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Thread: Amplifier/sub question...

  1. #1
    Newbie technopsych's Avatar
    Join Date
    Mar 2004

    Amplifier/sub question...

    I could use a little help with set-up on my system. Thanks in advance for any suggestions - sorry for the long post.

    I have no head-unit, car pc with Soundblaster Live! 5.1 PCI card, two amplifiers - a 4 channel MTX and a 2 channel MTX, Polk Audio 6.5 component speakers in front, 6X9 stock speakers in back, two 10" MTX subs in back.

    So, the 4 channel amp runs the front and rear speakers via input from SB Live soundcard (green and black colored jacks respectively). After playing with the set-up in SB Live software and adjusting the gains, I was able to get good sound out of all four speakers.
    I tried running the subwoofer output from the SB Live (orange colored jack) to the input RCA's on the 2 channel amp and then from the amp's speaker 1 and speaker 2 output to my two subs.
    [SB Live orange jack --> phono jack cable --> RCA cable --> R&L RCA inputs on 2 channel amp --> speaker 1&2 (- and +) outputs --> sub 1&2 (- and +)]

    With the above set-up on the 2 channel amp I was getting no sub output. When I plugged in the jack to the orange connector on SB Live I got a thump, but then no sound from subs.

    I checked all the wiring on my subs - OK. The amp is powering up and looks to be working. I set up the SB software for the 5.1 setting and ran the test - still no sound from "center" speaker. Hooked different speakers to orange jack on SB Live (without amp)- got sound.

    Partial Fix:
    So, I saw a diagram online that prompted me to change the set-up. I ran a pair of RCA's from the "sum out" RCA jacks on the 4 channel amp to the "input" RCA's on the 2 channel amp. (So, in this set-up there is no input from SB Live orange jack.) I got both subs to work and they sounded fine.

    This may be how the system needs to be connected, but I thought folks were running sub input directly from the soundcard. I would rather have that set-up I think.

    1) So, is there a way with my set-up to run the subs with input coming from SB Live soundcard, not from the 4 channel amp's "sum out"?
    2) Are there advantages or disadvantages to either set-up?
    3) If not, and my set-up as mentioned above is correct:
    a- Does anyone know what the settings in SB software should be (5.1 vs 4 speaker)?
    b- Does anyone know what the cross-over settings on my MTX amps should be? The 2 channel has a switch that has three cross-over settings; "LP", "FR", and "HP"? The 4 channel front has cross-over settings of "FR" and "HP" and the 4 channel rear has cross-over settings of "LP", "FR", and "HP".

  2. #2
    Maximum Bitrate Altimat's Avatar
    Join Date
    Oct 2003
    Use the "sum out" outputs on the 4-channel amp to send the signal to the two channel amp. Don't bother with 5.1 or the orange sub output on the sound card, and set your sound card for two speaker stereo. The two channel amp should be set to LP, or low pass, and the four channel amp to either FR full range or HP high pass, but use HP only if you can adjust the crossover frequency. If your sub is <70Hz, then limiting the <70Hz frequencies from the front speakers may have some advantages. Most likely you'll want to use FR on the 4-channel.

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