is but that was comical. The impedance of a speaker has little to do with the driver but instead the cross-over network and how complicated it is. In short; what HDTVChallenged said.
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post #9 of 21 Old 10-08-2010, 04:07 PM - Thread Starter
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Quote:
Originally Posted by borntoboggie View Post

MichaelJHuman:

I'm not sure who Paul Scarpelli is but that was comical. The impedance of a speaker has little to do with the driver but instead the cross-over network and how complicated it is. In short; what HDTVChallenged said.

I thought driver had a nominal impedance. I had assumed that was a function of how their windings are designed.

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post #10 of 21 Old 10-08-2010, 05:44 PM
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Quote:
Originally Posted by MichaelJHuman View Post

I thought driver had a nominal impedance. I had assumed that was a function of how their windings are designed.

They have an impedance rating, and a DCR rating which are usually quite different. But it's the XO that combines all the drivers and determines the impedance presented to an amp. Which, incidentally, is all over the place - not a fixed value.
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post #11 of 21 Old 10-08-2010, 05:56 PM - Thread Starter
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The enclosure also plays a role, altering system resonance. I have seen speakers with multiple Q's, and multiple impedance dips.

So I think it can be said that the system as a whole defines the impedance behavior.

"But this one goes up to 11"
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post #12 of 21 Old 10-08-2010, 06:30 PM
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The lower impedance, the easier you can damp resonances electrically. It may reduce requirements for mechanical and acoustical design.
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post #13 of 21 Old 10-08-2010, 06:55 PM
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No Micheal,

I think it can be said you ask questions with opinionated answers of your own and then reformulate. You need to brush up bro. Why ask a question if you already have the answer?
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post #14 of 21 Old 10-08-2010, 07:13 PM - Thread Starter
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I didn't say I have the answer. I think you will find I am quite open minded about audio with a few exceptions. Telling me power cords can improve sound seems to defiy the laws of physics, for example. So I will be skeptical of such claims, until I see some proof.

And no offense, but I am not your bro...

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post #15 of 21 Old 10-08-2010, 08:28 PM
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People answered your question yet you still drew your own conclusions against the answers presented. Which makes one wonder...why did you ask the question if you think people are wrong.
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post #16 of 21 Old 10-08-2010, 08:47 PM - Thread Starter
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I drew no conclusions. All I have done is discussed various aspects of the topic.

"But this one goes up to 11"
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post #17 of 21 Old 10-08-2010, 09:05 PM
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Quote:
Originally Posted by JEFFakaMAX View Post

This will surely ruffle some feathers, but what the hell.
http://forum.ecoustics.com/bbs/messa...79/556241.html

Sounds like this guy in your link is trying to water down the facts rather than explain anything. I'm sure he knows that sensitivity is usually measured at 2.8v and that is why 4ohm speakers measure higher. However your average AVR does not double it's power at 4 ohms. If you feed a 4 ohm speaker from a given manufacturer 1w and the 8 ohm version 1w you will often find the actual 1w/1m sensitivity is very similar. Generally for me, if I'm looking to run an AVR, I would rather run 8 ohm speakers. That goes for most HT multi-channel amps as well. When I was a newb I always found myself seduced by 4 ohm wattage ratings, the honeymoon is over now.
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post #18 of 21 Old 10-08-2010, 10:00 PM
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Soundstage use to have the graphs for most popular speakers showing the impedance
varying with the Hz some more so than most but a lot dipping close to 2ohm around
200 to 75Hz and a spike in the 1KHz and sometimes in the 18KHz but each brand varried
and the crossovers had to play a role in it if certain brands used the same drivers.
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post #19 of 21 Old 10-08-2010, 10:10 PM - Thread Starter
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I have wondered how big a deal these impedance dips are? What happens with a real signal.

A real signal is not going to emit pure tones near resonance very often if at all. I wonder if these dips simply don't cause problems in the real world very often?

Douglas Self discusses this in his amplifier handbook, but though I read some of the chapter, I don't fully understand the math/conclusions.

"But this one goes up to 11"
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post #20 of 21 Old 10-08-2010, 10:39 PM
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I can't help but think you started this thread based on my post the other day. As I said the other day, the speakers I have that I "Love" were speakers I bought WELL before I even thought about buying a new receiver AND I already owned a 200 w/ch into 4 ohms Parasound HCA-1000 amp to drive them. I already owned BOTH of these pieces before I even considered a new receiver. A big reason why I went Yamaha was because they offer preouts so I can use the Parasound amp. I have been curious to know how the 667 might handle the Dynaudio's, that was the extent of it.

When you get out of these forums focused around receivers and AV and into the more 2 channel realm, 4 ohm speakers are much more commonplace, as I'm sure you know, and those that are interested in them, those that buy them, those that design them, know that they might require more power and when it's accepted in their mind then it resigns the thought to complain about it. And further, I believe that the thought that you NEED out-of-this-world-power to drive a 4 ohm speaker is a bit overstated a lot of the time. My little 45 w/ch into 4 ohms Parasound Zamp (spare amp) drives my Dyanudio, 4 ohm speakers amazingly well - in my medium to large sized room. That said, it's torroidal power supply and high current capability has a lot to do with that and I'm no way trying make a case for receivers - I know their limitations.
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post #21 of 21 Old 10-08-2010, 11:36 PM - Thread Starter
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Likely wasn't you. Just an accumulation of posts where people want AVRs to power 4 ohm speakers. Me personally, I would just avoid 4 ohm speakers. But certainly if you love some speakers which happen to be 4 ohm I can understand being in a situation where you need an AVR to drive them.

Note that I am not suggesting you need a lot of power for 4 ohm speakers. You need an amp/receiver which can deal with them regardless of power. Any amp designed only for 8 ohm loads might have issues because of the higher current. Higher current means more heat to deal with. And the output transistors need to be handle the voltage/current. In the case of your 45 watt amp, I would guess it was designed for 4 ohm loads seeing how it's from Parasound (guess.)

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