Operation of RMX850a Amplifier

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Fri Mar 31, 2017 12:34 pm

  • Greetings!

    I am interested in purchasing this year 3 power amps for bi-amping the three front channels in my home theatre system.

    I am aware of the solid reputation which your company has in the pro audio market, and of the sturdy construction of your products.

    The RMX850a would be suitable for my needs. I have knowledge about the various classes of amplifier output stages and designs.

    However, I have a few questions concerning the operation of that amplifier:

    1. As a Class AB amplifier, does it operate totally in Class AB at all power levels, or does it operate also as a Class A amp for low power levels?
    2. If it does work in Class A at low outputs, up to how many watts will it run in Class A with a 4 ohm load, before functioning in Class AB?
    3. I will be sitting in front of one such unit where I'll be about 5 feet away from the variable speed cooling fan. This would be used to bi-amp the front center channel.
    I listen to music at an average volume level (75-80dB), mostly classical and opera. I know the fan operates at variable speeds. If I don't listen at high volumes, I assume that the fan would operate at a rather slow speed. What would be its noise level in dB then?
    4. Finally, if I did find the fan noise annoying, do you have a quieter fan that I could use to replace the one supplied with the unit? If so, what is the catalogue number for it?

    Thank you in advance.
    Bellinut
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Fri Mar 31, 2017 4:22 pm

  • A class AB amp is class B, but biased so there is overlap between the positive and negative voltage swings, and therefore a smooth glitch-free handoff between them.

    The cooling fan in an RMX amp is noticeably louder than the fan used in, say, the PLX, Powerlight 3, DCA, etc. amp lines. Whether its noise is objectionable is subjective and depends on many factors, including ambient temperature, mounting, etc.

    Sorry, but we do not offer a quieter fan for the RMX series.
    Bob Lee
    Technical Communications Developer
    QSC, LLC
    Fellow, Audio Engineering Society
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    Bob Lee
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Fri Mar 31, 2017 7:08 pm

  • Thank you Bob, for your early reply.

    Why does the RMX Series amps use a noisier fan than some other amps then? I know it is usually used for live sound but I don't understand why it should necessarily put out more noise. In my situation, I wouldn't be using the amplifier to its clipping state because I listen to music in a living room of 12 by 18 feet at an average SPL of 75-80 dB at the most.
    BTW, you haven't answered my question about the dB noise level at the fan's lowest speed. It must have been measured at one point. Companies like AC Infinity publish noise levels for their fans, and I don't see why you shouldn't.
    I know that many owners of the RMX Series complain or report about the noise of their fans, and this makes a lot of prospective buyers to opt for a different product from another manufacturer instead. Maybe your company should look into that problem.
    I live in Montreal and there doesn't seem to be any possibility of renting one RMX850a for a few days to experiment and see if the fan noise would be objectionable. Of course, if that was possible, I would be pleased to try one.
    I am indeed interested in the RMX850a because it only takes a 2U rack space,has outstanding specs and has ample power for my needs. One of my Audioholics friends was suggesting that I go for the RMX1450a instead as he is under the impression that its fan would not run as fast because the more powerful output transistors might not heat as much.
    Looking forward to receiving your comments and a favorable reply.
    Bellinut
    Bellinut
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Tue Apr 04, 2017 12:07 pm

  • When the RMX series was first introduced, they were the new entry-level range of power amps from QSC. Minimizing fan noise was a desirable goal but not a primary one.

    Fan noise in an amplifier is a function not only of the fan but also of the structure of the amp. About a decade ago the manufacturer of the fan used in the 2RU RMX amps changed its design a little bit. In free air the difference in noise between the old fan design and the new was almost unnoticeable, but when the fan was mounted in the amp chassis, the new design had a pronounced and very audible whir. We prevailed upon the manufacturer to revert to the old design, which has been in use ever since.

    On an RMX amp the fan is mounted just inside the rear panel of the chassis. The noise level therefore will be lower in front of the amp than in back of it. Also, mounting amps and other gear in an equipment rack will help baffle the fan noise, especially if there is no short, direct air path between the front and the back.
    Bob Lee
    Technical Communications Developer
    QSC, LLC
    Fellow, Audio Engineering Society
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    "If it sounds good, it is good." —Duke Ellington
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    Bob Lee
    QSC Audio Products
     
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    Joined: Thu Mar 06, 2003 2:01 am
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