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If an amp has 4 ohm taps

(and I am specifically referring to a tube amp) would there be any reason to avoid speakers with a listed nominal impedance of 4 ohms?

I realize this subject has been discussed in some form before, but I cannot seem to get a tight grasp on what makes a speaker an easy or hard load to drive.  What I THINK I have gotten out of it so far is that the lowest impedance a speaker presents to the amp may be more critical than the listed sensitivity?  (And also that for best performance the tap on the amp should match the nominal impedance of the speaker.)  

So I would think that generally speaking, if one had an amp with only 8 ohm taps (which is probably includes quite a few amps) one shouldn't even consider speakers with 4 ohms listed as nominal impedance.

 Generally speaking, is a 4 ohm nominal impedance  a tougher load than an 8 ohm nominal impedance?  But if the (tube) amp has 4 ohm taps, does this mean that using those taps it should be able to deal with a speaker with a nominal impedance of 4 ohms as well as it would deal with a 8 ohm speaker using the 8 ohm taps?

And here is another discussion that I found here on A'gon, but I am not sure if it answers my question:

https://forum.audiogon.com/discussions/amplifier-circuitry-4-ohm-vs-8-ohm

 

 

immatthewj
re-lar-kvothe's avatar
re-lar-kvothe
184 posts
 

The impedance curve at the link provided indicates the lowest it goes is approximately 3.5-ohms. Using the either the 4 or 8 ohm taps will be fine. Plug your speakers into each set of taps one at a time and see which ones sound better. 

alan60's avatar
alan60
189 posts
 

I've had tube amps with 4 and 8 ohm taps, irrespective of what speaker I had or it's relative impedance I never found an amp that sounded better using the 4 ohm taps, over the 8's.

allenf1963
372 posts
 

For those way more knowledgeable than me, where should I connect my KLH Model 5 speakers to my McIntosh 225 amp?  The 4 ohms tap?

 

THE AMP: Stereo or Twin Amp: (Center tap internally grounded) 600 ohms (One side internally grounded) 4 ohms, 8 ohms, 16 ohms, 150 ohms, 200 ohms

 

THE SPEAKERS: KLH Model Five speaker has a nominal impedance rating of 6 ohms with a minimum impedance of around 3.5 ohms at 140Hz

panzrwagn's avatar
panzrwagn
564 posts
 

Speakers are a complex load with resistive, inductive and capacitive components, as well as a back EMF generated by the speaker and damped by the amps out stage. More sensitive speakers have greater back EMF. All that rolls up into a goat rodeo called complex impedance. Serial inductance causes current and voltage to become out of phase, so when that happens, impedance drops - current draw increases as voltage drops. That’s the definition of a low impedance. For reasons I don’t fully understand most modern loudspeakers in terms of current requirements (certain LS/35a models notwithstanding) behave more like a 4 Ohm - or even lower - load.

Vacuum tube amps use output transformers to convert voltage to current. He 4,8, and 16 Ohm taps represent the transformer designers best notion of how to best match the speaker impedance load. That’s an oversimplification,, of course, there’s much more to the art, science, and craft of transformers, but impedance matching is the heart of it. And that’s why it’s difficult to generalize about things like which tap to use with what speaker. Best to try your options. You won’t hurt anything, but it may not be ooptimal choice.

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