If you like the sound with the 6080's , What about upping the gain with an input transformer?
. . . . . or doing a shunt cascode circuit where the gain of the stage comes from its gm not the mu? The 6080 has plenty of that.
. . . . . or doing a shunt cascode circuit where the gain of the stage comes from its gm not the mu? The 6080 has plenty of that.
Right T. I’ve tried all three of them and like the 6080 best….. for my system of course. Great design!!!
Ok, I decided to build the 4S Universal tube preamp. Here is what I have to work with. Hopefully I can use some of it.
I built something like that 4S Universal Preamp, complete with volume control on the output, a few years before the commercial internet came into being. I built it using a 5687 so it would have a decently low output impedance. The problem was that the volume control on the output meant the tube's grid was always seeing the full blast output from the source, so distortion was always high, even at low volume settings. It sounded nice and tubey that way, and the volume control attenuated most of the noise. But I eventually got tired of that kind of sound.
I would redesign that 4S preamp to put the volume control at the input (where it belongs) and add a cathode follower on the output so it can drive cable runs and solid state amplifiers. I would use a negative feedback loop to adjust the gain as needed.
Or you could always build the 12B4A preamp from right here on diyAudio. Mu on the data sheet is 6.5, so I'd expect about 5x gain in real life. Low output impedance without negative feedback too.
https://www.diyaudio.com/community/...the-12b4-better-than-the-grounded-grid.68267/
https://www.diyaudio.com/community/threads/12b4-preamp.137771/
https://www.diyaudio.com/community/threads/diy-12b4-preamp-any-good.317693/
https://www.diyaudio.com/community/threads/12b4-preamp-design-pre-build-questions.287521/
https://www.diyaudio.com/community/threads/12b4-line-stage-amp.141025/
Other low-mu and low Zout possibilities would be 4P1L in triode, 12P17L in triode, or even EL86 in triode.
Salas's 6V6 triode preamp is popular too.
https://www.diyaudio.com/community/threads/6v6-line-preamp.102352/page-209#post-7204481
I would redesign that 4S preamp to put the volume control at the input (where it belongs) and add a cathode follower on the output so it can drive cable runs and solid state amplifiers. I would use a negative feedback loop to adjust the gain as needed.
Or you could always build the 12B4A preamp from right here on diyAudio. Mu on the data sheet is 6.5, so I'd expect about 5x gain in real life. Low output impedance without negative feedback too.
https://www.diyaudio.com/community/...the-12b4-better-than-the-grounded-grid.68267/
https://www.diyaudio.com/community/threads/12b4-preamp.137771/
https://www.diyaudio.com/community/threads/diy-12b4-preamp-any-good.317693/
https://www.diyaudio.com/community/threads/12b4-preamp-design-pre-build-questions.287521/
https://www.diyaudio.com/community/threads/12b4-line-stage-amp.141025/
Other low-mu and low Zout possibilities would be 4P1L in triode, 12P17L in triode, or even EL86 in triode.
Salas's 6V6 triode preamp is popular too.
https://www.diyaudio.com/community/threads/6v6-line-preamp.102352/page-209#post-7204481
If too much gain, just add an extra resistor between the regular cathode resistor and ground for some degenerative feedback to knock down some gain, similar to the input stage of ARC SP6, Obviously you need an input cap to block DC reaching the input pot. This way you don't need to wrap global feedback around the gain stage and will have less of that feedback dryness sound. I'm not against feedback but personally I try not having global negative feedback wrap around the first gain stage. Another way to reduce gain is to use step down input transformer.
Cathode degeneration is another form of negative feedback, but this time applied by current feedback, which increases output impedance while lowering gain. It also decreases the transconductance of the tube you apply it to. All that also can apply a dryness to the sound.
Global negative feedback is feedback applied around multiple (>2) stages.
Negative feedback applied around one stage is 'local' negative feedback.
