Beginner impedance confusion

[gronker]

I’m nearing completion of my first speaker project and want to build a matching amplifier next. First of all - thanks to all the contributors on this forum who have provided so much info to get beginners like me started, now my question:

How do I calculate the impedance the amp will see for an MTM speaker wired in parallel? I have two woofers with a nominal 8 ohm (closer to 12 at the 3kHz crossover point) in parallel to provide a nominal 4 ohm (or 6 ohm at 3kHz) and a tweeter with a nominal 8 ohm (but closer to 6 at the crossover point). As the impedance varies a lot with frequency do I:

1. Use the nominal impedance of the speakers combined in parallel, which gives a frighteningly low 2.7 ohms;
2. Consider the woofers and tweeters separately, in which case if I use the nominal impedance the woofers are 4 ohm and tweeter 8 ohm;
3. Use the impedance at the crossover point;
4. Design for the minimum possible impedance, probably around 2 ohms;
5. Whatever, just build an 8ohm amp, plug it in and watch for smoke (this is my default choice).

I’m also thinking of changing the crossover to be in series instead of parallel, is this a good/bad/indifferent thing to do? By my uninformed reckoning that would give an impedance somewhere between 6 and 12 ohms, which seems better than somewhere between 2 and 8 ohms.

I measured it with a multimeter, this would be a DC current and the impedance it sees dances between 2.5 and 6 ohm, so that didn’t really help me, except for some extra confusion.

The speakers are my first proper speaker project – they are some Morels I have in temporary open baffles while building boxes for them and testing the crossover. They sound fantastic to me in the open baffles so I hope I don’t mess it up with the boxes but I’m also building a SKA kit amp and need to decide if it is to be a 4 or 8 ohm amplifier?

Thanks

[paulb]

The crossover adds a lot of impedance to the woofer at high frequencies, and to the tweeter at low frequencies. So #2 is pretty safe.

[gronker]

Yep, thanks. That makes sense, I was just getting lost in all those ohms.

[Svante]

Long ago* the definition of nominal impedance was "an impedance that the actual impedance is never less than 80% of". Bad wording of mine, but that is the essence of it. That is, the impedance of an 8 ohm speaker should never go below 6.4 ohms, and the impedance of a 4 ohm speaker should never go below 3.2 ohms.

For your system, where you have paralleled two 8 ohm woofers, nominal impedance would be 4 ohms, and minimum impedance >3.2 ohms. The tweeter's impedance will never go below 6.4 ohms, still as the woofers may, the system as a whole is a 4 ohm system. The woofer and tweeter never loads the amplifier fully at the same frequency (if the crossover isn't terribly poorly designed) so you do not need to calculate the parallel impedance of all three.

FYI, the impedance also typically peaks at other frequencies with values of 10-20 ohms or more. This is normal and has nearly no negative effects for normal amplifiers.

* Nowadays it happens that impedance is actually even lower than 80%, but that is not really good even if the amplifer survives.