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Live monitor question - ohms and wattage...

Discussion in 'Amps and Cabs [BG]' started by Dkerwood, Sep 14, 2005.


  1. Dkerwood

    Dkerwood

    Aug 5, 2005
    Midwest
    I know that this doesn't really belong here, but I didn't know where else to put it.

    I'm looking at getting some monitors for my band. The only free monitor I have in my PA is a speaker rated for 300W/channel at 4 ohms. Only one channel is working - the other is blown (which is why it's not in service right now).

    So I found these Kustom monitors on Musician's Friend. They are switchable to 16 ohm, so I could run 4 of them in parallel. Here's my question. The 10" monitors are rated for 75W RMS/150W peak. The 15" monitors are rated for 125W RMS/250W peak. If I plug all four monitors into this amp, how much wattage will it deliver? 75W per monitor at full blast? If that's the case, do you think that the 15" monitors would be totally underdriven?
     
  2. Petebass

    Petebass

    Dec 22, 2002
    QLD Australia
    I'm going to move this to where all the technical people hang out - Amps!
     
  3. billfitzmaurice

    billfitzmaurice Commercial User

    Sep 15, 2004
    New Hampshire
    Owner, Bill Fitzmaurice Loudspeaker Design
    You'll never drive monitors at 'full blast' because of this little thing called 'feedback'.
    The main difference between a 10" versus 15" monitor is the same as that between 10" versus 15" bass cabs: frequency response. Unless you plan on running the bass through the moinitor system get the tens. Vocals don't need fifteens.
     
  4. DaveMcLain

    DaveMcLain

    Jun 19, 2005
    Cuba MO
    The monitors will NOT feedback because of having too much power, that's a falicy. They will feed back worse and at a lower SPL with less headroom. Here is the reason, microphones feed back because they can only tolerate so much gain before feedback therefore if the system feeds back with 60db of gain it makes no difference if you've got 60 or 600 watts it happens at the same gain level. Therefore if you are out of headroom on the 60 watt amp and the mic feeds back increasing to 600 watts will give you about 6db more headroom before it feeds back again.

    Also, every enclosure that you add with the same mix drops the amount of level you can achieve before feedback by 3db, same for every microphone you add, you lose 3db before feedback.

    You can never have too much power for the monitors, right now with my band we use 4 biamped enclosures each with a 12 inch and a horn. We run 1000watts on the highs, 1200 up and 1400 watts on the lows 1200 down to 250hz where we roll off the system.

    We have found the best results with a monitor system that is very midrange heavy, no bass at all. Depending on the stage we always roll out below 250-400Hz, there just isn't anything helpful down there that doesn't get in the way of the vocal coming through the monitors. It also helps to run as few things as possible through the monitors, generally we run only vocals, keyboards and once in a while a small amount of rhythm guitar into my mix that is on the opposite side of the stage. There is no kick drum or bass in the monitors for us ever.

    I think you would be better off with fewer enclosures and more power if you are limited on the number of channels you've got with your PA setup.
     
  5. billfitzmaurice

    billfitzmaurice Commercial User

    Sep 15, 2004
    New Hampshire
    Owner, Bill Fitzmaurice Loudspeaker Design
    Sorry, that makes no sense. The mic will feedback when sound from the speaker reaching the mic exceeds the mic's sensitivity threshold and is then recirculated through the system. Your amps headroom has nothing to do with it, all that matters is how loud the monitor is. Too loud=feedback. The cure? Turn it down. That means putting less power into the speaker, not more.
    True, which is why fifteens are a waste of cartage, not to mention money.
    The opposite is true. Since feedback is caused by excessive sound levels at the mic you want to use a lot of enclosures running at lower levels rather than a few at high levels. That way you can reduce the overall sound level on stage with fairly uniform coverage rather than have a series of hot and cold spots.
     
  6. Dkerwood

    Dkerwood

    Aug 5, 2005
    Midwest
    billfitz- "Full blast" was referring to the amplifier. Of course, most of my power amps are run at 10 all the time anyway. That way I can control levels from the board. I use a tight compressor to cap the peaks (mainly to protect my speakers).

    My new mixer has only 2 monitor channels, although we could cheat with creative fading to achieve 2 more post fade monitor mixes. The band is a three piece right now, and I figured I'd take a monitor and share the mix with the bassist, since we both sing. Then we can run the other 2 monitors back on either side of the drummer.

