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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.


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almarg's avatar
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almarg
9,670 posts
 
To add to my previous posts on this subject, an illustration of what a clipped waveform looks like:

https://www.mtx.com/i/mtxcom/clipped-signal.jpg

As you can see it alternates (going both positive and negative), as does the original signal of course. So it is not DC, which the series capacitor that is commonly part of the high-pass section of a speaker’s crossover network would block from reaching the tweeter even if it were.

+Vcc and -Vcc shown in the figure represent the maximum positive and negative output voltages the amp can provide into the particular speaker load.

Also, note the sharp discontinuity between the smoothly rising or falling parts of the waveform and the flat top and bottom. The frequency components corresponding to that sharp discontinuity contain the excess high frequency energy I referred to. (Examining the clipped waveform with a spectrum analyzer, which indicates the various frequency components of the signal, would show that). Most of that excess energy will tend to be routed to the tweeter by the crossover network, since it is at high frequencies.

Regards,
-- Al

ieales's avatar
ieales
1,201 posts
 
@djones51

A square wave is an infinite series of odd harmonics above the fundamental. When an amp clips, the waveform stops at the power supply limit and creates an odd multiple series of the clipped fundamental. Below is an abreviated list of 200Hz odd harmonics to 20kHz. They continue forever. As you can see, they cover all driver ranges. Note that if only a portion of the wave is clipped, that period is the fundamental and the odd harmonic progression starts there. 1% clipping @ 200Hz starts @ 20kHz and continues upward. On a 200wpc amp, that’s 200w into the tweeter for a few µs. 200Hz 10% clipping starts @ 2kHz and supplies full power for a dozen plus full power harmonics continuing to the amplifier bandwidth.

H# Hz
003 600
005 1000
007 1400
009 1800
011 2200
...
091 18200
093 18600
095 19000
097 19400
099 19800
101 20200

Music has an approximately 50% power ratio per octave and acoustic instrument harmonics are only a fraction of the fundamental, so there is very little power actually sent to the HF driver. [Electronic instruments, buzz guitar and heavily Eq’d program can have a grossly skewed driver eating power spectrum.]

Hz   Pwr %
200 100%
400 50%
800 25%
1600 13%
3200 6%
6400 3%
12800 2%
25600 1%

When a tube amp distorts, there is a fair amount of lower even [2nd & 4th] harmonic distortion which is a good indication the amp is losing control and gives fair warning. A good SS amp with robust power supply gives no warning and dumps the full spectrum willingly. By the time user hears it, it can be too late for the HF driver.

Hence a low power SS amp into inefficient speakers is more of a risk.
twoleftears's avatar
twoleftears
5,318 posts
 

Yes, the Nighthawk's specs are 20 watts per channel: (8Ω, 1% THD).

If you follow John Atkinson's measurements in Stereophile, he often has to allow higher levels of distortion, e.g. 3%, to get a tested tube amp to achieve its manufacturer-specified output.  The amp will put out more than 20W but as almarg says with increasing levels of distortion.

atmasphere's avatar
atmasphere
11,901 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. 
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