150W Subwoofer

Initial Idea

I had always wanted a good subwoofer just to compliment the bass lines of both music and movies, just for the added enjoyment!

Over time, I managed to get a large array of components, such as those for the power supply. Some were brought for other projects in larger quantities and others were brought because they were cheap, or needed to make up the minimum £20 order...

The main reason I wanted to make a subwoofer was to put to use the P3A amplifier board I brought from Rod Eliott some time back in order to build and complete my 5 channel bi-amplified power amp project.

Being a stereo amp naturally, there were several options available to me...

  • Drive to speakers from each half of the amp. With 4 ohms this would be up to 100W into each speaker, and around 60W into each speaker if they were 8 ohms.
  • Drive one speaker by bridging both halves of the amplifier. The speaker CANNOT be 4 ohms in this case as it would represent a 2 ohm load to each amp, but it can be 8 ohms, which represents 4 ohms to each amp half, giving about 200W.
  • Use a dual voice coil speaker and have each amp half driving each voice coil

At this stage, I knew that I would be using the P3A amplifier because if I did not, then it would be highly unlikely that I would use it for some time, and that would be a waste!

So after buying bits and pieces, I thought about what I really need from this subwoofer. I wanted nice sounding, controlled bass, rather unlike most car subwoofers and cheap home cinema ones that just manage some intermittant vibrations. Being an Indie/Rock/Alternative listener in general too meant that low frequencies in the music could also contain bass guitar lines rather then pounding of a drum.

I thought about my room and the rather odd location where I could only put my subwoofer and soon realised that some frequencies would be dead and others would resonate terribly, drowning out the main system.

Input processing

Control over the frequencies my subwoofer would produce would be essential, and this is where again, Rod Elliott has a solution.

Project 84 is an 8-band graphic equaliser for subwoofers. With component values suggested, frequency centres are around 20Hz, 25Hz, 32Hz, 40Hz, 50Hz, 63Hz, 80Hz and 100Hz. Any frequencies above 100Hz would be removed using a simple low-pass filter. Frequencies higher than 100Hz would be left to the main speakers, since when frequencies go above 100Hz it is easier to tell where the sound is coming from.

So on the input to my subwoofer I would have a low pass filter to remove anything above 100Hz and that would be followed by an 8-band equaliser so I can control eight octaves below (and including) 100Hz.


My dad had some spare 10" speakers that he used in his earlier Jamo hi-fi subwoofer, which was sold at a later date with the cheaper speakers in it.

These R.C.L. speakers cost around £30 each and are of good quality. With a maximum power input of 150W they could run at high power too!

One unfotunately has a hole in the rubber surround though, making it unsuitable for use in pretty much all cabinets.

The good one however I put to use, despite knowing the fact that would would need to find the Thiele/Small parameters of the driver to put it into the right case! Sadly R.C.L. have not made these speakers for a long time and did not know any further details themselves.


It is either inconvenient, or just too much damn effort to have to reach round to the subwoofer in order to switch it on or off. The planned amplifier is expected to provide a large power on thump too...

Ideally, I wanted control from my preamp, of power on/off state, a low voltage power "button" on the front (saves running mains cable) and also output muting for the speaker.

With knowledge of the PIC micro processors now, it really would not take much effort to use a small PIC processor to handle the power control, and I'll be able to add some "state" LEDs to the front too.

The program would be simple to write (especially in C), and will therefore be made freely available for anyone to use or modify for their subwoofer system.