My comments in italics
Impedance matching has no application in low frequency transmission (i.e. audio frequencies).
Impedance matching refers to output impedance = load impedance.
i'm aware of that, thats what I was inferring when using RF terms, Erno Borbely often used terminated lines in his audio amps too including using BNC connectors, but this was probably more about being a perfectionist than anything. the effects are still there, but due to the reflections being less of an issue with audio its hardly worth bothering about IMO
sounds like "Ernie" uses this between the pre-amp and the power amp? Since we are not worried about power efficiency between pre-amp and power amp I guess we can let Erno be a perfectionist!
I got curious and pulled out one of my reference textbooks: "Ben Duncan" High Performance Audio Power Amplifiers. Ben says that at audio frequncies and at reasonable cable lengths, cables look like the respective sums of resistance, inductance and capacitance. As he points out, most loudspeakers are looking for a low output impedance voltage source (the power amp) to drive them.
In a power amp you want output impedance to be much lower than load impedance for most efficient power transfer and accurate voltage transfer.
In actual practice it is impossible to build an amp without feedback, local feedback is inherent to all transistors and tubes, for example, without feedback a tube or transistor would have infinite bandwidth.
I agree, degeneration is Local Feedback.
I'm sure the "no feedback in this amp" thing is another usless marketing term.
Anyone have any examples of a "No Feedback" amp?
yeah i'm doubtful too, but I dont know much about tubes at all, so I didnt rule it out and just said 'very very rare' as in i've never seen one, it would seem to me to be a ticking time bomb if it did exist. I was under the impression that in general tubes were very wide band devices, so would have to have some at all times
I'm really not an expert in tube power amps either, but I know more than a little about solid state amps.
I think I have to eat my words, Ben Duncan refers to single ended, transformer coupled amps running in Class A with low "overall" (global) feedback I guess the next step may be NO global feedback. He also comments on Class A amps without feedback..
I agree, in general tubes are very wide bandwidth. In tube power amps the bandwidth is limited by the output transformer. Of course there are guys like Julius Futterman and his vacuum tube OTL power amps.
Generally speaking, loudspeakers are designed to be driven by a voltage source.
The voltage is applied to the speaker, which causes current to flow, the change in current creates a varying magnetic field, which makes the voice coil move.
Generally speaking yes, but they operate in the current domain and there is definitely a section of audio using current buffers to drive horn speakers. not to my taste but its definitely a reality and I respect Nelson and other engineers at DIYA who are playing with it and swear by it enough to think there must be something to it, just another flavor I guess
I agree, Nelson knows his amps!
Back to my Ben Duncan textbook!
"Ben says" loudspeakers are normally designed to be driven by a low impedance voltage source.
What's up with the current sources? Can you send me a link? Is this a current buffer between an SET and a loudspeaker to make the SET look like a high output current source to the speaker? Is it a solid state amp with a voltage gain of 1?
OK, lemme try again:
match tube output impedance to transformer primary impedance
Transformer output impedance (secondary impedance) is less than load impedance.
hes talking about current transfer not voltage transfer as I understand it
I think I may have (finally!) figured it out, we were discussing this from different points of view.
Since tubes output high voltage & low current, then we use a transformer to step the voltage down and step the current up to a drive the loudspeaker.
At the output tubes, the transformer shows the tubes a load of (for example) 2,000 ohms or so. Or you could say the transformer turns an 8 ohm load into a 2,000 ohm load.
From the loudspeakers point of view (and from the point of view of a power engineer like me) the loudspeaker is driven by a fairly low impedance source. From the loudspeakers point of view the loudspeaker is driven by a voltage source with a fairly low output impedance. Generally speaking, transfomers usually have a source impedance between 5-10% of the load impedance. So lets say maybe 0.8 ohms for an 8 ohm tap.
The contradictions are ironed out............I think. So it is optimizing voltage and current, and therefore power transfer.
Originally Posted by qusp
aha, so you do mean global feedback and the cathode resistor is the degeneration. same thing. so no such thing as no feedback. without some form of feedback the device would not have any..well..feedback.. or reference to be able to refer its adjustments to
I agree, it's the same thing.
As I define it, or as the standard definition goes.......Global Feedback is often called Loop Feedback, from output back to input of the complete power amp (or preamp or whatever).
The cathode resistor his friend Jack used is often called degeneration and often referred to as Local feedback.
I have a vacuum tube pre-amp which has no loop feedback and has no degeneration at Cathode. It is the Counterpoint SA-5.1. But since it does not have infinite bandwidth, there is parasitic capaciance between grid and plate limiting it's bandwidth. The parasitic capacitance is the feedback. I have the schematic if you are interested.
So no such thing as "no feedback". It just may not be apparent.
Edited by Chris J - 4/11/12 at 5:50pm