Building a home sound amplifier, Part 1

Late last year we ran a series on installing speakers in the floor and walls of a house. Despite having nothing to do with cars, the series (starts at Sound in the Lounge, Part 1) was very popular – there was a lot of enthusiasm for having dual 15-inch subwoofers under the floor and 8-inch two-way speakers inside the walls! Building on that series is this one – how to construct an amplifier to run the speakers. The speakers could be like those in the walls and floor, or they could be designs that are more conventional. The good news is that building such an amp is these days a series of ‘verys’ – it’s very easy to get very good sound at a very low price. And you don’t even have to solder anything…

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.

Starting points

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

8.     box

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.

2.     Heatsinks

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.

4.     Transformer

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?

Transformer

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.

How much power do you need? When you are building an amplifier you rapidly realise that more power = more cost… and the dollars can start going up very fast. So how much power do you need? There are some very important points to keep in mind when making that decision. ·       Speaker efficiency? The first step in sizing an amp is to consider how efficient your speakers are – that is, how loud they are for a given input power. This figure is often shown in speaker specs as dB at 1 watt, 1 metre – ie Sound Pressure Level (SPL) at 1 metre distance from the speaker when it is powered by 1 watt. This figure is vital because a 3dB reduction in efficiency will need twice the amplifier power to achieve the same SPL. Typical speaker efficiencies vary from 85dB (poor) to 95dB (much better). To emphasize the point, if you use speakers with 92dB efficiency, you will get the same loudness with a 15 watt amplifier as someone listening to 86dB speakers and using a 60 watt amplifier! More efficient speakers typically use a ported bass reflex type of enclosure, and have larger drivers and enclosures. Of course, there are other relevant factors as well - but it’s hard to make an efficient speaker by using small drivers mounted in a sealed enclosure. ·       Real watts Finally, how real are those watts? When I was researching the amp build I looked carefully at the specs for a variety of currently available, expensive multi-channel amps. And I couldn’t believe how bad some of those specs were. Firstly, many manufacturers quote the maximum power with only one channel running (boy, does that make it easy for the power supply to keep up!). Secondly, many were quoting max power figures at 10 per cent distortion! (That’s 100 times worse than the full-power distortion of the cheap modules I selected for my amp.) Ten per cent distortion reminds me of the bad old days of car sound, before anyone starting building decent equipment. There is simply no way you could ever have the amplifier cranked up that far and still be happy listening to it. So when in this series I talk about – say – 50 watts per channel, that’s 50 watts at distortion levels you cannot hear. To put that another way, you should be able to run a 50 watt amplifier at full power on all channels without audible distortion from the amp. ·       How loud? So how loud do you want the system? If you have efficient speakers, a relatively small listening room, don’t want to be deafened, and have an amplifier producing distortion-free watts, it’s very likely that 50 watts/channel will be heaps. But if you really want to make your ears ring, or have poor efficiency speakers, or listen in a really big room, then that power requirement may have to increase a lot. Don’t forget that doubling amplifier power makes only a small difference to how loud the sound is – to get a big jump in apparent loudness, you will typically have to triple or quadruple amp power. (You can hear this by disconnecting one channel of a two-channel system – it nowhere halves in apparent loudness, yet the delivered power has halved.)

My approach

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.

How many channels? There’s nothing stopping you using exactly these techniques to build an amp with as many channels as you want – whether that’s two channels (stereo), three channels (stereo plus subwoofer), four channels (perhaps bi-amping of a stereo pair), five channels, six channels, seven channels, eight channels – whatever. Just keep adding pairs of modules and their rectifier/filter boards and transformers (or use one or two very large transformers).

Conclusion

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