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The LM3886 Quad Amp Project


betocool

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I've had this itch for a while, to build a power amp based on the LM3886. I have one on my guitar amp, and it does a very good job, although in a very different category. I also have some PCBs I had printed some time ago, which work very well, so I really only need to get some wire and some components.

 

Oh... and a transformer. And a case. And a nice power on/off switch. Luckily for me, I have most of those parts at hand, in some form or another.

 

Last year, a colleague and I decomissioned a microscope from one of the local hospitals, and as luck would have it, I got to keep a massive 240VA toroidal transformer rated at 23V 8A, German made. It's been sitting on the shelf for far too long, and what a wonderful transformer it is. The case is a former Sony TA2640 which had seen better days, it was a good companion, but sadly, it was distorting at moderate volume. I might have tried and repaired it, but then again, I need the case, so the amp has been stripped of its innards, and there I have a fully metallic case for my project. The most expensive parts then I have.

 

This will take a while though. I will start with a soft-start controller based on a Discovery board. On the OFF mode, everything shall be disconnected from the power. I'm not a big fan of standby mode. A 9V battery will power the controller and activate the main relay, which will then provide power from the grid to two transformers, one for the controller and the power transformer. The power transformer will be connected at the beginning over two big 76 Ohm resistors to avoid a big inrush current. After about 1 sec I will bypass the resistors. Last, after about 5 seconds, when all the voltages have settled and the LM3886 is on it's operating point, I will activate two relays to power the speaker outputs.

 

Each individual output will be driven by a single LM3886. That is, four in total, 2xA and 2xB. Overkill? Maybe, but might come in handy if I decide to bi-amp some day.

 

For turn off, the controller shall disable the speaker output first and then swiftly kill itself by removing power from the main relay and the secondary relay. All of it (hopefully) with the push of a button.

 

Nothing else, no volume control, no fancy switches, no A/B speakers, no mute. Just a simple power amp as power amps should be. I'm thinking 40000 µF per rail should do fine.

 

I started today, power transformer in place, that's about it. Will do some drawings to have the wiring check and get to work on that too. And the controller.

 

Fun times ahead! As if I didn't have any other things cooking (like, lasagna for tonight... yum!)

 

Cheers,

 

Alberto

 

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Little progress today. Very little. And yet it took ages! Never underestimate wiring!

 

Put in the mini transformer for the power controller and wired the mains to it through the relay. Since I was in the vicinity, I also screwed the heat sink and amp PCB holder to the side. That will limit my space even more... it will be a tight squeeze. Just tested the relay with 9V, and lo and behold... it relays!

 

IMG_20161030_145351.jpg

 

 

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  • 3 months later...

Hi, thanks!

 

It's still on the bench! Still need to get some parts, like the Audio Amp Chips, and program the power logic, but some things came in between that have a higher priority at the moment, like my digital phono stage and low voltage power distributor. I'll definitively finish it this financial year... I think...

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