Ext speaker ohms question

[Jack Knife]

I have a 2x10 combo that runs the speakers at 4 ohms. If I plug a 16 ohm ext cab in, the total load will be 3.2 ohms, close enough.

Is this OK to do? It won't hurt the amp? I heard a comment that the power will be distributed unevenly to the two sets of speakers. How do you figure that out?

Thanks

[EMan]

If this is a typical tube amp, you can use about any EXT speaker you want. The amp is already loaded with the combo speakers, and most anything else you hang in conjunction with those speakers is fundamentally inconsequential.

The distribution of power will not be the same across all speakers if the loads vary. If all the speakers end up in parallel, you'll have an 8, 8, and 16 ohm speaker that 'sees' the same output voltage. Ohm's Lae - E = I*R and P = I*E says that E^2/R = P ( E squared divided by R ). You can see that if the E remains constant, and you change the R...the P will change accordingly.

It may not be enough to be all that noticable, depending on the various efficiency ratings of the speakers. It will sound different...and that may be a good thing depending on your taste.

[Jack Knife]

Thanks, I think. So which will get the most power delivered, the 4 ohm internal load or the 16 ohm external load?

[EMan]

If we take a number like 10 to be the voltage, then 10^2 = 100.

E^2/R = P, so -

100/8 = 12.5 Watts
100/8 = 12.5 Watts
100/16 = 6.25 Watts

[mistermullens]

Well, since we're on the subject, are there any adverse affects on an amp if its got a 4 ohm output into an 8 ohm speaker?

[Dacious]

No. You will slightly reduce power and output, which may be compensated for by more speaker area or higher efficiency speakers depending on what you are plugging in to. The tonality will probably slightly change, as the impedance will be reflected into ouput section of either SS or tubes. It might be slightly more bassy especially with tube amps.

[Jack Knife]

Originally Posted by EMan

If we take a number like 10 to be the voltage, then 10^2 = 100.

E^2/R = P, so -

100/8 = 12.5 Watts
100/8 = 12.5 Watts
100/16 = 6.25 Watts

Got it. Thanks

[scooteraz]

In your case, 80% of the power will go through the 4 ohm speakers, and 20% will go through the 16 ohm speakers. Unless the 16 ohm speakers are really efficient, not much to gain here by way of volume. And, with that 4:1 ratio, I am not sure how well you will hear the extension in the mix.

From the standpoint of 4 ohm output vs 3.2, probably not a huge deal, but your output tubes and transformer will work harder.

[EMan]

Looking at this application - there are three speakers in parallel....two 8 ohm speakres and one 16 ohm speaker.

Of the two 8 ohm speakers -
Speaker A gets 40% of the total power.
Speaker B gets 40% of the total power.
The 16 ohm speaker, Speaker C get's 20% of the total power.

In this case, the 16 ohm extension speaker will have roughly half as much power as either of the other two 8 ohm speakers.

Relatively speaking, reducing the power to a speaker by half doesn't make a huge difference in volume.

[Jack Knife]

Actually, the extension cab in question has 2-10" speakers 8 ohms/ea wired in series. Well, there's a switch on the cab to go between series and parallel wiring, 4 ohm / 16 ohm. The amp builder did not recommend running the amp at the 2 ohm total that two 4 ohm loads would impose.

How does that change the results, if any?

[EMan]

If this were a old Fender amp, I'd say run the cab at 4 ohms. If your amp OEM doesn't like the 4 ohm cab hanging on their OT, then run it at 16 ohms. You'll have to determine if you think it sounds okay that way.

The other alternative is to re-wire the combo for 16 ohms, and run the cab at 16 ohms. This will even out the power distribution, but you wouldn't be able to run the amp without the cab.

[studiodunn]

Great info here.
My situation is i just picked up a 50w 1x12 combo, running at 4 ohms. Their is a switch between the internal speaker jack and the external speaker jack that forces 8 ohms for external speakers. I have a 4x12 cabinet(two green backs and two G12h) running at 16 ohms, that has two input jacks. Can i remove the jumper between the unput jacks, making each one run 2 of the speakers and be at 8 ohms per input jack?

[studiodunn]

So, 4 x 12 running at 16 ohms = 16 ohm speakers wired in series/parallel.

Now if i wire up each input to one set of speakers in parallel, each input now carries 8 ohms..........right?

[EMan]

studiodunn - without knowing how your amp's output switching is wired, it's hard to make a suggestion.

Some amps use a 4/8 ohm OT where the selector sets the EXT and MAIN speaker jacks in series in the 8 ohm position. This way, you can run the 4 ohm combo speaker, and a 4 ohm cab.

If that were the case, you could just wire your 4X16 ohm speakers in the cab in parallel, and make it a 4 ohm cab.

[jjmantele]

You should also verify that the 2 cabs are in phase with each other. If they are not, and you don't fix it, running both will sound worse than one alone.

The battery test works on this.