After I built several LM3875 and LM3886 gainclone amplifiers, I was totally impressed by their audiophile sound quality. Then I saw the Jeff Rowland Power Amplifiers that use LM3886, I decided to create my own version - the BPA300. My design goal is to create a audio power amplifier that can deliver 300W into my 4-ohm DIY speaker with low distortion. I want it to produce deep, tight and punchy bass while keeping the excellent mids and highs from my other gainclones.
My design uses a PCB to hold 3 paralleled 3886s (i.e. PA150), and then I use the DRV134 to bridge 2 of the PA150 PCB boards. The function of DRV134 is to convert the un-balanced input signal to a balanced signal, so that the non-inverted signal is fed to one PA150, and the inverted signal is fed the another PA150. One of the PA150 is connected to the speaker's positive input, and the other PA150 is connected to the speaker's negative input. Because of this push-pull configuration, the total gain of the amplifier is doubled. Each PA150 has a gain of 20, so the gain of the BPA300 is 40.
[UPDATE : Sept 20th, 2007] I have tried using the LL1545a input transformer in the BPA300 to replace the DRV134, see the BPA300v2 page for details.
I used the LM3886 in the inverted configuration, the reason is that I can use a trim pot to adjust the DC offset of each 3886. National's application note AN1192 suggested to use servo to set the DC offset to zero, but I prefer this simpler solution. After some adjustment, I can get the DC offset of each 3886 to about 9mV, because this is a push pull configuration, the DC offset at speaker terminal would be very close to 0mV.
PA150 board Schematic :
The PCB is designed in a way that the center area is filled with ground. Which the +30V supply line is at the outer area of the back of the PCB. The -30V supply line is at the outer area of the front of the PCB.
The PA150 PCB - The resistors R2 and R3 are 0.1% parts, you could also buy 1% parts and hand match them to 0.1% accuracy. For the R7 0.2ohm 2W resistor, the PCB is designed to accept either SMD or thru hole component.
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