SS Sound vs. Tube Sound (for the umpteenth time)

[opus111]

Negative feedback is supposed to lower output impedance, but that is an example of a myth. It has no effect on actual output impedance at all.

Does with opamps, so why wouldn't it with power amps? Or is this some special meaning of the word 'actual' ? Like not meaning 'measured' ? Feedback (including positive feedback) does all kinds of wonderful things with measured output impedance, including making it negative.

<edit> I wrote the above before digesting the subsequent text in your post. ISTM you're conflating load driving ability with output impedance. They're not the same at all, quite orthogonal. Output impedance is a small signal parameter.

 

[Soundminded]

We make tube preamps that can drive cables of over 100 feet with ease. Further, all ESLs have widly variable impedance curves- that comes with the territory. But that does not seem to be a problem with tubes so long as the impedance does not get too low (below about 1.5 ohms @ 20KHz). See http://www.atma-sphere.com/Resources/Paradigms_in_Amplifier_Design.php for why.



You might want to look at the link above too. That will make it easier to understand the answer, as the Voltage Paradigm has a different definition for the term 'output impedance' than seems to be used elsewhere in electronics. It is also measured in a different way. In the Power Paradigm, the output impedance is the actual impedance of the output section and FWIW what we measure usually falls within 5-10% of what the formula predicts. So with the S-30 its about 7 ohms, the M-60 about 4 ohms, the MA-1 about 2.3 ohms, the MA-2 about 1.75 ohms and the MA-3 about 0.8 ohms.

Negative feedback is supposed to lower output impedance, but that is an example of a myth. It has no effect on actual output impedance at all.

Here is why: If an amplifier has an output impedance of X, then it can drive a certain load that will be X times Y. If the output impedance is cut in half, the amplifier output impedance would then be 1/2X and could drive a load that is 1/2(X times Y) with the same power. However in reality we find this is not true. If you have a tube amp with an output transformer that can drive an 8 ohm load with 35 watts, if you then connect it using the same tap to a 4 ohm load it will be seen to have less power. If you add feedback the power level will not increase to the level it was with the 8 ohm load, although steady-state distortion may drop.

The only way to get more power into that lower impedance is to have a different tap, or more power tubes, bigger power supplies, more heatsink area, more output transistors etc. There is something called Kirchoff's Law, which is a basic principle of electricity, which cannot be violated and is why you can't simply add feedback to get more power into a lower impedance! Thus we see that the Voltage Paradigm is using a different definition (whether admitted or not) than is seen elsewhere in the electronics industry. I use the word 'Paradigm' as those schooled in its principles will attack anything outside that platform of thought as heretical. We see that all the time in this industry- it is the source of much debate- tubes/transistors, objectivist/subjectivist, etc.

I recommend that you review the maximum power transfer theorem which states that the source will deliver maximum power to the load when the load impedance is equal to the source impedance. BTW, a high output stage source impedance translates into low electrical damping factors, a real problem for woofer/enclosure combinations with an underdamped mechanical resonant peak. It's especially troublesome where the moving mass of the woofer is considerable. Reverse emf from the voice coil drives current back at the source which is not quicky damped out by converting it to heat. Instead it's refelected back and forth between the voice coil and the plate circuit over and over again as a spurious resonance. Most solid state power amplifiers have very low output impedance and therefore very high damping factors. One manufacturer Crown advertised one model that has a damping factor of 20,000 at low frequencies.

 

[Gregadd]

"babbling on", "blahb ah" Are those technical terms?

 

[Gregadd]

Humor is so much better than ridicule.

 

[hvbias]

We make tube preamps that can drive cables of over 100 feet with ease. Further, all ESLs have widly variable impedance curves- that comes with the territory. But that does not seem to be a problem with tubes so long as the impedance does not get too low (below about 1.5 ohms @ 20KHz). See http://www.atma-sphere.com/Resources/Paradigms_in_Amplifier_Design.php for why.



You might want to look at the link above too. That will make it easier to understand the answer, as the Voltage Paradigm has a different definition for the term 'output impedance' than seems to be used elsewhere in electronics. It is also measured in a different way. In the Power Paradigm, the output impedance is the actual impedance of the output section and FWIW what we measure usually falls within 5-10% of what the formula predicts. So with the S-30 its about 7 ohms, the M-60 about 4 ohms, the MA-1 about 2.3 ohms, the MA-2 about 1.75 ohms and the MA-3 about 0.8 ohms.

Negative feedback is supposed to lower output impedance, but that is an example of a myth. It has no effect on actual output impedance at all.

Here is why: If an amplifier has an output impedance of X, then it can drive a certain load that will be X times Y. If the output impedance is cut in half, the amplifier output impedance would then be 1/2X and could drive a load that is 1/2(X times Y) with the same power. However in reality we find this is not true. If you have a tube amp with an output transformer that can drive an 8 ohm load with 35 watts, if you then connect it using the same tap to a 4 ohm load it will be seen to have less power. If you add feedback the power level will not increase to the level it was with the 8 ohm load, although steady-state distortion may drop.

The only way to get more power into that lower impedance is to have a different tap, or more power tubes, bigger power supplies, more heatsink area, more output transistors etc. There is something called Kirchoff's Law, which is a basic principle of electricity, which cannot be violated and is why you can't simply add feedback to get more power into a lower impedance! Thus we see that the Voltage Paradigm is using a different definition (whether admitted or not) than is seen elsewhere in the electronics industry. I use the word 'Paradigm' as those schooled in its principles will attack anything outside that platform of thought as heretical. We see that all the time in this industry- it is the source of much debate- tubes/transistors, objectivist/subjectivist, etc.

Thanks for the explanation Ralph. I will look over those white papers when I have some free time, though my guess is a lot of it will go over my head; my electronics knowledge ends abruptly after the first Morgan Jones tube book. I will see about hearing your OTLs with Sound Labs when I'm reading to buy.