Understanding Loudspeaker and Amplifier Impedance

Published: 06th April 2006
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This article gives information on understanding loudspeaker impedance (resistance) which is measured in ohms, and how to match loudspeakers to a suitable power amplifier.

The information is relevant to any music system, including in car entertainment, Home Cinema Systems, Band pa rigs and Mobile Discos

A Channel on an Amplifier refers to the individual module or circuit inside the amplifier which takes in a signal mono low level signal and boosts it to drive a loudspeaker(s).

All Stereo amplifiers have two Channels, one channel handles the left hand signal and speaker, the other obviously does the same for the right. Other than some amplifiers sharing the same power supply, the internal Left and right channels or circuit boards are seperate, which means in the event of a loudspeaker problem or component failure the opposite channel should continue to work.

Some Amplifiers used in home entertainment or Home Cinema systems have four or five output channels. These essentially work just like stereo (2 channel) amplifiers, but use additional outputs to drive front and rear loudspeaker pairs, and either a subwoofer or center channel for surround sound applications.

Okay - the load which determines how much power your amplifier provides to the speakers is dependant on the actual number of speakers connected and their individual impedance.

The load is called the impedence, and the level of impedence is measured in ohms. Most hifi and low power PA amplifiers will only work to a minimum of 4 Ohms however some professional PA or Disco amplifiers such as Peavey, and QSC etc and some powerful car amplifiers will work into a 2 Ohms load (often refered to by the manufacturers as being 2 Ohms Stable. However this application is rarely encountered since it would need 4x 8 Ohm Speakers to be connected to each channel to acheive this.

Working out Ohms:-

Most Music, Hi-Fi, Disco & P.A loudspeakers are rated at 8 ohms, so connecting one 8 ohm speaker to one channel of the amplifier will present a load of 8 ohms to it.

However most automotive loudspeakers are rated at 4 ohms.

Plugging in an additional 8 ohm loudspeaker into either the existing connected speaker or the second socket on the same channel of the amplifier (if fitted) will equal 8 ohm + 8 ohm = 4 Ohms load to the amplifier.

To get 2 ohms you would need to connect:-

8 Ohms + 8 Ohms + 8 Ohms + 8 Ohms = a 2 Ohm Load, so to get your amplifiers 2 Ohm rating you need to connect (4x) 8 ohm speakers to it.

Given the fact that each channel is identical and seperate, your Amplifier has the capability of running 4 x 8 ohm speakers per channel! - a total of 8 speakers - which is why I said to ignore 2 ohm ratings when buying an Amplifier and concentrate on the 4 or 8 ohm wattage ratings!.

2 Ohms of load on the Amplifier can also be acheived by connecting (2x) 4 Ohm loudspeaker cabs to the same channel.

In Hi-Fi Applications, it is also common to find 6 ohm and 4 ohm loudspeakers for sale in the marketplace. If your amplifier is rated at 4 ohms minimum load, then it is wise to just connect a single speaker to each channel to avoid overload.

So some common speaker combinations are below

8 Ohms load can be acheived by connecting either

1 x 8 ohm speaker

or

2x 16 ohm speakers (16 ohm speakers are very rare nowadays)

4 Ohms load can be acheived by connecting:-

1x 4 Ohm speaker

or

2x 8 ohm loudspeakers

or

4x 16 ohm loudspeakers

or

1x 8 ohm and 2x 16 ohm loudspeakers


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