Series/Parallel speaker impedence logic question

MattBanshee

So, I have my eye on a nice little 2x10, 400w 4ohm bass cab on eBay. I already own a 1x15 250W 8ohm cab. I would like to use the 2x10 on its own for band practice, but potentially stack it with the 1x15 for gigs. I have an Ampeg transistor head, rated at 4ohms minimum.

I appreciate that, as stock, I can't run the two cabs together using the amp's parallel outputs, as this will drop the impedence to 2.6ohms and therefore be unsuitably low.

However; if I were to install a series/parallel switch between the two speakers in the 2x10, this should give me a series option of 16ohms, which when paralleled with the 1x15 8ohms, gives a total of 5.3ohms, which the amp would be happy with... 

Have I got that right?

ICBM

Yes .

It also means that when using both cabs, 2/3 of the power will go to the 15 and 1/3 to the 2x10, which is probably what you want for the fullest sound.

jpfamps

I never think that having switching on the back of speakers is a great idea.

ICBM

jpfamps said:

I never think that having switching on the back of speakers is a great idea.

I'm not sure about it either, but it does solve a problem here. It's pretty safe if you use a heavy-duty DPDT - assuming you remember to set it correctly. (There is also a good way and a bad way to wire them.)

What I really don't like is using the jack switch contacts for it - they're not reliable or high-current enough, and if they fail they will often leave the amp with no load. eg the ones on modern Marshall 4x12"s...

jpfamps

ICBM said:

jpfamps said:

I never think that having switching on the back of speakers is a great idea.

I'm not sure about it either, but it does solve a problem here. It's pretty safe if you use a heavy-duty DPDT - assuming you remember to set it correctly. (There is also a good way and a bad way to wire them.)

What I really don't like is using the jack switch contacts for it - they're not reliable or high-current enough, and if they fail they will often leave the amp with no load. eg the ones on modern Marshall 4x12"s...

You should definitely wire it so there is no break in continuity when switched, however I still don't like the idea of having a switch on your gear that when switched could destroy your amp. 

MattBanshee

As an alternative to a switch, is there a way to wire it with two separate jack sockets, one for 4ohm parallel and one for 16ohm series?

ICBM

MattBanshee said:

As an alternative to a switch, is there a way to wire it with two separate jack sockets, one for 4ohm parallel and one for 16ohm series?

Yes, but it’s a bad idea because jack switch contacts are inherently unreliable compared to proper switches.

MattBanshee

ICBM said:

MattBanshee said:

As an alternative to a switch, is there a way to wire it with two separate jack sockets, one for 4ohm parallel and one for 16ohm series?

Yes, but it’s a bad idea because jack switch contacts are inherently unreliable compared to proper switches.

Would it need to use jack switching then? Can't be done with just discrete wiring?

ICBM

MattBanshee said:

Would it need to use jack switching then? Can't be done with just discrete wiring?

No, it needs switching.