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Power supply for amps


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You won't have anything to limit the power to only 40W. You won't even know when the power reaches only 40W. If the supply voltage is too high for the amplifier's load then the current will also be too high. Voltage times current= power. Power makes the amplifier IC get hot. The heatsink with a fan can take away only as much heat as per the power rating of the IC.

One inexpensive 25-0-25 transformer is perfect for two LM3876 amplifiers driving 8 ohm speakers. If you use 4 ohm speakers then either parallel ICs are needed and a single transformer per channel, or a lower voltage transformer with two LM3886 amplifiers.

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Ok. Will this transformer do the trick?

230V to 18-0-18V 2x4.44A 160VA

Or must I get another one?

Can I still use Rod's design for the LM3886 just as it is with the PSU I mentioned earlier with two 4700uF's and a 400V 35A bridge rectifier?

I also have another question about pots. What is the difference between a linear and log pot and which one will work best for this project?

Thanks.

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Two 50W amplifiers will need a transformer of at least 143VA so the 160VA transformer will be fine.

Two 4700uF capacitors would be the minimum value for 1 amplifier. Use two 10,000uF capacitors for the stereo amplifier.

The voltage to the bridge rectifier is the peak of 18V which is only 25.5V. It is a waste to use a bridge rectifier rated for 300V. A 50V or 100V bridge rectifier would be fine.

A linear pot is for adjusting a DC voltage. A logarithmic (audio taper) pot is a volume control and it matches the logarithmic response to loudness of our hearing.

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The noise from these excellent IC amplifiers is so low that you would hardly hear it, and it would be all the time, not just with signal. The gain is only 23 and is recommended.

The pcb layout and supply bypass capacitors are very important to prevent the IC from oscillating, which could occur with signal. The pcb ground trace from the speaker should not flow into the input ground trace.

Frequently the datasheet has a recommended pcb layout but not this one. Maybe its applications note does or maybe there is a project with it on the web.

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Yeah. I am actually disappointed in velleman for that.

I have another question: What is the purpose of the 2R7 resistor and the 100nF cap on the output?

Also, what would you recommend I use for a low-pass crossover for a sub? An inductor or a electronic crossover. If an inductor, how do I make one?

Thanks in advance!

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Hi Francois,
Speakers are inductive which causes them to have a high impedance at high frequencies. The RC networks at the amplifier outputs are "zobel networks" that are a low impedance at high frequencies to prevent the amplifiers from oscillating.

An inductor and capacitors to make a passive crossover network for a sub would be huge and have a high resistance which would ruin damping of the sub.
Use an active crossover circuit instead. There is a second-order sub crossover project in our projects section, but I think a crossover should be 3rd-order instead. Rod Elliot has a 3rd-order crossover project but it isn't for a sub, it is for 3-way speakers. You could re-calculate its parts for your sub. You should also have a highpass for the main speakers as part of the crossover network.

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An active crossover circuit for a car might use a single 12V supply and an output coupling capacitor. When power is turned on or off then the coupling capacitor will charge or discharge and make a loud "BANG" in the sound system.
A home active crossover circuit will probably use a positive and negative supply, so its coupling capacitor won't charge and discharge and won't make a noise.

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A guy called Gevv joined our forum a few months ago and posted many projects with the RAR file type, and having a password I think is gevv. I saved some of his projects and many are amplifiers using the LM3886, some paralleled.

EDIT: This file is without a password and has a 100W into 4 ohms amplifier project using two LM3886 ICs. Its pcb is double-sided.

PA100_2x_LM3886_in_parallel.rar

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