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#355766 - 09/29/11 09:33 AM Question about 4 ohms
mattbrid Offline
newbie

Registered: 09/27/11
Posts: 2
I have an Onkyo tx-nr809 that is 135w/channel into 8 ohms. The manual says it can do 4 ohms, but doesn't specify how much wattage it would put out in that mode. Does anyone have any experience with this (or similar) receivers driving the M80s? Is there a difference in quality of signal, heat, or otherwise driving 4 ohms. Thanks!

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#355767 - 09/29/11 09:42 AM Re: Question about 4 ohms [Re: mattbrid]
bdpf Offline
aficionado

Registered: 06/28/08
Posts: 769
Loc: Toronto, Canada
Matt, welcome!

I wouldn't worry about the wattage. Most likely your Onkyo will be fine unless you listen at extremely loud levels for very long period of time. My Denon is not even rated for 4 ohm and I've watched movies at the 0dB mark driving M80s and a VP180. While the receiver got hot, it never shut down on me.
If you're listening at very loud levels, the only thing that you have to watch out for with AVRs is that you don't drive them into clipping which could potentially damage your tweeters.
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M80s/VP180/QS8s/EP600/AVR-890
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#355773 - 09/29/11 10:58 AM Re: Question about 4 ohms [Re: bdpf]
Murph Offline
axiomite

Registered: 10/05/06
Posts: 6955
Loc: PEI, Canada
The short answer to question is that if you cut the ohms in half (resistance) you will double the wattage. More specifically,

Amplifier Output = Amplifier Watts x (the ohm rating used / Speaker ohms)
So your max output would be 137 x (8/4) = 274

Bruno's points are the important factors though.
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#355796 - 09/29/11 03:19 PM Re: Question about 4 ohms [Re: mattbrid]
alan Offline

connoisseur

Registered: 01/29/02
Posts: 3258
Loc: Toronto/New York/parry Sound
Hi mattbrid and welcome to the forum.

In general, all solid-state amplifiers operating into a 4-ohm load will pass more current through the output transistors (because the resistance is less, 4 ohms as opposed to 8 ohms), so the output section will always run hotter than with an average 8-ohm system. Also, if you measure amplifier distortion with a 4-ohm load, it will always measure higher, but it will still be far below audibility unless the amp approaches clipping or clips.

All AV receivers contain some form of protection circuitry; most have thermal monitors on the output stage so if it gets too hot, the receiver will shut down before the output devices get damaged. There is usually a current-flow monitor as well, so if current flow becomes excessive, the protection circuit will either shut down the receiver or greatly reduce power output (current flow) to protect the output stage. Onkyo receivers a decade ago had overly touchy protection circuitry that would shut them down with 4-ohm loads and I would never recommend them for driving the Axiom M80s. Recent Onkyo models are much better, but since your owner's manual doesn't specify what the amp's power output is into 4 ohms, I suspect that there would be some type of current-limiting that kicks in to reduce power output into 4-ohm loads.

As other posts have noted, be careful about high-volume playback with 4-ohm speaker loads and if you hear any distortion or harsh, edgy quality in the sound, immediately turn down the volume.

Regards,
Alan
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#355820 - 09/29/11 09:50 PM Re: Question about 4 ohms [Re: mattbrid]
JohnK Offline
shareholder in the making

Registered: 05/11/02
Posts: 10621
Matt, welcome. The reason that the 809(along with most other receivers)doesn't have an "official" power rating at 4 ohms is that under the FTC power amplifier regulations it would have to operate at the full rated power into a 4 ohm test load for at least five continuous minutes. Few are designed to do that without overheating and shutting down. Outside the testing lab in the real world of home listening these receivers nevertheless put out substantial power into 4 ohms during times in music when the load happens to be at that number, and no problems occur.

There doesn't appear to be a published lab test on the 809, but similar units show about 50% higher numbers at 4 ohms for the brief periods that actually occur in music. These units are being used every day for speakers similar to or more difficult to power than the M80(it's slightly above average in sensitivity and requires a little less power). There's no good reason to expect that the 809 wouldn't be fine with the M80s at all safe home listening levels.
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#355833 - 09/30/11 12:04 AM Re: Question about 4 ohms [Re: JohnK]
mattbrid Offline
newbie

Registered: 09/27/11
Posts: 2
Thanks for all the info. I noticed that the vp-150 is 6 ohms and the qs-8 is 8 ohms. Since I'm considering getting those with the m80s, does it matter to the receiver that they are all different? I assume not since I haven't read about any real issues regarding this. I also assume that lots of people with axiom HT systems have receivers like mine and have no issues. Thanks again.

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#355835 - 09/30/11 12:30 AM Re: Question about 4 ohms [Re: mattbrid]
JohnK Offline
shareholder in the making

Registered: 05/11/02
Posts: 10621
Speakers with different impedance ratings aren't a problem. The receiver handles them independently. Each speaker itself usually varies widely in impedance as the frequencies played vary, and the rated impedance of a different speaker isn't of any greater significance than this.
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Enjoy the music, not the equipment.



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#355984 - 10/02/11 11:23 AM Re: Question about 4 ohms [Re: mattbrid]
BlueJays1 Offline
connoisseur

Registered: 09/19/08
Posts: 4116
Loc: Porch,enjoying Bombay Sapphire
Originally Posted By: mattbrid
Thanks for all the info. I noticed that the vp-150 is 6 ohms and the qs-8 is 8 ohms. Since I'm considering getting those with the m80s, does it matter to the receiver that they are all different? I assume not since I haven't read about any real issues regarding this. I also assume that lots of people with axiom HT systems have receivers like mine and have no issues. Thanks again.


The different impedance rating of 4, 6 or 8 ohms doesn't mean much. Despite each speakers rated impedance rating, if you look at the impedance graph of any speaker the impedance varies which each frequency. Impedance is not constant. So different impedance ratings will not harm anything.

The only harm impedance can cause is when a speakers impedance minimum is very low in the 2-4 ohm range in a limited range of frequencies (usually below 400hz) and where the phase angle is drawing current above what the impedance is at that frequency. At loud volumes and dynamic content, this might cause current limiting and overheating with some receivers.

That's not the case for the QS8's and VP150.
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