Chipamp Dual Mono LM3886 Gainclone Speaker Amp

I'll be dipping my feet in the chip amp waters again. I enjoyed the results of my previous Chipamp.com LM3886 build, so I decided to build another, more serious version. You can visit that post to read a little bit about the history of the chipamp, also known as the Gainclone, based on 47Labs Gaincard topology.

As you are likely aware, DIYers have created chip-based builds using a variety of power opamp or audio opamp chips including National Semiconductor LM1875, LM3875, LM3886, and LM4780. The most popular chips being the LM3886 and LM3875. You'll find a number of these designs at Chipamp Electronics.

This more serious build features a pair of Antek 300VA torroids, each with their own shield, integrated amp functionality with an Alps Blue Velvet and multiple selectable inputs, film capacitors on the driver board, and higher end parts like Nichicon Gold Tune power caps, Cardas binding posts, and Cardas RCA connectors.

Cardas wire, RCA connectors and binding posts

Other nice additions to the build include a 6 position Grayhill selector switch and the Alps Blue Velvet RK27 (100K) volume potentiometer, both with the corresponding ChipAmp BrianGT PCBs. The RK27 board is handy as you don't have to discern which pin is which (the white screen tells you which wire goes where). It's also a little easier that soldering to the little pins. The Grayhill switch's pins are so minuscule that you really have to use the board, it's not like an Elma or Goldpoint with larger gold solder lugs. The nice thing is the combo is a fraction of the price of the fancier selector switches.

  

Chipamp's PCBs for pre-amps / integrated amps

This project would require a larger box than the Bud 7" x 12" x 3" aluminum box I used previously. Rather than build my own gravity mount chassis, I wanted something a little different this time. I turned to Horace Atkinson of www.iagaudio.com for a robust 17.5" W x 10"D x 3.75"H chassis. I stumbled upon his chassis work on eBay: it's a nice combination of aluminum and wood panels for sides, as well as attractive wood accents on the bottom. The metal used in these chassis is a thicker gauge, one piece .125" aluminum tubing, so it can support heavy transformers without any bend or flex. (FYI IAG offers thicknesses up to .188"). The bottom features perforated sheet metal for adequate air ventilation reaching the heatsinks cooling the LM3886. A unique feature is the wood side panels actually act as feet, holding the aluminum portion up higher to allow for ventilation, very cool!

IAG Audio aluminum and wood chassis 

A view of the bottom with the perforated sheet metal removed. Note the bottom of the chassis is not completely open due to the rigid, tubular design, however underneath the wood side panels, there are removable metal panels that can be removed by unscrewing two flush mounted screws to give you adequate access for drilling, etc.

Chassis interior

As noted, the wood panels are all easily removable if you decide you want to access the sides of the chassis interior or change the wood stain, etc. Horace found some beautiful figured maple for me, as you can see below:

Figured maple side panels and accents

After much internal debate, I decided the chassis would be colored to match some of my other equipment with a red and cream color combo, so the wood was stripped down and stained a deep red hue, then given a number of generous coats of MinWax Satin Clear Urethane.  

Red stain and urethane on Maple

Now that the wood is my a nice shiny red, it's time to drill the chassis. After planning out the layout in Photoshop, the chassis is prepped with a ruler and sharpie and taken to the drill press. Many of the holes will be countersunk for this project. The others will be deburred using a Skaviv deburring tool with Cobalt bits. The IEC hole is trimmed using the typical combo of drilling four holes in the corners and using a Dremel cutting wheel to remove the rest. 

Chassis prep

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. I installed 1" aluminum standoffs on the heatsinks so I could mount them to the chassis. Small drilled venting holes will be above them so when heat rises it can escape the chassis. 

Below is the chassis after a nice cream colored powdercoat. 

Since the amp is dual mono, it gets not one, but two chunky toroids. The transformers are always what weigh down an amp (big magnets with large quantities of copper wire spun around them aren't the lightest of materials). The pair, with steel cases, weigh in at a healthy 22 pounds. These are 300VA rated units from Antek.

Below is one of the toroids in a steel case. I had these powdercoated brick red as it makes a nice accent color for the cream. Typically the steel cases are used as a shield within a chassis, however I'm using them as both a shield and external transformer covers. I don't think I've ever seen a solid state amp with external transformers (usually they're just hulking boxes) so this will be a unique design choice.  

I'm adding the Power Supply Soft Start Board (or SSB for short) from DIYAudio for this build, this allows a gentle inflow of power on startup, which protects the power caps and should offer a more reliable service life for the amp. 

