Edgar

I do not. Some resistors of a few hundred ohms, permanently attached to the tube amp outputs, are often used as protection against accidental open-circuit operation. But basically, "Don't do that."

Search for "audio amplifier switch box". Just remember that some amplifiers cannot tolerate their (-) outputs being connected together, others cannot tolerate them being connected to ground. EDIT: And tube amps cannot tolerate open-circuit outputs.

It's all about context, and I can lose sight of that as easily as anyone, I guess.

While differences in the sounds of amplifiers tend to be very subtle, one area where those differences are occasionally very apparent is in sibilance. I have used perfectly good amplifiers to drive perfectly good loudspeakers, and found the sibilance to be so pronounced as to be painful. Mate that amp with a different speaker, or that speaker with a different amp, and the sound was fine. Some amps just don't work well with some speakers, and you can't know about it until you try.

It occurs to me that the problem may be in the definition of terms. I am an engineer who specializes in signal processing. So I think of AC and DC in terms of the associated mathematics: a DC signal contains only components at 0 Hz, an AC signal contains at least one component that is not 0 Hz. That may not align with the popular definitions, where DC means that the signal is either always positive or always negative, and AC means that it crosses 0V at least sometimes.

Yes, it's AC with a DC offset.

Perhaps a thought experiment will help. Suppose you have a signal that switches from -1V to +1V, then a half second later switches from +1V to -1V, then a half second after that it switches again from -1V to +1V, and so on forever. Is that DC or AC? Easy -- it's a 1 Hz square wave, so it's AC. Now change the frequency so that instead of switching once every half second, it switches once every 50 years. Is that DC or AC? Also easy -- it's a square wave with a period of 100 years. Now change the frequency so that instead of switching once every 50 years, it switches once every lifetime of the universe. You may witness one of the switches, but you'll never witness the following switch back. But it's still AC! The point is, once the level switches at least once, it's no longer DC. It doesn't matter how long it takes to switch back (if ever), it's still AC.

What you describe is an impedance problem, but it's not because of a mismatch per se. In most line-level audio cases, it is desired to have as low an output impedance as possible driving as high an input impedance as possible. Ideally the output impedance is zero and the input impedance is infinite, so in that case a huge impedance mismatch is a good thing. By comparison, in some telephone systems a 600Ω impedance match is maintained for maximum power transfer. In home audio we don't have to worry about that.

Buffering and impedance matching are two different things. But yes, buffering can be an important function of a preamp. If a device has a high output impedance, and it's trying to drive a low input impedance (especially one that is not purely resistive), then problems will occur. Those problems may range from low signal level to poor frequency response to increased noise. A high impedance buffer will solve all of them. The buffer may alter the sound somewhat, but in many cases the tradeoff will be worth it.

Don't fret. I've never been invited, either. Seriously, they're good speakers, and as good speakers, they're fine for both of the situations that you mentioned, and everything in-between.

I understand that people also like to set drinks on top of them at parties. 😠

Here in the USA we have this. I don't know if an equivalent is available in Germany.

I think we lost the last one when my dad died three three years ago. All of his equipment sits in my basement, in boxes. I hope someday to fire it up and see what's out there, but even he complained that the airwaves have been silent for many years. It's so frustrating because I have the service manual and know how to use the test equipment ... I just don't have the test equipment and I don't know anyone who does. So a truly fine tuner sits in a box, refurbished electronically and cosmetically but out of alignment, and there is very little that I can do about it. It was the first piece of high-end gear that I bought, way back in 1979. I was still in college, and FM was still good.

Does anybody have experience and/or opinions about the Sangean HDT-20 tuner? I'm most interested in conventional FM reception at 40-45 mile range. I'll be using a Stellar Labs 30-2460 antenna mounted in the attic at about 10' AGL. I also have a Yamaha T-1 tuner that is superb, but in need of realignment. The alignment would cost me about as much as the Sangean, and in the end I would still have a 43 year old tuner. However, if anyone has recommendations for someone who can perform that alignment, I'd appreciate that, too. I could do it myself if I had the test equipment, but I don't. Thanks.

Well, the analogy does break down at some point. It is pretty good for visualization, though.

Well I've been from Tucson to Tucumcari, Tehachapi to Tonopah. Driven every kind of rig that's ever been made ... Local weather says it's headed my way next.

I guess in Class A push-pull two runners hold the same baton for the entire race.

Joseph Fourier would disagree. A step function has infinite frequency content.

There can be Class AB push-pull amps, and there can also be Class A push-pull amps.

Well, that's a bit of an oversimplification. Ideally, one device will amplify the positive half of the waveform, and nothing but the positive half. Another will amplify the negative half and nothing but the negative half. It is probably obvious that this would require one device to turn off instantly and the other to turn on instantly, which is basically impossible in the real world. So in Class AB there is a small region of overlap where both devices are amplifying; one is in the process of turning off while the other is in the process of turning on. This region where both are operating is often referred to as "operating in Class A", but as I said, that's an oversimplification. Yes. Again, in this region one is turning on and the other is turning off, so some distortion can result. Sort of, see above. If you have efficient speakers and you want to operate in Class A, then use a low-power Class A amplifier, IMHO.

You can find generalizations about all of the classes just by searching. Like anything else, the magic is as much in the execution as in the design. One advantage of Class A is that the amplifying device(s) never turn off, so there is no anomaly associated with switching on and off. One disadvantage is that Class A dissipates a tremendous amount of energy as heat. Don't even bother with Class B. It is intentionally nonlinear, and is seldom if ever used for high fidelity audio. One advantage of Class AB is that it is far more efficient than Class A. One disadvantage is that it can suffer from crossover distortion when the device(s) switch on and off. One advantage of Class D is extreme efficiency. One disadvantage is that an inherently nonlinear process is linearized (approximately) by passive filters. Those filters interact with the loudspeaker impedance in ways that are not always agreeable. Which one sounds best? It all depends upon the design, the implementation, and the application. All of the advantages listed above can be ruined with a bad design. All of the disadvantages can be minimized with a good design.

Yes.

DC isn't DC if it changes. Going from 0 Volts to 12 Volts is a change.

That is an ill-posed question. Is a Jeep inherently inferior to a Ferrari? If you're going to the racetrack, then yes. But if you're navigating the Rubicon Trail, then the answer changes. Each topology has its advantages and disadvantages. If one topology was the clear winner in all categories, then the others would have disappeared by now.

That may be the most beautiful Klipschorn I have ever seen.