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Thread: DBX DriveRack 4800

  1. #46
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    Okay, thanks. Now I know what you mean. (This is already at 0 though).

    But the image I linked too. How would you set this for the output? The input should be set, so full scale on the Onkyo results in "almost clipping". Do I connect the amps to the DBX and do the same clipping test?

  2. #47
    Senior Member 1audiohack's Avatar
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    Quote Originally Posted by Jonas_h View Post
    Okay, thanks. Now I know what you mean. (This is already at 0 though) Do I connect the amps to the DBX and do the same clipping test?
    You could do that if you needed maximum output from the amps, but chances are, and hopes are that you don't. If for instance the HF amp input attenuator knob needs only to be at say 10 o'clock for the HF part of the system to play as loud as you would ever play it, then having it there gives you all the volume you need, leaving all the remaining headroom available for dynamics with the minimum amplification of noise. If it is turned up any higher, it only raises the noise floor.

    The LF amps may very well require the input attenuator be much higher, like say 3 o'clock or more but the same principal applies, advance the attenuator only as far as needed to achieve the desired maximum loudness. In the event that you don't get enough output with the amp attenuators wide open, (check the amp for switchable input sensitivity) you would then have to increase the 4800's output level to those channels that are not loud enough. At least woofers don't hiss.
    If we knew what the hell we were doing, we wouldn't call it research would we.

  3. #48
    Senior Member Lee in Montreal's Avatar
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    One test would be to set all atenuators on the amps to zero and the volume control on the preamp at max. Then raise the amps' attenuators until the maximum sound level to be sustained at home is reached. This is most likely where the system will produce the least hiss...

  4. #49
    Senior Member Eaulive's Avatar
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    Quote Originally Posted by Lee in Montreal View Post
    One test would be to set all atenuators on the amps to zero and the volume control on the preamp at max. Then raise the amps' attenuators until the maximum sound level to be sustained at home is reached. This is most likely where the system will produce the least hiss...
    But the problem, and as I understand, the concern of Jonas, is to know exactly where to set the levels on the amplifiers for the different drivers.

    Example, for my 4520 setup, I have to have the MF set 7dB lower than the LF, and the HF 11dB lower than LF.
    It's easy to do with the driverack, but if you want to preserve headroom it's better to do it directly on the amps.
    Now the problem is to know where to set the amplifier attenuator to have exactly this value.

    What I do in this case is that I disconnect the speakers, drive the system right into the onset of clipping with the correct attenuation set on the driverack, then I reset the driverack to 0dB and lower the gain on the amp to return back to the onset of clipping.

  5. #50
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    Quote Originally Posted by Eaulive View Post
    But the problem, and as I understand, the concern of Jonas, is to know exactly where to set the levels on the amplifiers for the different drivers.
    Yes, exactly. I found this in the Crown manual though: http://dl.dropbox.com/u/10759510/ForumPics/ctsLevel.gif

    If the listings are precise, I can se exactly how much I'm lowering the gain on each channel.

    In regards to the "analog gain structure" function on the DBX4800 I linked to earlier, can anyone explain this? Not much info in the manual, and I am a little confused if lower or higher values are preferable. With lower output-clip values, I am able to raise the gain on the amp and be closer to the 3/4 gain setting recommended by Crown.

  6. #51
    Senior Member 1audiohack's Avatar
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    I'll see what I can find.
    If we knew what the hell we were doing, we wouldn't call it research would we.

  7. #52
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    Quote Originally Posted by 1audiohack View Post
    I'll see what I can find.
    I would really appreciate that - thank you

  8. #53
    Senior Member 1audiohack's Avatar
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    Is that analog gain structure screen shown on your DBX 4800 screen? Or on a PC with the GUI open? I don't have a 4800 here to check. Are you working to cal an RTA mic to the 4800 or is this something else?

    Sorry I am losing focus on what we are trying to do, but I am easily confused.☺

    Do you have a mic for the DBX RTA? After you get the gain settings sorted out in the DBX you could certainly use the RTA to get the amp levels set as long as you stay out of the region where the room stores energy, RTA is time blind and will lie to you in that region.
    If we knew what the hell we were doing, we wouldn't call it research would we.

  9. #54
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    Yes, the analog screen is shown on the PC. I have not used the psysical buttons yet, as System Architect is much easier. When I change the settings, a higher output clip value results in louder volume. But I dont know what the goal should be.. High amp gains and low output clip or the other way around. I will be home this weekend (have been travelling because of my work) and I will play with the settings while playing full scale pink noise and find the settings where "almost" clipping appears and set them amp gains for that.

    Someone told me once though, that the Crown's would not come alive if its gain was set very low.

