Axiom Home Page Forums General Discussion Technical Questions ROTEL RECEIVER not RATED FOR 4 Ohm?,

I read a review today indicating that the new Rotel 1550 AVR is rated at 75 watts per channel into 8 ohms. The review goes on to say that the receiver is not rated for 4 ohm use. Does this mean that its OK to use this receiver with 4 ohm speakers but the receiver wasn't tested for them and thus is still unrated? I believe that this receiver is capable of driving M80's. What is your opnion? See http://www.rotel.com/content/reviews/15%20series/rsx1550-equip_rotel_cm.pdf Thanks...Lee

Lee, everything I've read suggest that yes you can drive the 4 ohm 80s with the 1550 without any problems and I personally believe there should not be any issues but you may want to ask the question over on the Club Rotel forums...

Club Rotel

This looks like a very nice AVR, I like it!

No where in the manual of the RB1080 does it say it's rated at 4 ohm but it sure does a nice job with the 80s. I'm sure the 1550 will work fine.

Lee, the reason that the 1550 and so many other receivers that can handle 4 ohm rated speakers with no problem in real-world home use can't claim an "official" 4 ohm rating is the severity of the tests they would have to pass. The FTC(Federal Trade Commission)has regulations for amplifier power ratings which have to be followed for any units sold in the U.S(and which, as a practical matter, are the same as those sold in Canada), and the UL(Underwriters Laboratory)tests for overheating. In brief, for an official 4 ohm rating the unit would have to be operated at the full rated power for at least five continuous minutes into a 4 ohm load. Most units aren't designed for such an ordeal without overheating and having their protective circuits shut them down.

Nevertheless, for actual home use the 1550 and many other receivers are serving their owners well driving speakers, including the M80s, which are rated at 4 ohms. Incidentally, you might also note that the M80s are 8 ohm speakers over the majority of their frequency range.

That's not what I see when looking at the M80 impedance graph John. While a majority of the graph shows the M80s above their 4 ohm rating it certainly does not show them to be "8 ohm speakers over the majority of their frequency range".

Here is the graph for reference. Looks like it is 8ohm or above from around 700hz - 2K, and 5K to 20K Hz.

Yes it is from 700hz - 2K but it is not up to 20K, it shows it from 5k to around 12k. Put a straight edge on the graph and it will show it. The point being it does not show the 80s as being "8 ohm speakers over the majority of their frequency range".

Which is why I showed the graph, to confirm your statement. Didn't take the time to use a straight edge, to lazy.

Got ya.

Speakers are rated on an average of their impedances, not the minimum. The NRC graph does in fact show that the M80s are 8 ohm speakers over the majority of their frequency range.

Hi Rick and all,

As I explained in a PM to Rick, impedance ratings by the designer are "nominal"--arrived at by eyeballing the overall impedance curve and picking a number that is kind of an average. Since the M80 impedance curve does touch 4 ohms at several places, and remains below 8 ohms up to nearly 1 kHz, for the sake of clarity, Axiom refers to the M8os as "4-ohm speakers". Actually, Ian Colquhoun, the M80s designer says they are closer to 5 ohms.

The point is that in the frequency range where the greatest amount of current is drawn, the M80s' impedance is at 4 ohms or slightly above that figure, so that's why we give it a nominal impedance rating of 5 ohms. To avoid confusion in the specs we call it 4 ohms.

Regards,
Alan

Aww, you could call them 5 ohms and give people another random reason to hate them!

I don't see what chart John is looking at.

LMAO!!

So, to conclude...the M80s are considered 4(5?) ohm speakers, for the most part, because they touch 4 ohms at lower freq's where they draw more power? If the charts showed they dropped to 4 ohms but much higher up in the fq's, then could they be considered 6-8 ohm speakers? If it is an average, wouldn't/shouldn't the lower fq's take precedent over higher ones?

Adrian,

Yes, yes, and yes.

Alan

Do you think you should be shouting Adrian's name followed by "Yes, Yes, Yes" ?