The 3SB System

September 2024

The 3SB Loudspeaker System ("System"), is a set of active loudspeakers, and the components required to play 2-channel stereo. 

The system consists of two types of speakers.

The first type ("Keystone") is designed as as small-ish stand-mount speaker that can be used in small-to-medium sized rooms domestically.

The Keystone was designed first, and generally achieves its design goal above. However for larger rooms, a second type of speaker was developed ("Bedrock") to increase bass for medium-to-large rooms.

Both the Keystone and Bedrock utilise cardioid bass. Simply speaking, cardioid bass results in significant cancellation at the rear of the speaker, and moderate cancellation at the top, bottom, and sides of the speaker. The net result is bass that is primarily towards the listener. 

Cardioid bass was chosen primarily to reduce the intensity of room related bass peaks/nulls, based on reducing bounce from the back, ceiling, floor and side walls. 

The use of cardioid bass is an example of using techniques and components used in pro-audio into the design of the system.

The overall system borrows heavily from pro-audio, although the system was not designed for the studio per-se. 

From the early prototype scribbles on paper, it became apparent that a passive or hybrid-passive design was not a going to be the preferred way to based the system.  

To cut a long story short, the following design decisions were incorporated into the specification for the system.

The system would be based on:

  • Open source tools
    • Open standards
      • Free tools
        • Cost effective proprietary tools
          • Manufacture casework via CNC
            • Casework made from Baltic Birch Plywood or MDF
              • Drivers available directly to the public
                • Multichannel DAC
                  • Active, Linear Phase XO
                    • FIR filtering
                      • DSP via FIR convolution engine

                        Based on the above, two additional components were designed for the system.

                        An 8ch amplifier to drive each speaker was decided. As you will see, this isn't strictly required. I chose to dedicate an 8ch amplifier to each speaker because I wanted to be able to dedicate each driver with an amplification and DAC channel individually.

                        The Keystone is logically a 5 way loudspeaker. There is a Tweeter, Midrange, Midwoofer, a Front and Back Woofer. As we will see later, the Front and Back Woofer are different signals because of cardioid.

                        The Bedrock is logically a 2 way loudspeaker. There is simply Front and Back Woofers.

                        The Keystone has 7 physical drivers, and the Bedrock has 8. I chose to dedicate an 8ch amplifier to each speaker for simplicity sake, but also to decouple amplification design from loudspeaker. This enables me to re-use the amplifiers on other speaker designs in the future. 

                        Similarly for the DAC. It turned out that the simples approach was to use a single, high density multi-channel DAC. This is another example where we use components from pro-audio.

                        I needed 30 physical DAC channels, 7 + 7 + 8 + 8 (for Keystone Left and Right, and Bedrock Left and Right).