    I'd really like to do this without spending any more money on amplifiers. (I just realized that I seriously messed up the original post. The only AMPLIFIER I have free has one channel that's rated for 300W @ 4 ohms). If that's not possible, I can buy another cheap 900W amplifier and have 3 total channels for monitors.

    We've done some different things to make ourselves heard onstage. Right now we're running a QSC 1450 amp into 2 Nady 15" 500W (8ohm) cabs, and 2 Peavey 4x12" column speakers (wattage unknown) (4 ohms - which is why the other channel of my free amp is blown... lol). We've run only one of the 4x12s and put it behind the drummer. We've tried turning the 4x12s for use as side fills, but the sound from them is so nice and warm... I hate to not use them FOH. Oh, and we've also put them on the floor on their sides tilted back as floor monitors. Now THAT was a lot of monitor!

    At the last gig, the bassist and I just heard ourselves from the FOH mix. The drummer needed a little more, so we ran a monitor channel out of the board into an older Peavey guitar amp (Preamp in, of course). It worked, but we were still left guessing on a few pitches, and my guitar was all but lost onstage (I mic'd up my guitar amp through a tube preamp, and then turned down to "lower stage volume"... tell that to my drummer).

    So yeah, I guess I'm just grasping at straws for cheap options for monitors. The band plays on average once a month, and usually we only make 50-100 dollars per gig, which then goes toward gas money. So we don't have a lot of money to put towards PA. Any advice about monitors or about my PA setup would be appreciated.
     
  7. DaveMcLain

    DaveMcLain

    Jun 19, 2005
    Cuba MO
    The monitor placement is also a big issue, the enclosures should always play into the null of the mic and on a cardioid pattern mic like an SM58 or something similar that's toward the rear or cord end.

    I would make sure to always use the same mic at each position for best results and esspecially on mics which will share the same mix.

    Again, the microphone will NOT feedback because of excessive VOLUME, only from excessive gain, so be careful if you are using a compressor in the setup. Try using the compressor as stated earlier as a peak stop limiter only.

    Excessive gain concentrated into a certain very narrow band will also cause feedback. This is caused by uneven frequency response from the enclosures and microphone and also from any comb filtering that is occuring due to the stage environment. Having an EQ, preferably a 1/3 octave per mix is an excellent way to chop down some of the peaks in the response, help the tonality and get the volume higher as needed.

    The Sabine FBX boxes can be a big help too esspecially if you need something simple and you don't have a lot of time to mess around with the gear before the show.

    Our monitor chain is as follows: 2 mixes, 2 enclosures each mix. for a total of 4. They are Peavey monitors 1 12 Black Widow with a horn 22TI driver. The microphones are Shure Beta 58's into a Mackie 16x4 board. The signal comes out goes to a Peavey 215 EQ(2 15 band graphics 15 bands per channel) after that it goes through a Sabine FBX box on each mix. Then it travels to the stage, the signal is balanced from the board to the EQ through the FBX and then down the snake to the stage. At the stage it breaks out of the snake and it goes into the monitor power amp rack. In there it hits a DBX stereo compressor(one side for each mix) that is set as a peak stopping limiter, hard limiting to control the signal to the amps. After the compressor it goes into a Peavey CS 1000 where each mix is split using a PL 1200. This makes the split between bass and trebble at 1200Hz, the lows are fed into a Mackie 1400 power amp and the highs into the CS 1000. Each mix gets one channel from each amp.

    Now the signal is sent to the enclosures down two 4 conductor Speakon cables which make biamping a snap. It's 4 conductor so it handles the signal from the high amp and the low amp on one cord. I modified each enclosure to have two speakon connectors so that I can daisy chain from one cabinet to the next, remember there are two cabinets per monitor mix. Speakons cut down on setup time a bunch and they also elimate patching errors when biamping(a good thing on a dark stage when you are in a hurry). The internal crossovers in the enclosures are not used.

    This setup works great for us and the monitors will play incredibly loud if needed. It works good on lots of terrible stages too. There are things I would like to make better but it works fine, it's been very reliable and it fits in the truck. We've been using this basic setup since about 1997 or so..
     
  8. billfitzmaurice

    billfitzmaurice Commercial User

    Sep 15, 2004
    New Hampshire
    Owner, Bill Fitzmaurice Loudspeaker Design
    One way to get around the amp thing is with onboard amplification. One of my customers did that with some monitors he built and is quite happy with the result. The cost of the amp was like $125 and he's driving four monitors with it. Here's one of the threads:
    http://www.audioroundtable.com/BillFitzmaurice/messages/8958.html
    If you're interested ask Leland for some help, he's always quite amenable.