Matching Ohms on Passive Speakers and Power Amplifiers

Matching ohms can be a complicated process! If you get it wrong, you run the risk of blowing your speakers or your power amp.

When choosing the power amplifier for your passive speakers you need to ensure you match the power of each correctly.

First, look at the technical spec of your passive speakers. Note whether they're rated at 4 Ohms or 8 Ohms (it will most often be 8). Now find the power rating of the speaker in RMS (not total or peak).

If you are using two passive speakers, 200 watts RMS per speaker at 8 Ohms, you will need a stereo power amplifier rated at 200 watts per channel RMS at 8 Ohms or just over. it should not be under. Some people prefer to give it a little more head room, and it will depend a little on the make of the equipment too. Sometimes the speaker manual will give a recommendation for the correct model of the same brand power amp so it's worth checking there first.

 If you are connecting more than two speakers to the power amplifier this changes. If you plan to run four passive speakers from one power amplifier, you sum the power of the speakers each side (e.g. 200+200 watts RMS = 400 watts RMS) and you halve the resistance (e.g. 8 Ohms/2 = 4 Ohms). So you will need a stereo power amplifier capable of 400 watts RMS @ 4 Ohms per channel, maybe a little more, but never under.

In this example the speakers must be connected in parallel. You would connect the left output from the power amplifier into left speaker one, and out from left speaker one's output into left speaker two. Then repeat on the right.

If the speakers are connected in series, then their Ohm ratings are summed which will result in very different power amplifier requirements.

To make it a bit easier for you, we've put together some Passive Speaker Packages with the correct power amplifier to power them safely. If they don't fit the bill just contact us via enquiries@whatpa.co.uk or fill in our simple enquiry form and we can put together a package just for you.

More reading ...

4 Ohms into 8 Ohms does go ...

Most stereo power amps operate at 4 Ohms per channel. If you connect a 4 Ohm speaker to each channel then you will get the full wattage of the power amp.

However, if you connect 8 Ohm speakers to each channel, the wattage of the power amp will be reduced by around 30%. In terms of perceived loudness, the amplifier will seem about 15% quieter. For example:
Power Amp Speakers Delivered Watts Perceived Loudness
2 x 300W at 4 Ohms 2 x 300W 4 Ohm 600W 100%
2 x 300W at 4 Ohms 2 x 300W 8 Ohm 420W 85%

... but 8 Ohms into 4 Ohms NEVER GOES!


Never, ever connect a PA amplifier into speakers with a lower Ohm rating - this will result in either the amp or speakers (or both) being damaged.
 



Still confused? Email enquiries@whatpa.co.uk and we'll try to help!