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In Reply to: RE: -And they still haven't- OTLs don't need feedback! posted by Naz on July 24, 2012 at 16:35:22:
Agree that any amp can be built with no global NFB but in practical terms that means more tubes with OTL and most use quite a bit.
Actually this statement is not correct.
There is confusion in the industry about the term 'output impedance'! Take a look at the link below- under the Voltage Paradigm, the term 'output impedance' does not refer to the actual output impedance of the amplifier, it refers to the servo gain of the feedback circuit.
If adding feedback really did lower the output impedance, we could easily drive lower impedance speakers with greater power simply by adding feedback. This would of course violate Kirchhoff's Law, one of the most fundamental laws of electricity. Obviously you can't do that :) The reality is that to get more power, you need more tubes, bigger transformers (or more transistors, bigger heatsinks, etc.) and the like.
So it is obvious that the term 'output impedance' has become conflated with the use of negative feedback but in the real world the two are quite different. Under the Power paradigm, output impedance is the actual impedance of the output circuit, which will be exactly the same whether feedback is applied or not. IOW, this has a lot to do with how much power the amplifier will make and into what load.
To give you a practical example, we have our M-60 which makes 60 watts into 8 ohms using eight (10-watt) power triodes to do it. The amp makes about 40-45 watts into 4 ohms. Now if you add 30db of negative feedback to it, those power levels don't change at all. If the output impedance were indeed lowered, the 4 ohm figure would be seen to rise. But is doesn't, so we know that the output impedance is unchanged.
This is true of all amplifiers and what we can take from this is that there are special charged terms in the Voltage Paradigm that are used in a way that does not exist elsewhere in the electronics industry (and so it should not be surprising that audiophiles and engineers alike are confused as a result); 'output impedance' is one of them ('sensitivity' in loudspeakers is another). However in the world of audio, the Voltage Paradigm is the predominate means used to do test and measurement- that is what you see in the pages of Stereophile and other magazines when amps and speakers are tested.
I am very much convinced that the use of the Voltage Paradigm has set the audio industry back as it seeks to create numbers that look good on paper rather than create specs that relate directly to human hearing/perceptual rules. In seeking to 'look good' (one of the most powerful human motivators BTW) it literally throws the baby out with the bathwater; anyone who has looked at amplifier specs knows what I mean- you can't tell much about how the amp will sound from the specs.
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Follow Ups
- the effect of feedback on output impedance - Ralph 08:09:51 07/25/12 (4)
- RE: the effect of feedback on output impedance - Naz 17:36:16 07/27/12 (3)
- RE: the effect of feedback on output impedance - Ralph 13:34:44 07/30/12 (2)
- 7242 for higher mu? - Lew 12:48:49 07/31/12 (1)
- RE: 7242 for higher mu? - Ralph 13:03:52 07/31/12 (0)
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