A Quick Guide to Passive Speakers

by Tom Thackwray

When shopping for passive (non-amplified) speakers for stereo music systems powered by a separate amplifier like the Sonos Amp or Bluesound Powernode, it's important to understand the differences between speaker types and the limitations of the amplifier itself. The following will help you choose the right speakers from our extensive range.

Firstly, all speakers need some sort of cabling. Active speakers have built-in amplifiers but as a result require main power. Whereas traditional passive speakers (the subject of this article) must be powered from a separate amplifier via a 2-core (or in some cases 4-core) speaker cable of suitable gauge (thickness). For most applications we recommend 16 gauge speaker cable.

With any stereo amplifier, it's important that both channels (left and right) are connected to the same number of speakers, and that this load (impedance) is balanced. Running an amplifier with only one channel connected, or with two speakers on left and one on right for example, will eventually damage the amplifier.

Passive speakers are sold in three main categories...

Single Mono - Mono (single channel) speakers are sold individually and must be purchased in pairs for use with stereo amplifiers.

Stereo Pair - Some manufacturers (such as B&W) box their speakers in pairs. A stereo pair consists of two identical mono speakers.

Single Stereo - A single stereo is a special type of speaker that has two separate bass driver voice coils and two tweeters. This makes it possible to use this type of speaker individually on a stereo amplifier for smaller spaces such as bathrooms. PLEASE NOTE: Single stereo speakers must be treated like stereo pairs when cabling. Although one unit, they require separate left/right inputs and therefore need 4-core cable or 2x 2-core cables.

One Amp How Many Speakers? - Most amplifiers are designed to operate with a load of between 4-16 ohms. Modern speakers are typically 4-8 ohms. The maths for load matching multiple speakers is a little complicated (see here), but the effect of simply doubling up the number of speakers on the amplifier output terminals (2 into left +/- & 2 into right +/-) actually roughly halves the perceived load. One might expect the load to simply double, but wired like this in parallel the effect is the opposite.

Why is this important? Well... reducing the load (impedance) that the amplifier is connected to makes the output current flow more easily from positive to negative. This low impedance can easily make the amp overheat, especially at high volumes, causing thermal cut-out and potentially damaging the amplifier. As a result it is important to be aware of the total impedance... for example: 4x 8 ohm speakers would create (roughly) a 4 ohm load per channel... all good... but 4x 4 ohm speakers would create (roughly) a 2 ohm load per channel... bad! A solution to this problem is to use a speaker switch (see below)

Speaker Switches - When connecting more than 4 speakers to a stereo amplifier we recommend using a speaker switch. This will not only simplify the connections and provide on/off control of each speaker pair, but it will also ensure that the total speaker load (impedance) at any one time does not drop below the minimum 4 ohms. This is possible thanks to the way that speaker switches use a mixture a parallel and series switching connections to balance the load. PLEASE NOTE: Most speaker switches only provide on/off control and do not allow different volumes between different pairs. Additionally, when more speakers are switch on, the amplifier power is shared between all the speakers, therefore the relative volume of each pair will drop as speakers are switched on.