Matching ohms can be a complicated process!

As the simplest example, take the number of watts RMS of one passive speaker, note the ohmage, and find a power amplifier that matches the speaker watts RMS output per stereo channel at the same number of Ohms. E.g. your passive speaker is 250 watts at 8 Ohms so you need a power amplifieClick to view passive speaker packagesr with 250 watts per stereo channel at 8 Ohms. If you are connecting more than two speakers to the power amplifier this changes, so please read below about running more than two speakers from a power amplifier.

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.

 

4 Ohms into 8 Ohms does go ...

Most stereo power amps operate at 4 Ohms per channel. If you connect 4 Ohm speakers 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.

 

Running more than 2 speakers from a power amplifier

If you double up speakers connected to each channel of your power amp, then how you connect them together will affect the combined number of Ohms.

If two speakers (of the same rating) are connected in parallel then their Ohm ratings are halved. Working on the example at the top, if your speakers are 250 watts RMS each at 8 Ohms, then connecting two in parallel will change the load to 500 watts at 4 Ohms per side, so you have to find a power amplifier to match that.

If they are connected in series, then their Ohm ratings are summed.
 

How do I connect 2 speaker cabinets in Parallel?

Most passive speaker cabinets have 2 jack plugs at the back. If you connect the amplifier to speaker 1 via the first jack and then connect speaker 1 to speaker 2 via a jack to jack lead, then the 2 speakers will be connected in parallel. If each speaker is rated at 8 Ohms then the combined rating is now 4 Ohms.

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