Serious stereo tube amp questions.

The Chinese tube amps you are citing are mere toys.

Ornaments or conversation pieces for oyur desk or mantelpiece.

..... Phil

It is determined by how much power goes to the speakers. That, in turn, is usually dependent on the voltage the amp is running on, by ohms law. Power (watts) is the amps times the volts. If you have 8 ohm speakers and are pushing one amp through them you have 8 watts of power and are using 8 volts to do it. Really high power transistor amps use power supplies capable of 200 volts and can produce a few thousand watts at the speakers.

The voltage a tube amp runs on is not as important because you generally have an output transformer and aren't driving speakers directly. But the power is still important and it still takes more iron, copper and money to produce more power in conventional tube amps.

Class A produces heat because when the amp is just sitting there with the sound muted it is still dissipating the full power through the tubes or transistors, class AB is static power is determined by the bias point (how much current is allowed to flow with no sound out - higher bias = more heat and better sound due to less cross-over distortion)

Everything gets larger in conventional (linear) amplifiers as power increases. More copper, more iron, bigger filter caps, more money. At some point, if you really like music and have golden ears, the money is better spent developing a good music room rather than shelling out more for amps. There is such a thing as a law of diminishing returns. For $50 you can build a pretty decent amp but to make it sound significantly better might take $500....

We had some audio power amps in the navy that were used to drive a deflection yoke in a direction finding display. Old tube technology. The amp used direct drive and had a whole chassis of nothing but big tubes with graphite plates running cherry red and producing huge amounts of heat to push the relatively low voltage through the deflection coils. It was all done to produce a very low distortion signal so the display would show a perfect circular light trace on the screen. Any deviation from perfection resulted in a DF bearing being off by a degree or two. (which translates into miles of error)

They were replaced with solid state devices and better technology.

I bought a couple of the 5 watt or so switching amps form China to play with. Put them on a scope and it was interisting. Instead of seeing a sine wave, it looked like a bunch of pulses. It sounded ok to my untrained ear for voice quality.

I remember a 555 based class D amplifier in Elementary Electronicsi n the mid-seventies. And it relied on the speaker, and the ear, to do the filtering. So of course on a scope it would look digital, but might sound 'fine".

I've not paid enough attention to more serious work to know how much relies on the speaker to do the filtering and how much is doen by deliberate filters.

Michael

Sometimes it takes some magic to get a good reading with a scope and digital sound source. A ground loop or sometimes just the way the amp is laid out and shielded will allow switching transients through... and other times it is just not filtered all that well, particularly those elcheapo Chinese amps where most of the design effort is in marketing bling rather than quality components like low ESR caps and suitable magnetic components.

Thanks for the replies. The amps I have been considering are all class A amps. I am not interested in hybrid amps. And even though I listen to MP3 players every day and will be playing an MP3 player through whatever amp I get I still want a purely analog amp. Eric

you can trim those: