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dbx 231 floor noise?

Discussion in 'Sound, Music, and Intercom' started by jkowtko, Dec 8, 2007.

  1. jkowtko

    jkowtko Active Member

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    Do the DBX EQs, and more specifically the 131/231 models, have audible floor noise? I have two 231s and one 131 hooked up to our FOH speakers (SRM450), and with the outs on the mixing board turned down all the way, I'm hearing a very low hiss coming out -- i.e. on the order of -60db ... very, very low, but audible in a quiet house of all five speakers.
    Hitting the bypass switches on the EQs only seemed to increase the noise a bit and change it's texture a bit. Changing the output volume control on the EQs didn't seem to have any effect at all.
    Granted, with a loud night club musical like Smokey Joe's Cafe there is no chance of hearing this noise during the show, but for non-musicals where the house needs to be dead quiet, I may be able to hear this.
    If in fact these EQs do have a small amount of floor noise, then I can always pull them out for non-musicals. However if this is a known issue and there are other units that are quieter, I might be interested in upgrading:)

    For example, I just read up on the DBX 2231 ... obviously a much more expensive unit, but the advertised S/N ratios are much higher than the 231, and it also has a noise reduction circuit in it that adds another 8db S/N ... am I just going to have to bite the bullet and start upgrading to these?

    Also, I noticed the 2231 has a different S/N ratio posted for +-6db fader range vs +-15db ... on my units I'm at the wider range on all channels. I'll try switch to the lower range to see if that has an effect.

    Thanks. John
     
    Last edited: Dec 8, 2007
  2. museav

    museav CBMod CB Mods Departed Member

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    Every electronic device has a noise floor and less expensive units often have higher noise floors or lower S/N ratios. However, before making any equipment changes I would first make sure that the EQs are the only problem and that you have all the existing settings and gain structure optimized. With a S/N ratio of 90dB, the 131 and 231 are not as quiet as the 2031 and 2231, but should not be that much of a problem.

    I believe that the dbx units do not have an output level control, it is actually an input level control. So your saying that adjusting this makes no difference indicates that the the noise is likely not part of the incoming signal. And the comment that activating the bypass causes the noise to go up and change indicates that the noise is being affected by the actual EQ circuits. It also indicates that you may have a lot more cut then boost which is not bad practice by itself but could be adding to the noise issue. A couple of quick checks would be to disconnect first the inputs from the EQ and then the outputs of the EQ and see how these changes affect the noise. This should allow you to confirm whether it is the EQ that is the only source of the problem.

    Also look at the output levels you normally run and verify that you are geting reasonable levels at the output of the EQ. I have seen many systems where an overall noise problem is the result of too low a signal level at one point then being compensated later in the signal chain.

    I don't know how your units are set or what cut and boost are applied, but the EQ section can be set to either +/-6dB or +/-12dB. If you have no boost or cuts greater than 6dB then I would operate it in the +/-6dB mode as that may help with the noise and allow for more finite adjustment.
     
  3. jkowtko

    jkowtko Active Member

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    Okay, thanks for the helpful hints.

    Yes, the EQs are all set at +- 12db range, and yes, all of the adjustments are cuts, no boost. And, yes, a handful of the cuts go below what would be the 6db level.

    For starters, if I can get to the theater early enough I'll pull the EQs out of the loop real quick and verify that it's the EQ that is generating the noise.

    Assuming that's the case, then which do you think would have more beneficial impact -- raising all faders up a consistent amount so that average adjustment is neutral instead of moderate cut, or drop to 6db range?

    Thanks. John
     
  4. jkowtko

    jkowtko Active Member

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    Alright, we tested, and it's the first five faders (20 thru 50 Hz) set to the bottom that was causing the problem. When I centered them the noise went away.

    We also verified:
    - the bypass switch didn't help -- in fact it made the noise a bit worse
    - switching between +-6 and +-12 had no effect
    - and of course, bypassing the EQ unit entire by cabling around it also eliminated the noise.
    - I also noticed that raising the faders for the top few frequencies loudened the noise, but dropping them below unity didn't eliminate it.

    And the low cut switch didn't bring the noise back, so I left those in, and that more-or-less covers the five faders which we adjusted.

