RMS tricks? [Archive] - Audio & Video Forums


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02-22-2006, 02:01 PM
Hi. I recently got an older (I think... 1982-83) Sony integrated amp from my uncle, as well as as a bunch of components. (Mostly broken, ended up putting them in the 'pile of broken audio stuff' corner.) Anyway there it sat for a good year, and a few days ago, on a whim, I hooked it up in my main stereo system, not expecting to be very impressed.

To my surprise, despite being as old as I am, and the fact that I don't think it was ever anything but entry-level gear, it both sounded and powered my system quite well... in some ways it sounded louder/more powerful than my newer reciever, which is a JVC home theater reciever.

I then thought back to before we got rid of my dad's old Sansui, which was rated quite low compared to modern gear but still seemed to pack a punch. (Though the last time I remember it working correctly I was ten.)

The power rating on the Sony is 155w divided 2 or about ~75 watts per channel.
The JVC has the typical 100 watts per channel, 5 channels, that I see commonly on home theater recievers.

I've read that audio manufacturers today use various tricks to make their systems more powerful on paper than they really are in practice, making the RMS numbers pretty meaningless - I thought that RMS was supposed to prevent that. Anyway, what're some of the tricks that they use to do that (if possible to explain in lamens terms), and when did the practice start?

02-22-2006, 02:24 PM
I think this stuff's been going on since at least the early 80's. RMS is just a way of calculating a form of "average" watts. It came in response to people using "peak watts" to rate amps.

Most manufacturers abide by the industry standards for rating amps/receivers. Multichannel receivers are what cause the most confusion. The front 2 main channels are rated the same as other amps...honest for the most part. The additional channels are rated by the max power RMS that they can deliver, but don't take into consideration the other channels...that is all-channels driven simultaneously.

Many argue that's deceptive. Truth is its a much more accurate, and practical measurement than the "all channels driven" measurement that tests the max power with all 5, 6, or 7 speaker channels driven to their limit.
Some companies even just add the maximum instantaneous power a channel can deliver with the same figure from all the other channels, something like 100 X 5 for 500 watts...

Other tricks include the limit on distortion (THD) or impedance of the load in the test.