Don't confuse shunt negative feedback around a single stage with global loop NFB applied around an entire amplifier of multiple stages including an output transformer. That's a lot more complicated than this ultra-simple shunt NFB around the one stage (plate to grid), and there's no phase/resonance problem like you'd get with an audio transformer inside the FB loop.
If no negative feedback can be tolerated then a fully bypassed 6080 common cathode amplifier should do. It will have low gain and low Zout, but will require a lot of plate current (40mA or more per channel) and a lot of heater current (6.3V 2.5A).
I threw together this simple common cathode 6AS7 in a simulation and it looks like it would do the job.
Global negative feedback is feedback applied around multiple (>2) stages.
Negative feedback applied around one stage is 'local' negative feedback.
Don't confuse shunt negative feedback around a single stage with global loop NFB applied around an entire amplifier of multiple stages including an output transformer. That's a lot more complicated than this ultra-simple shunt NFB around the one stage (plate to grid), and there's no phase/resonance problem like you'd get with an audio transformer inside the FB loop.
If no negative feedback can be tolerated then a fully bypassed 6080 common cathode amplifier should do. It will have low gain and low Zout, but will require a lot of plate current (40mA or more per channel) and a lot of heater current (6.3V 2.5A).
I threw together this simple common cathode 6AS7 in a simulation and it looks like it would do the job.
Geez, I feel like a complete idiot. I was looking at all my bits and found this half done pre amp built I forgot all about. Maybe I should just finish this build!!
https://audiokarma.org/forums/index.php?threads/what-do-you-think-of-the-aikido-pre-amp.899465/
https://audiokarma.org/forums/index.php?threads/what-do-you-think-of-the-aikido-pre-amp.899465/
If an entire preamp or device has only one gain stage and you wrap feedback around it or take from output to the input, why can't you call it GLOBAL negative feedback?
This is not a power amp with output transformer. Of course I highly recommend using output transformer to be driven by cathode follower to step down the gain but without the transformer in the feedback loop if feedback is used at all.
For NFB around a single-stage amplifier you could call it 'global', but that would not follow standard practice. In general, 'global' negative feedback means NFB applied around multiple stages, while 'local' NFB means NFB applied around a single stage.
If you take a single common cathode amplifier and put shunt NFB from plate (output) to grid (input) then I'd call that 'local' NFB.
If you put a buffer on the output of that common cathode amplifier (either a cathode follower or a source follower) then are you buffering that single stage or is that a 'second' stage? I'd just call the buffer a buffer, meaning it's still a single voltage gain stage amplifier. If I then wrap NFB from the output (cathode) of the cathode follower buffer back to the input (grid) of the voltage amplifier (common cathode) stage, then I'd say it's still 'local' NFB. Like this:
For a global NFB design, I'd be thinking of something like the 'ring of two' or the 'ring of three' circuits, in which you have two cascaded voltage gain stages with NFB from the output to the cathode of the first voltage gain stage. This is a phono preamp in the 'ring of three' topology:
Note that there are two voltage gain stages inside the feedback loop. I'd call that global NFB, although of a minimal sort.
My personal thinking is that if the feedback is wrap around the entire device whether it's single stage, like an anode follower, or two or multiple stages, it's called global feedback. I understand what you're trying to say. Local feedback is within a single stage through degeneration and typically doesn't involve a time constant. It comes down to does calling something "global" feedback have to have more than one gain stage? I'll let others to decide.
I don't believe the shunt cascode (as called by Coleman, Moglia et al) is limited that way . Moglia has written a page about it here.
(Still on the theme of getting more gain from the 6080.)
If you look in the electronics textbooks, you'll see that cathode degeneration is a form of current feedback, and not voltage feedback. When the triode draws current because signal at the grid causes the plate to swing negative, the current being drawn through the unbypassed cathode resistor causes both a DC and AC voltage increase through the cathode resistor, decreasing the plate current, causing the plate to swing more positive (against the current draw causing the plate to go negative). It's the current through the triode that is being resisted by the voltage increase (both DC and AC) through the unbypassed cathode resistor. In this case the feedback mechanism is changing the biasing of the triode (the amount of plate current drawn through the triode).
