Speaker impedance mismatch question

BlackbirdCabs

I have a stupid itch to scratch.......

I've heard good things about the sound of old Gibson amps that combine a 12" and an 8" speaker in the same cab. Seeing as 8" speakers are pretty cheap it feels like something I can try fairly easily.

The issue I foresee is that the 12" will drown out the 8" (the 12" I have in mind is rated at 97dB sensitivity, the 8" at 95dB).

The 12" is 8 ohms.

How about I get a 4 ohm 8" speaker and wire them up in parallel.

I end up with a 2.67 ohm overall impedance and the 8" gets twice as much power as the 12". Stupid or actually not a bad idea?

My amps are all valve amps with selectable output impedance so I can set their outputs to 4 ohms.

Cirrus

Not a great idea - stated speaker impedances are just nominal minimum impedances, the actual impedance is frequency dependent. Just for example, a celestion 70 80;



So in practice, with mismatched speakers, the power distribution is harder to predict and will be frequency dependent*. The frequency output from a guitar amp is constantly changing depending on what you're actually playing...


I wouldn't be so hung up on the relative speaker sensitivity. If a speaker is doing something the other speaker isn't, it'll add to the blend even if it's 3dB quieter on paper.


* especially since the resonant frequencies of a 12" and 8" speaker are likely to be so different!

BlackbirdCabs

Nice, thanks @Cirrus - I figured there had to be a reason not to do it!

ICBM

It will work fine, but as Cirrus said it’s actually likely to sound less good than having them both 8 ohms and allowing the very different tones to take care of the balance - especially as the 8” is likely to be much more midrangy and hence audible over the top of a nominally louder but more scooped speaker.

This is also true with mismatched 12”s, eg the G12M-25 and the G12H-30 - there’s a 3dB difference, but in a cab together they sound exactly the same volume because the M is much more middy and fuzzy, and the H clearer and slightly scooped.

I had a Mesa bass cab once with a 15”, a 10” and two 6”s - no crossover, and despite the huge sensitivity difference between the 15” and the 6”s, everything sounded balanced and even.

octatonic

I have a question (apologies for the threadjack)

My new Matchless combo has an 8ohm output for the internal speaker and a 4ohm output for an external.
If I plug an 8ohm cab into the 4ohm output as well as the internal speaker does that bring everything down to 4 ohms?
I am guessing not?

Not entirely sure why they went with 4 ohms for the second output, other than their extension cabs are all 4 ohms.
All my external cabs are 8 ohms (three 2x12's) and one1x12) and I don't fancy buying a Matchless extension cab.

I can't think of any way to rewire a 2x12 with 2x 16 ohm speakers to give me 4 ohms.
Will be annoyed to have to replace speakers that are already nicely broken in. 

Any suggestions?

ICBM

octatonic said:

I have a question (apologies for the threadjack)

My new Matchless combo has an 8ohm output for the internal speaker and a 4ohm output for an external.
If I plug an 8ohm cab into the 4ohm output as well as the internal speaker does that bring everything down to 4 ohms?
I am guessing not?

Not entirely sure why they went with 4 ohms for the second output, other than their extension cabs are all 4 ohms.
All my external cabs are 8 ohms (three 2x12's) and one1x12) and I don't fancy buying a Matchless extension cab.

I can't think of any way to rewire a 2x12 with 2x 16 ohm speakers to give me 4 ohms.
Will be annoyed to have to replace speakers that are already nicely broken in. 

Any suggestions?

I'm not sure how the outputs are arranged, but if the jacks are either a single jack for each impedance tap, or the extension one switches the amp to 4 ohms, the same solution will work - you need a splitter so you can plug both the internal and extension speakers into the 4-ohm jack.

You can't rewire a 16-ohm 2x12" cab to 4 ohms - 8 or 32 ohms are your only options - but you won't need to, or replace the speakers, since an 8-ohm cab is fine even if you connect the internal speaker to the 8-ohm output and the cab to the 4 - it's a mismatch, but within the safe range.

Plugging into both jacks at the same time if they're simply connected to different taps on the transformer isn't ideal, since that creates a low mismatch, but in fact an 8-ohm cab connected to the 4-ohm tap reduces this problem.

octatonic

Thanks @ICBM.

octatonic

Just to follow up on this, in case someone reads this down the line.

Matchless have said that their speaker outputs cannot be used simultaneously.
Either use the 8 ohm output or the 4 ohm output, not both.
So if you have two 8 ohm cabinets then you should chain them or use something like the Palmer 'Cab M' passive cab merger in parallel and use the 4 ohm output.