Sorry I got to this late.
It seems there are some things that need clarifying. If I am not beating a dead horse too much...
Bifwynne, by adding negative feedback to an amplifier you do indeed move the amp towards the Voltage Paradigm. This is because as you surmised the 'output impedance' is lowered. I paraphrased the term because it is misunderstood however, and that is where the clarification comes in. Unsound, you might want to pay attention to this.
Under the Voltage Paradigm, you have the term 'output impedance'. The term has a definition which is not intuitive. It refers to the amount of servo gain that the amplifier has which allows it to react to a load. It does not refer to the actual output impedance of the amp, as measured by any other field of endeavor in the world of electronics.
How can we know this? The answer is simple. If the output impedance were indeed lowered, the amp could drive a progressively lower and lower impedance. It might even make more power. But we see by adding feedback to an amplifier that the output power into lower impedances does not change.
IOW, what is happening is that the feedback gives the amp the ability to adapt to its load within certain limits by taking samples of its performance and using that as an error correction. The only way you can really get a lower output impedance is with bigger output transformers, more tubes or more transistors. The extra ability to drive a lower impedance does not come out of thin air or feedback- to do so would violate a fundamental rule of electronics known as Kirchoff's Law.
The difference between the Voltage and Power Paradigms has more to do with feedback then tubes/transistors.
With regards to Current Source, Power Source and Voltage Source:
If the amp has a lot of voltage feedback it could become a Voltage Source. Most Transistor amps attempt to be Voltage Sources (and most of them run a lot of feedback). What this means is given a sine wave input signal, and a load of variable impedance (like a typical box speaker), the output voltage will always be the same. Note that with this model (the Voltage Paradigm) the *current* will vary as the load impedance varies. This is how such an amp can double power as impedance is cut in half, as with half the impedance the current will be double.
Under the Power Paradigm the amplifier will attempt to make constant power into any load rather than constant voltage (there is a little math of note here, Power = Voltage X Current). IOW a Power Source. This is the behavior of a tube amp without voltage feedback. Depending on the load variation, both output voltage and output current will vary.
Under the Power Paradigm, the output impedance of the amplifier is in fact the actual impedance of the output section of the amp.
An amplifier can also be a Current Source, although there has never been a paradigm of design, test and measurement that has evolved around it. This type of operation is done by using current feedback rather than voltage feedback. Constant Current amps tend to have a very high output impedance by any measure, often well above 50 ohms. This does not mean that they cannot work with conventional speakers. Nelson Pass as done a lot of work in this area in recent years.
It seems there are some things that need clarifying. If I am not beating a dead horse too much...
Bifwynne, by adding negative feedback to an amplifier you do indeed move the amp towards the Voltage Paradigm. This is because as you surmised the 'output impedance' is lowered. I paraphrased the term because it is misunderstood however, and that is where the clarification comes in. Unsound, you might want to pay attention to this.
Under the Voltage Paradigm, you have the term 'output impedance'. The term has a definition which is not intuitive. It refers to the amount of servo gain that the amplifier has which allows it to react to a load. It does not refer to the actual output impedance of the amp, as measured by any other field of endeavor in the world of electronics.
How can we know this? The answer is simple. If the output impedance were indeed lowered, the amp could drive a progressively lower and lower impedance. It might even make more power. But we see by adding feedback to an amplifier that the output power into lower impedances does not change.
IOW, what is happening is that the feedback gives the amp the ability to adapt to its load within certain limits by taking samples of its performance and using that as an error correction. The only way you can really get a lower output impedance is with bigger output transformers, more tubes or more transistors. The extra ability to drive a lower impedance does not come out of thin air or feedback- to do so would violate a fundamental rule of electronics known as Kirchoff's Law.
The difference between the Voltage and Power Paradigms has more to do with feedback then tubes/transistors.
With regards to Current Source, Power Source and Voltage Source:
If the amp has a lot of voltage feedback it could become a Voltage Source. Most Transistor amps attempt to be Voltage Sources (and most of them run a lot of feedback). What this means is given a sine wave input signal, and a load of variable impedance (like a typical box speaker), the output voltage will always be the same. Note that with this model (the Voltage Paradigm) the *current* will vary as the load impedance varies. This is how such an amp can double power as impedance is cut in half, as with half the impedance the current will be double.
Under the Power Paradigm the amplifier will attempt to make constant power into any load rather than constant voltage (there is a little math of note here, Power = Voltage X Current). IOW a Power Source. This is the behavior of a tube amp without voltage feedback. Depending on the load variation, both output voltage and output current will vary.
Under the Power Paradigm, the output impedance of the amplifier is in fact the actual impedance of the output section of the amp.
An amplifier can also be a Current Source, although there has never been a paradigm of design, test and measurement that has evolved around it. This type of operation is done by using current feedback rather than voltage feedback. Constant Current amps tend to have a very high output impedance by any measure, often well above 50 ohms. This does not mean that they cannot work with conventional speakers. Nelson Pass as done a lot of work in this area in recent years.