Impact of amp/speaker impedance selection

My amp allows me to select either 4/6/8 ohm impedance with the turn of a dial. My other amps that have this either enable it by “under the hood” changes or different taps. When I first got it I went with 6 ohms as my Harbeth C7’s are listed as such.

What is the impact of switching it—less from a sound perspective but I’d be interested in that context too—more from a tube life and driving my speakers perspective? On my last amp I went with 8 because it sounded good and changing it was a PITA so never thought of it again.

If I play around and like the 4 or 8 ohm settting are there any implications I should be aware of? The dial lets me rotate it to make the change but does it take instant effect like changing the resistive load on an MC cart at the phono stage?

Thanks for any context you can share.

 

Tubes should stay biased as they are, but the output transformers will be asked to deliver more current at the higher impedance settings.

Your speakers have an impedance curve that changes at different frequencies. You want the amp set below or at the lowest dip in impedance the speakers exhibit. If you set it too high, the amp will be less able to control the drivers, so motion in the drivers and also the stopping of the motion will be more difficult. The output transformers will start putting off more heat.

Think of it like a vehicle with a drive and braking system. Low impedance speakers are like adding weight to that vehicle, which makes it harder to get going and harder to stop. At some point the engine and brakes may simply overheat and fail.

 

So if I understand you correctly and continue your analogy, you are saying that using the 8 ohm setting with 6 ohm speakers is going to make it harder for the amp to control the speaker and put off more transformer heat?

Amp auto-biases btw

 

Yes

 

Yes, it is immediate. You are changing the transformer output taps.

 

You should get better bass response at 4 ohms than 6 or 8 ohms. V=I × R due to more current. I generally preferred the 4 ohm taps on my tube amps.

 

I've often wondered what the correct tabs are for 6 Ohm speakers. I've always had them hooked to the 8 Ohm taps - would 4 Ohm be better?

Now, my 12 Omh Zu DW's I have hooked to the 8 Ohm taps of my PrimaLuna tube amp. Zu people told me it would be better than 4 Ohm, and an easy load for the PL to drive.

 

I'm currently running 6ohm speakers, and I have them in the 4ohm taps of my tube amp. Sounds excellent with great bass control. I assume a 6ohm nominal is hitting down to at least 4 ohms at various points. Possibly lower.

But like so many things, it seems like there is no hard and fast rules. It's worth trying both taps just to see what works best for you.

 

Thanks to all for the input...

Well interestingly, I played a bit yesterday and the differences are not subtle between 4 and 8. 4 ohm setting is louder (?) and has way more air/sparkle/life but it's not bright (I HATE bright). Bass is different but hard to say better--think I need more time comparing but I'm not sure I'll even bother because of how good 4 ohms sounds right now. One thing I'll need to keep an eye on is how sometimes that air/life CAN become annoying over time to me--after the initial excitement wears off.

When I went back to the 6 ohm setting, it wasn't as dramatic as going from 8 to 4 but it was there. 6 ohm really did seem to split the difference. 4-6 and 6-8 not that much different from each other but 4-8 was a definite difference. I only wish the differences on cart loading was so apparent to me (because it's not and much harder for me to focus on differences there).

So I'm staying with 4 ohms for now. If my speakers implode on themselves I'll blame all of you in this thread.