5 ch Vs 2 ch Rotel amps

Discussion in 'Archived Threads 2001-2004' started by JohnSC, Feb 16, 2002.

  1. JohnSC

    JohnSC Stunt Coordinator

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    I am considering either purchasing the ROTEL RB-1070 2-ch or ROTEL RMB-1075 5-ch amp. I noticed that generally speaking the 2-ch specs are better than the 5-ch. My question is whether when listening to 2-ch stereo will it make much of a difference? Here are some examples of the spec differences....
    Damping factor - 1070 = 500, 1075 = 180
    S/N Ration - 1070 = 120db, 1075 = 115db
    Power - 1070 = 2x130w (8 ohms), 1075 = 5x120w (8 ohms)
    Other than getting additional channels from the 1075, what do you think?
    Monkey any opinions?
    [​IMG]
     
  2. Dennis B

    Dennis B Stunt Coordinator

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    John,

    I went through the exact same thinking as you. I still have a 1070 at home which I'll return to my dealer next week when I pick up the 1075, and it's just a great piece of equipment (the 1070, I mean).

    I don't know the answer to your question, but I decided for the 1075 because I listen to music maybe 40-50% of the time and the rest is HT. I also like to listen to some live recordings in 6-ch stereo so I guess the 1075 was my choice.

    My other option would be 3 1070s...

    But I really hope the other guys who have a lot more experience than me chime in to help us.

    Rgds.
     
  3. JohnSC

    JohnSC Stunt Coordinator

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    Dennis B,

    I'd be interested in what you think after you get the 1075. The the biggest difference (on paper) is the damping factor. I imagine this should produce tighter bass from the 1070. Whether it translates that way in practice remains to be seen (heard actually!).
     
  4. BillG

    BillG Agent

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    I have a Rotel RB-985 amp, 5x100. The sound with my NHT 2.9 speakers is excellent. If I change to 2-channel amps, it will be for convenience - that will let me put the rear channel amps with the rear speakers.
    My opinion: The difference between the two amps you are looking at is, at best, extremely subtle. The differences in power and noise are not relevant. You did not list distortion, but knowing Rotel, both amps run in the 0.03% range which is inaudible.
    The only spec that seems to be major different is the damping factor. You have to take damping factors with a big grain of salt. Yes, it is important and is one indicator (among many) of a good amp design.
    Definition: The damping factor is defined as the ratio of the amplifier load to the reverse impedance of the output stage. In other words, it is the impedance of the speaker divided by the impedance of the amplifier output stage. When published as a specification, the speaker impedance is assumed to be an 8 ohm resistive load. The thing you hav to be careful of is the added impedance of the speaker wire. Example:
    If an amp is rated for damping factor of 180, that means its reverse impedance is 0.044444 ohms(8/180). Pure copper solid 12 gauge wire has a resistance of 1.588 ohms per 1000 feet (Handbook of Physics and Chemistry, 55th edition). I could not find a resistance spec for stranded wire, but it is probably slightly higher. If you have ten feet of speaker wire between your amp and speaker, you have added 0.03 ohms of impedance to the circuit (20 feet of wire at .001588 ohms per foot). From the viewpoint of the amp, the speaker load goes from 8 ohms to 8.03 ohms, an irrelevant change.
    However, from the speaker's viewpoint the load goes from .04444 to .07444 ohms, nearly doubling! The damping factor at the end of the speaker wire is therefore (8 / .07444) = 107.
    With an amp of very high damping factor, the difference is even more startling. Assume an amp of damping factor 500. At the speaker, the true damping factor is (8 / .046) = 174, less than 1/3 of the amplifier rating. With a damping factor rating of 1000, the true damping factor is (8 / 0.38) = 210. Diminishing returns ...
    If you change to 16 gauge speaker wire, you go to 4.016 ohms per 1000 feet. With this size of wire, the wire impedance makes the amplifier's reverse impedance almost irrelevant.
    In my opinion, damping factors over about 100 mean almost nothing. Between these two amps, the major differences probably have to do with how well they handle highly reactive, low-impedance loads. Such loads are often seen on electrostatic speakers but are very rare elsewhere.
     
  5. Kyle_Y

    Kyle_Y Stunt Coordinator

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    BillG,

    Wow! Those are some cool calculations you did there. They are very helpful to all of us. I think I get what you are saying, that is you have a damping factor of above 150 or 200, it makes almost an insignificat difference right? I was wondering about this exact topic, and now all my dampinf factor questions have been answered, thank you!
     
  6. JohnSC

    JohnSC Stunt Coordinator

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    BillG,

    Great response. You have put in into perspective for me.

    I wasn't aware of the rate of diminishing returns. Interesting stuff.
     
  7. BillG

    BillG Agent

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    Thanks, guys. I hope someone else who knows more about amps than I will chime in and verify both my calculations and my conclusions.
     
  8. Russ_K

    Russ_K Auditioning

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    BillG,
    [​IMG]
    Great way to look at this... You have me convinced that the 1075 is the way to go over the 1095 for surrounds and center. I was a bit worried about the headroom of this amp, but not any more.
    Thanks,
    Russ
     
  9. Dennis B

    Dennis B Stunt Coordinator

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    JohnSC,

    The RMB-1075 is home, and I only had a chance to listen to HT with it, no music yet. Now talk about an improvement! I haven't listen to 2-ch music to compare it directly with the 1070 yet, which I'll do in the next couple of days, but for HT there was a huge difference between the Rotel and my now pre/pro Marantz 7200. Sort of the same thing I felt when listen to music with and without the 1070. More to come.

    Dennis
     
  10. JohnSC

    JohnSC Stunt Coordinator

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    Dennis B,

    Great stuff. Let me know how you go. I ordered the 1075 today. I should get it next Friday. Let me know how you go.

    John
     

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