Impedance matching and power output of second cab

[gio]

Just bought a Peavey Encore 65 - Vintage Tube Series for my son at a pawn shop for $85. Works fine and sounds ok. But, I've read on some other forums that the Scorpion 12 inch that it comes with isn't all that and that this amp sounds great with a Celestion Vintage 30. The Scorpion is a 4 ohm speaker and the specs on the amp says it can take 4 or 2 ohm loads. I don't want to replace the Scorpion so what I'm thinking is I'd like to get a second 2x12 cabinet and put 2 Celestion 8 ohm V-30's in parallel.

I really don't know what I'm doing but it sounds simple enough.

Here's what the manual I got from the Peavey site says about the speaker jacks.

"A special output transformer allows the Encore 65 to deliver its full 65 watt RMS output into either a four or two ohm speaker load. The internal speaker load from the factory is four ohms and should be plugged into the main jack.
When a four ohm extension (stack) cabinet is employed in addition to the normal four ohm load, it should be plugged into the auxillary jack. The auxillary jack is a switching jack which activates the two ohm tap on the output transformer allowing full power output into both speaker systems."

"Rated Power and Load: 65 W RMS into 4 or 2 ohms
Power @ Clipping:
70 W RMS into 4 ohms
65 W RMS into 2 ohms"

So, two 8 ohm Celestion's in parallel should be a 4 ohm load same as the factory speaker.

If I use the main jack with the new Celestion cabinet what happens to the output? Am I still at 65 watts power to each speaker or is it halved? I just don't understand enough about this stuff to know what the result is. Do two 8ohm 12 inch speakers deliver the same output as one 4ohm? If I understand this correctly if I plug the new cab into the main jack at 4ohms then the output would be split between the two 60W V-30's right? So, I would only get 30 watts to each. Is the decibal level of two 30 watt speakers the same as one 60 watt?

Now, if I use the aux jack for the new cab and main jack for the internal speaker what happens to the power for each speaker?
It says 65 W RMS into 2ohms in this configuration. Does this mean I'm distributing 65 watts across both channels? Or do both channels get 65 watts now since it says "the two ohm tap on the output transformer allowing full power output into both speaker systems"

I'm sure this is pretty academic for you guys but if you could help I'd feel a lot better about trying this. Thanks.

Gio

[ChrisK]

Since you are creating a 2 Ohm load, you get 65 Watts across 2 Ohms. This means that, since the internal speaker and the external speakers are all in parallel, the output voltage is the same across each. This would be 11.4 VAC at 5.7 Amps total (I used magic to figure this out).

In essence, you have 11.4 VAC across the internal 4 Ohm speaker for P=(V^2)/R or 130/4 or 32.5 Watts.

You have 11.4 VAC across each external 8 Ohm speaker for P=(V^2)/R or 130/8 or 32.5 Watts or 16.25 Watts.

The sum of these is 65 Watts.


magic

W=IV
W=(V^2)/R.....65R=V^2.....V^2=130.....V=11.4
W=I^2R.....P/R=I^2.....32.5=I^2.....I=5.7

proof.....64.98(W)=5.7(I)*11.4(V)

/magic

[sumgai]

gio,

Hi, and welcome to the NutzHouse! ;D

Your cup does indeed 'runneth over' with questions! What I took away from all of that were the germane questions:

"Do two 8ohm speakers deliver the same output as one 4ohm?"

"Is the decibal level of two 30 watt speakers the same as one 60 watt?"

The main answer is 'it depends'. We're no longer talking about power output of the amp, but instead we're speaking about speaker efficiency. In the most basic sense, if all other parameters were equal, then Chris's answer above is correct. (As complicated as it looks, it's really just Ohm's Law!) But when you factor in the speaker's ability to use those watts that are fed to it, you get a whole new picture.

For your questions to be answered, one would need to physically listen to the speakers in question, and make a judgement call based on reality. I don't believe that we can make any tests that accurately tell each and every person the same thing, it's always subjective. That said, I quite heartily disagree with your other forum's readers - the Scorpion is vastly superior in both power handling and overall tonality compared to any Celestion ever made, but that opinion is mine, and is not shared amongst very many other listeners/users/owners.

If you have the wherewithall, I suggest you experiment. Sounds simple, I know, but......... take notes!! If you can, make recordings, just for future reference. You're the owner/operator, so the bottom line is up to you, but ask others for their opinion too, it can't hurt to get an occasional reality check.

And always keep in mind that if you buy used, and you don't like it/wanna keep it, you can always sell it again as..... used. Remember, craigslist is your friend! ;D

HTH



sumgai

[gio]

Wow. Thanks guys! Both sets of answers are germane.

The amp sounds good to me but we/I are just starting out on our musical endeavours so I don't really have a discerning ear yet. I think I'll still try the Celestions just for fun and to see for myself what the differences are.

Regarding the Ohmic magic. So, if the amp says X Watts then X Watts is what you get in any configuration of speakers.

"You have 11.4 VAC across each external 8 Ohm speaker for P=(V^2)/R or 130/8 or 32.5 Watts or 16.25 Watts."
16.25 Watts each, right?

So, if I plug the two parallel 8 ohm Celestions into the 4 ohm main jack then I will get 32.5 Watts each.

Makes sense now.

Thanks again!