Introduction: DIY HiFi Gainclone Power Amplifier (2x68w, Class AB-A, LM3886)
I've made gainclone amplifier that uses a LM3886 chip. It's one of the best compact HiFi amplifiers. It is a Class AB-A (conjugate) amplifier that has a fully symmetrical structure (push-pull), this mean that the sine waves produced, will produce a + , - output. A 25 volt (+/- Dual Rail), 5 Ampere, toroidal transformer was used to supply electricity for the amp. There's also a PCB Layout provided for the project. I found an old CPU processor attached to a gorgeous heatsink & fan, I recycled both of them and drilled holes for it to be mounted on the LM3886 chips.
- Maximum Output Power: 68W RMS - 108W Peak
- Frequency Response: 7Hz-25kHz (Filtered ⇒Linkwitz)
- THD: %0.03 at 60W
- SNR: 110dB at 60 W - 92.5dB at 1W
- Output Class: AB-A (Conjugate)
- Auxiliary Features: DC/AC Short circuit protection and thermal protection.
- Working Voltage/Power: 12-94 Volts (Dual Rail), 1-10 Amperes
- Audio Features: Input Mute Funtion (100% silent = no input)
Too bad I had a late documentation, that's why I only have a few pictures of the steps and procedures. Due to the late documentation, I had to get images from the original source. Some of the pictures are not mine, there are no claims that the PCB designs are mine nor the diagrams, although there where some modifications made by me for the amp.
What Is A Gainclone?
Back in 1999, relatively unknown manufacturer 47-Labs released the "Gaincard" to rave reviews. It immediately caused controversy because it was based around a $5 power amplifier IC (Integrated Circuit), yet a typical setup would cost you around $3300. A Gainclone in the other hand refers to any DIY amplifier that uses a LM3875/ LM3886 as it's main component.
What's So Special About It?
The gainclone only uses small amounts of space, it's built with a minimal supply of components, yet giving a great HiFi result. It beats the hell out of those modern HiFi amps, it produces 2x68 watts of power, with a Total Harmonic Distortion of 0.03%THD and built on a 2x3 PCB board! So why doesn't the audio industry use it? I'm not really sure. All of these sounds "too good to be true", I recommend this project for those who are still starting on their DIY HiFi amp hobby, since it only uses some few components.
What's A Power Amp?
A power amp is a amplifier with no preamp, volume control, tone control, or any auxiliary devices attached to it. It's just pure simplicity.
Coming Soon: DIY HiFi Preamp With Tone Control (Works With The Gainclone)
Step 1: Parts, Materials, & Tools
I can tell that parts in the US are highly expensive. Here in the Philippines a typical resistor would cost 0.25 centavos ($0.0006), while in america, a typical resistor would cost $1 each. There's a large difference. Also, parts are available here almost everywhere. That's the beauty of living here when you have a hobby of electronics. My project cost me P870.00 ($21.75) all in all.
- LM3886 amplifier chip (2pcs.)
- 25 Volt, Dual Rail Transformer (Toroidal/ IE)
- Rectifier Diodes 6 Amperes (4pcs.)
- 2x3 PCB board (2pcs.)
- Large Heatsink (2pcs.)
- Cooling Fan (1pc.)
- Plastic/ Metal Enclosure(1pc.)
- Binding Posts
- Female RCA Plugs
- AC Cable
- 20k 1/4 watt Resistor (4pcs.)
- 10k 1/4 watt Resistor (2pcs.)
- 15k 1/4 watt Resistor (2pcs.)
- 1k 1/4 watt Resistor (4pcs.)
- 2.2k 1/4 watt Resistor (2pcs.)
- 10,000uF 25v Electrolytic (2pcs.)
- 10uF 25v Electrolytic (4pcs.)
- 2.2uF 25v Electrolytic (2pcs.)
- 470nF Mylar/ Ceramic (2pcs.)
- 150nF Mylar/ Ceramic (2pcs.)
- 100nF Mylar/ Ceramic (2pcs.)
Step 2: Getting to Know Your LM3886 (I.C./Chip)
The LM3886 is the brother of the LM3875. It's the improved version. It produces a higher output wattage, and thermal and mute features are added too.
There are two types of LM3886 IC. One of them is unisolated LM3886T and the other one is isolated LM3886TF. Be careful not to mess with the isolation. The LM3886T requires a smaller heatsink, it's plate is conductive, a negative current flows through it, two LM3886T chips cannot be mounted on a single heatink without using a mica insulator, nylon rings and other insulation devices. A LM3886TF on the other hand, can be mounted on a common heatsink without insulation devices
The LM3886 is a high-performance audio power amplifier capable of delivering 68W of continuous average power to a 4Ω load and 38W into 8Ω with 0.1% THD+N from 20Hz–20kHz. The performance of the LM3886, utilizing its Self Peak Instantaneous Temperature (°Ke) (SPiKe™) protection circuitry, puts it in a class above discrete and hybrid amplifiers by providing an inherently, dynamically protected Safe Operating Area (SOA). SPiKe protection means that these parts are completely safeguarded at the output against overvoltage, undervoltage, overloads, including shorts to the supplies, thermal runaway, and instantaneous temperature peaks. The LM3886 maintains an excellent signal-to-noise ratio of greater than 92dB with a typical low noise floor of 2.0μV. It exhibits extremely low THD+N values of 0.03% at the rated output into the rated load over the audio spectrum, and provides excellent linearity with an IMD (SMPTE) typical rating of 0.004%.
