I need help with the crossover on my plate amp.

Discussion in 'Archived Threads 2001-2004' started by Scott Parker, Nov 20, 2002.

  1. Scott Parker

    Scott Parker Agent

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    I've got the 250watt plate amp from parts express and I was doing some experimenting with it the other day. Here's my equimpent:
    Denon 1802 (80hz crossover)
    2 Le Amps
    PE 250w plate for my sub
    nOrh 4.0's all around
    DIY 340L EBS Tempest (sonotube)

    I was trying to see how my nOrh's sounded crossed over at 60hz. So I took the preouts from my receiver into my Le Amps and then into the high level in on the plate amp. I hooked the nOrh's up to the high level out. So I figured if I crossed the plate amp over to 60hz this should work right? Well, the bass from my subwoofer dropped to about nothing, and the nOrh's sounded about the same. Just out of curiousity I played some test tones from 100z to 600hz and I could still hear a 600hz tone from my sub loud and clear with the nOrh's shut off and the plate amp crossed at 40z. I was like man that sucks. So I decided to see if the crossover in my receiver's any better. So I hooked the plate amp up to my sub out in my receiver and set the crossover to max and I could still hear 250hz fairly well, but defintely nothing at 600hz. Then I crossed over my plate amp to 40hz (now double crossed) and I couldn't hear anything above 160hz, which seemed a whole lot better to me. But now I'm losing like half my signal through crossovers. So now I'm starting to look at preamps with good hi-pass and low-pass crossovers. Norh is coming out with one that's looking like just what I need for around $700. If I hook up my sub to the low-pass at like 60hz does that mean everything else will come out of the high-pass. If so, then why do they allow high-pass crossover settings from 1.2khz to 3khz. This would seem like I'd be missing everyting from 60hz-1.2khz. They also have a set of full range outputs, so would I use those. Sorry for all the questions guys, I'd just really like to understand all this for once :b
     
  2. Ryan Schnacke

    Ryan Schnacke Supporting Actor

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    As you learned, crossovers are not brick walls. The "cutoff" frequency is usually just the frequency where output is down by 3dB. From there the response will continue to fall off as frequencies get farther from the pass-band. The slop of the fall off is determined by the order of the filter. I'm pretty sure the PE plate amp has only a 2nd order filter (12dB/octave). But your receiver likely has a 4th order filter (24dB/octave). So even though you had a lower cutoff frequency with the plate amp, the receiver's filter quickly overcame that disadvantage. Not to mention the fact that you're cascading 2 filters for a 6th order rolloff above 150Hz.

    Another note about the PE plate amps: The crossover only affects the low-pass filter. The high-pass filter is at a fixed frequency (can't remember off the top of my head) and I think the high pass has a fairly undesirable Q - over 1.0 IIRC.

    I'm wondering why you'd want to run the norh 4's any lower. Mine sound great crossed at 80. They don't really have a lot of bass response anyway. Barnes claims extention to 55Hz but they start rolling off at 65Hz. And your Tempest should do a terrific job filling in.

    I don't know what norh preamp you're referring to, but it sounds like it would be used as an active XO between the midwoofer and tweeter in a 2-way speaker. Barnes is an active proponent of using all active XO's. He created the Multiamp for that specific purpose. It was designed to work specifically with the norh 9.0's once you removed the stock passive XO. Barnes is also not a big proponent of subwoofers so I'd double check that it has accomodations for a subwoofer output.

    There are other receivers and preamps with adjustable XO's. Outlaw and Sony ES come to mind.
     
  3. Scott Parker

    Scott Parker Agent

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    Thanx Ryan, that makes more sense. I was just experiementing with the nOrh's, yeah I know they don't have much output below 65hz [​IMG]. But I hope pretty soon that I'll have fronts that go to around 40z. But I'm still not clear on the high pass and low pass filters. If the high pass filters are at a fixed frequency, say 500hz how are you supposed to be able to cover the range from 80hz to 500hz. And why would anyone use the high pass ins and outs if this is the case? If I crossed the nOrh preamp over at 60hz and ran that to my sub where could I hook my fronts to get the rest of the frequency range (60hz - 22khz). Or is this even possible? It'd seem like the full range outputs would still be full range and the high-pass outputs would only be adjustable from 1.2khz - 3khz, hence missing the 60hz - 1.2khz range.
     
  4. Chris Tsutsui

    Chris Tsutsui Screenwriter

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    The high pass filter of your amp is 6db 125hz.
    This would mean you set the crossover on the plate amp to 125hz or less ( I think )
    The 125hz high pass is good for satellites or speakers that can't extend deep. I am currently using the 125hz high pass which is working VERY well for me. I am using stereo tempest subs that are placed right next to each main, so I get stereo bass that's integrated with the mains. This takes a load of bass from my mains to improve their sound quality and the sound stage is not affected.
    Another reason why I am using high pass is because my pre/pros (sound card on my pc) has a fixed crossover point at 540hz. That is a bit on the high side for a tempest wouldn't you think? [​IMG]
    High pass filters, if set to 500hz would simply require a low pass filter that can be set that high as well. If the high pass filter on my plate amp was 500hz, and all I have is my 40-160hz low pass then I'd have a gap.
     
  5. Scott Parker

    Scott Parker Agent

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    Thanx Chris, yeah it makes sense if the high pass is 125hz. What sound card are you using? I'm using the M-audio AP 24/96, it sounds pretty good. I also noticed much less bass coming from my sub when I used the high-pass ins as opposed to the the RCA line in. Have you had this problem? I think I've come up with a semi-solution. I'm going to set my mains to large and let them roll off naturally and then I'll adjust the plate amp crossover using an SPL meter to fill in the bottom end. I'm thinking it'll probably wind up around 60hz.
     
  6. Ryan Schnacke

    Ryan Schnacke Supporting Actor

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    Scott,

    "If I crossed the nOrh preamp over at 60hz and ran that to my sub where could I hook my fronts to get the rest of the frequency range (60hz - 22khz)"

    A true crossover will have both a high-pass filter and a low-pass filter working in concert to break the signal into parts without losing or modifying any part of the signal. How are you going to "cross" the norh preamp at 60Hz? Is this a function built into the preamp? If so, it should have accomodations for both the sub out and the main speaker outs.

    Chris,
    Did you mean to say the amp has a 6dB/octave filter with Fc = 125Hz? Where did you find this? Looking at the schematic I see a 2nd order (12dB/octave) Sallen-Key filter with C1=C2=0.1uF and R1=5.6kohm and R2=22kohm. This would have Fc=143Hz and Q=1 for 1.2dB of boost at 205Hz. This is what I meant in my previous post about "undesirable Q". Who wants boost at 205Hz?
     

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