The Project 68 & Friends Subwoofer
One very big boom in an unexpectedly small package.

This article has multiple pages: Page 1 | Page 2 | Page 3 | Page 4 | Page 5

A powered subwoofer was my initial motivation to explore the world of Do-It-Yourself audio. I knew I needed practice before diving right into a big subwoofer project, so I began with a simple headphone amplifier. Pleased with the results, basic though they were, I began planning and building my subwoofer in earnest.

I wanted to use Elliott Sound Products' Project 68 as the basis of the sub, and I wanted at least a 10" (250mm) driver, probably 12" (300mm). Actually, I had been eyeing the (now obsolete) Infinity Perfect series by Harman, but for various reasons, I ended up with something a bit different. More on that later.

I had been reviewing options from the ESP Project Pages for several months by the time I began planning the build. The overall design took a bit more time to solidify but the high-level electrical schematic was looking like this (click for a larger view):

Figure 1. Subwoofer operational diagram. (Click image for a larger view).

The conceptual design shown here became the electrical basis of the actual unit, as shown on the following page.

As noted in the diagram, the subwoofer would make use of the following ESP projects:

Project 23 - Clipping Indicator
"Know it before you blow it" - clipping sounds bad and damages drivers. Also useful as a visual aid during high-power sine tests.

Project 33 - Loudspeaker Protection & Muting
Always good to protect downstream systems from an upstream failure. Power-cycle muting is also nice.

Project 38 - Signal Detecting Power-On
Another standard feature of commercial subwoofers that I wanted to try out.

Project 39 - Soft-Start Circuit
Large transformers and large filter capacitors mean large inrush currents.

Project 48 - Subwoofer Equalizer
Big sound from a small box...or in other words, a subwoofer that isn't larger than a living room lamp table.

Project 68 - High-Power Subwoofer Amplifier
Big subwoofers require reliable power, and this project was within my price range.

The power supplies were dicated by available parts. I discovered up a 500VA, 120V:70V-CT transformer lurking in a junk box, and that became the high-voltage supply. It had been pulled from an old mainframe computer rack and was extremely well-built. I also found two 41,000µF, 50V-rated filters at a surplus store. The resulant 45V rails meant I would have to go with a 4Ω-nominal driver in order to get at least a 200W output. Since I was building most of the sub myself, I ended up making a few changes to suit my needs, such as combining P38 and P39 into single circuit.

It was an ambitious idea, but I was up for a challenge. Now, to see how it came together in practice.

< prev | next >

WARNING: HIGH VOLTAGE ELECTRICITY IS INHERENTLY DANGEROUS AND CAN CAUSE INJURY AND LOSS OF LIFE OR DESTRUCTION TO PROPERTY. The presentations on this website are given for informational purposes only and are not guaranteed for accuracy or fitness to any use or purpose. Consult your local standards and codes before building or modifying any mains-connected equipment.