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June 18, 2014

BrianGT LM3886 Chipamp aka Gainclone Amplifier

Given that I mostly enjoy building tube amps that could have been built in the 1960s, this is a huge departure; we are zooming along to 1999 technology! As you may have read, the well revered audio company 47Labs, who are known for their often simple, minimalist circuit topology, brought into the world the Gaincard, a $3,300 amplifier (with power supply) that would spark debate in the audio community for several reasons.

The amplifier made use of the National Semiconductor LM3875, a 56 watt chip and relatively low capacitance. The internal parts were rumored to cost under $100, and the amp garnered a number of great reviews, so it was only natural that DIYers would want to try and duplicate, or potentially improve such an intriguing circuit.

DIYers have created similar builds using a variety of power opamp or audio opamp chips including National Semiconductor LM1875, LM3875, LM3886, and LM4780. Likely the most popular chips are the LM3886 and LM3875. You'll find these in designs from Peter Daniel of AudioSector, BrianGT of Chipamp, DalAudio, and others.

After reading about the various kits available, I settled on the BrianGT design as I liked the idea of additional capacitance in the power supply.

The Chipamp LM3886 Stereo Kit

Aside for the boards and the limited number of parts, the build would require a transformer, a box, heatsinks, heatsink mounting compound, binding posts, RCAs, switches, an IEC inlet with fuse, wire, standoffs, etc. After roughly deciding on where each part would reside within Photoshop, I settled on a 7" x 12" x 3" aluminum box. Both Hammond and Bud offer boxes this size, I ended up going with Bud as it was in stock at Mouser. See the rough layout below.

I prefer to fit the components into a case where there is enough room to breath, but there isn't a significant waste of space at the same time, so I generally avoid the 17" and 19" wide rack style enclosures. For the Toroid, an Avel 250VA 25V + 25V was selected (Model Y236652). Other brand options out there are AnTek and Plitron. For the heatsinks, I found some reasonably priced ones on ebay that had mounting holes for screws. The instruction manual from Chipamp says you can also use a 3"x3"x1/2" piece of aluminum in free air in certain applications.

The first, and most time consuming step, is to prepare the chassis by laying out the components and punching/drilling the various holes. The Bud aluminum enclosure was quite thin and soft, which caused some mild chassis bending on the drill press. I don't think I would pick up one of these again, it's just too thin. Can't expect much for $30 though.

Prepped and sanded chassis

Once the chassis had the necessary holes, it was sanded down and prepped for paint. I used some Hammertone paint to give it a nice vintage patina with an interesting texture.

Hammertone Finish

With the chassis ready, I could concentrate on populating the PCBs. I purchased the kit from the Chipamp website, however decided to switch out a few of the parts for some brands I tend to trust, namely Wima and Elna. Wima polypros were used (and in higher values for coupling duty) where they fit, and Elma Silmic IIs were used for nearly all electrolytics (I had some of these already lying around, hence the 100v ones used on the signal boards). In addition, the resistor in the signal path was replaced with an equal value Kiwame. Surprisingly, the PCB holes were large enough to accommodate the thicker leads on this. As some of the parts were larger than the originals, they were mounted on the bottom of the PCB and long 1" standoffs were used.

Populated PCBs

More to come... of course!

The Fine Print:
Please remember that building circuits and performing circuit modifications can be dangerous to you and/or your surroundings and should only be performed by a certified technician. The owner of this blog and all associated parties can not / will not be held responsible if you attempt a build or modification posted above and cause physical harm to yourself or your surroundings. Many electronics contain high voltages that can kill, and mods, if performed improperly, can be a fire hazard. Please keep this in mind.