Axiom Home Page Forums General Discussion Technical Questions Maximum amperage for M50's?

Hello! I have the original? M50ti's -- about 5 years old, and I'm wondering what is the maximum safe amp power for these? Specifically, I'm thinking of getting the Onkyo TX-SR875, and running it in bridged mode (Onkyo manual says this doubles the power, and then can only run 2 channels instead of 7, and no lower than 8 ohms).

Can anyone spread some insight on this?

Those speakers are nominally rated at 200 Watt RMS so they would benefit from the additional power. Though I'm a bit suspicious about Onkyo's claims for doubling power based on their history of overstating rated amp power in their receivers.

I was taking a look at my brother's TX-SR875 when I was in LA this weekend and it runs very hot. So right away forget about the amp's rated output "140W minimum continuous power per channe, 8 ohm loads, 2 channel driven at 20-20khz" claimed in their specs. Also with one single transformer in that receiver, what they claim is biamping or bridging is more likely just biwiring.

Anyway you won't hurt the M50s and you never know there may actually be an improvement in sound so its worth a try.

Same power supply so it really is not true bi-amping, more like Buy Wiring.

Just hook up those 50's and enjoy, there is no way that receiver has to much power for them.

Why do you think you need more power for them? Is the receiver clipping, or simply not playing loud enough?

BTW, SirQ, apparently the thing has bridgeable amps, so it would actually be putting out more power--and the M50s only have a single pair of binding posts anyway!

awe I see, thanks for the clarification...

Yeah, I totally read it that way the first time around.

This BTL bridging feature sounds like marketing hype to me. I had an Arcam avr300 once which had a similar feature which didn't so much.


This BTL--Bridged Transless--design supposedly eliminates a capacitor in each amplifier circuit which may be a reason for the mediocre increase in power. I doubt one would detect much difference in performance and there is some argument it may add to distortion. Also eliminating capacitors has to impact damping factor and the ability of the amp to respond to quick demands for voltage. I don't see much benefit and actually see potential problems.

Jamin, you don't have to worry about damaging your M50s, because your ears will give out long before they will. A more significant question would be why you have the idea that you'd need more maximum power capacity than the 875 would provide in standard operating mode. The first point is that you can rely on the rated 140 watt figure as being accurate, since all amplifiers sold in the U.S. have to be rated in compliance with FTC regulations which require that the rated power with at least two channels driven be sustained for at least 5 minutes continuously(unrealistically severe for home use, of course). No manufacturer is foolish enough as to violate the law in regard to power ratings, so this is one of the very few things in audio where we should be confident that the advertising is true.

If you haven't studied it before, the S&V lab test of the 875 verifies its power capabilities.

Bridging an amplifier sends twice the voltage into the speaker, and because of Ohm's Law power is proportional to the square of the voltage, so theoretically, doubling the voltage would increase maximum power capacity by 4 times. Inefficiencies don't permit this in practice and roughly 3 times is said to be a typical result(note that the S&V test said only that it was over 300 watts).

Keep in mind that most of the time your M50s use about 1 watt at a comfortably loud listening level. Much more is used on brief peaks, of course, and enough headroom should be provided for as much as 20dB over the average level on highly dynamic source material, such as some classical recordings or an occasional pop item which hasn't been dynamically compressed. In the vast majority of setups receivers rated in the area of 100 watts provide plenty of headroom. Although it's merely the statement of a truism, sometimes there's a failure to realize that unused headroom is simply that: unused.

Thanks for that article John. Those power specs are inconsistent however.

At the end of the second paragraph the article describes "Bridged output at clipping was in excess of 300 watts, two channels driven into 8 ohms." while in the list below:

Output at clipping (1 kHz into 8/4 ohms)
1 channel driven: 201/322 watts (23/25.1 dBW)
5 channels driven (8 ohms): 141 watts (21.5 dBW)
7 channels driven (8 ohms): 128 watts

Also in the Stereo Performance section:

Output at clipping (1 kHz, both channels driven)
8/4 ohms: 185/320 watts (22.7/25.1 dBW)


Presumabley he meant bridged output at clipping as as in excess of 300W at 4 ohms not 8 ohms. It would have been more helpful if the author had posted power graphs. I also note that Onkyo's specs "160 watts minimum continuous power per channel, 8 ohm
loads, 2 channels driven at 1 kHz, with a maximum total
harmonic distortion of 0.7% (FTC)"

Note that 5 channels driven is 141wpc so the increase in power in 2 channel is only in the order of 19 watts at 8 ohms. That increase will translate into only a fraction of a decibel with somewhat higher distortion.

Thanks for the thoughts everyone.

Currently I'm running the m50's with a 2-channel NAD amp (2400), and the rest of the speakers with a multi-channel NAD amp (906), and was hoping to be able to get rid of an amp to save some space.

But if the Onkyo's amps didn't sound as good as the NAD amps, then I would just keep using the NAD amps.

Jamin, both the amplifier in the 875 and the amplifiers in the 2400 and 906 amplify transparently within their power limits, so there's no reason why any of them would sound any better than the others.

