Tube Amp for Martin Logan Speakers
Byfwinne, yes I agree. The internal amp Zout is not changed of course. Re: increased 3rd with nfb: that has been laid to rest many years ago. Most people know the (in)famous Baxandall graph showing increased harmonics (even harmonics that were not there without nfb) with increasing nfb. Then, when you continue to increase nfb, the harmonics eventually became invisible again - all of them. There are two sides to this. One, not everybody realised that the worst you could do in this situation, was to use moderate nfb. For lowest harmonics, in this situation (see below), use none, or use a lot! Second, this was a single, crummy FET stage. It has been shown that if you look at this with a whole amplifier rather than with a single crummy stage, the effect is not seen. If you start with a reasonable amplifier, applying nfb decreases ALL harmonics. |
Bifwynne, if I may, not trying to split any hairs, but the term SOA has a very specific meaning for ss devices. The Safe Operating Area defines the combinations of current and voltage across the device, that it can withstand for a certain time before being destroyed. For instance, a power transistor might be rated for 100W dissipation, but that's not the full story. 10 amps at 10V, being 100W, would be OK indefinitely. BUT, 1 amp at 100V would NOT be OK indefinitely, although it is also 100W. That 1A/100V combination can only be withstood for, say, 100 milliseconds, and then it will let out all the magic smoke*. Now over to amplifiers: they also have an area where they will operate as designed. As you noted, drive the level too high, they will clip. Drive too much level in too low a load and they may eventually overheat and break. One common accident on the test bench is full power at 20kHz - not many amps can hold out on that, and of course during music reproduction, they don't have to. But all this is not referred to as 'the SOA' of the amp. * The reason is that at higher voltages, the current tends to hog a small part of the transistor die, so that small spot will heat up much more that the rest of the die. That small spot cannot handle the 100W the whole transistor could handle. It blows. |
I would like to thank Jan Didden for laying this furfie "NF feedback DOES NOT effect Z=out or damping factor" out in the open, in his usual calm and palpable way. His is a bottomless pit of correct information over at the much more techincal forums, along with the usual others like Nelson Pass, John Curl, and many others, there should be more of them to police bad info on forums. http://www.wordsense.eu/furfie/ Cheers George |
George don't make me blush :-). Anyway. Atmaspere's statement that the INTERNAL Zout doesn't change has another side to it. It suggests that you can open up an amp and point to a component and say see, there's the output impedance. This is not the case. Apart from a few minor things like the resistance of the cabling from PC board to speaker connector, the Zout is sort of virtual. One cause for instance is the fact that the gain of the output transistors drops when you request more output current, so the output voltage drops a bit and that is seen as 'Zout'. As Bitwynne noted, what nfb does is counteract the drop in output voltage so it appears that Zout is now less. But the 'internal Zout' and the Zout with nfb are equally virtual. |
