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How to accurately gauge speaker sensitivity to match with tube amp?

I'm in the process of matching speakers to my amplifier and need a bit of advice. Most recently, I'm trying Focal 936 towers with my Quicksilver Mono 60w amp. They were sounding pretty decent until I experimented by hooking up my old Adcom 535L amp. All of a sudden, there was a giant jump in control, tautness in the bass, quickness in transients. The QS stuff was doing quite decently, but the Adcom really snapped these towers to attention. The mids and high ends, not to mention the soundstage, were worse with the Adcom — no question. But there was quite a difference with the other qualities just mentioned.

My question becomes one of sensitivity. The Focals self-rated as 92 db. Stereophile rated them as 89.5db. I realize that these are average measurements and a much bigger picture is told by the impedance graph (and other factors).

As I continue to search for the right match of speaker (I have a couple contenders), I'm sure one piece of advice is to look for speakers with higher sensitivity averages. But what else should I look for to help make a guesstimate about whether the amp will drive the speakers with the kind of control they are capable of? [Specs for this amp are here: http://quicksilveraudio.com/products/sixty-watt-mono-amp/ ]

I realize I need to hear speakers, in my house, with my gear, etc. to get a sense of them. I’m working in exactly this way. Your advice can help me eliminate candidate speakers that would pose similar challenges to my amp as these Focals have.

And I just bought the amp, so I don't want to change it.

Thank you for any thoughts. 

P.S. Anyone who has has had great success with this amp or similar, please shout it out.


128x128hilde45
atmasphere's avatar
atmasphere
11,800 posts
 
For this reason, the argument that only speakers with an impedance of 8 ohms or higher will work on tubes is incorrect. There are many tube friendly speakers whose nominal impedance is 4 ohms. If the speaker’s impedance is relatively flat and consistently hovers around 4 ohms, and if the phase angles are only slightly capacitive, or better yet inductive, there is no reason why a 4 ohm speaker cannot perform well on a tube amplifier. Some of our favorite tube friendly speakers are 4 ohms!"
I missed this earlier but the fly in the ointment here is that all amps regardless of technology make more distortion into lower impedances- and that distortion is audible- the difference being, if you just want a good stereo, 4 ohms is fine, if you want it to sound real then higher impedances will help out by keeping amplifier distortion down.  Tube amps in particular like higher impedance loads for another reason as well- if the amp has an output transformer, it will make deeper bass into higher impedances- the lower the load impedance, the less efficient the output transformer becomes and it will roll off faster on the bottom, by as much as an octave! This can result in phase shift, robbing the amp of impact.
So the tweeter getting to hot from excess energy would cause failure? If occasional mild clipping wouldn't cause any damage, how easily would it be to detect mild clipping in LF leaking into the HF area of the tweeter? Could this type of clipping be easier to not notice in lower powered tube amps causing not occasional clipping but a lot?
Tube amps have a soft clipping character that solid state amps do not. Essentially you get less higher ordered distortion, which is what damages the tweeter. Since its the higher ordered distortion of powerful low notes that usually overloads an amp, the distortion has some power too. The crossover won't stop these harmonics since they are the frequencies that are supposed to pass. So the tweeter gets fried. Generally speaking, its solid state amp clipping that damages tweeters. You can usually get away with it with tube amps unless you clip them really hard. 
djones51
4,947 posts
 
Thanks for the information, just goes to show it's best not to repeat what one has heard for years. I read about 2 hours last night, articles on clipping and tweeter distruction. I think everything both almarg and ieales explained was mentioned though it's hard to remember everything I read. Anyway I apologize for hijacking the thread, back to speaker and amp matching. 
b_limo's avatar
b_limo
2,274 posts
 
Look at all of Fritz’s speakers impedance curves.  Absolutely outstanding.  I would have to assume that it is due to his exceptional crossover design.  It’s also why some people report that his speakers have sounded really good driven by 8 watts.  It’s not always about sensitivity.

I am absolutely smitten with his Carreras.  Fantastic speakers that are making me upgrade the rest of my system because the music they
are making in my listening room is intoxicating, addictive and downright therapeutic.  

Anyone in the Colorado Springs area is more than welcome to stop by once I get my new integrated and hear these wonderful speakers!
wolf_garcia
6,695 posts
 
You can clip a tube amp all day and no harm will come to drivers...ask any guitar player.
ieales's avatar
ieales
1,201 posts
 
guitar cabinets typically don't have tweeters or crossovers
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