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  1. #1
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    Amp change due to feedback.

    Hello: Excuse me if this is not the right forum for a newbie wih this question.
    My knowledge on sound systems is more common sense instead of technical.
    The situation is a small older 175 seat auditorium with 20 (8in) ceiling speakers with 25Vxtrans tapped at 2.5 watts. The amp is a Grommes Precision Amp, GTE Series 60W. At any given time we can have 1 to 4 live mics.

    We have very little headroom. Itís hard to control the volume of the person speaking without some feedback or ringing coming in. I can't even reach 70db. Voice quality, in my opinion is poor. Likewise when we play music from a MP3 or even a CD player the quality is not there.

    I bought one EV-PL80a microphone to test out. It helped a lot. Iím going to buy two more for the stage.
    I came across an EV MA-1212 Mixer Amp, 120 watt amp on criagslist.
    I want to test it out. These are my questions:

    When I install the EV MA-1212:
    Should I plug and play leaving the speakers tapped at 2.5 watts?
    What will be the results?
    Should I tap them at 5 watts?
    If so what will be the results?
    Can the speakers be a factor? They do have a back cover.
    If so being that it is mostly speaking (toast masters type) vocals and some CD music, what will be the best yet economical speaker?
    Should it be coaxial or dual cone? BTW I have to stay at a 25v system.

    Any advise will be appreciated.
    Cayetano

  2. #2
    Hi Cayetano,

    If you leave the speakers tapped @ 2.5 watts, altogether you will burn
    20*2.5 = 50 watts, and with a 120 watt amp that is not good. Tap them at 5 watts to give you 20*5 = 125 watts. If they have 10 watt taps that may be even better to protect them from being overdriven.

    As far as dual cone v. coaxial, coaxial will give you a better frequency response, but they cost a little more. I kinda doubt it would help much.

    It seems from your description that even a bigger amp won't help much with the feedback problem, though, since feedback is not a function of headroom or even efficiency, but of the acoustics of the room, speaker placement, mic placement and how it is used, and the speaker volume. It may help some if it was caused by clipping from the speakers being overdriven. You could always put a multiband EQ in there; The EV has a means on the back of it to do that. Or a feedback control device.

    Hope this helps!
    'I think my intimate relationship with electronics started as a child when I was playing with a screwdriver and a wall plug, Doc, and...'

  3. #3
    Need a lot more information.

    The situation is a small older 175 seat auditorium...

    How wide, how deep, how tall? Live acoustics, dead acoustics, or in between?

    ...with 20 (8in) ceiling speakers with 25Vxtrans tapped at 2.5 watts.

    Aha!

    The amp is a Grommes Precision Amp, GTE Series 60W. At any given time we can have 1 to 4 live mics.

    If you ask somebody who's good with EQ to tune the graphic equalizer that comes with the Grommes, that will help your feedback situation a lot.

    We have very little headroom.


    "Headroom" could mean two things - that you can't turn the volume up because the amp runs out of power and starts clipping, or you can't turn the amp up because you get feedback. Which do you mean? (FYI, "headroom" properly means the former. The expression for the latter is "gain before feedback.")

    Itís hard to control the volume of the person speaking without some feedback or ringing coming in. I can't even reach 70db.

    How are you measuring 70dB?

    Voice quality, in my opinion is poor. Likewise when we play music from a MP3 or even a CD player the quality is not there.

    No surprise there, older ceiling speakers are garbage.

    I bought one EV-PL80a microphone to test out. It helped a lot. Iím going to buy two more for the stage.

    What mikes are they replacing?

    I came across an EV MA-1212 Mixer Amp, 120 watt amp on criagslist.
    I want to test it out.


    Don't buy it. The EV is a good amp, but the Grommes has a graphic equalizer and the EV does not. Ceiling speakers typically have huge response peaks and dips, and the peaks are where feedback occurs first. If you can get the graphic EQ set right, you will get at least 6dB extra gain before feedback compared to no EQ. Also, the EV does not have a 25V output.

    These are my questions:

    When I install the EV MA-1212:
    Should I plug and play leaving the speakers tapped at 2.5 watts?
    What will be the results?
    Should I tap them at 5 watts?


    What is the highest tap on the transformer? (See below.)

    If so what will be the results?


    If you do get a different amp (not the EV) the speakers should generally be tapped so 80-100% of the amp's available power is used up. Otherwise, you're no better off.

