Hurst Audio - building powerful speaker systems

Line array speakers can be very impressive, due to their high power handling capacity, reduced intermodulation distortion and solid bass output. This gives them a characteristic "big" sound. However, this level of performance can come at a cost, in both size and expense, even when scaled down for domestic use. So we use bookshelf speakers as low cost pre-assembled modules, which can be turned into line arrays with the minimum of fuss and bother.

You need four pairs of compact/bookshelf speakers to build a pair of line arrays, so you may wish to check the availability of speakers in your area, either from dealers (if you can afford to purchase new), or second-hand from internet auction sites. It is generally preferable to settle on one model built by one manufacturer, because the results are more predictable. Different speaker designs have differing advantages. E.g. Speakers that have rectangular cases (like the vintage Wharfedale Diamond I's in the above illustration) have the advantage that you just have to stack them four units high, wire them up, and you have your line arrays! (You can worry about a more permanent construction later). Whilst the Ariston Acoustics MSX-07's (in the lower illustration) need a wooden frame to mount the speakers on. But, even with the frames, the MSX-07s are smaller, and hence are less likely to dominate the room.

Four Speaker Series-Parallel Line Arrays

The way that the speakers are wired together electrically is crucial to achieving a well behaved system. This is because amplifiers are generally designed to work best with a load impedance of 6 Ohms to 8 Ohms per channel, and rarely lower than 4 Ohms. However, if the speakers are connected in parallel, they will generally present a load of 1.5 to 2.0 Ohms, which could be catastrophic. Whilst if they are connected in series, they would generally present a load of 24ohms to 32 ohms, which is very inefficient. So, instead, the four speakers must be connected in a series-parallel array (as below). This ensures that each stack of speakers has the same impedance and sensitivity, and is as safe and as easy to drive, as a single speaker.

This is then repeated for the other stack(s)...

Achieving a Wider Stereo Field

Another aspect of a "big" sound is the size of the stereo field. Stereo recordings can capture sounds from a horizontal arc up to 180 degrees. However, the stereo field that can be produced by two speakers is limited to the distance between them. So, if the speakers are placed 1.2m (4ft) apart, the stereo field produced is 1.2m (4ft) wide (or less).

With suitable multi-channel recordings, AV systems can easily beat the width of the stereo field produced by the pair of speakers. Furthermore, if you wanted to use line arrays to enhance the home cinema experience of an AV system, you would be hard put to find a better choice at the price than the MSX-07 stacks.

But if you are looking to widen the stereo field of stereo recordings, you have a problem. If you simply increase the spacing between adjacent speakers, the central images between the speakers become increasingly bloated. So instead of widening the speaker-to-speaker spacing we add a second pair of stacks beside them. When spaced 1.2m apart the four stacks can double the width of the stereo field to 2.4m which is a dramatic improvement, and draw you into the imagery.

The stacks should generally be placed along the longer wall of the room. The outer pair may be turned inwards, whilst the inner pair face forwards. The listener should be at least as far from the speaker line as the speaker-to-speaker distance, whilst some listeners may prefer being up to three times that distance.

Note: Each stack requires a power amplification channel. You therefore need to connect the left channel of your stereo source via a "Y" adapter to the left inner and outer power amplifier channels. Similarly, you need to connect the right channel of the stereo source via a "Y" adapter to the right inner and outer power amplifier channels. No crossfeed between the left and right channels is required or desired. Similarly, the only difference between the signals from the inner and outer channels is the optional ability to vary the volume of the outer channels in the range +0 to -6dB.

Requests for further information on building the stacks, and any other enquiries, should be addressed to tlhurst@hotmail.co.uk

Last update: 24th June 2022.