Question about impedance matching

[bluez]

On the rear of my Supway Blues it looks like this :

The Question is, are the two 4Ohm inputs in parallel ?Does it mean that i can connect two 4 ohm speaker cabinets for a total of 4Ohm if i also disconnect the 8Ohm built in speaker?

 

[domct203]

The 4 ohm jacks are both connected to the 4 ohm tap on the output transformer, and they are primarily used to connect two 8 ohm cabinets in parallel which is a 4 ohm load.

If you connect two 4 ohm cabs the amp will be seeing a 2 ohm load, too low for the output transformer.

Dom

 

[DECEMBER]

The two 4 ohm outputs can take one 4 ohm cab or two 8 ohm cabs.
To use the internal speaker plus a cab, move the 8 ohm speaker plug to a 4 ohm jack and plug another 8 ohm cab into the other 4 ohm jack.
The jacks are in parallel, but it's 4 ohms total for both jacks.
Mesa/Boogie amp manuals usually say that a cab of higher impedance than the amp output tap is a "safe mismatch", i.e.
16 ohm cab -> 8 ohm tap
8 ohm cab -> 4 ohm tap
and that you can run a 16 ohm cab and an 8 ohm cab in parallel by connecting them to the 8 ohm & 4 ohm taps, respectively.
Always check your amp's manual to be sure before trying any impedance mismatch. And remember that a cab with lower impedance than the amp output tap is never a safe mismatch.

 

[Nathan]

Hi guys I want to piggyback on this thread because I am in the same situation - I want to play through 2 different cabs, one 8 ohm and one 4 ohm. Combined they would be 2.67 ohms. Do I simply plug both into the 4 ohm tap??

 

[DECEMBER]

Hi guys I want to piggyback on this thread because I am in the same situation - I want to play through 2 different cabs, one 8 ohm and one 4 ohm. Combined they would be 2.67 ohms. Do I simply plug both into the 4 ohm tap??

That's no good... All of the "safe mismatches" listed in the Mark V manual are for cab impedance being higher than the amp output impedance, i.e.
4 ohm tap -> 8 ohm cab or
8 ohm tap -> 16 ohm cab
or both at the same time, but it never says it's safe to put a cab(s) with lower impedance than the output.
How many speakers are in the 4 ohm cab? You should be able to rewire it in series to make it 16 ohms. Then you can plug the 8 ohm cab into the 4 ohm output and the 16 ohm cab into the 8 ohm output.
But don't plug a 2.67 ohm load in.

 

[domct203]

Hi guys I want to piggyback on this thread because I am in the same situation - I want to play through 2 different cabs, one 8 ohm and one 4 ohm. Combined they would be 2.67 ohms. Do I simply plug both into the 4 ohm tap??

If you put the cabs in series you’ll have a 12 ohm load that you can safely run from the 8 ohm tap.

You can easily build a series box with three jacks and a pedal enclosure.

Dom

 

[Nathan]

That's no good... All of the "safe mismatches" listed in the Mark V manual are for cab impedance being higher than the amp output impedance, i.e.
4 ohm tap -> 8 ohm cab or
8 ohm tap -> 16 ohm cab
or both at the same time, but it never says it's safe to put a cab(s) with lower impedance than the output.
How many speakers are in the 4 ohm cab? You should be able to rewire it in series to make it 16 ohms. Then you can plug the 8 ohm cab into the 4 ohm output and the 16 ohm cab into the 8 ohm output.
But don't plug a 2.67 ohm load in.

This turned out to be accurate!
Previously (8+4 ohm using the two 4 ohm jacks) the 2x12 sounded more quiet.
I rewired the 2x10 for 16 ohm operation and tried each plugged into the appropriate jacks.
Works like a charm and no imbalance!

Thank you so much Dec!