Electronics enthusiasts have been building their own amplifiers for decades. Typically, doing so involved buying an electronic kit and then soldering the components to a printed circuit board – some kits even included the box and power supply. But in all cases, you had to have the skills to build printed circuit electronic modules – for example, not making any component or soldering mistakes. Make a single mistake and the result was quite likely to be a lump that did nothing!
But these days, the situation has radically hanged.
Available now out of China via eBay are fully built amplifier modules. And, not only that, but you can also buy pre-built rectifier/filters, remote volume controls, switchable input modules – nearly everything to build a full amplifier just by connecting the modules together. That’s right: no soldering, just connecting the terminal blocks.
And the modules are cheap, cheap, cheap.
So are they cheap and nasty? Not at all. Most use well-proven designs incorporating amplifier chips that have been around for a long time. And the fascinating thing is that when you look at the performance specs of these modules, in many cases they are far better than large and expensive multi-channel amplifiers being sold in shops all around the country.
But like most things, there are traps for the beginner. So let’s take a look at how to do it, culminating next issue in a four channel, high performance amplifier that, depending a little on how you build it, will cost from AUD$300. And that’s for an amplifier with an easy 60 watts/channel (even with all four channels being driven at full power), 92dB signal/noise ratio, and 0.1 per cent distortion at max power.
To build a working sound amplifier, the following parts are needed. We will assume at this point that the amplifier will have just two channels and will be fed by a device that has its own volume control (like an MP3 player or a pre-amp). The parts are:
1. two amplifier modules
2. heatsinks to suit amplifier modules
3. one rectifier / filter module to suit amplifier modules
4. one transformer
5. power cord with plug
6. mains power switch
7. hook-up wire
On this list it’s the first four parts that are most important. Let’s look at each in turn.
1. Amplifier modules
The amplifier modules, as their name suggests, are the electronic bits that make the input signals louder. Each module has connections for:
· line level inputs (so they can be connected directly to an MP3 player, TV audio out, CD player, etc)
· speaker outputs (that connect straight to the speakers)
· power supply (typically something like plus/minus 28V DC – more on this in a moment)
So each amplifier module is a self-contained bit of gear that does nearly all of the hard work. Some of these modules are single channel units, but you can also buy ones that feature multiple channels. Most don't have volume controls, although the one shown here does have one.
Decent single channel amp modules are available from about AUD$15 each.
Audio amplifier chips get very hot under sustained high power use. They therefore must be fitted with large and effective heatsinks that work through natural convection and radiation.
Another approach is to use much smaller heatsinks but to fan cool them – the problem with that is it’s very hard to make a fan-cooled amplifier that has zero fan noise. (And you want zero amp noise between songs and in really quiet passages.)
Heatsinks are one area where you can save a lot of money by buying second-hand or using innovative approaches (like using multiple ex-CPU heatsinks, for example). Without trying very hard, you should be able to source adequate heatsinking for around AUD$20.
Good heatsinking is one of the keys to building an amplifier that will be strong and reliable.
3. Rectifier / Filter module
The rectifier / filter module (sometimes just termed the power supply unit [PSU] ) has a number of functions.
Firstly, it turns the alternating current (AC) provided by the transformer into direct current (DC) – this is done by the rectifier. Secondly, it smooths the DC so that there is less ripple. Thirdly, it provides storage of power for feeding to the amp modules doing short-term, high current requirements. These latter functions are performed by the bank of big capacitors on the board – usually, the greater the capacitance, the better.
Typically, a single rectifier / filter module can feed two amplifier modules.
Each rectifier / filter module has terminal blocks for:
· connection to the transformer (three terminals)
· connection to the amplifier boards (again, three terminals)
It makes sense to buy a rectifier / filter module that is sold as being appropriate for a particular pair of amplifier modules.
Note that some amp modules place the rectifier / filter capacitors on the amplifier board itself. There’s nothing wrong with that, except that typically, these combined boards use only a couple of smallish capacitors and so the results aren’t as good as achieved by a separate board using lots of big capacitors.
Rectifier / filter modules are available from around AUD$25.