    So, don't know exactly why the noise. I assume it was some side-effect of summing the EQ offsets at those low frequencies. Or maybe it's just poor circuitry. If someone has a 2231 I'd be curious to see if it shows the same behavior when you drop the first five faders all the way down.

    Thanks. John
     
  5. BNBSound

    BNBSound Active Member

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    It would help to know what the rest of the gear is. If any of it says Mackie on it that's a dead give away as to where the hiss is coming from. But apart from that, you asked about adjusting faders and such. The best policy is to set your gain pot with the faders at unity and mix from there. Keep the groups and master at unity as well. If your speakers and amps are correctly sized for the room you should be in good shape. If your system is a little much for the room, keep mixing at unity and reduce the inputs on the amps. That keeps the signal as clean as possible.

    Any time you depart from a unity gain setting you alter the signal in some way. Gain greater than unity can add hiss. Gain settings that are too low aren't necessarily a bad thing, but they can cause you to compensate later on in the signal chain and introduce noise. Mixing in the "fat" of the fader, that is +/- 5 or 10 dB from unity, is not only convenient as the logarithmic taper is the easiest to control there, but it makes you pay more attention to the input gain, which is where the money is.
     
  6. jkowtko

    jkowtko Active Member

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    Yeah, you guessed it -- everything else is Mackie :)

    We use SRM450s for all the FOH speakers -- at least the five that are being EQ'd. And our board currently is a CFX-20.

    However, pretty much everything is set to unity or slightly below. Input channel faders are running in the -10 to 0 range during songs. I use the group outs for my output channels to the speakers, and they are between 0 and -10 depending on the house level settings. The EQ master levels are at U right now, with very slight adjustment to remove hotness from certain seats in the house. And the speaker amp gains are at 12 o'clock.

    I know the SRM450s put out a little bit of noise, but that's my baseline. This extra noise was in addition and could not be EQ'd out ... the only way to remove it completely was to remove the EQ from the signal path. So I would assume it was the EQ adding the additional noise.

    (If the noise was coming from the board I should be able to EQ it out, right? And if the noise was coming from the speakers there would be nothing I could really do about it ... right?)

    I do know what you're talking about with proper level setting though ... on my Mackie CFX board the input channels need to be set at a certain level -- almost too hot -- for "best" throughput of sound ... and I've noticed that when I change the mix of input channel to master fader from show to show I can easily affect how difficult it is to get a good clean volume.

    I am planning to buy a Yamaha 01v96 to use in place of the Mackie whenever I do musicals, so I hope that end of the signal path will get cleaner and more manageable.

    On the other end I don't know that we'll be replacing the Mackie speakers very soon -- there is really no other low-cost alternative and our theater has almost no budget for these types of capital expenditures. I am trying to work with Meyer Sound (just down the "street") to see if we can put together a donation package of their equipment ... that would be sweet, but I'm not counting on it anytime soon either ...

    So in the meantime I just want to understand and improve the "house system" equipment as best I can. I'm definitely learning a lot as I go, and the sound in the house has improved tremendously over the past year.

    Thanks. John
     
  7. soundman1024

    soundman1024 Active Member

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    I don't know if this is the case or not, but the 450 Mackie speakers could have a gate on them that would kill expected noise from a Mackie mixer. After the signal goes through the dBX it may be high enough to open up the gate. I realize gain shouldn't be added by the EQ if your settings are at unity, but I wonder if there is some sort of gate action going on.
     
  8. BNBSound

    BNBSound Active Member

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    Check into the cable you're using to patch the EQs in. If it's unbalanced cable on a balanced connection, that's not necessarily a bad thing, but you'll have about 6dB loss. That can cause you to run things hotter down the line. I replaced the cables feeding my monitor EQs a couple years ago and instantly I was mixing in sane territory again and the red lights were off.
     
  9. jkowtko

    jkowtko Active Member

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    I used all balanced cables except for the inserts where I can't. And pretty much all of my cables or new or at most one year old. All the Mackie board outs are balanced as well. No, it's the EQ ... as I mentioned before, if I pull the XLR-in and XLR-out cables out of the EQ and plug them into each other, this problem noise disappears.
     

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