Subwoofer Project – Part 1, My First DIY Amplifier

This write up is largely just to set the scene for my later forays into audio projects which I will progress onto in later sections but it does ramble a bit!

Some time ago back when I was still a student I realised that most reasonably priced speaker systems consisted almost exclusively of a series of tiny satellite speakers and a main sub which was often comparatively overpowered in order to mask the inadequacies in the satellite speakers mid range. Now there are systems about which actually use pretty well performing satellites with a decent frequency range but these are usually quite expensive making them well beyond my reach as a student and still a bit lacking and so I started working out what I could do.

My first project was based around some amplifier kits I found at a local electronics store on offer so being student with little money I decided they would be a good place to start so I bought a pair of them to provide both left and right channels with the heady heights of 7W per channel! Shortly afterwards I went on eBay to find a cheap and reliable way to power them from the mains to make a self contained unit with minimal risk of death! I ended up buying a job lot of 5 transformers each rated at 12VAC at about 5A, or about four times what both amplifiers would use at peak output! But they were cheap and well insulated (much safer than cutting up a wall adapter and fitting it in the case). Some may ask why I didn’t just use a normal 12VDC wall adapter and just have the amplifier modules in a case – basically I hate the things but it was also because I wanted to be totally sure the power supply wasn’t the limiting factor!

Note : Since writing this post the company I bought the kits from (Maplin) has ceased trading and so the link above no longer works. The kit in question is Velleman K4001 7W Mono Amplifier details can be found here.

Going off on a tangent….

Now all this was going well except one thing, at the time I had virtually nothing in the way of tools I needed to build this apart from the soldering iron – an item my dad happened to see in a skip when a local college was upgrading their equipment back in probably the late 90’s and an item that has been used for every electronics project I’ve done ever since! If anyone is curious it’s actually one of these Xytronic XY9-60A  but it’s been branded as a Rapid Electronics unit and is orange rather than the blue shown.

These were about £100 new and my advice is if you know anyone who is starting to show an interest in soldering buy them something like this! There are plenty of even cheaper, decent soldering stations around now due to the greater number of people doing hobby electronics projects at home and it makes these projects considerably less frustrating!

One key thing for me was the temperature control which being an analog control on mine isn’t terribly precise but it’s still a huge improvement over very basic soldering irons and good enough for the vast majority of hobby projects. The second thing is the soldering tip is small and also interchangeable. A soldering iron that looks like a screwdriver isn’t terribly helpful in most cases because it makes even easy joints much harder to get access too. Back during the early 2000’s when the old Nokia phones were popular there was a brief craze of changing the tiny keypad and screen backlight LED’s out for different colours and I actually did several of these with a soldering tip I filed narrower for the purpose!

Now back to the point…

Having no tools I decided to buy something that would cover as many tasks for as little money as possible, so I bought a £20 fake Dremel and an aluminium enclosure and got to it! Some might describe what happened to that case as butchery but at the time I was using what I had. So the case looked a bit scruffy but it did the job! In the process of using the fake Dremel with an abrasive disk the disk shattered and one of the bits flew of and made a mark in my then virtually new monitor but by luck missed my face. Wear safety goggles – they really are worth it!

Taken from Reddit:

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I soldered up the kits as per the instructions and then soldered a diode rectifier and a capacitor onto the transformer output to produce DC (like this) and connected this to the two amplifier modules. I used a dual logarithmic potentiometer (variable resistor) for the volume control (one potentiometer wired to each amplifier). This is wired between the audio source and the amplifier input.

Potentiometer Wiring

A single pot usually has three legs, in the diagram the box represents the pot. The audio in+ and audio in- go to the two outer legs (doesn’t matter which way) with the audio to amp+ being taken from the middle leg. Audio in- and Audio to Amp- are both connected to the same leg.

It’s probably best to point out here while it doesn’t electrically matter which way round you wire the In+ and In- just make sure when doing a dual pot make both channels the same otherwise when you turn the pot one side will get louder as the other gets quieter and you have accidentally made a balance control!

Power came through a panel mounted IEC connector. If you can’t cut panel holes perfectly accurate then I recommend buying these as bolt in types, they normally need M3 countersunk bolts and then a matching nut on back but they offer a much greater tolerance than push fit ones and you wont pull them out! As a bonus they can also be fitted to any thickness of material with either long bolts or if you’re trying to mount into a decent thickness of wood even small screws in a sensible length can be used.

The finished product looked like this:

Basic Amp

It’s moved house with me maybe six times and has suffered a bit but it still works ten years on!

As a final note on this, this amplifier is as basic as it gets. The Velleman kits use a TDA2003 amplifier chip for all their functionality and the boards are basically just filtering capacitors for it. These are ok but the main limitation I found is at very low volume they seem to either limit the output power or the frequency response is terrible and all the bass on the audio drops away. As you turn it up the bass comes back which is a bit odd but in some ways actually ok when I was a student as it meant the low frequencies which tend to disturb people at night weren’t so prevalent! I never saw this as a major problem though.

Another point it’s worth considering because very few people actually appreciate it is that 7W per channel doesn’t sound much but it’s quite surprising what it can do – the volume control has lived at 25-30% for most of its life and with the source on maximum you will almost certainly go well above what most would consider a comfortable listening volume. Speakers are rated for sensitivity which is the sound level they generate per Watt of supplier power at 1m distance. The very cheapest speaker drivers should achieve 82db for 1W at 1m so for just 1W of input power we’re talking a sound level similar to a food blender! The issue is that the sound level vs power is not linear, doubling the sound level (+3db) actually requires four times as much power but we have 7W watts and that only takes us up to 4W but clearly another 3dB is beyond the limits of the amplifier but at 85dB we’re at the point where in the UK companies have to supply workers hearing protection. Better speakers can achieve sensitivities  of 90dB with 1W at 1m which puts at noise levels comparable to petrol lawnmowers using 4W of power! Higher power outputs amplifiers have their uses but the next time you hear about someones 1kW+ amplifier it’s worth being *very* dubious!

If you want to understand what this is all about I recommend reading  where Rod Elliot describes the concept of power vs sound level in much greater detail. It gets quite involved but will explain the limitations and realities!

Also for a bit of a laugh have a look at   where he describes what a 1.5kW amplifier would actually look like and involve. Rod is amazingly knowledgable and I have built some of his projects using his PCB’s and used them in my subwoofer project so these will appear in later sections.