Help with frequency roll off on QSC please

Discussion in 'Amps and Cabs [BG]' started by Sanctum, Jun 13, 2002.

  1. Ok, my QSC 1850HD poweramp has frequency roll off switches. Here is the manual quote about them:

    "The low-frequency (LF) filter rolls off signals below either 30hz or 50hz. This improves bass performance by limiting sub-audio cone motion, making more power available for the speakers' rated frequency range."

    It goes on to say the 50hz filter is good for most full range cabs. Here are the specs on my Eden cabs:

    210XLT +/- 2db 48hz to 14khz       -6db @ 35hz
    115XLT +/- 2db 46hz to 14khz        -6db @ 38hz

    So I had the filter set for 50hz as this seems to fit the range, but what about those last numbers? I'm a bit confused. Everything sounds ok to me, but as I'm new to this kind of option, I thought I'd ask. QSC says the 30hz filter is for subwoofers, which I dont consider these cabs to be.

  2. ZuluFunk

    ZuluFunk Supporting Member

    Apr 14, 2001
    I'm not sure.

    I think the freq of a low B is just over 30hz and a low E is like 41hz. I could be wrong and I'm sure to get jumped soon if I am.

    Does this mean if I'm flat on my low B with the 30hz low pass filter engaged, I will hear nothing? No. I think it just filters the subharmonics.

    I have mine set now with the 210 side cut off at 50hz and the other with my 115 cut off at 30hz.
    Using the low pass makes the signal tighter. Try doing a comparison and you probably won't be able to "hear" the difference. It's supposed to be better for your cabs to focus them on the frequency range of your fundamentals.
  3. Thanks man, I was just thinking of switching the 1x15 to 30hz tonite.

  4. VicDamone


    Jun 25, 2000
    I agree with ZuFu, and if we're wrong I'm sure will hear about it. I've got the 50Hz filter on going to the 2-10" which seemed to give it more focus. On the other hand the 1-18" subwoofer is wide open.
  5. geshel


    Oct 2, 2001
    The 30Hz high-pass would be good to get rid of really low frequency gunk like your fingers hitting the strings etc (of course you might want this, if the cabs can handle it). Using the 50Hz cut on a 210 sounds like a good idea too, unless it's something like an Acme that is tuned below 50Hz.
  6. geshel


    Oct 2, 2001
    It's a high-pass filter, BTW. :)
  7. I'd been considering buying that same amp, and wondered about how I would set the cutoff freq., myself, if I did get one.

    The low B string does, in fact, produce a fundamental of 31 hz., if it's tuned properly. (E = 41hz., A = 55hz., D = 74, & G = 98.)

    As others here have suggested, I would pick 30 hz. for a 15" cab, a 50 hz. for a 210. However, that would just be my "first guess." Definitely try it both ways to see what sounds better to you, rather than pick the one that seems to make sense based upon the fundamental frequency of the note that you're using.

    You see, what people often forget is that, while a B string will produce a fundamental freq. of 31 hz., that is not the only freq. it produces and, from what I understand, isn't even the most important frequency. Often, some of the harmonics are actually louder and play a more important role in the sound than the fundamental, especially on notes that are nearing the bottom limit of human hearing. I've heard of bass players rolling off their signal at around 100 hz., and reporting that it makes it sound "better" to them (tighter, better definition, etc...). As you can see, this would be cutting back on the fundamentals of all the open strings. However, you can still hear the notes, because the harmonics are still there.

    [edit for typo]
  8. chucko58


    Jan 17, 2002
    Silicon Valley, CA, USA
    I paid for all my gear myself. Well, me and MasterCard.
    Gee, where's Bob Lee when you need him?

    The -6 dB numbers should be considered the lowest frequency those cabs can reproduce.

    Setting the filter at 50 Hz with these cabs is a bit of a compromise; you could physically damage the speakers if you fed them a steady diet of 30 Hz, but the 50 Hz cutoff leaves off a bit of low end that (IMHO) the cabs could handle safely.

    Playing a low B with the 50 Hz filter engaged shouldn't be a problem. Bass guitar signals are mostly harmonics anyway; the fundamental is usually weaker than the 2nd harmonic, so it's not like you're cutting out the biggest part of the sound. And for low notes, human hearing synthesizes the fundamental from the harmonics. That's why little tiny computer speakers don't drive you bonkers with their lack of bass!

    I have a pair of Acme Low B-2s, so I set my RMX's filter at 30 Hz. Works like a champ.
  9. Jerry J

    Jerry J Supporting Member

    Mar 27, 2000
    P-town, OR
    Actually the -6dB figure is indicating that at 35Hz that frequency is down 6 decibels from flat, on the 2x10 cab.

    You know it's funny but I almost never play an open B string on any song that I play. Usually the lowest note that I play on my B string is the C and that's even rare.
  10. I also hardly ever play a low B, but boy, that B-string sure does make a nice thumbrest ;-) !!!!
  11. Bob Lee (QSC)

    Bob Lee (QSC) In case you missed it, I work for QSC Audio! Gold Supporting Member Commercial User

    Jul 3, 2001
    Costa Mesa, Calif.
    Technical Communications Developer, QSC Audio
    Especially if you play a 5-string with a low B, the 30 Hz setting would probably be better than the 50 Hz.

    You don't have to worry about subharmonics, because there are none unless you use a subharmonic generator of some kind.

    When you get down to the 31 Hz neighborhood of the fundamental of low B, most bass cabs have reduced output, and the sensitivity of human hearing is also greatly reduced. You hear proportionally more harmonics--2nd, 3rd, 4th, 5th, and so on--than on higher notes, but from a psychoacoustic standpoint, your brain seems to recognize the fundamental fairly well anyway.

    The biggest danger in putting very low frequencies at relatively high power into a speaker cabinet is overexcursion (assuming that the voice coil and other components are capable of handling the power), because cone excursion is inversely proportional to frequency and below the cabinet's frequency, the air loading on the cone decreases--that is, the air in the cabinet becomes more compliant because it can move in and out through the port with less resistance. As a result, you're more likely to make the cone(s) bottom out. The low-frequency filters are supposed to help guard against that.

    If you like the sound of the 30 Hz setting (or no LF filtering at all) better than the 50 Hz, by all means use it, and keep it if you get the sound levels you want without making the speaker cone(s) bottom out. But if you ever hear the unmistakable whack of a cone bottoming out, back off immediately on your levels to avoid damage, and consider your options: changing your filter or EQ settings (which will alter your sound), reducing your volume (which will alter your sound, too, and might alter your playing style), or getting a beefier speaker system (will probably alter your sound, but might expand your options).
  12. ZuluFunk

    ZuluFunk Supporting Member

    Apr 14, 2001
    This was a great thread.

    Thanks Bob and Din
  13. I agree. I always look forward to reading posts from Bob Lee. There is always lots of good, useful info.