Ohms...

[bigdawg]

they confuse me...

so previously Ive been running 2 16ohm cabs via the two 8ohm outputs of my amp.

now Ive (nearly) got a 2x12 with both speakers rated at 8ohm do I run this out of the 16ohm plug??

[MattANJH]

Generally a cab should be connected to the matching impedance socket on the amp, unless otherwise instructed by the manual.

Depends on how the cab is wired really.

Here's a bit of a guide. XX" represents either 8", 10" or 12" speaker.

Parallel means the speakers are wired to the jack socket seperately, there's a wire going to the + of each speaker and then one from the - back to the jack. Serial means they're in one big circuit, so jack to + on one speaker, - to + on next speaker and so on until the last speaker, where the - goes back to the jack.

1xXX" cab
If it's an 8 ohm speaker, plug it into the 8 ohm output.
If it's an 16 ohm speaker, plug it into the 16 ohm output.

2xXX" cab
Two 8 ohm speakers wired in parallel. 4 ohm cabinet. Plug into 4 ohm output. If no 4 ohm exists, I'd rewire the cab.
Two 8 ohm speakers wired in serial. 16 ohm cabinet. Plug into 16 ohm output.
Two 16 ohm speakers wired in parallel. 8 ohm cabinet. Plug into 8 ohm output.
Two 16 ohm speakers wired in serial. 32 ohm cabinet. You've been screwed, beat the person who sold it to you around the head and rewire to give 8 ohm output.

3xXX" cab
Dodgy ones. They're possible to do, but not that common AFAIK and you need to be careful. You'll be getting mixed circuits. Possible combinations are.
Possibility 1
Circuit 1) Two 8 ohm speakers wire in serial. 16 ohms.
Circuit 2) One 16 ohm speaker. 16 ohms.
Two circuits will be wired in parallel to give 8 ohm ouput.
Possibility 2
Circuit 1) Two 16 ohm speakers wired in parallel. 8 ohms.
Circuit 2) One 8 ohm speaker. 8 ohms.
Two circuits could be wired in series (would be a bit of a comples wiring set up!!) giving a 16 ohm output.

4xXX" cab
A few different possibilities with this one.
1) Four 16 ohm speakers wired in parallel to give 4 ohm output.
2) Four 16 ohm speakers. Two lots of serial circuits wired in parallel to give 16 ohm output.
3) Four 8 ohm speakers. Two lots of serial circuits wired in parallel to give 8 ohm output.

For the ultimate in loudness, it would be possible to get a pair of 4xXX" cabs loaded with 8 ohm speakers all wired in parallel to give a 2 ohm rating. Connect these to a 4 ohm output with a serial Y cable and you've effectively got a 4 ohm 8xXX", but who really wants that sort of power volume?!

Other cabs will be possible, I've seen 8xXX" bass cabs before now, but are uncommon so probably not worth noting unless a specific example is needed.

[bigdawg]

cool, I kinda got it right, thanks

[manicguitarist2]

Folks should pay attention in physics lessons to the bit on adding resistances...

[ecc83]

True Manic, true.

Basic bottom line A saving data:

Valve amps: NEVER no load!

Better a lower load than set for than higher, Thus if the amp is set at 8R a 4R cab will eat your valves a bit faster but a 16R cab MIGHT just make them arc over and also wreck an op tranny. So, worse case, set for 4, load with 16? NO!

Tranny amps: just the opposite: never a short (happens a lot with jacks, hate 'em for speakers!).

Higher load than set, just gives less power but then an impedance selector on a tranny amp almost does not exist and if it did valve rules would apply.

Bridge mode trannies: NEVER go below rated load which is usually twice the single channel minumum.

In THEORY a transistor amp can drive its ***s off into an open circuit. Hmm, been caught, don't do it!

Dave.

[mattbanshee]

I'd always been told that using a cab with a lower impedence than the amp (e.g. 8ohm amp into 4ohm cab) could damage the speakers if they aren't powerful enough, so it's best to use one to match or with a higher value than the amp. Most modern transistor amps I've seen are simply marked "Minimum speaker impedence 4ohms", as using a higher-rated cab would just reduce the volume.

[ecc83]

Speaker power handling capabilities are an entirely DIFFERENT issue and quite complex to boot!

Well, you wouldn't want this all to be too easy would you?

Dave.

[bigdawg]

Orange amps have multiple outs (two 8ohm & one 16ohm) - so be quick before I blow myself up - 16ohm out into 2x12 8ohm cab

[ecc83]

bigdawg: Rewire the cab to 2x 8R!

Dave.

[bigdawg]

sorry that was 2 12'' 8ohm speakers wired in serial...

is that what 8R means??