LM3886 Based Stereo Amplifier
Several years ago, National Semiconductor came out with some very high performance, easy to use audio power amplifier ICs. I was in need of an extra amplifier so I could biamp some of my home-built electrostatic loudspeakers so I decided to give the LM3886 chip a try.
I chose this part because of the ease of use, power output, turn-on and off thump suppression, low distortion, and built-in protection against shorts and thermal runaway. There isn't much more to ask of a power amp than that. When driving electrostatic speakers, you can't have too much protection!
You can download a data sheet and app notes for the chip here.
Now there are some folks out there who claim to have "golden ears" and feel that no application note supplied schematic diagram is ever good enough so they insist on "tweaking" to make "improvements". The problem is most of the time these folks are not engineers and have no idea what the potential consequences of their "improvements" may be.
For example, for a few years when these chips first became popular with the audiophile crowd, it was all the rage to use minimal power supply filter caps for "best sound". We're talking about 500 mF on each rail of the supply for each amplifier chip used. This is woefully inadequate, and leads to distortion at fairly low volume levels as the power supply sags under the load. The problem was that some of these people saw the IC's great power supply rejection ratio spec and figured that meant they could tolerate a 10V of ripple in the supply. Well, I am pleased to say the pendulum has swung the other way and now many audiophiles are putting proper amounts of energy storage into the power supply.
Another thing I used to see a lot of was the elimination of the "Zobel" network at the output of the amplifier because it "sounded bad". After a few people burned up some expensive speakers they came to realize what the network is for and that dead speakers sound pretty bad, so these days it seems most people are using the Zobel network. I have used my amp to drive electrostatic speakers, a notoriously difficult load due to the capacitive reactance, and never had ANY stability problems whatsoever.
When I built the amplifier I wasn't using a preamp because I didn't need any tone controls and didn't need to switch multiple inputs. I just went from the CD player to the power amp to the speakers. My schematic is pulled right off the NS data sheet, with the power supply added.
When building something like this, the housing is often the most expensive part of the amp. It has to dissipate heat from the amp ICs and power transformer, it has to look acceptable, and it has to be large enough to hold everything. I found the housing at a scrap yard in Dallas, where I was able to buy it for $1 per pound- about $15. The box originally housed a cell phone diversity antenna amplifier.
As you can see, the entire top surface of the box is covered with heat sink fins, so dissipating heat will be no problem!
The side walls are 1/4" thick welded aluminum which contributes greatly to the sturdiness of the box as well as heat dissipation.
There were a bunch of connectors going through holes in the sides of the box. I was able to use some of them, but others were not useful, so I had to figure out a way to fill them. I went to the local auto parts store and bought a can of Bondo auto body filler and used it. It has worked out quite well and I recommend this technique to anyone who is recycling surplus enclosures. First I taped some scraps of PCB material to the inside of the box then smeared the Bondo into the hole from the outside. After the Bondo hardened, I sanded the outside surface flush with the rest of the aluminum.
I added a few more holes where I need them, and enlarged a couple of the existing holes and was ready to apply the finish.
One of my other hobbies is restoring antique radios. I have one very special radio in my collection- a 1927 Neutrowound Super Six. This radio must have looked like it came from outer space back in 1927. At that time most radios were plain, rectangular wood boxes. This one was is a steel box with a very unusual look provided by the nickel plated tube covers that protrude through the top, and the exotic painted finish- cracked blue over orange!
I liked the finish on that radio a lot so I decided to try to duplicate it on my amplifier. I selected a beautiful metallic blue (not quite the same as the radio) and a bright orange for the base coat.
The finishing steps were primer, orange acrylic base coat, acrylic crackle medium, metallic blue acrylic, and finally an acrylic clear coat for protection.
Primer
Base coat
Final coat
Next came construction of the amplifier. I selected suitable components and hay-wired the first channel because so few parts were required. I didn't like the look of it much, so for the second channel I used some perf board. The plan was to eventually go to printed circuit boards (more on that later). You can see I am a firm believer in the "star-ground" concept.
I used a 320VA power transformer- a bit overkill since I am only able to get about 30W per channel out of the amp with the voltage I have in the supply...
Interior as originally built
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The filter caps in the power supply total about 100,000 mF, more than enough, but at such a low voltage, capacitance is cheap, so why not?
The volume control is mounted close to the input jacks on the rear panel to keep the signal wires short and far away from the power transformer. I had to figure out how to make a shaft extension to the front panel, so I used a piece of 1/4" diameter brass tubing. My alignment of the volume control shaft and the front panel hole were not perfect so I needed a flexible coupling. I made it from a piece of urethane air tubing and two tie-wraps. It works well- if you turn the knob past the stop in the pot, it slips a little, limiting the stress on the pot.
Shaft coupler
Eventually I got around to ordering some printed circuit boards and rebuilt the two amplifier channels. I prefer PCBs because I am always afraid that something that is hay-wired together may develop short circuits if something moves unexpectedly.
Final interior with amp chips on printed circuit boards.
Back side of finished amplifier
Front side of the finished amplifier
Vital statistics:
Weight: 18 lbs, 2 oz.
Size: 13" x 10" x 5"
Power transformer: 2x 18V @ 8.9A
Caps: 4x 27,000 mF @ 35V
Supply Voltage (no signal) +/- 26.5V
Volume control: ALPS dual 20K
