Crown vs Rotel

Discussion in 'Amps, Pre-Pros & Receivers' started by Keith raufer, Mar 18, 2017.

  1. Verdinut Audioholic

    Verdinut
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    I plan to purchase three QSC RMX850a power amps to bi-amp my 3 front speakers and use the Marantz SR5010 as a preamp-pro. I already have a pair of EV 7100's for driving the surround speakers in a 7 channel configuration.
    My HT system will have a lot more power than with the AVR, but before, I was driving Altec A7 components with a 99dB sensitivity which I replaced with the following three-way systems (87dB sens.). Finally, I will have less SPL available than with Altecs, but I don't listen to very high sound levels, mostly classical and jazz.
    Actually, I have built 3 identical 4 ohm three-way speakers for the front channels consisting each of one Dayton RSS390HF-4 sub, two Peerless HDS P830991 mid-woofers and one Airborne RT-4001 Air Motion Ribbon.
    The enclosures have a net internal volume for the subs of 7.5 cf and they're tuned to 16 Hz for an F3 of 20 Hz. Active crossovers will be at 190 Hz using DBX223XS and passive x-over will be at 3600 Hz.

    Back to the QSC amps, a friend of mine plays with a band and has used many brands of pro amps, and the only brand that he couldn't kill was the QSC. That's a good plus for them. When I have got acquainted with the new amps, I will surely let you know how they perform. This should be before the end of this year, my fellow Canadian friend.
    Last edited: Mar 19, 2017 at 6:43 PM
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  2. PENG Audioholic Warlord

    PENG
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    I read up a little more about the RMX series and am impressed with the details they provided in the specs. If you are dropping from 99 to 87 dB sensitivity, you may want to go with the RMX1450a to get a couple more dB of headroom, and hopefully that should keep the fans off most of the time.

    Thank you in advance for your year end report.
    PENG,
  3. Verdinut Audioholic

    Verdinut
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    The situation is that I am going to use those inefficient amps in an apartment with a limited number of AC circuits, only one 15 amp line for the full living room. I am planning to use the bedroom circuit for two of the 850's and the living room one for the center channel. Note that, according to the indications by QSC, at 1/8 rated power with pink noise which represents typical program with occasional clipping, with a 4 ohm load, this amplifier draws (both ch. driven) 4.5 amps. Imagine with the 1450, the power consumption is up to 6A.
    I know that electricity is cheaper in Montreal than in Ontario, but the electrical supply limitation in my apartment has to rule. I don't intend to use the kitchen stove circuit !:)
    With regard to the fans, I read that QSC would have quieter fans available at a very reasonable price. Should I be annoyed with the noise, I will surely look into that possibility of replacement.
    Cheers,
    Last edited: Mar 19, 2017 at 5:03 PM
  4. PENG Audioholic Warlord

    PENG
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    That must be peak current not rms, the 1450 at 1/8 rated output 4 ohms should draw no more than 2A rms both channel driven. I doubt your average power for normal listening level will be any where need 1/8 of the rated 400W, probably closer to 1/100, but for other reasons I do agree a 15A cct is more suited to three 850.
    PENG,
  5. panteragstk Audioholic General

    panteragstk
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    I'll be interested to hear your listening impressions. QSC makes a great amp. Especially the series you are looking at.

    I am curious though, did you consider the Crown amps at all and if now why?
  6. Verdinut Audioholic

    Verdinut
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    To tell you frankly, I didn't consider the Crown amps because I prefer Class A/B amps. Also, one of my friends, who plays in a band, was telling me that he tried many brands of pro amps and the only ones that he couldn't kill were the QSC's. That says all.

    With regard to my impressions on the QSC RMX Series amps, I shall post a new thread with details about my listening experience with them toward the end of the year.
    Cheers,
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  7. slipperybidness Audioholic Ninja

    slipperybidness
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    I think you are confused here. Crossover distortion has nothing to do with harmonic distortion.

    It has to do with the point where the signal waveform moves from the + to the - voltage. In a pure class B amp, you get terrible crossover distortion, basically because 1 transistor is shut off (+) before the transistor for the other half of the waveform (-) is turned on.

    Thus, to get around this problem for audio applications, you bias the transistors slightly into class A operation.

    You should get a thorough read and understanding of the DC Load Line for Transistors, then proceed to read up on Class AB amps. This all comes back to 2 key concepts: Transistor Bias and Transistor Load Lines (and biasing the load line between cutoff and saturation).

    Edit: Just quickly browsing that article that you posted in post #16, it seems to cover most of this info. But, again, crossover distortion is a completely different beast than harmonic distortion.
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  8. slipperybidness Audioholic Ninja

    slipperybidness
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    The QSC engineers have published some articles in one of the (few) DIY electronics building mags.

    Reading their white papers, I have very high confidence in QSC. I think Crown has a longer history and is a more recognizable name, but I think QSC is right on their heels.
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  9. PENG Audioholic Warlord

    PENG
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    I have done some reading too since, and I am now also quite comfortable of their class AB amps. The thing about class AB is, as I mentioned before, one can choose the quiescent point to barely qualify to be called class AB. It is much more complicated to control the bias to the point where the amp can actually operate like class A at least for the first few watts, or higher in the higher end ones.
    PENG,
  10. Verdinut Audioholic

    Verdinut
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    I did some calculations which indicate that, with the RMX850a at full rated power of 280W, I would get a 101.5dB SPL at 10 feet out of the subwoofer, and if I add 3dB in view of the bi-amplification effect, and an additional 2dB headroom w/o clipping according to specs, that would mean 106.5dB.

