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In Reply to: AVR s and speakers compatibility posted by Nick P on November 17, 2001 at 11:07:37:
With a 4 ohm speaker the receiver would have to deliver twice the current and therefore power to the load (your speakers) as that of an 8 ohm speaker. From an electronics standpoint, there are 3 things you need to think about when ensuring your receiver or standalone amplifier is compatible with your speakers:1. Check the sensitivity rating of your speakers. Let's say for example that they are rated at 86dBSPL/W/m - a common rating. This means that a 1 Watt output from your amp will produce 86dBSPL at 1 meter from the speaker measured on-axis. 2 Watts are needed for 89dbSPL, 4 are needed for 92dBSPL, etc..Basically your amp needs to be able to double its output power for every 3dBSPL increase in acoustic output from your speakers. You can now figure out how much power your amp will need to be able to provide in order to get the high home theater output levels which often exceed 110dBSPL. For every 3dB increase in speaker sensitivity your amp only needs to be half as powerful.
2. Now that you know what your amp will need to produce in terms of output power you have to check that it will meet its rating for the given load. So from the previous example, your amp would need to be able to produce 16 Watts to produce 101dBSPL/W/m. Now you need to take the 101Watts and divide this by the speaker's minimum impedance rating. Note that speakers are often rated with a nominal impedance rating (4, 6, 8Ohms are common) but everyone knows that speaker's impedance varies with frequency so a 6 ohm speaker may drop to 2 ohms and be as high as 10ohms at specific frequencies! Anyway, let's say that your speaker has a minimum impedance of 4 ohms. In this case you will need 101Watts/4ohms = ~25amps to produce this high ouptut level. In reality your home's circuit breaker would blow (at 20amps) but you get the idea. Now you need to check if your amplifier's rating meets this requirement. 99% of the mass market receivers don't have the ability to double the output power when the speaker's impedance is halved so you'll see specs like: 120W x 2 (8ohms), 200W x 2 (4ohms).
The first case would mean that the amp can deliver: 120/8 = 15 amps and the second: 200/4 = 50amps. So this amp would be 'suitable' from a power perspective.3. OK, sorry for the long post but I think that this info is important. Check to make sure that the specs. for your amplifier/receiver are for all channels being driven simultaneously.
-JackOfFewTrades
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- Re: AVR s and speakers compatibility - JackOfFewTrades 19:12:51 11/17/01 (0)
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