I'm not sure if this makes a difference regarding the 4 ohms vs 8 ohms thing, but the 875 manual on page 24 states in bold:

• Use only front speakers with an impedance of 8
ohms or higher for bridging. Failure to do so may
seriously damage the AV receiver.

It also states the following (which is why I made my original post):

• When using bridging, make sure that your front speak-
ers can handle the additional power.

Since that S&V test says "over 300W" in bridged mode, then it sounds like it would NOT be safe to run the m50's with the 875 bridged... correct?

Thanks JohnK! It sounds like I can replace both NAD amps with the receiver! Very exciting. I just hope it sounds at least as good as the NAD pre/pro (S170). I'm mainly getting it because I want uncompressed audio from my Blu-ray's via HDMI.

I think you'd probably be ok with the M50s; it's not likely that you're going to be hitting that 300W very often (if at all). I know that Axiom tends to be very conservative with its speaker ratings--although the M80s are rated at 400W maximum, they have been tested (by Axiom) at well above 1000W!

Jamin, the 875 has plenty of maximum power capacity in its standard operating mode, as the S&V lab tests indicate, so you shouldn't be worrying about still more maximum capacity from bridging(incidentally, the S&V article correctly mentioned that the bridging was at 8 ohms; bridging can't change the impedance of a speaker or other load which is being driven). But no, if you did it, the M50s can handle brief bursts of maximum power in excess of the conservative number given in the Axiom specs. So again, you should worry about your ears more than your M50s.

I just didn't want to chance ever damaging my precious Axioms! You've been a big help -- thank you very much.

If budget is the main concern what you are doing makes sense. I've experimented with that receiver and it clips at not that high volumes and at loud transients. If you are planning on using it at low to moderate levels its OK.

However if you are asking are you giving up sound quality by selling the separate NAD amp and relying on the very hot amp inside the onkyo receiver box then the answer is that it would be a mistake. Do not believe the over-simplistic statements that a receiver amp is the same sonically as a capable separate amplifier. The high heat (hot to touch) along with the radiation from the amp inside the box will affect sound quality.

When bridging which I do with my Bryston and McIntosh amps, the power output for 4ohm loads will be different than with 8ohm loads. With well designed efficient solid state amps the 4ohm power output will be nearly twice the power output with an 8 ohm load. That's why its important to know the ratings at both those impedances or ideally have a graph. The published data is inconsistent. 300w output at 8ohms bridged implies around 600w bridged at 4ohms. No way with that amp. 300w at 4ohm bridged is believable though I doubt even that is achievable given the high heat that receiver generates.

No it would be safe. The rule of thumb I have always used when matching speakers and amps is to use a minimum amp power of 2X the speakers nominal power handling. That way you avo.d clipping in all but the loudest passages.

Another thing to keep in mind with that receiver is to NOT set the speaker impedance in the menu to 4 ohms as it will limit power output to 55 wpc all channels driven. No reason given for this except I suspect its related to its different design with few capacitors and kept low in order to get certified. That means it will clip more easily. I don't like the amp in that receiver primarily for that reason. Capacitors and how they are arrayed are important determinants to how fast the amp can respond to voltage demands and how much spare power is held in reserve.

Uggh... all this is so contradictory and a lot to think about. One guy says the amp doesn't matter, one guy says it does... everywhere I read, it's always something different.

I've never had my current system so loud that it clipped; it always sounds great & gets plenty loud. And right now my mains are 'only' powered by 100W, the center with 90W and the surrounds with 30W. Before I went separates I had an Onkyo 787 receiver and it never ran out of power either (and honestly, after going separates the improved audio quality was only noticed with SACD / DVD-Audio, and then only marginally).

So I guess I just don't see how this new Onkyo 875 would not have enough power. Can it really be any hotter than these NAD amps now? They get pretty darn warm, but I've never felt I needed to measure their temps.

Thanks again for everyone's input!

If you never notice clipping and you don't listen at loud levels then get the Onkyo and be happy. Just keep it ventilated because it does get quite hot.

Hello again everyone. After spending about 2 hours researching every night, I've decided against the Onkyo 875 or 905. I don't like the possible heat issues (which seems to be caused by the Reon HQV video chipset), and the audio 'pausing' for a few seconds when switching channels.

So I'm almost 100% decided on the Denon 3808. Anyone have any experiences with this unit? I've been very happy with Denon in the past.

A fair number of us on here have the 3808 and I love mine. The GUI is very nice and the network capabilities for streaming directly to it is great. I also like that I can listen to internet radio stations through it.

Just to pile on, I had the same question about 2 weeks ago and the folks here more than convinced me on the 3808. Mine is in the mail now, so I can't comment on performance, but after going through the same decision process you did I can tell you I don't feel like I'm losing much going to the 3808.

When I really started to think about the amount of SDTV I watch (next to none) the Reon upscaling became much less of an issue for me. On the flip side, I know I will use the network streaming and love the fact that firmware updates can be done in-home without dealer involvement. In an industry that has so many changing standards and evolving audio formats, knowing I'm a little more future proof is an added bonus.

Well I went over the pluses & minuses of the 2808, 3808 and the 4308 with the wife, and she liked the 4308 the best. Will be delivered tomorrow!

Hey, very nice. You'll have to tell us all how the 4308 works out.

I think I'm in love!