    One caveat - advisory to you too, Mr. pennies - when a typical cheapo speaker transformer is tapped at its highest wattage, high-power low-frequency signals will cause the transformer to saturate, which is a very bad thing. (Basically, the transformer runs out of iron to be magnetized, twice each cycle.) The rule of thumb is, if you're going to be playing music with a lot of bass, don't use the highest power tap. And, since those speakers are so junky, you'll probably be tempted to boost the bass on the amp, which makes matters worse. Rule of thumb here too: attenuate everything below 50Hz.

    Can the speakers be a factor? They do have a back cover.

    The speakers are the largest factor. That, and the fact that they're probably placed so a lot of their sound spills back onto the stage. That's where the feedback is coming from.


    If so being that it is mostly speaking (toast masters type) vocals and some CD music, what will be the best yet economical speaker?
    Should it be coaxial or dual cone?


    Strictly speaking, a coaxial is a dual cone - one woofer, one tweeter - unless you mean a speaker with the little paper "whizzer" cone. Those are junk. Once you tell us the dimensions of the room and how it's used, we can advise on the next step.

    BTW I have to stay at a 25v system.

    How come?

    Awaiting your answers with 'bated breath.

  4. #4
    I looked at the topic title again _ "amp change due to feedback" - and I have an additional comment.

    Unless the mikes, speakers and installation (deployment) are good, the quality of the amplifier is irrelevant. The crummiest amp in the world has specs that are an order of magnitude better than an old ceiling speaker.

    Feedback is due to sound from the speakers getting back into the microphones, which is an issue of speaker and microphone choice, placement and aiming, and to frequency response peaks. Amplifiers do not have frequency response peaks. This is not where your problem is originating.

    Here endeth the sermon.

  5. #5
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    Thank you for your replies.
    Like I said my knowledge is mostly common sense not technical.
    I agree I need to do more home work. I will investigate the speakers and acustics more.
    As far as the db's I'm using a Radio Shack Sound Level Meter. As far as being Mr. Pennies it's not because I want to be, it's because I am.
    If I had the money I would hire somebody to come in. Anyway "I'll be back" with more info so maybe somebody can help.
    Thanks
    Cayetano:

  6. #6
    Actually "mr. pennies" meant penniesfromheaven, I wanted to alert him to my comment about transformer saturation and power taps.

    Okay, so you can't get 70dB SPL (Sound Pressure Level) on the meter. I agree, that's not good. 70dB is just a little louder than normal conversation, so less than that is really marginal for a sound system. 75dB is desirable, 80dB is getting a bit obnoxiously loud. So we will want to get you to the 70-75dB range when we're done with this.

  7. #7
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    Thank you.
    I have a feeling I need to look into the stage acoustics. The stage is small. It has two side walls that come in at angles. Wider up front narrower toward the back of the stage. The podium is in the center. I can imagine sound bouncing off a wall back into the main mike. I will either need to diffuse or absorb sound from those two walls. Correct?
    Thanks
    Cayetano

  8. #8
    Diffusing/absorbing is one way to do it. Just for a quick test, hang blankets a few inches from the walls.

    The other way is to arrange the speakers so not much of their sound reaches the stage to begin with. If you're sticking with the ceiling speakers, you haven't got much choice how they're arranged. But it would be nice to know how they're laid out. Are some of the speakers over the stage? Are some of them right in front of it? Is the ceiling flat or slanted? (In other words, are the speakers aimed straight down or not?) You might be able to disconnect the closest speakers or put them at a lower power tap.

    'Nother question - when you get feedback, what general frequency range does it occur in? Voice range, or treble, or bass?

  9. #9
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    Hello:
    There are 16 (4 rows of 4) ceiling speakers in the main hall facing straight down. The ceiling is a suspended ceiling, it's low about 12 feet. The first row of speakers are about 12 feet from the stage. I will be back in the building next week and will measure the main hall, get a speaker layout and double check the speaker condition. Good idea on testing the acoustics with a blanket/curtain. There are no speakers/monitors on the stage. The stage ceiling is only 10 feet high. I just found out the speakers are 19 years old. Is that old for a speaker?

    Thank you for your interest. I will return with better info.

  10. #10
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    Oh one more thing.
    The feed back occurs in the voice range.

    Thanks

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