LM3886 amplifier assembled boards are available on EBay and Amazon for under $10 US, assembled. For near field monitors, non-OEM IBM/Lenovo power supply bricks, (available on EBay for under $10 US, each,) can provide +/- 20 V DC rails at about 4.5 Amps, for a power output of about 20 Watts per channel, for an under $50 US stereo power amplifier suitable for near field, or computer audio systems.

However, there are a couple of issues:

• The input offset voltage of the LM3886 is 10 mV, and, with a gain of 10, the speakers will have 100 mV DC across the terminals, (the prevailing wisdom is 2 mV, maximum.)
• The IMD is 0.01%, and the THD is 0.03%

Both issues can be corrected by green wire bridging the frequency contouring passive components on the LM3886 board, and adding a DC servo loop and gain at the input.

To faciliate design, a tape archive, PA.tar.gz, which contains the simulations and printed circuit board, (PCB,) development database, (including industry standard Gerber format files, connectivity "rat nests", component vendor and bill of materials, cost estimates, etc.,) is available.

Figure I, (1600X1200,) is an example schematic. A suitable iterated design methodology would be something like:

• Chose the value of R202.
• Chose the value of R203, such that U1 does not saturate at very low input frequencies, (i.e., less than 1 Hz.)
• Calculate the effective value of R201 for the desired gain. Reduce the effective conductance of R201 by the conductance of R203.
• Calculate the value of C207 required for dominant pole compensation of the amplifier.

U1, (the DC servo loop integrator,) is fairly pricy and can be replaced with almost any commodity op-amp, (the LM833 is a good choice,) with a low Vos-less than 2 mV at 25 C, (typically, +/- 6 mV, 0 to 70 C.) including Ios effects.

Note that there is about 20 dB excess open loop gain at 20 KHz., reducing the amplifier IMD to about 0.0001% and THD to about 0.0003%, meaning that the distortion specification is the almost the same as U201, (i.e., dominated by the low distortion NE5534.) U1 can be replaced with an LM833, also, which has a respectable distortion specification.

Also note that the inverting configuration of the power amplifier has the advantage that the open loop high frequency zero is eliminated from the transfer function, and an overall gain less than 10 can be obtained-possibly eliminating the necessity of the DC servo loop, (the output Vos would be controlled by U201's Vos, multiplied by the closed loop gain.)

License

A license is hereby granted to reproduce this software for personal, non-commercial use.

THIS PROGRAM IS PROVIDED "AS IS". THE AUTHOR PROVIDES NO WARRANTIES WHATSOEVER, EXPRESSED OR IMPLIED, INCLUDING WARRANTIES OF MERCHANTABILITY, TITLE, OR FITNESS FOR ANY PARTICULAR PURPOSE. THE AUTHOR DOES NOT WARRANT THAT USE OF THIS PROGRAM DOES NOT INFRINGE THE INTELLECTUAL PROPERTY RIGHTS OF ANY THIRD PARTY IN ANY COUNTRY.

So there.

Copyright © 1992-2016, John Conover, All Rights Reserved.

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