Showcase - DIY LM3886 100W+ bridge/parallel build

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meshsmith

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Hi everyone - fairly new to the forum. Just thought I'd show the results of my latest (successful!) amplifier build. This is one of the few that won't be destined to spend its days in the amplifier graveyard that is my workshop.

I got my hands on 10 original, fresh-from-Texas-Instruments LM3886 Class A/B amplifier chips, and wanted to put as many of them as I could into a single stereo amplifier. I ended up on a parallel + bridged design that uses 8 of the chips in total, leaving me 2 to experiment with and ultimately destroy.

I designed the PCBs and laid them out in Altium, with components calculated and chosen to deliver around ~60W per channel into 8 ohms. They run off +/-28 VDC, with each board capable of handling around 3.5A of current at a maximum.

Each PCB has two LM3886es in parallel, which is best suited for 4 ohm loads. However, each channel uses two of the PCBs bridged in a balanced configuration, one board for the positive rail and one for the negative. This brings the output power up to just over 100W per channel into 8 ohms. The final stereo amplifier will therefore have four of these populated boards - two per speaker - and be able to drive 4 or 8 ohm loads comfortably with adequate (read: monster) heatsinking!

Each board accepts one half of a balanced audio signal. To convert an unbalanced line input to a balanced output, I put together a simple DRV134 based board, with integrated 15VDC regulated converter that accepts 36-72V as an input. This means the balanced driver can be powered from the same PSU that drives the amplifiers (28V +28V = 56V) with very high efficiency. Alternatively they can also be driven with a native balanced audio input if available, and this conversion can be skipped.

The final build will have a separate power supply per channel - high currents mean it will be a little less stressful on the PCBs if they only have to handle half of the load. And of course a pair of monster toroidal transformers will take care mains conversion - I got the fine folks at Swiftheat.co.za to roll me up some 500VA 22-0-22V dipped coils.

So far subjective audio tests are very promising, but have yet to hook it up to the scope and scrape some THD values. I doubt there will be any surprises, but fingers crossed!

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