8 Ohm or 16 Ohm into 12 Ohm speakers

Still using a restored Fisher 400 with Moth Cicada's, which are 12 Ohm speakers. The Fisher has taps for 4, 8 & 16 ohms but I've only ever used the 8 ohm taps. Knowing vintage gear can be finicky, I figured I'd best ask if there would be any potential issues in trying the 16 ohm taps, and perhaps what sound difference I might hear.

Thanks all.

 

I think you're going to have better bass control out of the 16 Ohm taps...

 

I believe 8s are going to better cover low swings in impedance that the speakers show. 12 nominal means certain frequencies are going to swing down below 16 and you will lose some ability of the amp to control the drivers in those ranges. 8 will probably give you more controlled bass and less forward presentation. I had a 6.7 Ohms speaker for a long time, and I always went back to the 4 Ohm output. It didn’t get as loud, but what you got was much better.

 

I could be wrong, but I'd think the opposite. The lower ohm taps should have a lower output impedance, which would give a higher damping factor.

I don't think there's a problem with trying all of the taps, as long as reasonable levels are maintained. Yes, the 12 ohms is likely an average, or possibly even in the high range of impedance the speakers get to, unless they are known for being uniformly high or you have an actual curve. I would agree the 8 ohm tap would be the 'default' but try the others, you may like one of them better.

Edit: huh! the impedance does remain above 11 ohms over the whole range:
http://www.stereophile.com/content/moth-audio-cicada-loudspeaker-measurements
I'd still guess that the 8 ohm tap would be the 'default', but try the others ... these are rather high sensitivity speakers, so the amp shouldn't have any trouble driving them to pretty loud levels without strain.

 

What Rick said....

 

Nominal impedance is usually what is given as a spec. 110 to 1100 sags down below 12 Ohms. 8 might be the best bet on paper.

 

Cool, thank for the tips. The amp has no problem driving these but I'll try the 16 ohm taps just to see how the sound changes, if at all to my ears.

 

I don't know if there's a real correlation between damping factor ("DF") and bass control in this combination, but I think it's worth trying the 4 ohm tap as well. I am guessing, but say the 4 ohm tap provides a 1 ohm output impedance ("OI"), the DF would be 12/1 or 12. For the 8 ohm tap, if the OI is 2 ohms, the DF would be 12/2 or 6 ... 16 ohm tap, OI = 4, DF = 3. For these low values (solid state amps usually have much lower OI/higher DF, something like DF = 100 or more) it may make a difference? I don't know. I would guess with a SS amp, if the DF is 100 or 400, it may not sound much different, unless the speaker/woofer is really sensitive to that parameter.

Actually, the real OI values may be different and not result in DFs as I guessed above ... but there may still be effects that are heard.

Maybe with the 4 ohm tap, the bass may sound a little more tight/solid/tuneful but not seem as deep or loud, which may actually be a result of a little 'overhang' or looseness? and for the 16 ohm tap things may sound a little 'bassier' but not quite as 'tight'? Interesting experiment! There may be mid/treble effects as well, and/or soundstaging/depth effects. I think overall, whichever sounds the best to YOU is the correct one! I don't believe there's a chance of 'hurting' anything by using any of them, especially with the high sensitivity speakers at at moderate volume levels.

Actually, do be careful to not 'short' the wiring or run the amp without a load connected. Some tube amps (possibly not the Fisher 400, I don't know) can be damaged if run without a load connected - in case you didn't know.

 

There will be slightly more power available at the 16 ohm taps but will also be slightly more taxing on the amplifier; less power at the 8 ohm tap but will also be easier on the amplifier.

Not familiar with that loudspeaker but if it's highly efficient (ie >95 dB SPL @1w/1m), odds are you'll never push the outputs anywhere near maximum safe limits therefore of little consequence regardless of taps chosen.

I don't believe the damping factor would be affected by taps chosen.