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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.


hilde45

Showing 3 responses by atmasphere

atmasphere's avatar
atmasphere
12,099 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. 
atmasphere's avatar
atmasphere
12,099 posts
 
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.
@hilde45 This quote from VTL is largely false. The diameter of the driver has **nothing whatsoever** to do with the ability of the amplifier to ’control’ it (although I will concede this is a popular myth)! What **does** matter is the relationship between the amp and driver.


To give you can example, my speakers at home employ dual 15" woofers, which by most accounts are large drivers. Yet they are easily controlled by our little S-30 amplifier which has a very low damping factor. This is because the designer of the speaker prefers smaller tube amps, and our S-30 has full power bandwidth to 2Hz. The size of the drivers (TAD 1602s for anyone wondering) is irrelevant. That same manufacturer (Classic Audio Loudspeakers) features an 18" driver in their next model up, yet no worries controlling it with the same amp! In fact, our MP-1 **preamp** by most accounts would seem to have an output impedance too high and power too low to be able to drive a loudspeaker, but it does more than a passable job on both of these speakers as long as you don’t need more than a conversational volume level- and the bass is just fine!

So don’t be fooled by the high damping thing- it isn’t real.

Well, it seems that audiokinesis put me in my place. Hey guys, it's just been my experience that woofers are quicker and tighter with a higher damping factor amp.

'Tight bass' is a coloration that does not seem to exist in the real world. You only hear it or hear about it when dealing with audio reproduction. Admittedly, its popular, but probably in the same way that turning up the bass tone control can be pleasing- pleasing, but not natural and not neutral.
atmasphere's avatar
atmasphere
12,099 posts
 
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'.
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