Yes, the intrinsic emitter resistance does constitute local feedback, but I was trying not to get too technical.
Ok, speaking of emitters and such, the development of "ring emitter" transistors lead to a radical change in transistor design. Back when I started getting serious about amp design, you had 2 choices: RCA and Motorola. RCA (for whatever reason) did not make high-power PNP devices. Motorola did. With those, we had amps like the SWTP "Tiger" series. A bit unstable, but probably the first step towards modern amp design. Even then, the transistors were made with diffused processes, and were not the most rugged in the world. Eventually, they learned how to make epitaxial processes, and things started to take off. Some firms, Bedini as example, stuck to using only NPN devices in the outputs.(The RCA approach.) But most everyone else went to complementary devices. However, out of that grew the 0.000001% THD wars, and the resultant bad sound.
(Looking back......in hindsight......there may not be a convincing reason to use complementary devices in closed loop amps. Remember, the little 20 watt Bedini did sound good.)
I suspect that the guys who came up with the ring-emitter concept were probably used to designing RF transistors. Sanken, Toshiba, and Fujitsu all had strong contenders. Linear, fairly rugged, and perhaps most important: low capacitance. This allowed designers to push the bandwidth higher, as we were all concerned with TIM, SID, and a host of other "new" mechanisms that we were becoming convinced explained why our amps all sounded like doo-doo. Somewhere, things had gone horribly wrong.
To me, the thing that really got my attention was not only the linearity and low capacitance, but the new packing concept. WOW! You can bolt the transistor to the heat sink, on the inside, bend the leads 90 degrees, and hook it right to the PCB! No more drilling hole through the heat sinks, using nasty sockets, steel cases with screws going through them to make electrical contact, etc.
But let me pause and give praise to the guy who may have been the first to "think outside of the box", when it came to using ring-emitters, and in an entirely different manner.
John Iverson.
Not only did he incorporate the new transistors, mounted in a different manner, but he came up with an usual input stage, followed by an even more unusual gain stage. I had not seen anything like it before. He refined the gain stage somewhat in the later versions of the Eagle amps.
OK......what was so great about it?
Some will argue, but transistors are basically current controlled devices. (Yes, you have to create a voltage to have current......not the point here.) If you think as the input/control signal as a current, and design with current linearity, not necessarily voltage linearity, as the parameter to optimise, you come up with ideas that have not been used before.
At least not in audio power amplifiers. I suspect John may have worked on some military/government electronics somewhere in his career. Regardless, guys who thought like him gave us things like folded cascodes, and other techniques that increased both linearity and bandwidth.
The more linear it is to start with, the better it will sound if you use feedback to lower it. Likewise with bandwidth: the higher you can get it, the more stable an amp should be.
So, a lot of factors came along that made it easier to build amps that were inherently more linear than the junk we designed in the 70s. Some of us decided that designing by specs was even more meaningless than the rest of the crowd, and we got rid of all the loop feedback. But none of it would have possible 25-30 years ago. The semiconductors did not exist, we had our head(s) screwed on backwards, and it took some cock-eyed ideas (which may have been invalid!) to get them oriented back in the right direction.
Actually......now that I think about it......Audio Research was on the leading edge in SS design with the notoriously unreliable D-100. It used a "zero-feedback" output stage....what was it...mid 70s?......long before anyone else thought of that concept. (The problem was mostly a heat sink issue. The amp could have been reliable with about 2, maybe 4, times the heat sink surface area.) The input stage may have been bad.........I don't know, the modules were potted, but the output stage concept was a good one. I know..........I have used it the last 10+ years. With decent transistors on much larger heat sinks.
OK.....that ought to be enough to digest for a while. I appreciate the encouraging e-mails. Thanks.
Ok, speaking of emitters and such, the development of "ring emitter" transistors lead to a radical change in transistor design. Back when I started getting serious about amp design, you had 2 choices: RCA and Motorola. RCA (for whatever reason) did not make high-power PNP devices. Motorola did. With those, we had amps like the SWTP "Tiger" series. A bit unstable, but probably the first step towards modern amp design. Even then, the transistors were made with diffused processes, and were not the most rugged in the world. Eventually, they learned how to make epitaxial processes, and things started to take off. Some firms, Bedini as example, stuck to using only NPN devices in the outputs.(The RCA approach.) But most everyone else went to complementary devices. However, out of that grew the 0.000001% THD wars, and the resultant bad sound.
(Looking back......in hindsight......there may not be a convincing reason to use complementary devices in closed loop amps. Remember, the little 20 watt Bedini did sound good.)
I suspect that the guys who came up with the ring-emitter concept were probably used to designing RF transistors. Sanken, Toshiba, and Fujitsu all had strong contenders. Linear, fairly rugged, and perhaps most important: low capacitance. This allowed designers to push the bandwidth higher, as we were all concerned with TIM, SID, and a host of other "new" mechanisms that we were becoming convinced explained why our amps all sounded like doo-doo. Somewhere, things had gone horribly wrong.
To me, the thing that really got my attention was not only the linearity and low capacitance, but the new packing concept. WOW! You can bolt the transistor to the heat sink, on the inside, bend the leads 90 degrees, and hook it right to the PCB! No more drilling hole through the heat sinks, using nasty sockets, steel cases with screws going through them to make electrical contact, etc.
But let me pause and give praise to the guy who may have been the first to "think outside of the box", when it came to using ring-emitters, and in an entirely different manner.
John Iverson.
Not only did he incorporate the new transistors, mounted in a different manner, but he came up with an usual input stage, followed by an even more unusual gain stage. I had not seen anything like it before. He refined the gain stage somewhat in the later versions of the Eagle amps.
OK......what was so great about it?
Some will argue, but transistors are basically current controlled devices. (Yes, you have to create a voltage to have current......not the point here.) If you think as the input/control signal as a current, and design with current linearity, not necessarily voltage linearity, as the parameter to optimise, you come up with ideas that have not been used before.
At least not in audio power amplifiers. I suspect John may have worked on some military/government electronics somewhere in his career. Regardless, guys who thought like him gave us things like folded cascodes, and other techniques that increased both linearity and bandwidth.
The more linear it is to start with, the better it will sound if you use feedback to lower it. Likewise with bandwidth: the higher you can get it, the more stable an amp should be.
So, a lot of factors came along that made it easier to build amps that were inherently more linear than the junk we designed in the 70s. Some of us decided that designing by specs was even more meaningless than the rest of the crowd, and we got rid of all the loop feedback. But none of it would have possible 25-30 years ago. The semiconductors did not exist, we had our head(s) screwed on backwards, and it took some cock-eyed ideas (which may have been invalid!) to get them oriented back in the right direction.
Actually......now that I think about it......Audio Research was on the leading edge in SS design with the notoriously unreliable D-100. It used a "zero-feedback" output stage....what was it...mid 70s?......long before anyone else thought of that concept. (The problem was mostly a heat sink issue. The amp could have been reliable with about 2, maybe 4, times the heat sink surface area.) The input stage may have been bad.........I don't know, the modules were potted, but the output stage concept was a good one. I know..........I have used it the last 10+ years. With decent transistors on much larger heat sinks.
OK.....that ought to be enough to digest for a while. I appreciate the encouraging e-mails. Thanks.