OK, I'm game.
If I look at that circuit, I think that the two transistors in the middle-right are acting as a Hawksford correction: The output signal is at their emitters (through the diode offsets) and the input signal is at their bases (through the left-hand transistors BE and diode offsets).
So, assuming the output falls too low, Ic of the top/middle/right transistor increases. That would pull down the upper gate only to aggravate the too low output. BUT, it also lowers the base voltage at the input stage upper transistor (IF it's the input stage, nothing seems conventional here), and that lowers the input stage Ic, therefore allowing the upper gate to rise through the current source. So, the ratio of the resistor string appears to by crucial for the exact times-1 correction.
The diodes in the output stage appear to be there to avoid cut-off (or, more correctly, reverse biasing). They are then also needed in the input stage for correct DC bias throughout the amp.
Jan Didden
If I look at that circuit, I think that the two transistors in the middle-right are acting as a Hawksford correction: The output signal is at their emitters (through the diode offsets) and the input signal is at their bases (through the left-hand transistors BE and diode offsets).
So, assuming the output falls too low, Ic of the top/middle/right transistor increases. That would pull down the upper gate only to aggravate the too low output. BUT, it also lowers the base voltage at the input stage upper transistor (IF it's the input stage, nothing seems conventional here), and that lowers the input stage Ic, therefore allowing the upper gate to rise through the current source. So, the ratio of the resistor string appears to by crucial for the exact times-1 correction.
The diodes in the output stage appear to be there to avoid cut-off (or, more correctly, reverse biasing). They are then also needed in the input stage for correct DC bias throughout the amp.
Jan Didden
Nelson Pass said:
Just simulating. I will have no chance to catch the time sooner than a week later.
"...So the idea of putting collector resistors to ground at the VAS output only degrades the theoretical performance with respect to phase modulation below 20 kHz, and keeps it about the same above 20 kHz. The original Leach amplifier in the Audio magazine article in February 1976 http://users.ece.gatech.edu/~mleach...b77articles.pdf had these resistors (R20 and R21 in Figure 2). However, later versions http://users.ece.gatech.edu/~mleach...raphics/ckt.pdf omit them. This issue was the subject of debate back in the late '70s and Dr. Leach changed the design. He specifically mentioned at that time that what was really important is not the open-loop bandwidth, but the gain-bandwidth product. Removal of the resistors brought about improved conventional distortion performance without sacrificing transient capability..."
Maybe it is not necessarily bad to have the collector resistors to ground at the VAS output? Is it possible that a properly selected value for these resistors could "stabilize" the open loop VAS gain so it is not affected by the load under high power conditions?
Fab
Maybe it is not necessarily bad to have the collector resistors to ground at the VAS output? Is it possible that a properly selected value for these resistors could "stabilize" the open loop VAS gain so it is not affected by the load under high power conditions?
Fab
Loading the Vas with resistance to ground is a perfectly good
technique, used in the Aleph 0, 0s, and 1.0. Yes, it increased
the distortion figure slightly by anchoring down the load
impedance that the VAS saw, and thus limiting the open loop
gain figure, but I liked what I heard, which was more spectral
consistency and less variation with load impedance.
It's not always appropriate, but I always at least try it to see.
😎
technique, used in the Aleph 0, 0s, and 1.0. Yes, it increased
the distortion figure slightly by anchoring down the load
impedance that the VAS saw, and thus limiting the open loop
gain figure, but I liked what I heard, which was more spectral
consistency and less variation with load impedance.
It's not always appropriate, but I always at least try it to see.
😎
Nelson Pass said:
Somehow i don't think you can 'hear more spectral consistency'....or 'less variation with load impedance'.....
In any case, these dubious quantities cannot rationally be 'improved' by making the design more susceptible to supply line hash....which is what happens when one arbitrarily reduces LF foward path gain by resistively loading down the transimpedance stage (TIS!!!)....
Moreover, the later increases TIS distortion, while simultaneously depriving the major feedback loop of the gain required to reduce it.....an altogether horrendous faux pas....
😱
Theory versus Practice
As usual, theory is not always born out in practice. 😀
As usual, theory is not always born out in practice. 😀
mikeks said:
Maybe you can't, but Nelson can.
Maybe that's why he's been one of the most innovative and successful designers for over 30 years.
Re: Theory versus Practice
Rather patronising i dare say.....If by 'practice' you mean the 'evidence' of your ears in unscientific listening 'tests', then give me well reasoned, and proven theory....any day.....
Charles Hansen said:
Rather patronising i dare say.....If by 'practice' you mean the 'evidence' of your ears in unscientific listening 'tests', then give me well reasoned, and proven theory....any day.....
Re: Theory versus Practice
It's expected that theory won't in general be born out by non-scientific practice. And I don't think Nelson's making the claim that his final decision was based on a scientific approach. He just went with what sounded better to him. Nothing wrong with that. If you consider the scientific portion of what he said though, namely:
then the theory was born out in practice. Loading down the VAS increased the distortion consistent with the theory.
Charles Hansen said:
It's expected that theory won't in general be born out by non-scientific practice. And I don't think Nelson's making the claim that his final decision was based on a scientific approach. He just went with what sounded better to him. Nothing wrong with that. If you consider the scientific portion of what he said though, namely:
then the theory was born out in practice. Loading down the VAS increased the distortion consistent with the theory.
How do you know what Nelson can or can not do?
I know one thing that he can do:
Make amplifiers that have met the acceptance of many consumers, over a considerable period of time.
Not an easy feat.
And his critics have done..............?.......what........?...is that silence that I detect?
Oh, yeah.....I know.....write articles on how he doesn't know squat about SOA.
Jocko
I know one thing that he can do:
Make amplifiers that have met the acceptance of many consumers, over a considerable period of time.
Not an easy feat.
And his critics have done..............?.......what........?...is that silence that I detect?
Oh, yeah.....I know.....write articles on how he doesn't know squat about SOA.
Jocko
The folly of youth...
Actually, I'd be willing to bet that you're the one doing the "patronising". Why? Because I doubt you've taken the time to do the listening tests yourself.
I know where you are coming from because I've been there myself. Luckily, I was disabused of that particular arrogance at a very early age, while still a teenager.
All it took was an afternoon. We rounded up all the best preamps of the day, me the "scientific" skeptic and my friend the "hands-on" audiophile. All of them had what was considered then to be "perfect" specs -- frequency response +/- 0.25 dB (this for the phono input of course, as that's all there was back then for high quality sources) and distortion less than 0.1%.
To my complete shock and surprise, the audible differences were laughably obvious.
Try it sometime. You might learn something. It's like they say, "the mind is like a parachute -- it works better when it's open."
mikeks said:
Actually, I'd be willing to bet that you're the one doing the "patronising". Why? Because I doubt you've taken the time to do the listening tests yourself.
I know where you are coming from because I've been there myself. Luckily, I was disabused of that particular arrogance at a very early age, while still a teenager.
All it took was an afternoon. We rounded up all the best preamps of the day, me the "scientific" skeptic and my friend the "hands-on" audiophile. All of them had what was considered then to be "perfect" specs -- frequency response +/- 0.25 dB (this for the phono input of course, as that's all there was back then for high quality sources) and distortion less than 0.1%.
To my complete shock and surprise, the audible differences were laughably obvious.
Try it sometime. You might learn something. It's like they say, "the mind is like a parachute -- it works better when it's open."
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