- 68W cont. avg. output power into 4Ω at VCC = ±28V
- 50W cont. avg. output power into 8Ω at VCC = ±35V
- 38W cont. avg. output power into 8Ω at VCC = ±28V
Step 3: The Schematic Diagrams (for Amp and Power Supply)
LM3886 Gainclone Schematics: (source: circuit-projects)
As you can see, negative feedback line is revised. In this way, about 7dB is gained at 30Hz-70Hz band, and reached about 34db at peak point at 30Hz.
When 26v DC supply is applied to the given design, with 4 ohm speaker load and 2Vp-p input signal it gives about 65W RMS power. For users who want to use 8 ohm, we recommend +/- 38V voltage supply. Of course less than this value may be applied too.
Supply circuit is shown below, It is simple too. There are two KBU series (6A) bridge diodes and 2x10000 uF capacitors. As a general rule, 2200uF stabilization capacitor is used per ampere but we are used to use 4700uF per ampere. In our circuit, 2 Amperes will be drawn per line and we use 10000uF capacitors. We recommend you to use 2x10000uF capacitors per each line if you will use the amplifier in party mode.
Dual Rail Power Supply PCB:
It's so simple that you don't need to use a custom PCB board, you can use a regular old universal PCB. All you need are four 6 ampere, 50 volts rectifier diodes and two large electrolytic capacitors for filtering (the larger the value the better/ cleaner/less buzzing), 4,700uF is enough to filter the line.
Since this is an audiophile setup, I've been using custom winded toroidal transformers. Toroidal transformers has a lesser magnetic flux leakage, this means less buzzing thus you can put the transformer close to the amplifier circuit without hearing any buzzing sound from the speakers/ output.
I've placed the capacitors inside the toroidal transformer's core to conserve space (shown in the picture).
Step 4: The PCB Layout (for LM3886 Amp)
Here's the PCB Layout of the amplifier. There's a PDF file provided below, the height and width is set, don't worry of resizing the image. If you wan't to double check the printed PCB layout, get a ruler and it should have a measurement of 2x3 inches or something close.
There's a flaw in the PCB labeling (ex: there's no C15), I will replace the schematic diagram soon, but for now, please use the schematic diagram from step 3 as your reference, please analyze the parts carefully. It will work, it just a bit confusing.
If you don't know how to make a custom PCB, you can either use a universal PCB board (perfboard) or you can read my other instructable about making a DIY PCB. The guide is useful, it got featured last September 3, 2011. You can click for the guide below.
Step 5: Soldering the Parts (PCB Assembly)
As I said a while ago, there are some flaws in the part labeling. No worries, we're just luck that the values are correctly labeled it's just that the schematic diagram, did not match the PCB layout component map. I managed to make it work. Just analyze the foil patterns and the connections found in the schematic diagram and you will do just fine!
As you can see that there are small tantalum and metal film capacitors soldered below the PCB, the original author did this because he ran out of space on the non conductive part of the PCB. Don't worry those tiny capacitors found below aren't really that important it's only used for line filtering (less buzzing), it's connected to the positive and negative power lines. As much as possible, try to buy those capacitors with a tolerance of 1%, this is done to achieve a better balance between both channels.
Step 6: Mounting the Parts in the Plastic Enclosure
Transformer/ Power Supply:
Try to conserve space as much as possible. I've mounted my line filter capacitors inside my toroidal transformer's core. Since the transformer doesn't heat up that much and the enclosure is tightly occupied, I just hot glued it to the enclosure. I also added a bright blue LED inside to create a cool professional glowing effect in the amp.
Heatsink With Fan:
Since the voltage applied is 25 volts and the fan requires only 12 volts of electricity, I used a 7812 regulator chip. If your not framiliar with it, you can just Google it, it is extremely common and easy to construct. As you can see, I've bent the heatsink in a "Y" shape for the screws to fit and to be mounted with the LM3886 chips.
Binding Posts And RCA Plugs:
This step is only a matter common sense. Just drill holes for the binding posts and RCA jacks to fit snuggly and tightly on the rear panel. I used a thin shielded coaxial wire for the RCA inputs and some gauge #16 (99% oxygen free) speaker wires for the binding post output.
I rotated the whole circuit upside-down to conserve space for some future regulators or modules. It's better to add spacers to occupy hollow spaces as a support for your circuit board. I stuck a used/ recycled spool and glued a foam on top of it, for it to cushion the amp circuit.
Step 7: You're Done! It's Finished!
You're Done! It's Finished! It only took 6 steps to finish a Gainclone. I assure you that his amp will give you astounding results. It gives off a very neutral audio response. All you need to do is to make a preamp and tone control circuit (next project).
I'm currently documenting the tone control/ preamp circuit project. Only this time it's fully documented, instructions would be more specific, there will be more details.