Building a home sound amplifier, Part 2

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 lounge room 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…

Last issue in Building a home sound amplifier, Part 1 we introduced the idea of building your own home sound amplifier using the pre-built modules now cheaply available on eBay. As was described in that story, with this approach you can get low distortion (up to 100 times better than many brand name amps!), whatever number of channels you want, and achieve all this at a low cost.

It’s also a project that’s more about mechanical engineering that electronics – the modules are pre-built and wiring them together is very easy. What is not so easy is building an enclosure, achieving adequate heat-sinking – and then cooling those heatsinks.

Now, let’s do it!

Parts

The four channel amplifier built in this story has these characteristics:

·       Four channels, achieved by the use of four LM3886 modules, each capable of 68 watts into 4 ohms at a max of 0.1 per cent distortion at full power (eBay - modules prebuilt)

·       Two rectifier / filter boards (eBay - boards prebuilt)

·       Two 160VA 25V + 25V toroidal transformers (Jaycar Electronics)

In addition, the following were used:

·       Aluminium heatsink, 1.2kg  (eBay - second-hand)

·       Mains power switch (salvaged from other equipment)

·       IEC power socket, fuse and filter (salvaged from other equipment)

·       Box (salvaged from another amplifier)

·       Front panel (eBay – new)

Obviously, if you’re building something yourself, it makes sense to use second-hand and/or free stuff whenever you can get it.

Note that this amplifier does not use volume, tone or input switching controls. These functions could all have been added through low-cost prebuilt eBay modules - however, in my case, I am using a Clarion EQS746 car unit pre-amp (that already has tone and volume controls) and a switching box (that allows me to switch-in different inputs). Incidentally, this pre-amp has excellent specs and a very low price.

In addition to the above specs, I chose to add:

·       Digital temp controller displaying heatsink temp (eBay)

·       Dual cooling fans switched by temp controller (the fans salvaged from other equipment)

·       Four-channel speaker protection  (eBay – two prebuilt boards)

·       Internally mounted crossovers for the low frequency speakers

·       Blue LED lighting! (Jaycar Electronics)

The total cost of the new parts was about $350.

Note that if you were to build a simple two channel amplifier with brand new amp modules, rectifier/filter and transformer, and second-hand heatsink and box, it would cost you around $100 all-up - and the amp could be sized as shown by the yellow rectangle.

Design

As shown last issue when I tested the modules at full load in a ‘bread board’ configuration, having adequate heatsinking is a high priority. The heatsink needs to:

·       Have high thermal conductivity (aluminium is most commonly used)

·       Use a large surface area (so explaining the use of lots of fins)

·       Be exposed to sufficient natural airflow and/or use fan cooling

·       Be thermally connected to the electronic components needing to be cooled

To achieve these aims, I bought on eBay a large, second-hand, aluminium heatsink. It cost $20. Slightly cut down, it is mounted on the rear of the box.

The enclosure uses a 19 inch rack mount box that I bought at the shop at the local rubbish tip. It came in the form of an old professional audio monitor and cost $5. I replaced the front panel with a new 19-inch rack panel bought on eBay for $15.

Full-load testing had previously shown that the rectifiers on the power supplies got hot very fast. In fact, the rectifiers (arrowed) supplied on the power supply modules are rather small and are not of the metal-case types that will both cope with higher power levels and are also readily attached to heatsinks. Furthermore, on these modules the location of a terminal strip right next to the rectifier makes attaching a heatsink to the supplied rectifier rather difficult.

So, despite the ‘no soldering’ bullet point in the lead-in to this story, I decided to bolt two new, larger rectifiers to the main heatsink and connect them to the power supply boards via soldered wires. Note that the rectifiers are clearly marked as to what terminal does what, so this change is easy to make.

Warning! The step-by-step instructions for wiring the mains switch, adequately earthing the case, ensuring that no mains power wiring is exposed, fitting a mains fuse, and providing a secure anchor for the mains cord (or using an IEC socket) are not specifically covered in this story. It is easy to build a metal-cased, mains-powered amplifier that is dangerous – and could potentially electrocute you or a member of your family. If you do not know what you are doing with the mains wiring, give the amplifier to a qualified electrician or electronics technician to perform these steps.

The Build

The build is easiest shown in pics.

Starting at the bottom-left and working clockwise around the outside, you can see:

·       Front panel power switch

·       Bass speaker crossovers

·       Speaker protection boards (they mount on the output terminals)

·       Huge rear heatsink with four amplifier modules and two rectifiers mounted on it

·       Small conventional transformer that powers the fans, speaker protection and temp display

·       Large mains filter

·       Fan (on clear plastic lid)

·       Front panel temp display

·       Two toroidal transformers (one for each pair of amplifier modules)

Inside the main body of the case you can see:

·       Two filter/rectifiers modules (one for each pair of amp modules)

·       Filter / rectifier module for the secondary power supply that powers fans, etc

In more detail…

The huge heatsink on the back of the amplifier is bolted to a thick aluminium plate on which the ICs and rectifiers are mounted. The amp uses a clear top panel with a cooling fan mounted within it.

The speaker terminals (attached internally directly to the speaker protection boards) are mounted on a piece of thick scrap plastic.

The four channel amp is used to drive two underfloor 15 inch woofers in addition to two 8-inch two-way wall speakers. Here are the crossovers for the underfloor woofers – again they are eBay prebuilt modules.

The programmable LED temperature display (seen here from its rear) is cheaply available on eBay. It monitors heatsink temperature via a remote probe, the probe bolted to the heatsink via a copper ‘eye’ terminal.

This transformer (and its associated rectifier/filter) were salvaged from other equipment. This power supply runs the fans, temp display and speaker protectors (that latter requiring 12V AC).

This and the fan in the top clear panel are switched on when the heatsink temperature exceeds 38 degrees C. The fan and its enclosure were salvaged from an old photo printer. The 24V fans are run at about 14V to give quieter operation at reduced airflow.

These prebuilt modules protect the speakers if any fault should develop in the amplifier. They mount directly to the rear of the speaker terminals.

A LED lightbar run off the fan power supply provides blue illumination whenever the amp is switched on….just for fun!

Conclusion

So am I happy? Yes!

Driving my underfloor 15 inch bass speakers and the 8-inch in-wall two ways, the amp develops plenty of clean power. With these relatively efficient speakers, I’d never want it any louder, and distortion at full noise isn’t audible to my ears – if you want, you can drive it at full volume for hour after hour.

And the amp copes with that type of full-power delivery just fine. Even in 30 degree ambient temps, it cycles the cooling fans on and off at only 5 minute intervals, the heatsink temp never exceeding 43 degrees C. At lower listening levels, or in cooler weather, the fans do not operate at all.

To be completely honest with you, I am amazed at how good it sounds