With everything ready, the chassis can start to get filled in. Shiny Cardas RCAs adorn the back of the unit, the binding posts would have been a bit tight on the back, so they were mounted to the top of the chassis right behind the transformers. Four large film caps will be residing in this chassis as well, so you'll notice some zip ties within the chassis as well.

Getting everything in place was a tricky predicament, as the sides of the chassis which held the heatsinks were screwed in place, then the wood panels were screwed to them, but the capacitors would have to be slid in to be held and secured, so order of operations took precedent. 

The power capacitors were a bit tall, so I had to use some minuscule PCB standoffs so they wouldn't come in contact with the chassis. The film caps, as you can see, were a perfect fit. I made a few small turret boards to keep the wiring neat. Cardas wiring handles all the signal work and thicker power wires as well. If you look carefully you'll notice one of the blue wires exists after the potentiometer to the RCAs to handle subwoofer duty. 

And the final product, see what you think:

Final weight was about 40 lbs. thanks to the hefty transformers and very solid tubular chassis. It's a unique take on a solid state amp, not the typical big black box with heatsinks on the side, but a more vintage tube-amp looking chassis. The sound is excellent, will be looking forward to putting it through the burn-in process and most more impressions.

The Fine Print:

Please remember that building/modifying circuits 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. 

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, Allan at Chipamp Electronics, DalAudio, and others.

After reading about the various kits available, I settled on the LM3886 from Chipamp Electronics 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.

Chassis layout created in Photoshop

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.

Chassis with parts in place

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. I installed 1" aluminum standoffs on the heatsinks so I could mount them to the chassis with the fins facing the vent holes I planned to drill. 

Pair of black heatsinks

To create the vent hole pattern before drilling, I used a ruler and marked the chassis with an extra fine Sharpie. The holes for the heatsinks were also marked.

The next, and most time consuming step, is to prepare the chassis by punching/drilling the various holes for the components. 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

Bottom of the unit with EAR feet

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

I needed to implement a USB DAC into the build so I could pull some computer audio through without relying on the PC's sound card. As we were concentrating on low-cost, high quality for the build, I decided to implement the GrubDAC, designed by Erik Soosalu. Beezar is currently offering the latest Rev. F of this design. The board and kit is only $35 plus shipping. SMD soldering is required, which requires a lot of solder flux, and a lot of patience ;)

Beezar Rev F GrubDAC kit

I ended up using one of the older green GrubDAC boards in this build and ran Belkin gold USB cable to a Switchcraft D planel mount USB connector for the back (part number EHUSBBABX). The board was mounted with standoffs to the side of the unit. A shielded Cardas 2x24 cable caries the signal to the front of the unit.

GrubDAC mounted on the side of the unit

Below is a photo of the completed interior of the unit. The LM3886s were fastened to the heatsinks with some Antec silver thermal paste between them. On the front of the unit is the power switch and an input switch that switches between the front D panel RCAs and the USB input. Power is carried with a twisted pair of 16 gauge silver plated copper wire in PTFE. Speaker output wire is shielded Cardas 21.5g x 2 wire. All signal wiring is run through 23.5g Cardas wire. The ubiquitous Alps Blue Velvet handles volume duty. Extremely small signal paths, so the wiring costs are minimal despite using "boutique" Cardas wire.

Interior of the unit with GrubDAC mounted

Front of the unit with switches, volume and D Panel RCAs

Side of the unit with heatsink vent holes

I used some generic yet very solid gold plated binding posts for the rear. The IEC inlet is mounted in a slightly odd position, but was necessary considering the small form factor. 

Rear of unit with binding posts and D panel USB input

The purpose of this little build was to go into a PC-based arcade cabinet without taking up much space. You can check out that build here. I did spend a good amount of time testing the unit as I was very curious about sound quality considering the low cost and minimal parts. I was VERY surprised at how good this unit sounded. I haven't heard anything this good for this cheap. It's quick and robustly detailed but doesn't have that typical solid state presentation that I stray away from. I may build another unit with a dual mono power supply and even nicer parts in the future.

Allan at Chipamp Electronics has the LM3886 kits and numerous other accessories including a nice selector switch on PCB, potentiometers, upgraded caps and other items you'd need to build a great sounding amp on the cheap.

As always, if you are in the market for some beautifully built custom audio cables, please check out Zynsonix Audio. They offer the highest quality parts and their prices are easily affordable.

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.