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

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

@djones51, not sure what I can add in response to your questions other than the generalities I stated at the end of my previous post. Basically, the more severe the clipping is the more likely it is to be noticeable, and the more likely it is to cause damage.

Regards,
-- Al

@nakam @djones51

When the amp clips the current doesn’t alternate but behaves like DC which the capacitor in the crossover will pass, especially bad for tweeters

While I of course agree that overdriving an amp to the point of clipping, or at least severe clipping, can and will often damage tweeters, and while many audiophiles apparently believe the quoted explanation, that explanation is not correct.

Capacitors are commonly used to **block** DC. Consider for example the coupling capacitors that are used at the outputs of many tube-based preamps or other line-level components, or that are used to couple signals between active stages within those components. They are there to block DC, perhaps among other reasons depending on the particular design. Also, the impedance presented by a capacitor **increases** as frequency decreases, and DC is zero Hertz.

Following is an explanation of why a clipped amp can damage tweeters, quoted from a post I made in a thread back in 2011:

The reason that speakers are commonly damaged by clipping of underpowered amplifiers is essentially as follows: Clipping occurs when the amp is asked to deliver a larger voltage swing at any instant of time than it is capable of delivering. That will usually occur at bass or mid-bass frequencies, where music typically contains its highest energy levels. Clipping means that the positive and negative peaks of what would normally be a smoothly varying waveform will abruptly transition to a flat, essentially constant output level corresponding to the maximum positive and negative voltages that the amp is capable of delivering.

The ABRUPTNESS of those transition points corresponds to high frequency spectral components being present in the output signal that are not present in the original waveform. In other words, the clipped waveform contains excess high frequency energy, which the speaker’s crossover will duly route to the tweeter. That can occur even if there is no high frequency energy at all in the original signal, as a result of the clipping of bass frequencies. Tweeters are ordinarily able to handle much less energy than lower frequency drivers, and can therefore be damaged by that excess energy.

Solid state amplifiers will typically clip more abruptly than tube amplifiers, therefore making them more likely to cause that kind of damage. Although if a tube amp is clipped severely enough, the same damage can result.

Severe clipping will be immediately obvious, because the sound will be horribly distorted. The onset of clipping may be characterized by mild distortion or slight popping sounds on musical peaks. I doubt that occasional mild clipping would cause any damage.

@nakam, with regard to your specific question note particularly the last paragraph above.

Regards,
-- Al

Again, David, keep in mind that the QS’s damping factor is higher than it is for many and I’m pretty certain most high quality tube amps. Assuming the spec is accurate, for the 4 ohm tap a damping factor of 20 means an output impedance of 4/20 = 0.2 ohms, not too far from solid state territory.

Although many audiophiles believe differently, Ralph ( @atmasphere ) and some other technically knowledgeable members (e.g. @kijanki) have explained in past threads that no speaker in existence requires a damping factor greater than the mid to high double digits, and perhaps even lower. Perceived differences in bass response and control between damping factors of say 100 and 2000 are due to other differences in the designs of the amplifiers, IMO and theirs.

Best regards,
-- Al

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