This is series current feedback, series applied:
https://www.eeeguide.com/current-series-feedback-amplifier-circuit/
To implement voltage feedback one needs to take a portion of the output signal and apply it to the input, independent of the triode's biasing or plate current. It's all done with AC signal voltage, not by directly manipulating the biasing of the gain stage. In the anode follower, the voltage feedback is in parallel, so is 'shunt' voltage feedback. That's the same idea as when you adjust the gain of an inverting op-amp using its feedback loop.
https://www.eeeguide.com/voltage-shunt-feedback-amplifier-circuit/
In your definition, all voltage feedback, whether shunt or series applied, would be 'global', while only current feedback could be considered 'local'. However, that's not how it's described in the standard texts.
Do you have a copy of the Radiotron Designer's Handbook 4th Edition?
The most gain you can get from a single 6AS7G or 6080 stage would be from using it in a mu-follower configuration, as in the schematic in post #7.
You could get more gain from this tube by applying positive feedback, but that would have unwanted side effects for audio amplification.
Using a 6AS7G or 6080 in a cascode would increase gain, but would also increase output impedance. Perhaps you could use a 2k plate load resistor for Zout of approx. 2k ohms?
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I was NOT referring to the cathode degeneration feedback I suggested to DJN and that is NOT global feedback. That has nothing to do with our discussion. I was referring to your quote in the post and the circuit you included in below.
Again, if you wrap feedback AROUND the entire device from output to input I call it GLOBAL feedback and that's what the above circuit is to me.
Again, does feedback needs to be more one GAIN stage to be called global feedback?
P.S., Please keep in mind the OP has TWO circuits to deal with: the 6080 circuit has not enough gain and the universal 12A*7 circuit has too much gain. We will end up barking the wrong trees. My point is that wrapping feedback to reduce gain (the 12A*7 circuit) from output to input via a cathode follower and cap is global feedback.
Again, if you wrap feedback AROUND the entire device from output to input I call it GLOBAL feedback and that's what the above circuit is to me.
Again, does feedback needs to be more one GAIN stage to be called global feedback?
P.S., Please keep in mind the OP has TWO circuits to deal with: the 6080 circuit has not enough gain and the universal 12A*7 circuit has too much gain. We will end up barking the wrong trees. My point is that wrapping feedback to reduce gain (the 12A*7 circuit) from output to input via a cathode follower and cap is global feedback.
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The reason I brought up cathode degeneration is that you posted it as an example of local feedback, but then posted that shunt voltage NFB around a single stage is 'global' feedback.
As I posted earlier, it is possible to have a local voltage feedback loop, but that requires a loop of signal from the output of the inverting gain stage back to its input. If I understood you correctly, you stated that this is an example of 'global' NFB because it goes around from the output back to the input. I disagree with you, and I'm explaining why. Therefore, cathode degeneration has everything to do with the point I'm trying to make here.
Yes.
NFB around a single gain stage = local NFB
NFB around cascaded gain stages = global NFB
This is the basic anode follower, which is considered to be an example of a single voltage amplifying, inverting gain stage with local NFB.
As explained in my previous post, I consider the cathode follower or source follower to be a buffer, so I consider the circuit below to be an example of 'local' NFB, because it's merely the above circuit with an output buffer added (which improves the performance of the NFB loop):
John Broskie posted pretty much the exact same circuits in his discussion of the Anode Follower on tubecad.com.
https://www.tubecad.com/2019/03/blog0458.htm
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In addition there are amplifiers with nested NFB loops, where there is local NFB around a single gain stage within a larger circuit with global NFB around it which includes the single stage with local NFB.
https://www.angelfire.com/ab3/mjramp/errorfb.html
More complex example of nested NFB loops in a tube power amp:
https://rmsacoustics.nl/tubeamp/tuba_signal_path.html
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