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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
immatthewj OP
2,406 posts
 

@immatthewj If the best sound quality out of a tube amp is your goal then it is best to avoid lower impedance speakers. This is particularly true if the amp uses no feedback.

The lower impedance makes the speaker cables far more critical. They should be heavier gauge and also shorter for best results. Output transformers with a 4 Ohm tap tend to be less efficient than when they are driving 8 or 16 Ohm on the correct tap; the use of 4 Ohms can easily reduce the low frequency bandwidth by an octave. The transformer can often run warmer too and all that heat is amplifier power warming it up due to the (slightly) reduced efficiency.

All amplifiers make more distortion driving lower impedances. For this reason alone IMO 4 Ohms is a Bad Idea since keeping distortion down helps with reproducing low level detail and the like.

Thanks for that explanation, @atmasphere . I will leave the 4 ohm Totem Bisons (Twin Towers) that I was kind of sort of looking at alone.  And that also includes the 4 ohm Totem Arro's that I found on ebay for what I was considering a price that I could afford to experiment with.

immatthewj OP
2,406 posts
 

Thank you for the link, @newfzx7 .

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
192 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
373 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

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