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LM1875 for small P.A use
LM1875 for small P.A use
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Old 10th April 2013, 04:28 PM   #1
dannyjmf is offline dannyjmf  Saudi Arabia
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Default LM1875 for small P.A use


I already have a DIY LM1875 X4 channel gainclone amplilfier. I plan to use it for a medium sized P.A open space venue for my university event day.

I wonder if the LM1875 is up to the task to do it or not. It will be used in Bi-Amp configuration. It will also be accompanied by a 10in subwoofer with its own amplifier. What precautions i need to take when doing a P.A event with DIY gainclone amplifiers? Sorry for lots of question since it is first time i am using a DIY amp for event day ...

Any Advice are well welcomed ... Thanks ...
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Old 10th April 2013, 05:07 PM   #2
AndrewT is offline AndrewT  Scotland
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if you use a horn loaded 100dB/W speaker you might get sufficient volume to be heard outside at typical PA listening distances, say around 5 to 10m.
regards Andrew T.
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Old 10th April 2013, 05:27 PM   #3
picowallspeaker is offline picowallspeaker  Italy
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Usually I see ( from the specs ) that active speakers feature an 80 W amplifier for the woofer and a 20 W for the tw. Or anything
At least, an LM3886 for the wf/mid, it should be the minimum
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Old 13th April 2013, 02:37 PM   #4
ChristianThomas is offline ChristianThomas  United Kingdom
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I'm with Pico here. It's like putting a lawnmower engine in a tractor. It's much too puny for PA purposes, though AndrewT does make an interesting point that you might just get there with some large horns. Even the LM3886 must be considered marginal and would probably need paralleling into a 4 ohm speaker to do any sort of justice to the needs. You could get away without paralleling them in an active system if the subwoofer/woofer is up to the job and you have a high pass filter on the low/mid unit, but you would still need low impedance drivers to get the output you need.
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Old 5th May 2013, 03:37 PM   #5
Jsixis is offline Jsixis  United States
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at a minimum if you have drums and guitars involved you need 200 watts for the mains and 50 watts for the tweeters, acoustically you could get by with 50 watt mains, if your taliking just a voice PA can get by with 3 watts if you use a horn.
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Old 6th May 2013, 11:17 AM   #6
JMFahey is offline JMFahey  Argentina
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1) Short answer: very low power for open air PA stuff.
Might do inside a classroom or something similar which is neither open air nor large.

2) what speakers do you have available or can get for that day?

3) what will you reamplify?
Somebody delivering a speech?
An acoustic singer/songwriter/guitar player?
A Rock band?
A Dee Jay?
Low volume background music?
All possible, all with different needs.
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Old 7th May 2013, 07:57 PM   #7
danielwritesbac is offline danielwritesbac  United States
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The prospect can work if we admit that the system is small and then increase its efficiency towards outdoor use. Yeah, time to cheat.

1). Starting with the single 10" subwoofer which is the main limiting factor, dial its crossover up to terribly boomy, which is the only thing that will be effective to increase the output for distance impact outdoors. See also the Boominator thread on how to get significant bass outdoors with little 10" woofers.

2). And then see what size cap to put in series between the LM1875 amplifier and its speakers to block bass loading (conserve amp power) and the cap sized to match it up to the fairly high pitched subwoofer. Also consider reducing the input cap size on the LM1875 amplifier. Both input and output roll-offs may get you enough output power efficiency.

3). Now add a larger cap between the subwoofer's amplifier and 10" woofer to reduce X-max (if ported, then some port adjustment may be needed--shorter port tube). Also set your source EQ with the 50hz control down, not up. With only one modest woofer to use, try to block pitches lower than 90hz. If at first you don't succeed, seal off the port to Stop the X-max, fiddle the "conservation cap" size, and then eq the heck out of it. See also Bob Cordell's site on speaker bass.

Small systems that succeed outdoors are usually unpleasant indoors and vice versa. See also room gain or cabin gain topics, which help bass and aren't present outdoors. If you want to use it outdoors, you'll probably have to tune it outdoors, well before the event. With just 1 of 10" woofer, you're going to have to maximize the output at the expense of the quality (due to the absence of the normal amount of woofers for outdoor PA).
Your system should be able to get the drum impact fairly well, but your bass guitarist will have to bring his own amp, hopefully equipped with something more significant than one modest woofer.

See also SA Stereo Tool, a professional but free software that can be used for a boost and/or limit x-max, and well it might be quite useful. A small system converted for maximum outdoor output may not be very pretty, but you could possibly dial some pretty back in with SA Stereo Tool, if you have a rather quick laptop available. It may need the sound card software adjusted for input port type as some of them default to microphone but can be swapped to line in via the driver software. Some don't have line in at all and that would not be so handy. This all needs checked out before the event.
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Last edited by danielwritesbac; 7th May 2013 at 08:10 PM.
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Old 8th May 2013, 10:18 PM   #8
Arty is offline Arty
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i think it may be more than enough.
factors that can change that:
-efficiency of the speakers used
-the purpose. Does it have to be verry loud ?
-setup. Will there be a wall behind the speakers, or its a big big realy open field from all directions ?

Actualy, if the speakers have decent efficiency and You have a brick fence, a wall of a house or anything similar at the back of the speakers, You may actualy find the lm1875 even TOO powerfull.

on the otherhand, some 86-87 dBL sensitive speakers in the center of an open field will more than likely fial bigtime.

If the speakr's impedance is not too low to drive with the lm1875, then there is no problem driving them with it. Nothing will fail. That should get checked out.
Other than that You might add additional cooling to the chipamps, and prehaps add a fan to the psu. Should be fine.
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