I'm a noob to the world of DIY home audio, so please forgive any ignorance on my part.
From what I understand the SET amps are popular because the distortion adds to the pleasant sound, even though they are low wattage devices.
Knowing that most speaker blowout is caused from low wattage amps distorting the signal, what is the likelihood of speaker blowout on SET amps? And is there a way to determine a "red zone" for speakers when it comes to harmonic distortion and wattage?
Huh? No, speakers blow from exceeding ratings (note that most speakers rated "RMS" are damned if they can handle even half that, in real watts!).
You could add a circuit to detect when the output tube's cathode current (or grid voltage) starts drifting, coincident with drive signal peak coming near
0V (when the grid goes positive, it pulls current from the cathode and a DC voltage appears on the grid leak resistor, shifting bias). Or a peak detector when grid voltage peak approaches say, -5 to -1V with respect to cathode.
But that's useless, because it'll sound like shit by the time you reach that anyway. Consider music that has a dynamic range of 20dB: that means, from an average low level of say 0.1W, you have peaks up to 10W. This might barely be acceptable on a 5W amp, since the peaks are getting clipped only
3dB, for a dynamic range (at the output) of 17dB. If you want to play it twice as loud, you just crunched your headroom to 7dB and it *will* sound nasty (especially on bass, which needs a lot of power to register the same intensity as midrange, and especially on peaks, which need a lot of power for only a short duration).
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The "RMS" rating, which is wrongly named, usually refers to the correct average heating power rating of the speaker. Most "Pro Audio" speaker/cabinets, i.e. the ones used by live bands, from reputable manufactures, will handle their "rms" rating with continuous white noise and most music signals. If the amp does not clip, then the average average power outputed by the amp will actually be less than its rated output.
Kevin Aylward firstname.lastname@example.org
SuperSpice, a very affordable Mixed-Mode Windows Simulator with Schematic Capture, Waveform Display, FFT's and Filter Design.
The speaker blowout you are referring to from low wattage amps is where a clipped signal has so much harmonic content that it overdrives the tweeters. This is (or used to be) a problem in disco houses that tried to get away with using consumer speakers. (Consumer grade speakers typically have a tweeter power rating that is much lower than the woofer rating, since most music has much less energy at the high frequencies.)
But the "coloration" distortion from a single-ended triode (when not driven into clipping) is typically much milder, no danger to tweeters under anything remotely like normal listening.. Added to that, most of these amps are only capable of a few watts output anyway. You'd have a hard time blowing anything but the cheesiest speakers!
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