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#194957 - 02/04/08 02:49 PM Maximum amperage for M50's?
jamin3d Offline
hobbyist

Registered: 07/24/02
Posts: 23
Loc: GSP
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?


Edited by jamin3d (02/04/08 02:49 PM)

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#194966 - 02/04/08 04:33 PM Re: Maximum amperage for M50's? [Re: jamin3d]
jakeman Offline
aficionado

Registered: 10/03/05
Posts: 852
Loc: Toronto
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.
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#194982 - 02/04/08 07:27 PM Re: Maximum amperage for M50's? [Re: jakeman]
SirQuack Offline
shareholder in the making

Registered: 01/29/04
Posts: 13341
Loc: Iowa
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.
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#194985 - 02/04/08 07:49 PM Re: Maximum amperage for M50's? [Re: SirQuack]
Ken.C Offline
shareholder in the making

Registered: 05/03/03
Posts: 17789
Loc: NoVA
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!


Edited by kcarlile (02/04/08 07:51 PM)
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#194997 - 02/04/08 08:19 PM Re: Maximum amperage for M50's? [Re: Ken.C]
SirQuack Offline
shareholder in the making

Registered: 01/29/04
Posts: 13341
Loc: Iowa
awe I see, thanks for the clarification...
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#195000 - 02/04/08 08:26 PM Re: Maximum amperage for M50's? [Re: SirQuack]
Ken.C Offline
shareholder in the making

Registered: 05/03/03
Posts: 17789
Loc: NoVA
Yeah, I totally read it that way the first time around.
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#195028 - 02/04/08 11:10 PM Re: Maximum amperage for M50's? [Re: Ken.C]
jakeman Offline
aficionado

Registered: 10/03/05
Posts: 852
Loc: Toronto
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.
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#195032 - 02/04/08 11:23 PM Re: Maximum amperage for M50's? [Re: jamin3d]
JohnK Offline
shareholder in the making

Registered: 05/11/02
Posts: 10405
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.
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#195044 - 02/04/08 11:53 PM Re: Maximum amperage for M50's? [Re: JohnK]
jakeman Offline
aficionado

Registered: 10/03/05
Posts: 852
Loc: Toronto
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.
_________________________
John

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#195353 - 02/07/08 12:43 AM Re: Maximum amperage for M50's? [Re: jakeman]
jamin3d Offline
hobbyist

Registered: 07/24/02
Posts: 23
Loc: GSP
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.

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