I recently built a dual-mono LM3886 chip-amp / gainclone amplifier from a kit sold by Brian Bell, at chipamp.com. You might like to build one too, they are a really cool design. In this article, I discuss the history of the gainclone and how I built my first chip-amp!
The LM3886 is one of National Semiconductor’s single-chip monolithic amplifier integrated circuits. It’s almost an entire amplifier-in-a-chip, with only a few extra parts required to build a fully functional, high-performance amplifier. The LM3886 features low distortion (less than 0.1%) and respectable power output into reasonable loads. The chip will output around 50 watts per channel continuously into 8 ohms, with correct heatsinking. It delivers a little less power into 4 ohms, though this depends on the voltage you feed the chip.
The gainclone concept goes all the way back to 1999 and a company called 47 Labs. The designers at 47 Labs introduced an amplifier called the Gaincard, possibly alluding to the fact that it represented close to the ideal amplifier – a linear gain device. The Gaincard was well-reviewed at the time, but caused considerable controversy. It sounded great, despite not using much power supply filtering and hardly any parts. It cheekily flew in the face of conventional amplifier design. Most of the controversy however, came from the sealed 47 Labs chassis and what was hidden inside. This naturally made owners keener to see inside and when someone finally did, the shit hit the fan…
Enter the ‘Gainclone’
What the audio community discovered was that the Gaincard amplifier, which retailed for $USD3300, was nothing more than an LM3886 integrated circuit and a couple of dollars worth of extra parts. Sure, this was creative, ingenious even – the total signal path was only a few centimeters long. The chassis was beautiful, but owners felt duped, they paid thousands of dollars for just a few bucks worth of parts.
The DIY audio community naturally became interested at this point – suddenly people realised that they could create gainclones, with perhaps even better sound than this expensive and well-reviewed amplifier. And so was born one of the great DIY audio phenomena of recent years – the gainclone!
There are several gainclone kits available online and via eBay, but I like this particular set of boards and parts from chipamp.com. The PCB and parts quality are superb and, after extensive research, I found that this is the best value of all the kits out there.
Everything was very easy to assemble and the Chip Amp kits include quality 10,000uF Panasonic filter capacitors, great quality resistors and other passives. The traces are gold-plated and boards are of quality fiberglass construction.
I was going to use two spare 100VA transformers with dual 12V secondaries, wired in series, that I happened to have lying around. The problem was that the 12V secondaries of these 100VA transformers wouldn’t be ideal in this application, yielding around 17V DC, before diode losses. Instead, I decided to use a single 300VA transformer, with dual 25V secondaries. The amp is still configured as a dual mono layout, the secondary voltage was now correct for driving an 8 ohm load and the single toroidal transformer will mount nicely where the old one did in this chassis. The 25V secondaries rectify up to around 35V (25 x 1.414), minus diode losses.
I decided to use this old amplifier chassis I had lying around:
Performance and Ground Loops!
You can see from the above image that my LM3886 gainclone amplifier is up and running and she sounds great! Everything you’ve read about chip amps is true. This amplifier has air, tone and sounds smooth and relaxed. One observation is that perhaps the amplifier is a little bass-shy, compared to my Krell KSA-150. Realistically, in most installations, the LM3886 gainclone is a cracking amp.
One small issue came about through not thinking carefully enough about the wiring layout. I experienced some hum on initial testing, which surprised me. After thinking and reading, I decided to modify the ground wiring layout, as shown in the images below. I rewired the ground wiring to include a true star ground. This would avoid the multiple earth returns of my initial layout, which caused the hum.
Look at the image below, to the rear of the chassis near the inputs. I’ve added a thick buss bar, linking loudspeaker grounds, amplifier and power supply grounds. The new ground buss is connected to chassis-ground, where the line cord earth wire is terminated. This arrangement is totally silent, no ground loops to be found!