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Old 10th November 2012, 04:35 PM   #1
gio89 is offline gio89  United States
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Default 100watt mono amp find???

Hello I'm looking for a 12volt 100watt mono 8ohm amp to power a pa speaker. I plan to use this to make my own portable pa system. This is the speaker i plan on using Gemini GT-1004 10" PA Speaker | GuitarCenter

The speaker is about $50 and I'm hoping to find an amp around the same price.

Please help if you can, thank you.
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Old 10th November 2012, 04:48 PM   #2
AndrewT is offline AndrewT  Scotland
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Look for a car amp that actually meets specification @ 13.8V and use that to derive likely output power when fed with ~12V.

As always, I'll add a but
you will fare far better looking for affordable high efficiency speaker/s

100dB 8ohms speaker on 5W, will play as loud as a 80dB 4ohms car speaker using 250W
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Last edited by AndrewT; 10th November 2012 at 04:51 PM.
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Old 10th November 2012, 09:48 PM   #3
gio89 is offline gio89  United States
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OK, then what if I use a Lepai amp which can be run on 12v and put out 15watts at 8ohms.

Can i still use the Gemini speakers or do i have to look elsewhere? (are the Gemini's high efficient?)

And if not I would like a pair of high efficient pa speakers for cheap. $50 is why I liked the Gemini.

What I am trying to do is provide portable music for a school jog-a-thon.
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Old 11th November 2012, 10:30 AM   #4
AndrewT is offline AndrewT  Scotland
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Borrow or hire a PA speaker of very high efficiency.
Domestic "HiFi" speakers are generally very low efficiency especially if they are small.

If you are supporting a charity event you may find a volunteer who will do you some outdoor music in return for spreading about some advertising flyers.
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Old 11th November 2012, 11:00 AM   #5
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This 12V V amp Ypu are looking for, is it meant to run "all day" on a car battery without possibility to charge this battery?
Then a 100W amp would be impossible to use.

Let's see what watt in an amlifier is.
Continuos output power of an amplifier with 12V as powersource is around 3,5W in 8 Ohms.
(Max voltage swing is 12V - 2X0,7V, gives around 10,6V P-P, Equals 3,75V RMS.)

When two amps is coupled in brigded mode one gets the double effective voltage-swing, wich is capable of giving four times this effect Equals 15W in 8 Ohms.

As car-amps is designed to work into a 4 Ohm load, this power is doubled 30W.

This is given in around 10% distortion (probably not any problem here)

As soon as You are looking for any higher wattage than this, one must use a sterp-up powersupply wich makes it possible to make ths voltage swing higher.
Unfortunately this comes with a "double price tag".
Not will more power be required from the battery, but also more power will be dissipated as heat in the DC-DC-converter and amplifier.

But there is often "more than enough" still using an oldfassioned TDA2002-in brigded mode when using this amp to drive that speaker. You have the 15W continous power, and if the speaker have an efficiency around 96dB, you will get more than 100dB sound out of it.

Here is a good example of such brigded amp made from two TDA2002.

Click the image to open in full size.

Good luck
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Old 12th November 2012, 02:12 AM   #6
gio89 is offline gio89  United States
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OK, so i decided to use a Lepai amp and that amp will put out two channel 15w rms to 8ohm, and I have seen it run on a battery so I know the power and amp will work. And i can mod it so it performs better.

Does anyone have any suggestions on a pair of high efficient speakers, remember I'm talking as cheap as possible that will still sound good.

I want it to blow away this ion block rocker that they already use(they actually use the older white one).

Unless you think its a lost cause. Do those portable pa systems that they sell for $200 really sound good, or is building your own always better?
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Old 17th November 2012, 10:18 PM   #7
gio89 is offline gio89  United States
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What if i use two of these speakers (Goldwood GW-8004 8" Coaxial Speaker 290-378) and a lepai amp?

They are 87db efficient (87 dB 1W/1m)
so does that mean 87db x 15w = 1305 loudness?
And if so, is that loud enough for what i need?
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Old 18th November 2012, 06:04 AM   #8
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Default It ain't DIY but...

87 dB/1W is very low efficiency. As you have decided on a 15W amp solution (wisely, I think), you may have the cash to spend on party speakers which are relatively high efficiency. Here is an image of an inexpensive, locally available product that may suit. Search Biema for more clues as to what is available in the US.
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Last edited by Ian Finch; 18th November 2012 at 06:08 AM.
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Old 8th December 2012, 05:28 PM   #9
JMFahey is offline JMFahey  Argentina
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You didn't answer, I think you were stunned by the over U$250 price tag for the suggested speakers.
I would be also
Well, let's step down a little.
To begin with, that Block Rocker is *exactly* what we were suggesting (showing they are wise chaps too): a 15W 12V bridged chip amplifier, driving a single speaker plus tweeter.
But *matching* it is not enough, you want to *beat* it, so you must provide appreciably more .... and you can.
1) this is an important point: *can* you carry a larger battery?
Or tap a nearby parked car? (cigarette lighter plug or big crocodile clips).
If you can, you can safely have multiples of 15W.
If not, and you must rely on your own 12V 7A gel cell battery, 15W will run all afternoon; 2x15W will die in the middle of the party .... Unless you add more batteries.
2) build or buy a 15W bridged chip amplifier.
You may use 2x TDA2002/2003 , or single TDA2005 (2 of them inside a single, larger chip) or more modern options, all about the same, and generally TDAsomething.
So far, you have matched their power output. Not bad, but nothing to brag about.
3) Use a *guitar* speaker.
They are both louder (more efficient) than Hi Fi types and typically have some midrange peak which makes them *seem* even louder.
Not my choice to listen to Classic Music at home, but at a Pic Nic .... fine.
I suggest the MOD820 in 4 ohms.
Jensen MOD Series speakers - Jensen MOD8-20 8" speaker. 4 or 8 ohms. Great choice for lead guitar cabinets.
Not the King of efficiency, but honest 93.2dB is much louder than 87 dB .
As in 4x the efficiency, how's that? For all of U$22.70 .
Use louder Tweeters. The Rock Block uses a nice but very weak silk dome tweeter.
You use a couple Piezo horns.
The generic $1 type. Use 2 in parallel with your 8" speaker, but add a 10 ohm 1/2W resistor in series with them.
This speaker system will be already louder than what they use now, although less "polite", more raw sounding, which in this case is not bad.
4) the beauty of this is that you have just built a somewhat louder "Block Rocker" for around $50 each (not counting the battery and mounting in a cheap homemade chipboard or plywood box).
The double beauty is that for around $200, you'll have four (4 !! , count'em !!) powered cabinets which will absolutely *destroy* the one they are using now.
All fed from a car battery, of course.
In fact, I'm just *now* building a bridged TDA2002 amp (that's how I found this thread) to listen to my MP3 player while I'm sawing wood, spray painting, etc.
Stay tuned.
I'll post pictures when ready.
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Old 10th December 2012, 02:20 AM   #10
gio89 is offline gio89  United States
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wow! Thanks for all the info.
Speaking of using guitar speakers, I have an old fender amp that my dad blew something on the amplifier board by touching wires together. Maybe I can check to see how good of a speaker it is. Or post it up here and see what you guys think of it.
But I defiantly will use those tweeters you mentioned.

When you say more raw sounding, will it sound just as good or better then that block rocker?

But that was great information, thanks for all the help!
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