    In addition, since the power demand above 190 Hz will be much less than below, should there be an additional peak power demand on the low frequencies, additional headroom would be available with the reserve power which is not being used for the high frequency drivers, because the two channels operate with the same power supply.

    Consequently, I'm sure that I will have more than sufficient power available.
    Last edited: Mar 20, 2017 at 3:32 PM
  11. PENG Audioholic Warlord

    PENG
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    That seems low, you mentioned your speaker sensitivity was 87 dB, was that for 1W or 2.83V. Even on the safe side, it would be 84 dB, plugged in the 10 ft and 560W (peak) in the calculator below, you get much more than 101.5 dB.

    http://myhometheater.homestead.com/splcalculator.html

    I think you can listen at reference level of 85 dB SPL average, C weighted, and have at least 20 dB for the dynamic peaks found in movies and music without biamping. You average power output should be well below the 1/8 rated, so no worry about any clipping.
    Last edited: Mar 20, 2017 at 3:36 PM
    PENG,
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  12. Verdinut Audioholic

    Verdinut
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    I had calculated the SPL for the subwoofer only, by adding the high frequency output, we are very close in our calculations. The sub sensitivity at 87dB is for 1 watt input, while I calculated the sensitivity for the HF drivers at 90 +/-.

    You're right. I just indicated the output for only one enclosure and I have three in the front.
    Last edited: Mar 20, 2017 at 3:48 PM
  13. Verdinut Audioholic

    Verdinut
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    I went to the above site and I obtained an SPL of 116.5dB -By adding amplifier headrooms and the bi-amping factor, there would be a possibility of over 120dB.

    I prefer to biamplify to avoid additional inductor resistance on the subwoofer voice coil and resulting reduced damping factor.
    Last edited: Mar 21, 2017 at 3:20 PM
  14. PENG Audioholic Warlord

    PENG
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    The article linked below is right about most who use the online calculator will end up getting 2X the power the calculator shows because they forget to compare peak power to the peak spl. That's why I asked you enter 560W instead of 280W to get the peak spl.

    http://www.acousticfrontiers.com/2013322spl-calculator/

    It says: "Many people also make a simple mistake which effectively doubles the size of amplifier required. Using the online calculators they enter follow a process of trial and error to determine the amplifier size required for 105dB SPL. The issue is that our THX requirement is not for 105dB continuous output but 105dB peak output. More on this later."
    PENG,
  15. PENG Audioholic Warlord

    PENG
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    Do you have neighbors?:D
    PENG,
  16. Verdinut Audioholic

    Verdinut
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    Yes, I do. But the building has concrete floors and walls. Late at night, I put the volume down anyway.

    I made some tests with a SPL meter and the maximum level that I would use for music is roughly 88-90dB.
  17. Verdinut Audioholic

    Verdinut
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    Hi PENG,
    I recalculated the SPL and I arrive at a possibility of 121.5dB before clipping. I believe you will agree that is obviously sufficient. I will be able to adjust the gain controls on the amps anyway.

    With regard to the current solid state Class A/B amplifier designs, I assume that the crossover distortion problem is most likely a situation of the past, since it seems rather easy for a manufacturer to fix the proper bias high enough using the appropriate diodes.

    Refer to: http://www.electronics-tutorials.ws/amplifier/amp_7.html
  18. panteragstk Audioholic General

    panteragstk
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    One man's "sufficient" is another's "ouch, my ears hurt. What?"
  19. PENG Audioholic Warlord

    PENG
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    You may be able to endure 121.5 dB (other than very short burst), but not too many home audio speakers can output 121.5 dB average or even peak, so I would say if you have enough amp power to get that kind of spl from your speakers, then you definitely don't have to worry about the amp clipping. You just have to worry about protecting your hearing, speaker damage, and whether your 15A circuit is the new bottleneck.

    Now if you really only listen to reference level or below, i.e. <85 dB average SPL, and assuming the highest peaks in what you listen to is 20-30 dB (such as certain classical music), then you have nothing to worry about because even the 15A cct. should be able to cope with peak burst of less an 20 to 200ms.

    Well designed class AB amp should not suffer from excessive crossover distortions. It seems reasonable to assume QSC class AB amps are well designed.
    PENG,
  20. yepimonfire Audioholic Field Marshall

    yepimonfire
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    You can't even get more than a 20dB peak at reference level of 85dB. If the system is calibrated to 75dB using a -30dBfs pink noise the maximum peak at 0dBfs is 105dB. The only way you'd need more headroom is if you set the fronts to large and ran the LFE channel into the fronts, in which case you'd need 115dB of headroom, but good luck finding a floor standing speaker that can cope with 115dB of low frequency content without major distortion or physical damage to the drivers. Even most large subs fail to do that and it usually requires multiple subwoofers. Even Klipsch's thx ultra speaker system, which has actually been designed to play at reference level without distortion, includes dual subs.

    Sent from my SM-G360T1 using Tapatalk

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