tempest/shiva wiring question

Discussion in 'Archived Threads 2001-2004' started by CodyL, Sep 11, 2002.

  1. CodyL

    CodyL Extra

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    I plucked the following from a thread on another message board:

    "If you have a choice, wire your sub like I did. That is: preamp out from your A/R receiver or preamp, to the preamp ins on the sub amp. Then, preamp out from the sub amp back to the preamp ins on your A/R receiver or preamp. This will take care of all high pass filtering in the crossover, and will avoid cascading crossover conflict between the sub amp and the LFE channel on your A/V receiver or preamp. Set the subwoofer control in the base management set-up to sub woofer "off". All low frequency info, from the crossover set point down, will now go to the sub. When done this way, the sub will play a part in all your audio inputs, not just LFE channel input from DVD's. The bonus is your mains will now have more available power and less distortion in the upper bass and lower midrange. Bare in mind that the LFE channel may not have any info, thus your sub could languish while your mains struggle."

    Does anyone have any thoughts about whether this is a good/bad idea? Theory sounds good, about having the sub fill areas other than LFE, but I am new to this so will defer to you all. My apologies to the original poster.
     
  2. jeff lam

    jeff lam Screenwriter

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    I would only see this as a good option if it were a two channel only system and the mains were flat down to 55hz or lower.
    I don't really see how it would benefit you otherwise.
     
  3. Ryan Schnacke

    Ryan Schnacke Supporting Actor

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    The person who wrote that post was trying to tell you how to use the subwoofer's adjustable electronic crossover to select the desired crossover frequency rather than "settling" for the receiver's crossover which is usually 80Hz and not adjustable.
    To do this correctly you'd want to set your main speakers to large and your center/surround speakers to small and your sub to none. This way your receiver redirects all the bass to the main speakers pre-amp outputs. Then you connect those preouts to the sub for filtering. Then connect the high-passed preout from the sub to the main speakers' amplifier (which may be in the receiver).
    Its a good idea but there's a catch. It assumes that the crossover in the subwoofer will vary both the high-pass frequency and the low-pass frequency when you turn the frequency knob. And I can see from looking at the schematic for the Parts Express/Apex plate amps that this is NOT the case. The preamp output from the sub has a fixed high-pass crossover frequency. So when you turn the "crossover frequency" knob down low you may be creating a frequency response hole between where the low-pass cuts off and the high-pass picks up.
    We know that these things aren't brick-wall filters - especially when they're only 2nd order as in the PE amp. But the hole could still be significant. If I'm reading the schematic right (its so fuzzy its hard to read) then I'm seeing a 2nd order Sallen-Key filter around op-amps U2A and U2B with C= 0.1uF and R=22k and 5.6k. That gives you a cutoff frequency of 143Hz and an unexpected 1.2dB of gain at 205Hz.
    Yes I know the PE amp manual says the preouts are full range but it just ain't so. This was brought up on the PE forum.
    So basically the original idea - a fully adjustable crossover - is a good one. But there's a fair chance that your subwoofer's crossover is far from ideal. The PE amp's crossover is likely to be significantly less desirable than whatever's built into your receiver.
    Of course you could always modify this built-in crossovers. Just like modding the rumble filters except there's two of them - one for the left channel, one for the right.
     
  4. Chris Tsutsui

    Chris Tsutsui Screenwriter

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    So if the pre-outs of the amp arn't full range, then will it affect the sound if I connect a second PE amp to the first one and not use a Y-splitter from the receiver?

    I have two 250watt PE amps that will power 2 tempests. Will one sub sound different due to the passthrough of the other amp?

    I don't really like the built-in crossover for the PE amp that much, I'd like to build my own adjustable external crossover for it. Do people do this successfully?
     
  5. Ryan Schnacke

    Ryan Schnacke Supporting Actor

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    Yes, they'll sound different. You'll probably have to turn the gain on the 2nd amp up higher to get to the similar volume levels and it won't sound right because of the 12dB/octave slope of the signal from the first amp.

    It would help if you would describe how you are wanting to implement the external crossover. Are you using the receiver's subwoofer pre-amp output or will you set sub = none? Will you be using pre-amp signals and op-amp filters or speaker-level signals and passive filter? Or maybe some mix of the two?

    I think the difficult part of the DIY adjustable crossover would be synchronizing the crossover points for the left speaker hi-pass, right speaker hi-pass and subwoofer low-pass. This might be a good application for some digital potentiometers.
     

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