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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
hilde45's avatar
hilde45 OP
4,974 posts
 
@almarg Thanks. I didn’t think it was all that low, in comparison to other tube amps and QS has a good reputation for driving a lot of speakers. Thanks for adding some quality information for context.

@audiokinesis I didn’t follow the technical details in your post, but I get the final drift. There is something about the combination of the QS amp and the Focal speaker that is responsible for a lack of control compared to the Adcom, and I admit after reading your post I’m less sure it’s damping factor, but now I’m also less sure what factors to be on the lookout for that create the same issues.

@fritzspeaks @audiokinesis Thanks, all. None of the issues I noticed with the Focals were present in the Carbons. These curves were experienced by my ears as very consistent across the ranges.

@ohlala Thanks for the link to VTL. As I look at it, I see that they are pointing at damping factor again. I have to confess, as a non-technical person, I have no idea what to think.

VTL: "In our experience the system begins with the loudspeaker that will work properly and sound good in the desired acoustic space. Matching the power amplifier to the loudspeaker is usually a function of driver size, efficiency and budget. Efficiency is not the only determinant – even though they may be very efficient, many speakers with large drivers need tremendous amounts of damping factor and current capability to control the driver properly.

To get everything out of the loudspeaker that it was designed to do we suggest that you use the highest power amplifier that you can."

P.S. I listened again extensively to the Focals and Carbon today. The muddiness is still there in the Focals but it’s not a lot of muddiness; just at the bottom, basically lower than the Carbons go. For them, I let my sub do the work.
audite84
110 posts
 
Hi hilde45! I am not familiar with your amplifier, but have you tried tube rolling? I’m speaking from experience. I have a tube preamp driving a SS power amp. In my set up, the Rogers-branded Mullard 12AU7 has very good mids and sweet highs, but loose bass. On the other hand, RCA 12AU7A "clear top" has more defined, better bass but mid and high just below the level of the Mullard. As the loose bass really disturbed me, I ended up using the RCA’s until now. I’m planning to try Amperex or another Mullard for a change.

Even then, tube bass sounds different from SS bass. I occasionally hook up my well-reviewed SS preamp and yes, bass is tighter but tube clearly wins in the mid and treble departments. I have learned to embrace tube bass and I’m happy.
hilde45's avatar
hilde45 OP
4,974 posts
 
Hi Audite, I’ve done some tube rolling. I have KT88s in my Monos. Not sure why a tube roll would change the fundamentals, here. 
avanti1960
1,480 posts
 
The measurements of your Focals show an intersection of a negative phase angle trace and the impedance trace at 4ohms.  This is a legit challenge to amplifiers despite their sensitivity (measured at 89.5db).
To get the best, most linear and dynamic sound the amplifier should have high rated output at 4ohms as well as a robust capacitor store because the current requirements of your speakers will be demanding.  
Check the Rogue Audio Stereo 100, a very robust tube amp that has a dual mono layout, decent power at 4 ohms (use the 4-ohm taps)  and a seriously robust capacitor store.  I have one and it drives my large Harbeths very well in terms of dynamics and power output.  It will take good care of your Focals and would be a synergistic match.  
mesch
3,510 posts
 
Hi David, I think you made a great purchase in the Quicksilver Monos. Having the ability to listen to different speakers in your own room helps tremendously in the quest for audio knowledge. This was (is) a great thread.

The Focal speakers are for the most part better served by a SS amp. This is due to their impedance at lower frequencies. The Arcam amplifier is no match against the Quicksilvers at higher frequencies. 
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