Combo with an external Cabinet

[thalweg]

Hi All,

I've hooked up my combo with an external 2X12 cabinet loaded with 8 Ohm Celestian G12S-50's. They are wired in parallel for a 4 Ohm load. The tone I'm getting is great with some extra bottom end and nicer balanced mids.

Whenever hooking up my cabinet however I notice a 5-8 db reduction in volume. Unhook and it comes back up.

I don't think anyting is wrong as both setups sound good but am just curious to understand the "physical properties" of hooking up a 4ohm loaded cab with the 8ohm Combo speaker and why the combo speaker seems to loose a bit of volume when doing so.

Cheers

 

[LPJunky]

You're not supposed to connect your external 2x12 at the same time as the internal combo speaker (on the 8ohms output) and your external cab on the 4 ohms output. This is not described in the manual and is not supported by Mesa. I had a long winded discussion in an other thread because I wanted to use as much as I could out of the MkV but ultimately by Mesa that was NOT to be done.

If you keep it at low volume it is probably ok but at your own risk.

Here's the link to the older thread http://forum.grailtone.com/viewtopic.php?f=33&t=45637

 

[thalweg]

Wow...thats strange...and has me concerned.

If you look at diagram 7 in the wiring schemes it allows for a 8ohm connection and 4 ohm connection at the same time?

 

[thalweg]

I think I understand it now. You need an 8 Ohm cabinet coming from the 4 ohm and a 16 ohm cabnit or speaker coming from the 8ohm.

Or maybe I don't understand it.

Can you help describe what I need to do in order to get the combo speaker and my 2 X12 cabinet (two 8 ohm speakers wired in
parallel for 4 ohm load)

Thanks
Flav

 

[thalweg]

Another thought is...could I take the combo speaker and plug that into the 4 ohm out and my cabinet in the other available 4 ohm 1/4 inch plug?

 

[thalweg]

Nope sorry...dont think that will work...when you are using two or more outputs it needs to double your resistance. The combo speaker will work but my cab will have to be wired in series instead of parallel

 

[MrMarkIII]

Nope sorry...dont think that will work...when you are using two or more outputs it needs to double your resistance. The combo speaker will work but my cab will have to be wired in series instead of parallel

You are correct, sir!

 

[thalweg]

Thank-you for confirming that mr. Mark.

Can you offer a thumbs up to this scenario?

So in order to be able to use my cab:

1. I need to connect my 8 ohm combo speaker to a 4 ohm output

2. Rewire my speaker cab in SERIES so that two 8ohm speakers will now have a 16 ohm impedance and I can plug that into the 8ohm output (where my combo was once connected)?

I think that follows the logic in diagram 7 of the wiring chart.

Really appreciate the help. I'm lucky I have been only playing in 10w low volume for the last few days...with the wrong cab configuration. A blown transformer would have sucked BIG TIME!

 

[MrMarkIII]

Yep, with your cab wired for 16 ohms, added to the 8 ohm internal speaker, you get 5.33 ohms total. (Diagram #7).
With the cab at 4 ohms, you get 2.67 total.
5.33 is safer than 2.67.
The math is:
( 16 x 8 ) divided by ( 16 + 8 ) = 5.33
( 4 x 8 ) divided by ( 4 + 8 ) = 2.67
The reason the combo speaker goes down in volume is that the lower impedance cab (4 ohms) gets more juice because the lower impedance offers a lesser path of resistance.
Same thing's going to happen with the cab wired to 16 ohms, but in reverse.
Then the combo speaker will the path of least resistance (8 vs 16), so it will be louder than the cab.
Sorry, you can't repeal the laws of physics.

 

[thalweg]

Thank you so very kindly mr.Mark!

I've been forced to understand impedance after all these years of neglecting that principle. It doesn't cease to amaze me that the learning never stops and one step closer to even better sound!

Thanks again.

Cheers

 

[thalweg]

Sorry one last quick question.

How are the two 8ohm and two 4 ohm outputs interrelated? Are they wired together some how? Parallel? If my assumption is correct wouldn't wiring in series open up more options for safe mismatches.

 

[LPJunky]

This is how the output transformer looks like. The 8ohms and 4 ohms are electrically connected.


Output
Transformer
) ( -------------- 8ohms out
) (
) (
) (
) ( -------------- 4ohms out mid tap
) (
) (
) (
) (
) (---------------Ground


You have to work out the maths for power consumption and stay withing the 90 watts. That's why Mesa won't commit at anything else than whta you find in the manual.