The transformer drops the mains voltage to an appropriate level. Centre-tapped, dual secondary transformers of the toroidal (donut-shape) type are the best ones to use. These transformers are usually described in this way:
· 240V primary
· 25V + 25V secondary
...where in this particular case the mains voltage is 240V and the secondaries develop 25V AC in each winding. Transformers are available with a wide range of secondary voltages. (And if you live in a country with mains voltage other than 240V you would of course select a transformer that matches that supply.)
In addition to having the correct input and output voltages, the transformer must also have enough power capacity. Power is expressed in ‘VA’ – for example, the transformer may be rated at 160VA, 300VA or 500VA, where the larger the number, the larger the available power output from the transformer.
Expect to pay around AUD$55 for a 160VA transformer, $85 for a 300VA and $125 for a 500VA. As you can see, the transformer is the most expensive part of the amplifier build – so always be on the lookout for second-hand items.
Making things work together
Here is the overall layout of the amplifier. The transformer feeds AC to the rectifier / filter module via three wires (AC, centre tap, AC), and then each amp module is fed DC power, again with three wires to each module (minus, zero, plus). The amp modules are mounted on a heatsink. The speakers and the input signals connect directly to each module – these wires are not shown. Simple, huh?
The most important aspect in making the bits work together is in ensuring the transformer is appropriate for the amp modules. If the transformer’s voltage output is too great, you may blow the amp modules. If the voltage output is too low (especially under load when it will sag), the amp modules won’t deliver their rated power.
And here’s a trick.
A seller of an amplifier module might state it needs a “28V-0-28V DC supply”. This means that you have a minus 28V supply and a plus 28V supply – the ‘0’ is the ground terminal. (Note that therefore these modules won’t work from a ‘single ended’ supply like a plugpack or normal battery.)
Now you might be thinking that if the spec asks for a 28V-0-28V DC supply, you would need a transformer with 28V + 28V secondaries – but you’d be wrong! Instead, it is better to use a 25V + 25V transformer. But why? In short, because by the time the rectifier and the capacitor bank have done their work, the DC voltages are actually substantially higher than the AC input voltages.
So go for a transformer secondary voltage less than the figures stated as being required by the amp modules. (More on this later.)
And how much transformer power is needed – the VA figure? You can easily argue that bigger = better, but typically a 160VA is fine for powering two modules, each developing 70 watts a channel (into 4 ohms). If you will be driving a pair of more powerful modules, go for 300VA transformer. When you are talking a lot of power, use a 500VA unit.
I decided to do some testing with the following combination:
· Two LM3886 modules and a matching rectifier / filter module from eBay. At the time of writing, these cost about AUD$35, including postage to Australia where I live.
· Jaycar Electronics 25V + 25V 160VA toroidal transformer – cost AUD$55
· Large heatsink (that I already had) used on the amp modules, and a small heatsink on the rectifier.
As an experiment, I mounted these components on a scrap piece of particle board and drove some speakers.
The very cheap amplifier sounded clean and strong.
I then did some full-power testing, using the techniques covered in Low $ Amplifier Testing. The testing was to ensure that:
· the pair of amplifier modules were developing their rated power
· the power supply was working correctly, both on and off load
I also wanted to get a feel for the amount of heatsinking that would be needed.
On each channel, the measured maximum power before clipping was 67 watts into 4 ohms - that’s close enough to the spec’d 68W. The power supply voltage at no load was plus/minus 39V (the spec sheet on the LM3886 suggests a max of 42V – ie 84V across both rails – so that’s also OK) and at full load the supply voltage dropped to plus/minus 29 V – still above the 28V at which the data sheet suggests 68W into 4 ohms is available. So the power supply was working well.
And the required heatsinking? At high listening volumes, the very large heatsink that was I using on the two amp modules was running 20 degrees C above ambient, and the on-board rectifier was hot to touch. Clearly the finished design will not want to skimp on heatsinking for either the amp modules or rectifier.
With the positive results of the test, I ordered two more LM3886 modules, another rectifier/filter board, and bought another transformer. The aim? A four channel design.
This article has really only scratched the surface of what’s now available. But one thing’s for sure, with the availability of cheap, prebuilt modules, it’s starting to make some commercial amplifiers look very expensive for what you actually get. In this case, DIY can get you exactly the number of channels you want, give you good sound and at a low cost – and you can be proud that you built it yourself.
Next issue – constructing the four channel, 270 watt amplifier