Hugh,
Thanks for the explanation. I am aware of the nature of balanced signal feed as I have worked with pro audio gear. However I was unsure of the specifics of running a pair of amps in bridged configuration and as such I didn't know if it was a good idea for the Fetzilla. Thanks for pointing out what the effects would be.
Cheers,
PS. let me know if you and Laurie are heading down to safety beach anytime. I'm still keen on the V-Sonics, but have to clear space and save money before jumping in.
Tani.
Thanks for the explanation. I am aware of the nature of balanced signal feed as I have worked with pro audio gear. However I was unsure of the specifics of running a pair of amps in bridged configuration and as such I didn't know if it was a good idea for the Fetzilla. Thanks for pointing out what the effects would be.
Cheers,
PS. let me know if you and Laurie are heading down to safety beach anytime. I'm still keen on the V-Sonics, but have to clear space and save money before jumping in.
Tani.
No drawings; very simple.
Imagine a npn transistor, with its collector connected to the emitter of a cascoding npn transistor above.
Input taken to the base of the bottom npn, output from the collector of the upper npn.
Upper npn device has base held at a constant reference voltage; this defines a cascode of course.
Simply place a resistor between collector of lower npn transistor and emitter of upper transistor.
The resistor should drop 200mV for what ever current is chosen.
This is Poufou's 'magic resistor'.
Hugh
Imagine a npn transistor, with its collector connected to the emitter of a cascoding npn transistor above.
Input taken to the base of the bottom npn, output from the collector of the upper npn.
Upper npn device has base held at a constant reference voltage; this defines a cascode of course.
Simply place a resistor between collector of lower npn transistor and emitter of upper transistor.
The resistor should drop 200mV for what ever current is chosen.
This is Poufou's 'magic resistor'.
Hugh
Mike,
This was 15 years ago, hell, I'm unable to remember where I put my car keys half an hour ago, so a bit of an ask........
Broadly, I noticed better resolution, more ambient details, and somehow a more vivid soundstage, greater sense of space, ALL OTHER THINGS BEING EQUAL.
After a bit of simple math, I realised that what might be happening here was that we were linearising the re of the cascoding transistor. As you are aware, the Zout of a common emitter device is very, very different to the Zin of an EF, so, if the re were linearised by a resistor with a bridging ratio, then things might improve......
And so it was. Take some figures. If VAS and its cascode run at 6.5mA, then Zin of the cascode emitter is 4R (26/Ie). This corresponds to re, of course. If we interpose a resistor and drop 200mV across it, then the value becomes 200/Ie, or 7.7 times the value of Zin of the EF. This is 31R, npv of 33R. 7.7 is suspiciously close to the accepted engineering bridging ratio of 10.
It helps both with impedance transfer from collector to emitter and linearises the input impedance of the EF, drowning it in a constant value. There appears to be no penalty for this, since voltage drop is trivial and hardly affects rail efficiency.
Cheers,
Hugh
This was 15 years ago, hell, I'm unable to remember where I put my car keys half an hour ago, so a bit of an ask........
Broadly, I noticed better resolution, more ambient details, and somehow a more vivid soundstage, greater sense of space, ALL OTHER THINGS BEING EQUAL.
After a bit of simple math, I realised that what might be happening here was that we were linearising the re of the cascoding transistor. As you are aware, the Zout of a common emitter device is very, very different to the Zin of an EF, so, if the re were linearised by a resistor with a bridging ratio, then things might improve......
And so it was. Take some figures. If VAS and its cascode run at 6.5mA, then Zin of the cascode emitter is 4R (26/Ie). This corresponds to re, of course. If we interpose a resistor and drop 200mV across it, then the value becomes 200/Ie, or 7.7 times the value of Zin of the EF. This is 31R, npv of 33R. 7.7 is suspiciously close to the accepted engineering bridging ratio of 10.
It helps both with impedance transfer from collector to emitter and linearises the input impedance of the EF, drowning it in a constant value. There appears to be no penalty for this, since voltage drop is trivial and hardly affects rail efficiency.
Cheers,
Hugh
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Hello
Here's the link to Peufeu's cascode vas and magic resistor:
Memory Distortion Philosophies - Part 5 : Circuits, continued
Bye
Gaetan
Here's the link to Peufeu's cascode vas and magic resistor:
Memory Distortion Philosophies - Part 5 : Circuits, continued
Bye
Gaetan
Thanks for the info Hugh.
If you look at current mirrors operating with large signals, you'll see that the nonlinear Re of the transistors causes a derivative spike in the capacitive currents of the transistors, at the negative peak when the falling conductance causes a fast risetime across the B-E junction. This is even worse for advanced current mirrors such as the Wilson and so on. So it makes sense linearizing Re is a good idea.
But I don't understand this "bridging ratio" you talk about. I did a Google search and found nothing (whoah...). Can you please tell me what this means?
If you look at current mirrors operating with large signals, you'll see that the nonlinear Re of the transistors causes a derivative spike in the capacitive currents of the transistors, at the negative peak when the falling conductance causes a fast risetime across the B-E junction. This is even worse for advanced current mirrors such as the Wilson and so on. So it makes sense linearizing Re is a good idea.
But I don't understand this "bridging ratio" you talk about. I did a Google search and found nothing (whoah...). Can you please tell me what this means?
Not far away - one set of GB2 fetzillas waiting for the final couple of caps. Had some tricky bits with the fuse holders and the end diode bridges whos legs seemed too big for the PCB holes. Nothing worthwhile comes without challenges however, nothing 5 minutes with a dremel didn't sort out. I went for a pretty standard build with the Auricaps, was going to try some Mundorf supreme silver/gold and some V-caps TFTF I have but both were too big to not cause issues. I'm trialling a set of Atohm GT1.0's here at the moment and they are stunning with both my Plinius amps and a recent chip amp build - keen to see how Fetzilla steps up. Give me another week and I'll be posting some listening sessions. - AJ
Well maybe it's a job for the square-law class A guys, I'll bet Ed Stuart could throw something interesting together.
I just got done re-fitting my Fetzilla with the proper parts! Ah, after all this time... I will check it over and if everything seems fine, set it on fire. (destroyed a prototype of my own a week ago, degen resistors on fire!)
I just got done re-fitting my Fetzilla with the proper parts! Ah, after all this time... I will check it over and if everything seems fine, set it on fire. (destroyed a prototype of my own a week ago, degen resistors on fire!)
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