# Ohm and watt question

Discussion in 'Amps and Cabs [BG]' started by optic, Mar 16, 2006.

1. ### optic

Nov 18, 2002
Sweden
Ok, here is the deal

I have a Bassforce XXL combo at 4 ohm, 300 watt
Im gonna add a EBS NEO 110 at 8 ohm, 250 watt

As I have understood, I wont get as much power as I am used to, and my output/max volume will be lowered

is this correct
and if so

what will the final output/max volume be?

2. ### SEK

Feb 15, 2006
Downeast
I'm new here, but I'm gonna take a stab at answering your question, as I'm doing a lot of research lately regarding ohms.

If I'm not mistaken, I believe you might damage your combo by arbitrarily tacking on an 8 ohm extension cab. If your combo is rated at 4&#937;, adding an 8&#937; cab will present a resultant 2.67&#937; to the amp portion of your combo. Many, if not most, amps have minimum 4&#937; cabability. Your addition of the extension cab may exceed the limits of your combo.

Experts: If I'm way off here, please let me know

3. ### fretlessrock

Aug 8, 2002
Connecticut
You aren't way off, but the question is: what is the impedance of the internal driver in the bassforce combo? The head might be rated at 300w/4ohms, but still be driving an 8 ohm internal speaker. The fact that it can drive an extension cab makes me think that the internal driver is 8 ohms, and that would be fine with the 8 ohm extension cab (4 ohm total impedance).

If that is the deal: adding an 8 ohm extension cab + 8 ohm internal driver = your amp is now putting out its rated 300W insead of about 150W that it was pumping into the single 8 ohm internal driver. So the original assumption that it would mean less power is wrong. You should notice more volume and headroom.

4. ### deebee

Feb 2, 2006
Brisbane, Australia
Agree, provided the internal speaker _is_ 8 ohms.

5. ### fretlessrock

Aug 8, 2002
Connecticut
Ahhhhhhhh...
H&K has an online manual.

The internal speaker is 4 Ohms. The extension speaker circuis is a *series* circuit and that means that the amp would now see 12 Ohms and that would mean less power. About 200W or so is my guess. It still might sound better/louder depending on the efficency of the drivers.

from the manual: "SPEAKERS: Use this output to connect external speakers/cabinets. You have two options for connecting additional cabinets to the BassForce XXL. For one, the amp is equipped with a serial speaker circuit that lets you do just that&#8212;drive the internal speaker in series with a connected external speaker. Although this increases resistance and thus decreases power output, the surface area of the speakers is enlarged several times over."

There should also be an "external Speaker" jack that will power only the external cab, disconnecting the internal driver.

Hope that helps more than my last post.

6. ### SEK

Feb 15, 2006
Downeast
Interesting.

Would it be correct then to say that if the external speaker output was wired in parallel, he would be unable to add the 8&#937; cab?

7. ### optic

Nov 18, 2002
Sweden
thanks alot guys, especially you fretless

then Ill be safe with my new cab

I will uppgrade in the future thou, and sell my combo, prolly buy a EBS head or a Thunderfunk combined an additional 2*12 cab.

Once again, thanks alot guys.

8. ### optic

Nov 18, 2002
Sweden
Btw, is there any reason to think that the new cab will be damaged? The dude at the local musicshop talked about that, but I never got a good grip about what the heck he was talking about.

9. ### fretlessrock

Aug 8, 2002
Connecticut
You won't be risking damage to the new cab. That amp should be just as stable into 12 Ohms as it is into 8. And the external cab will be getting less power than the internal cab. Series wiring is not common, but there are good reasons for it. One consequence is that any external load always increases the impedance. That is a safety factor for the amp.

SEK, yes, that is true. Actually, since the head has a minimum 4 Ohm rating you wouldn be able to add any parallel load because the internal driver is already 4 ohms and any parallel load would lower it.