The not-a-gainclone LM3886 amplifier

A while back the Denon amplifier I’d had since 1991 decided that it had had enough and would no longer perform as an amplifier.  The idea of going to the local electronics retailer and buying a new one was distasteful, so I decided to build an amplifier around an LM3886 from National Semiconductor.

Disclaimer: What’s described below uses voltages on the mains side of the transformer that can cause serious injury or kill, do not disassemble your amplifier based upon the description herein.  Also note that this is not a finished product and close examination of the photos may find hot exposed wires.  e.g Danger.  Proceed with caution.

Early on I decided that I’d reuse the Denon’s chassis and that I’d just basically throw something together based upon the National datasheet.  e.g. no regard for what makes a gainclone (google gainclone) a gainclone. e.g no fancy expensive toroid transformer, little regard to capacitors (ok I did buy Panasonic FC caps from Digikey with the LM3886 order, but they weren’t exactly expensive), no regard to short feedback paths, etc…

The result is

The transformer is reused from the Denon.  I do not know what it’s amperage is but figured it must be enough.  There are two taps, I chose the taps that are about 20-0-20.  The other set of taps are around 30-0-30 and resulted in a rectified voltage at the absolute limits of the LM3886.  The 20-0-20 taps give about |V+| + |V-| = 52V.  Listening tests reveal this to be plenty loud at even half volume.

There are two bridge rectifiers but that’s because I screwed up with the initial implementation of the power supply.  The rectifiers were reworked such that the two are effectively just one (a waste of $2.00 in diodes) and |V+| = |V-| with the correct connection of common.

You can also see here why people go through the time and trouble to etch circuit boards.  The rectifier implementation results in a mess of wires.  It would be far cleaner on a PCB.

The transformer and a ground loop isolator to reduce hum.  Also note that, not wanting to kill myself with mains power I added a chassis ground.  The original build of the Denon did not include a grounded plug.  I cut the end off of a spare grounded power cord I had and used that instead on the power from the Denon cord entering the chassis.  For safety’s sake I also installed a GFI outlet at my bench power outlet and ensured that it was grounded properly.

The amplifier board itself at the bottom of this picture.  It’s simply a buildout on a protoboard of the example circuit in the LM3886 datasheet from National Semiconductor.  The input and volume pot are hanging off the right side, the speaker connections are wirewrapped on the left side.  Power is coming from the single DC supply on top, which is contrary to most examples I’ve seen which have separate DC supplies for each channel.  In my build, this really proves not necessary.

Note that I’ve already decided my heatsinks are not large enough.   I’ll have to build another with a layout that will accomodate larger sinks.  Listening to the Brahm’s violin concerto (there’s only one, unfortunately) at a moderately loud volume the heatsinks are uncomfortably warm.  Listening to Rush’s Signals album the sinks are “damn that hurt” hot.

Otherwise, for something that cost about $40 in parts, not including my free, reused, transformer,  and with little to no regard to the audiophile convention as to what should be done with this opamp, I am very, very happy with the results.  Yes, you can put this thing together in a rather haphazard fashion and it still sounds great.  I have to hand it to the engineers at National.  Not only for a great chip, but also for the fantastic datasheets that they are known for.

The ultimate goal is to enclose it in the stripped Denon skeleton.  I’ve just been having too much fun listening to it to disassemble it long enough to hide it in the chassis yet.

~ by skatefriday on December 16, 2010.