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In Reply to: RE: I have a different question about the circuit posted by Triode_Kingdom on April 24, 2015 at 20:47:45
According to an article I read once by Kairos and Haffler, about a 6550 Williamson circuit adaptation of theirs, the resistors help in coupling the bass frequency.
Tre'
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"Still Working the Problem"
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"the resistors help in coupling the bass frequency."
I simulated this to see if that's true. Not only does the 470K not improve bass, there's a very slight degradation. My theory on why the resistors are there actually has nothing to do with frequency response.
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The article I read is linked below,
"The output stage is coupled to the driver stage through a resistive capacity network which provides conventional RC coupling at signal frequencies and attenuated direct coupling at subsonic frequencies.
This again introduces an improved low frequency phase characteristic which adds to the stability margin of the amplifier."
Question, does your theory have anything to do with the charging and discharging of the coupling cap and the so called "blocking" that that can cause?
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Typically this circuit provides a response step function at subsonic frequencies for improved subsonic stability under closed loop conditions (controlled phase shift).
It may also reduce blocking under overdrive conditions but there are better means of achieving this.
I would think a truly revelatory simulation needs to include both real-world OPT characteristics AND the closed loop.
I simmed it again, this time down to 0.1 Hz. Phase is much better controlled below 10 Hz with the 470k resistors, and especially below 1 Hz. I also see that this would only be important in a system with significant NFB, where the amplitude of subsonic frequencies is greater. Otherwise, with no loop and no parallel resistor, signals below 1 Hz are so severely attenuated, it doesn't much matter what the phase is. In addition, without NFB, no instability can result from this.
OK, that tears my theory to shreds. I was thinking about what happens if the bias supply fails. The 470K resistors would bias the tubes on hard, instantly blowing the cathode fuses and protecting the outputs. I should have known the designer(s) weren't focused on that. The fuses would probably blow the instant the supply fails anyway. :)
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Buy Chinese. Bury freedom.
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Have Fun and Enjoy the Music
"Still Working the Problem"
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