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What Measurements can we hear? Can we measure sound quality?


Ar9Jim

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With internal tube resistance being hundreds to thousands of ohms, there's simply no way a tube can be a better voltage source than a transistor, whose ability to deliver current is at least two orders of magnitude greater than that of any common audio output tube.

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18 hours ago, jvandyke_texas said:

Oh, the voltage source amplifier has been practically attained for 45 years.  We measure distortion of (Vout/Vin) and it's very small.

But is that design what we want for the speakers we have?

 

I think we're mixing practical application with theory...such is to be expected in a discussion like this.

 

Voltage amplifiers are very successful, there is no question. But to say that pure voltage source is ideal for a loudspeaker, or even better than tube amps in general, is a theoretical stretch. The tube amp design has characteristics that are difficult to duplicate with solid state and vice versa, however, good design can bridge the two, and that is really the focus of my thought in all this. Designing a system to faithfully drive the total mess loudspeakers are, is an incredible challenge. It's one thing to test into dummy loads - quiet another when driving transducers.

 

16 hours ago, basspig said:

My experience with seeking out amplifiers for reproducing low frequencies has taught me that for power efficiency and woofer control with exceedingly low distortion, a transistor amp is the way to go. Nowadays, I run industrial amplifiers which can work all day into 1Ω loads without strain and damping factors in excess of 2000. And I do get Nelson's point about overdamping. I did lose some bass by going to the much higher gauge wire. But the bass I have now is more accurate without the boom.

 

Subwoofers benefit greatly from low output impedance, largely due to their back EMF which can not only wreck havoc on an output stage, but also ring and distort against a higher output impendance. Large voice coils require an immense amount of total energy, especially in light of the fact that they are very inefficient for a transducer. Solid state is the way to go, but the amp must be formidable with and output stage ready to take on a beating. Don't get me wrong, I love solid state amplifiers. However, when I listen to many non-Carver amps I feel something is always missing...the mid range and top end can get so smeared and tiring. Tube amps just don't ever tend to sound quite like that. And that's what got me thinking...

 

 

16 hours ago, basspig said:

If I were to try to recreate my present system with vacuum tubes, I would need 15 tons of air conditioning capacity and a direct feed from Niagara Falls. I simply could not even afford to turn on the system for one hour.

 

Nah - all you need are good transfer function generators on your SS tower of glory.

 

 

11 hours ago, jvandyke_texas said:

Rereading Greg's posts, I understand them and now agree with much of them.  Yes, inside the SS amp, it's not a good voltage source, does not have low enough impedance, and has efficiency issues.  Improvement on these problems has been the subject of amp development for the past 50 years.  However, with negative feedback, outside the black box amp, the load has the illusion of being driven with a near zero impedance source.  That is, until the internals of the amp operate out of the linear region, and can no longer respond to feedback to keep distortion low.  Then it is not longer an amp.

 

That's a good distillation of my thought process. Solid state amps get very complicated, and it is all due to the problem of efficiency and maintaining fidelity over a wide range of load parameters ( I often tell people 10% of the amp does amp stuff, the other 90% of the components are there to make sure the 10% behave). Negative feedback has its limits, and loudspeakers can bully it substantially. Moreover, transistor output impedance, even with negative feedback, modulates with the frequency (think transient response and IMD) - it's just not a good relationship. Even good design makes compromises here.  However, as Bob discovered, using current and voltage feedback, you suddenly peek into a transconductance amplifier, which is the true ideal to strive for.

 

Obviously, there are many designs that work very well - no reason to bring all the products up here though. 

 

The elephant in the room is that voice coils are a current controlled device...and so voltage sources become challenged by the laws of physics, which care not of our feelings. ?

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2 hours ago, Rod H said:

Well this has certainly drifted off-topic. 

 

Almost, but not yet.

 

The conclusion is you can't measure sound quality. However, you can measure, and predict, sound fidelity.

 

Sound 'quality' is subjective. Sound fidelity is objective.

 

Both fidelity and quality are attributes that power amps exhibit with various degrees at the same time. So...which amps 'sound the best'? We're just deep in the weeds about what amps are predictably faithful, and what topologies are known to have good qualities...

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