Other things to make the LM3875 kit work - diyAudio
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Old 30th January 2011, 08:05 PM   #1
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Default Other things to make the LM3875 kit work

I am going to be building the LM 3875 dual mono kit from AudioSector soon and I have a few questions about the other parts and pieces to make the kit work. I was planning on using the stereo setup Mr. Daniel mentions in his pdf guide. I want to power two bookshelf speakers with an 8 ohm impedance and sensitivity of 90 dB. I have several questions below.


First: transformer. I need a minimum of around 250VA. The output voltage can range between 18-25V, from what I have gleaned through other posters. How would the output voltage affect the sound though? Would 18V give me a softer sound, whereas 25V would be a stronger, more powerful sound, with any voltage in the middle of the range being a compromise of the two? I primarily listen to rock, so a more powerful sound would be ideal.


Second: heat dissipation. Logically, a higher voltage going into the ICs would lead to more heat. With a 25V transformer, would a separate heatsink be necessary, or would attaching the ICs to an aluminum plate be sufficient? Does the transformer produce any significant heat?


Third: interference. I think I have read that AC voltages can do funny things to signal lines and such because of the oscillation, but I do not fully understand how I would prevent this. Can this be avoided with proper spacing of components? I was planning on designing the smallest permittable housing to contain the transformer, power rectifier PCB and two IC PCBs. Is there any kind of shielding I need to put around the transformer and AC power lines, or a minimum spacing between AC and DC components?


Fourth: power supply. I want to use one transformer for two ICs. I saw Mr. Daniel mentions this in the pdf guide, but I didn’t see where he explained how to do it (I know it has to be out there somewhere, I just haven’t found it yet). What do I need to look for with the toroidal transformer I will be buying to make sure I can do this? I was thinking about getting the AN-3224 from AnTek (found here: https://www.antekinc.com/pdf/AN-3224.pdf), but I am unsure if the wiring will work for two mono amps.

I think those are all my questions. Thanks in advance, and please just bear with me if I have some questions that are obvious.
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Old 31st January 2011, 12:12 AM   #2
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In my experience lower voltage produces indeed softer, more relaxing sound, while higher voltage supply will introduce more liveliness and impact.

The generated heat will not depend that much on supply voltage, but mainly on the output volume. In other words, if you push the amp to the limits, the small heatsinks can easily reach 50* temp. The chip has built in thermal protection circuit, so even if your heasinks are not efficient enough that shouldn't damage the chip. Patek amp uses really small chassis footprint, but so far I never had a problem with thermal shutdown. Heat from transformer is negligible.

The AC voltages from transformer are also negligible, you should be able to place pcbs directly beside the transformer and expect no problems.

The main thing is to use transformer with dual secondary windings. I explained star ground wiring here: Commercial Gainclone kit- building instructions
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Old 31st January 2011, 12:52 AM   #3
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Excellent. I will be ordering a kit from you as soon as I decide how close to finals week I want to have this temptation.
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