Sub connections and phase

Discussion in 'Hi-Fi and General Audio' started by michaelab, Dec 10, 2003.

  1. michaelab

    michaelab desafinado

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    We've done the "absolute phase" debate here a number of times. My DAC64 inverts phase and after the last discussion I vowed to try hooking up my speakers the other way around just to see if I could hear a difference where I'd never heard one before :rolleyes:

    Well, I did hear a difference, but it has nothing to do with absolute phase. The bass suddenly became a lot fuller, bloated almost. At first I wondered what was going on but then I realised that of course my sub was still connected to the amp "correctly", that is to say, not switched around so the relative phase of the sub to the speakers was altered. For those about to make a smart arse remark, when I tried inverting phase before and heard nothing it was before I used my sub for music ;)

    Now, I connect my sub (MJA Pro 50) via the high level connection so it has 3 wires that go to the speaker terminals on my amp: one each for the +ve Left and Right and one to connect to one of the negative terminals (it doesn't matter which). Is it possible just to reverse those connections just like I've reversed the speaker connections? ie: hook the two "postivie" L & R wires to the respective negative speaker terminals and the "negative" wire to one of the positive terminals? I didn't want to just try that for fear of damaging something.

    My sub has phase adjustment from 0 to 180 deg but since I had to have it set to 90 deg before to get good integration I would need an adjustment to 270 deg to get the same effect - or wouldn't I?

    Previously using either 0 or 180 deg gave ridiculously bloated bass and 90 was perfect and since I changed the speaker phase it's more or less the other way around but I don't quite understand why. Ideally I'd to "wire up" my sub with inverted phase just like I have done my speakers and then I can go back to using the settings that I know work that took me ages to get right :(

    Michael.
     
    michaelab, Dec 10, 2003
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  2. michaelab

    technobear Ursine Audiophile

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    Nooooooooo! Don't do it. Sounds like a very bad idea to me. Just adjust your sub's phase control until it sounds right. You may need to adjust the sub level as well.
     
    technobear, Dec 11, 2003
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  3. michaelab

    Paul Ranson

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    If your power amp is conventional (IOW not 'bridged') then the -ve outputs are connected to earth. If your sub connects its -ve input to earth then connecting that to a +ve output on the amp will be bad. If it doesn't then you'll get an inverted left or right channel rather than a right way up sum of left and right.

    So don't do it....

    I always knew sub-woofers were a bad idea....

    Paul
     
    Paul Ranson, Dec 11, 2003
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  4. michaelab

    michaelab desafinado

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    Whilst I'm demoing amps this week (SimAudio i-5 since Tuesday and Krell KAV-400xi coming tonight) I'm going to leave the sub disconnected for music as it won't help at all in making valid comparisons.

    I'll revisit the issue later with my ETF analysis software at hand for proper integration :)

    Michael.
     
    michaelab, Dec 11, 2003
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  5. michaelab

    Paul Ranson

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    I had a play with the free ETF software but found it very impenetrable and inconsistent. I have no confidence in it, certainly not enough to warrant expenditure. The search continues....

    There are a bunch of old Linux bits ('wfir' I think) that look like they'll build into a simple room measurement system.

    Paul
     
    Paul Ranson, Dec 11, 2003
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  6. michaelab

    michaelab desafinado

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    With the right laptop (it has to have a stereo line-in and not just a mic input) and the right bits of phono cabling it works like a charm.

    Using MLS test sequences instead of test tones not only gets results much quicker but is far more accurate. I always do each measurement 2 or 3 times so I know it's repeatable and wasn't just a one off cock-up.

    The user interface is a bit fiddly and getting the levels right can test one's patience aswell but once it's setup it's pretty straightforward. I use a Radio Shack digital SPL meter as a microphone (sitting on my camera tripod :) ) as it has an RCA output.

    Certainly way more accurate than messing about with a test tone CD and an SPL meter. I felt getting the full version was money well spent. Being able to take lots of measurements in one go and save and compare them etc certainly makes it a lot easier to use.

    Michael.
     
    michaelab, Dec 11, 2003
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  7. michaelab

    Paul Ranson

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    I have the analogue Radio Shack SPM and sufficient cabling and tripods etc. So I thought I might get somewhere. Probably operator trouble. I tried my laptop which has line in/out and a proper PC with a decent M-Audio card.

    I may try again but the demo version annoyed me.

    The Linux derived code uses MLS sequences. There are separate apps to create a sound file, to record the result and to analyse the recording. All nice easy command line stuff....

    Paul
     
    Paul Ranson, Dec 11, 2003
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