Power output of tube amps compared to solid states

Atmasphere wrote: "What tubes bring to the table is the ability to build a low-distortion amplifier without loop feedback. With no loop feedback, time-domain distortions are 100% eliminated. With feedback, time-domain distortions become the name of the game."

My understanding is that the reason time-domain distortions are of audible significance has to do with the human auditory system. The ear has a characterstic called "masking" by which it ignores a low-level signal that is near (in frequency) to a high-level signal. Audio data compression algorithms (such as MP-3) take advantage of this and simply omit signals that would likely be "masked".

Masking works great in the frequency domain, but guess what - it fails miserably in the time domain! Unless the loud and soft signal happen at exactly the same time, the soft signal is not masked. Distortions that arrive slightly later in time, even if they are much lower in amplitidue, are far more audible than the same distortion which arrives simultaneously with a masking signal.

Duke

Unsound, amplifier/speaker synergy can definitely be used to advantage. This is how it's done with a high output impedance amp: The speaker designer uses impedance peaks to get the amp to deliver more power where he wants it. We almost always see impedance peaks in the bass region, so by playing with the enclosures's tuning frequency the designer can use those impedance peaks to extend the bass deeper than it otherwise would have gone. However if the amp's output impedance is too high, the bass will boom no matter what the tuning - so there is an "optimum" for a given speaker.

The reason this type of amp doesn't give good results with all speakers involves more than just the bass region. The speaker's impedance curve usually has peaks and valleys above the bass region, and a high output impedance (or current-source approximating) amp will tend to deliver more power into the peaks and less power into the valleys. A low output impedance (voltage source approximating) amp does the opposite. If a speaker has a smooth impedance curve above the bass region it can work well with both types, provided the bass tuning is adjusted accordingly. With Ralph's S-30, most of my speakers will exhibit roughly one-third to one-half octave greater bass extension than with a solid state amp, but I have to change the tuning frequency. That extra bass is pretty much a "free lunch". In practice I would say Ralph's amps are closer to a "constant-power source" rather than a "constant-current source", but that's still different enough from "constant voltage" to present unique challenges and opportunites for the speaker designer.

Now a designer can also take advantage of the "free lunch" to be had from a solid state amp, by dropping the impedance in the region where he needs more output. In that case, I'd parallel a second woofer in the bass region to drop the nominal impedance to 4 ohms, increasing the amp's output in that region. That calls for a second woofer and a larger enclosure, so it's maybe not as much of a "free lunch" as the first case.

Duke

My understanding is that a woofer's electrical damping is described by the electrical Q (Qes) and the mechanical damping is described by the mechanical Q (Qms). The Qes value given in a spec sheet assumes a true voltage source amplifier, and in practice is raised by the amplifier's output impedance. I think the equation is Qes' = Qes x (Re+Rampout)/Re, where Re = the voice coil's DC resistance and Rampout = the amplifier's output impedance.

Much as I like prosound woofers, most of them have too much electrical damping (too low Qes) to give good bass extension with a low output impedance (high damping factor) amplifier. In other words, most prosound woofers are overdamped for home audio usage when used with solid state amps.

Duke

Kirkus wrote:

"The problem with increasing the amplifier source impedance for this type of driver [prosound woofer] (especially in a domestic application) is that while this raises the Q and the bass output, it leaves the resonant frequency unaffected . . . the result is then simply wooly, boomy mid-bass and no increase in bass extension."

With any speaker system designed to be "flat" with a voltage-source amp, there will be a bump in the bass region when that speaker is used with an amp that has a high source impedance. This is not uniquely limited to prosound woofers speakers. In many cases lowering the port tuning frequency will smoothe out the bump while extending the bass.

Pubul57, I'd rather go offline with that to avoid taking the thread on a commercial tangent; I'll shoot you an e-mail.

Duke