    I have no plans on connecting a mic to the DBX. I have done some measurements in REW, but I would rather use the drivers sensitivity for level mathing rather than measurements. As you say, room acoustics play a big role in the measurements, and I will risk that a room mode gives me incorrect level matching between the HF/LF.

  10. #55
    Senior Member 1audiohack's Avatar
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    Hi Jonas;

    The region where the RTA is incapable of giving useful data is easily calculated. RTA is a usefull if somewhat limited tool and you have it so, at the very least you could use it to get the three haystacks balanced ie subs, bass and HF. If you don't mind sharing, what are your room dimensions?


    This whole gain structure concept is just like a compound mechanical linkage system with movable levers and fulcrums. I don't know about anyone else but once I got my head wrapped around it in a way that made sense to me, it seems fairly intuitive.

    If my understanding of the concept and the attempted explanation thereof is only confusing, please let me know.

    I figure it's only knobs and it is easy to return them to where you were before if you're unhappy with the results.
    If we knew what the hell we were doing, we wouldn't call it research would we.

  11. #56
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    Sorry I have been away from the thread. I have played a lot with the gain structure "issue" and read about it, and I think I have it dialed in. I set the sensitivity so it was almost clipping when getting full input from my processor. I then set the amp gain so no measurements should be done at the processor to reach reference level when set to 0dbFS.

    I measured the response with the amp gains set equal where the LF/HF were level matched on the DBX. Then I set the HF to unity on the DBX and lowered the amp gain to the HF until the measurements where identical. (Adjusted 10 "click" down, and lowered 2db on the DBX.). This should eliminate the influence of room gain on the measurements, as I used the "correct" level matching as reference.

    I am very satisfied with the results now. It sounds amazing and absolutely NO hiss. Next upgrade will be a larger room - but this requires a new home, so its far out in the future

    Thanks for all your help so far!

  12. #57
    Senior Member Eaulive's Avatar
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    Quote Originally Posted by Jonas_h View Post
    I am very satisfied with the results now. It sounds amazing and absolutely NO hiss
    After all is said and done, this is the most important thing!

    I'm glad you enjoy your setup

  13. #58
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    You're right!

    But I still have to get everything dialed in, in regards to EQ etc. I have used Audyssey Pro for the calibration so far, and it does an ok job.. But I feel that it "removes" punch and other things in the sound to hide possible room artifacts. The acoustic in my room is very good, so I think that simple PEQ filters to the <1000hz area will yield in a better result.

  14. #59
    Senior Member 1audiohack's Avatar
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    I only did a quick read on Audyssey and can not argue it's strengths or limitations. I doubt it's execution is bullet proof however.

    That said, if we are considering a reasonably standard acoustical space that does not have dimensions with severe aspect ratios or major openings into other rooms, the region where a room is mode dominated can be calculated easily as follows;

    Multiply the speed of sound by three, divide the answer by the rooms smallest dimension, eg 344 X 3 = 1032 / 2.44m = 422.9Hz

    Divide the speed of sound by two, divide the answer by the rooms largest dimension, eg 344 / 2 = 172 / 5.8m = 29.6Hz

    In the above example the region between 30Hz through 423Hz is where modes dominate and cannot be controlled by EQ. Lowering certain offending bands of energy put into the room can only help by not aggravating the beast, but you then have limited the dynamics of the system.

    RTA cannot give you good information in this region and EQ really can't help. Do the math and see if this is the region that you don't like what Audyssey is doing.
    If we knew what the hell we were doing, we wouldn't call it research would we.

  15. #60
    Senior Member Mctwins's Avatar
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    Quote Originally Posted by 1audiohack View Post
    I only did a quick read on Audyssey and can not argue it's strengths or limitations. I doubt it's execution is bullet proof however.

    That said, if we are considering a reasonably standard acoustical space that does not have dimensions with severe aspect ratios or major openings into other rooms, the region where a room is mode dominated can be calculated easily as follows;

    Multiply the speed of sound by three, divide the answer by the rooms smallest dimension, eg 344 X 3 = 1032 / 2.44m = 422.9Hz

    Divide the speed of sound by two, divide the answer by the rooms largest dimension, eg 344 / 2 = 172 / 5.8m = 29.6Hz

    In the above example the region between 30Hz through 423Hz is where modes dominate and cannot be controlled by EQ. Lowering certain offending bands of energy put into the room can only help by not aggravating the beast, but you then have limited the dynamics of the system.

    RTA cannot give you good information in this region and EQ really can't help. Do the math and see if this is the region that you don't like what Audyssey is doing.
    Why are you multiply the speed of sound by three?

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