impedance question?

[herman]

Hi all,
I've just got hold of an ampeg BA500 2x10 combo, Im told the combo will run at 2ohms with a 4ohm extension cab, would it cause any real problem if I used an 8ohm extension cab instead of a 4ohm? I was thinking of an ampeg svt 2x10 8ohm as the extension cab, as I ask, would I cause either the combo or cab any harm?

Thanks herman.

[Jack]

Nope, in fact you could run 2 x 8ohms cabs just fine if you wanted as well.

[flyfisher]

It's worth digging out the manuals and reading through them - they generally explain these things pretty well.

Basically your combo has a 4 ohm internal speaker ( two actually, but the important thing is that they combine to be 4 ohms). Adding an external cab will reduce this impedance, so it,s important to know the minimum safe value, which in this case is 2 ohms.

This means you can add a 4 ohms external cab to give an overall 2 ohms and get the maximum power from the amp.

If you connect an 8 ohm external cab, you'll get an overall impedance of 2.7 ohms, which will be fine but you'll get slightly less than the maximum power from the amp. In practice I'd be surprised if you hear any difference and the type of cab you connect will likely make a bigger difference than the extra 0.7 ohms.

So, ohm-wise, you can safely connect any external cab. Indeed, as jack points out, you could connect a second 8 ohm cab if you wished, which would give you a total of 2 ohms. This handy calculator can be useful to figure out the options: http://www.1728.org/resistrs.htm. Keep the overall result at 2 or more ohms and the combo amp will come to no harm.

Of course, the audible effect of multiple cabs is another matter altogether, but at least you'll do no harm while experimenting.

The other consideration is power. With a 4 ohm external cab, the available power will be split 50:50 between the combo and the external cab. With an 8 ohm external cab the power will be split 67:33, I.e 2/3rds to the combo and 1/3rd to the external cab. So assuming 500W with an 8 ohm external cab means about 167W to the external cab. So, again, almost any external cab should be fine.

[badboy1984]

Remember asking TC the same question but on a bh500 head. If you ar using TC cab you can run two 4ohm cab tha means yu can run your internal cab and a 4ohm TC cab.

I don't think you can run a non TC 4ohm cab because your amp is a 4ohm load. The magic only work on TC cab.

[icastle]

[quote name='badboy1984' timestamp='1398034643' post='2429808']
Remember asking TC the same question but on a bh500 head. If you ar using TC cab you can run two 4ohm cab tha means yu can run your internal cab and a 4ohm TC cab.

I don't think you can run a non TC 4ohm cab because your amp is a 4ohm load. The magic only work on TC cab.
[/quote]

TC amps are the exception, but this is an Ampeg head we're talking about here.

I'm not sure if there's any 'magic' involved in TC stuff or if the tolerances of the amp are perhaps a little tighter than other manufacturers.

Whichever it is, it certainly focusses TC owners attention on TC cabs.

[flyfisher]

[quote name='icastle' timestamp='1398035475' post='2429813']
. . . but this is an Ampeg head we're talking about here.
[/quote]

Exactly. My answer won't necessarily apply to other amps or combos.

[badboy1984]

Hahaha, it was late night and I was reading this thread on a iPad mini. Mis-read and thought we talking about the BH500 by TC lol. Sorry ignore my post.