Converting 4 ohm to xx ohm?

[SteveC]

Hi all,

Can I simply stick a (half decent wattage) resistor of, say, 8 ohms in series with a 4 ohm speaker to make it 12 ohms - so I can use the speakers on an amp which has 8-16 ohm output?

[MikeK]

Originally posted by SteveC
Hi all,

Can I simply stick a (half decent wattage) resistor of, say, 8 ohms in series with a 4 ohm speaker to make it 12 ohms - so I can use the speakers on an amp which has 8-16 ohm output?


You could, but unless you want to listen at very high volumes, there isn't really much point. Just use the speaker as it is - in it's 4ohm guise. Probably wise to make sure you aren't wiring it in parallel with other speakers though, as is often the case with second speaker set terminals (usually marked B)


The reason there isn't much point is this (simplified)

12V across a 4ohm speaker will draw 3A current.
3A x 12V = 36W power.

12V across a 4ohm speaker in series with an 4ohm resistor (8 ohm total) will draw 1.5A.
1.5A thru a 4ohm speaker will give 6V across that speaker.
6V x 1.5A = 9W

To get 36W out of the speaker, you'd have to up the amplifier voltage to 24V

24V across 8ohms total (4ohm speaker and 4 ohm resistor) will draw 3A.
3A thru a 4 ohm speaker will give 12V across the speaker.
3A x 12V = 36W

So, to get the same output level from the speaker, you'd have to drive the amp's voltage to twice the level it otherwise needs to be. There is little point in adding a series resistor and then simply upping the signal to compensate for the output drop - you may as well remove the resistor and turn the volume back down to get the same signal level.

[SteveC]

OK, that makes sense.

So, how can I run some 4 ohm speakers for my surrounds/centre speakers? Or shall I just bite the bullet any buy some proper speakers?

[MikeK]

Originally posted by SteveC
OK, that makes sense.

So, how can I run some 4 ohm speakers for my surrounds/centre speakers? Or shall I just bite the bullet any buy some proper speakers?


Just use them as you would any other speaker - just accept the fact that you probably won't be able to run the amp at full volume with these speakers without the over-current protection kicking in.

Roughly speaking, you can work out an amp's safe maximum load for speaker's it's not supposed to be able to drive.
If the amp is say 70W per channel into an 8ohm load (and it doesn't give figures for 4 ohm use),
then it's max voltage will be about 24V and max current about 3A (this is per channel, but in reality few multichannel amps can drive all 5 channels to max output simultaneously - their PSUs simply aren't up to it). So 24V x3A = 72W, which is close enough.
Using a 4ohm speaker at max current (3A), will give a voltage across the speaker of 12V (well inside the amp's capabilities). This gives an ouput power of 36W (or half the rated max)

So, if an amp will deliver 70W into an 8ohm load, then it's safe to use a 4 ohm speaker and drive it to at least 35W. Often it'll be safe to drive it higher too.
As speaker output isn't linear to the applied power, you may be surprised to find that 35W into a 4ohm/90db speaker and 70W into an 8ohm/90db speaker, won't sound massively different in terms of audible output.

[SteveC]

Ahhh! I see what you were trying to get at above now... Thank you MikeK!