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
You have a 60 WPC tube amp with KT88s, virtually the same amp I have run for decades. https://systems.audiogon.com/systems/8367 This is a Melody 50 wpc. Before that was an Aronov very similar 50 wpc. Between the two ran four very different speakers JBL L7, Linaeum Model 10, Vandersteen something or other (very briefly) and Talon Khorus. All these speakers were around or a little more than 90 dB. All played plenty loud in that 17x24 room. All sounded great, for what they were. You have plenty of power.  

Don't even ask me what the impedance curves looked at for any of these speakers. I haven't the foggiest. Used to pay attention to such things. Then I realized impedance curves, like frequency response and all the rest, are at best second derivatives. In other words there is the final result, what you hear, and then there are all the things that go into that. The sound you hear is derived from sensitivity, dispersion, impedance, etc. They are not what you want. They are only things we have learned to measure and derive from what really counts, the sound we get.  

Now considering you are having problems I have never had in 30 years you might want to stop and think about it. If a guy has never had a problem in 30 years, always picked great satisfying speakers, what is he doing? How is he doing this? It ain't by looking at curves on charts.

The only measurement you need is sensitivity greater than 90. Then from there you pay attention to listening impressions. Then when you find people describing a sound that matches what you like look for people who are hearing that sound with tube amps. 

This takes no more time than searching through impedance charts. Its no more easy to describe the sound than measure the impedance. Look how many times JA and others go into excruciating detail about the technical aspects of impedance. Then at best all you have is one more extremely crude measure of performance. It tells you nothing about how deep and wide and inviting the sound stage will be.

But on the other hand, maybe you don't care about such things as musical involvement. Maybe you do only care about tiny little technical details like how tight the bass is between 146.9 and 237.8 Hz. If so then, as Roseanne Rosanna Danna used to say, never mind. 
I think you may be better off asking people which amp they use with their Focal 936s or Focal speakers in general.  There may already be threads on this topic that you can consult.
Your advice can help me eliminate candidate speakers that would pose similar challenges to my amp as these Focals have.

Yes, David (@hilde45), as you realize and as I have said in a number of past threads that is precisely the main potential usefulness of specs and measurements. **Ruling out** candidates that are likely to be mismatches with either user requirements (e.g., desired peak volume capability) or with other parts of the system, thereby narrowing the field of candidates as well as reducing the likelihood of expensive mistakes.

Your Adcom and QS amps have similar power ratings, so it seems that for your purposes speaker sensitivity is not an issue, as long as it is in the area of 90 db or so or higher. If you already haven’t, though, do make sure that the brief dynamic peaks of recordings having wide dynamic range (i.e., large differences in volume between the loudest notes and the softest notes, such as in the case of many classical symphonic recordings) are reproduced cleanly, without noticeable distortion.

What should I notice in those curves? Just that they don’t dip down as you described -- down to... 2 ohms? For how far across the mid bass?

Erik has given you excellent advice re impedance curves, and as he alluded to it’s hard to provide a more specific answer that would have general applicability. But I agree that in this specific case the 936’s impedance curve raises a lot of concern about how optimal it would be for use with tube amps. Although the relatively high damping factor/low output impedance of your QS (for a tube amp) will tend to lessen the degree of adverse impedance interactions with this speaker than would be the case with many other tube amps.

The suggestion of researching what kinds of amplification others have used with specific speakers is of course also a good one, as I’m sure you realize, as is the suggestion of changing the output tap that is being used. Especially if you have been using the 8 ohm tap, which perhaps you have been doing since the speaker has an 8 ohm nominal rating.

On another note altogether:

... as Roseanne Rosanna Danna used to say, never mind.

That was actually said by Gilda Radner’s Emily Litella character. Her Roseanne Roseannadanna character’s famous line (and also the title of a book Gilda wrote, which I believe dealt extensively with her battle with cancer) was of course "it’s always something."

Regards,
-- Al


The Focal isn't crazy low for tubes but it will give them a hard time. You should use the 4 ohm tap with this speaker.
Generally speaking though if you want to get the best out of a tube amp then the speaker should not go much below 8 ohms especially in the bass region! If the amp has output transformers (and most do) its common for the output transformer to lose bandwidth by as much as an octave on the bottom when using the 4 ohm tap (and that's even when its properly loaded with a 4 ohm load). This can cause the amp to have additional phase shift and that is interpreted by the ear as a loss of impact.


So I see two ways out of this if the 4 ohm taps don't do it for you and you want to keep the amp. Either get a set of ZEROs (www.zeroimpedance.com) and see if they can help you out (by allowing you to use a higher tap on the amplifiers) or get a set of speakers that are an easier load.


Its really not a good idea to make an amp work hard for a living. They make a lot less distortion if they have it easy, and that translates directly to 'sounding more real'.