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Old 5th February 2010, 04:20 PM   #1
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Arrow Getting confused with LM4780 "parallel" and "bridged"

I'm embarking on my first amplifier build with two of the Audiosector LM4780 gainclone kits. Each kit contains 2 boards and 2 LM4780 chips.

My first kit is configured right now for stereo operation. That is, 2 channels of amplification from each chip/board for a total of 4 channels. The speakers are all 4 ohm impedance loads per channel. I planned to use a 400VA 18+18 transformer for this kit.

The second kit, I would like to maximize the available power, and use 2 channels output only. The reason for this is that they will drive woofers. Based on what I have been reading in the threads, each chip can be "paralleled" to run one channel instead of two. How much power does this yield? (Does it double the 60w rating, making 120w?)Also, does it require having a balanced source to run in parallel mode(which I dont have)?
I planned on using another 400VA 18+18 transformer for this as these are 4ohm loads as well.

Any advice or input you can offer is greatly appreciated. I've read through many of the threads but I havent found exactly what I was looking for. THANKS !
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Old 5th February 2010, 04:36 PM   #2
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Please read this article first: Amplifier Power Ratings

The amplifier's power will depend on supply voltage, available current and load impedance. Assuming first two parameters don't change, the power will double when moving from 8 ohm speakers to 4ohm.

To maximize power from LM4780 kit, you need to use voltage supply close to maximum allowable rating, which is +/-40V. While with a single amp inside LM4780 chip (half of the chip as used in stereo mode) that would be not a good idea as amp would not be able to produce enough current, connecting 2 amps in parallel should be fine and you will be able to get 120W. The transformer should be at least 25v + 25v.

Parallel is when you have inputs and outputs of both amps connected together, and for that you do not need balanced source.

Bridged is recommended for 8 ohm loads, and can be compared to both amps connected in series, where you feed each amp section with opposite polarity signal and the output is taken from "between" the amps with no reference to ground. Read page 7 of this manual for more info: http://www.firstwatt.com/downloads/f4_om.pdf
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File Type: jpg power.jpg (76.2 KB, 1257 views)
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Last edited by Peter Daniel; 5th February 2010 at 04:41 PM.
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Old 5th February 2010, 05:07 PM   #3
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Quote:
Originally Posted by Peter Daniel View Post
Please read this article first: Amplifier Power Ratings

The amplifier's power will depend on supply voltage, available current and load impedance. Assuming first two parameters don't change, the power will double when moving from 8 ohm speakers to 4ohm.

To maximize power from LM4780 kit, you need to use voltage supply close to maximum allowable rating, which is +/-40V. While with a single amp inside LM4780 chip (half of the chip as used in stereo mode) that would be not a good idea as amp would not be able to produce enough current, connecting 2 amps in parallel should be fine and you will be able to get 120W. The transformer should be at least 25v + 25v.

Man, I feel like a 3rd grader taking algebra Sorry for the beginner questions. I just want to be sure I complete this properly and use the right parts.

Ok, lets see if I have this right-

On each of your kits, there are two amp boards, one chip per board. On each board there is the option to have two outputs(Left and right), OR a single (parallel) output(mono), by following the wiring schematic. By wiring one of these boards to be parallel (mono) and by using 4ohm speaker impedance, I should expect to get 120 watts. Is that correct?

I will use the single 400VA 25+25 transformer as you recommended to supply both of these parallel (mono) boards.

Thank god its Friday. I need a beer.
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Old 5th February 2010, 05:14 PM   #4
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Yes, That's correct.
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“Do something really well. See how much time it takes. It might be a product, a work of art, who knows? Then give it away cheaply, just because you feel that it should not cost so much, even if it took a lot of time and expensive materials to make it.” - JC
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Old 5th February 2010, 05:40 PM   #5
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Thanks Peter.
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Old 5th February 2010, 06:33 PM   #6
taj is offline taj
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Quote:
Originally Posted by captainobvious View Post
Thank god its Friday. I need a beer.
You deserve a beer for graduating to 4th grade.

..todd
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Old 5th February 2010, 09:10 PM   #7
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Quote:
Originally Posted by taj View Post
You deserve a beer for graduating to 4th grade.

..todd



Im supposed to be in 5th...my parents held me back...honest




Click the image to open in full size.

Click the image to open in full size.

Click the image to open in full size.


Good to go. Now Im just waiting on the transformers and remaining parts.
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Old 5th February 2010, 10:56 PM   #8
taj is offline taj
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Nice blue boards, Peter. Can I trade in my old gold ones for blue ones? [kidding!]

..Todd
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Old 6th February 2010, 05:44 AM   #9
godfrey is offline godfrey  South Africa
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Looks like each amp board only has a 1500uF smoothing cap per rail.

For the intended use i.e. mono bass into a 4ohm load, wouldn't it be smart to increase that? Maybe an extra 4700uF or even 10000uF per rail connected between the rectifier board and the amp board?
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Old 6th February 2010, 02:58 PM   #10
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That's a good point and certainly worth trying. There are dedicated pads on rectifier board to mound additional caps. Some people also prefer to use snubbers when there's more capacitance and there's also space on board for those parts. However, using 1500uF is a good starting point and you may not necessarily like more caps.

Pictures show LM3875 PS board and for LM4780 the layout is very similar (although diodes mounting is reversed).
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File Type: jpg a1.jpg (62.0 KB, 1140 views)
File Type: jpg a2.jpg (68.1 KB, 361 views)
File Type: jpg snubber.jpg (90.6 KB, 389 views)
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