CS-Ultra BFD question

Discussion in 'Archived Threads 2001-2004' started by Shaun Thompson, May 27, 2002.

  1. Shaun Thompson

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    I recently purchased a BFD to help out with how my new SVS CS-Ultra sub was responding. I’ve felt that I just wasn’t getting enough oomph from this sub like I should be.
    Here is how my room is setup.
    [​IMG]
    The sub is connected as follows. Pioneer Elite receiver VSX-26TX with the crossover set at 80HZ --> SVS black box
    --> BFD --> Samson 1000. The sub has been calibrated with a radio shack SPL meter using Avia at about +7 over my mains or 92 db.
    Here is the graph using the Stryke zone CD measured at reference level from the listening position using only the CS-Ultra sub.
    [​IMG]
    Since I can only mess around with my equipment one day out of the week – my brother currently has all my stuff while I’m looking for a house – I wanted to get input from people that are familiar with calibrating using the BFD on some filters I could try out so I don’t spend a whole bunch of down time with my HT.
    Thanks for any input!
     
  2. Shaun Thompson

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    No one has any ideas on some filters to use???? I guess my main problem was figuring out the BW to use on the filters to smooth out the major dip and major peak.

    Thanks
     
  3. brucek

    brucek Second Unit

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    Shaun,

    I hope you don't mind if I pass on my two cents with regard to the positioning of your BFD / Black Box. If you're satisfied, that's great, but here's my feeling on the subject.

    It would be best to place the Black Box line amp between the BFD and the SVS Samson power amp for several reasons. Feeding the line amp from the processor isn't the best idea.....

    The BFD is not an amplifier, it's a unity gain device. Since it's digital and you're feeding an ADC/DAC chain you want to optimize its input level to take advantage of all the bits available. By this I mean to take maximum advantage of the dynamic range provided by the analog to digital converter in the BFD or any ADC for that matter, you need to provide it with a maximum input signal level that takes advantage of the greatest number of bits to represent the resultant voltage samples.

    If the range from your weakest signal to the strongest signal was only half the required maximum input level, you would be robbing yourself of dynamic range and subsequently increasing your signal to noise level. To get the most out of this system you would like the maximum signal sent to the BFD to "just" not turn on the red LED. If the yellow LED comes on in explosions etc, that would be fine. The BFD has no compressor or limiter on its input, so you can easily overdrive it and clip the output.

    This means once you've reached and set this level, you can't increase the level any more even if you wanted to. So it's the "output" analog signal from the BFD that you want to amplify with your black box line amp, not the input signal to the BFD which is easily set to optimum by your processor.

    The BFD's unity output (or less if filters are applied) can then be amplified to your hearts content with the line amp. If the line amp can create 15dB gain, then it will "all" be available to you, instead of being limited if you placed the line amp before the BFD.

    Anyway, as far as your response curve is concerned. I guess you've tried the other corners in your room to see if you can help your response and maybe get rid of that dip.

    Dips are sometimes hard to get rid of. You can throw more power into them and they stubbornly just sit there. Other times, they will respond very nicely. You'll have to see how that goes.

    Unfortunately, there is really no way to get around spending many hours fooling with this device - most owners will tell you that. It takes a lot of time.

    The bandwidth is the toughest concept to get a feel for. The most important thing to remember is that the bandwidth you enter in the front panel is the amount above "and" below at the 3dB down point that the filter will effect.

    In other words, if you enter a 20BW, this is 20/60th or 1/3 octave above and 1/3 octave below the center frequency at the 3dB point from zero. So the full effect will be still be felt even wider than what you enter until the filters effect is at 0dB. Also, the larger the cut or gain you enter, the wider the effect will be felt.

    So if I have a center frequency of 50Hz and enter a 20BW. This will be 1/3 octave above at 63Hz and a 1/3 octave below at 40Hz where the effect for example of a -12dB cut would still be effecting -3.5dB at 40hz and 63Hz and would reach zero effect a fair bit wider than that. So you can see that 20BW is fairly wide.

    My advice is to start small with a single filter and check its effect and then carry on from there.....

    Anyway, I suspect your peak from 40Hz - 68Hz will likely take two filters with bandwidths of around 10/60 or less centered around 47Hz and 57Hz with cuts of about 8dB to 10dB......That's a start.

    brucek
     
  4. Shaun Thompson

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    Thanks for the input. I'll try out the filters for starters and see how it affects my sub response. I wish I had easier access to my equipment but until I buy a house that's how it goes.

    I guess I moved the black box before the BFD in my setup because when I had placed the black box after the BFD and played a heavy bass scene - matrix helicopter explosion - at reference levels I could not get the yellow led light to even show. It would go up about 2 notches and that was about it.
     
  5. brucek

    brucek Second Unit

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    Shaun,

    Interesting, when the BFD level switch is in the -10dB position I don't think I've heard of anyone having input level problems - but if that's the case with your equipment, then your existing setup is necessary...

    brucek
     

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