Speaker impedence question

I've had my ST-120 up and running for over a week now, maybe about 40 hours on now. I'd like to understand the proper use of the speaker terminals. I have Polk Monitor 10's, rated at 6 ohms. I have them off the 8 ohm terminals. I haven't run them off the 4 ohm terminals. My understanding says it would be okay to run them from the 4 ohm terminals. So what is the rule for what speaker terminal to use?

I always felt it was better to tap lower than the actual impedance if there was a mismatch...from your 4 ohm tap for a 6 ohm speaker...but I have no scienticic basis for that. There are rarely exact matches I suspect and its all approximate anyway, try and do the best you can. I have done it all in sound reinforcement and nothing ever blew up, although frequency response may suffer...not audibly noticable in live gigs however.

Use the tap that sounds best.

Speakers are reactive loads and will vary in frequency, often dipping quite low. Your nominal 6-Ohm speakers will statically measure less, perhaps 4.75 or 5 Ohms, and will dip to 3-4 Ohms during certain conditions while being operated.

As a result, using your 4 Ohm tap on 6 Ohm speakers is probably safest. Tube amps, because of their output transformer, can be touchier about mismatch than solid state amps.

Plenty of non-technical reading available to understand this better:

http://hyperphysics.phy-astr.gsu.edu/hbase/Audio/imped.html
https://www.the-home-cinema-guide.com/speaker-impedance-matching.html
https://www.qacoustics.co.uk/blog/2017/08/14/beginners-guide-matching-speakers-amplifiers/
https://www.lifewire.com/speaker-impedance-3134705
http://www.audiogurus.com/learn/speakers/speaker-impedance-rating/1092

ramon68 wrote:Use the tap that sounds best.

That's always best, methinks.
I use the 4-ohm tap. Even on a nominally rated 8-ohm load, your amp is going to see 4 and lower load most any listening session.

I agree with using whichever one sounds best to you.

The 4 Ohm tap has a lower output impedance and therefore a higher damping factor. In principal it will control the bass a little better but you will lose some volume.

I run my 6 ohm Polk RT-11t speakers off the 4 ohm taps on my Stereo-70.
My room is small for the speakers so I can't crank it anyway, but I like the more controlled bass.

Your load (speakers) should always be higher than the amp output impedance. think of it as if you go way lower you would be approaching a near short. Dale

I've switched over to the 4ohm tap. Does it sound any different? I'm not sure. It does sound wonderful still.