My comparison of gyrator to plate choke was with the assumption that the choke would be of such inductance as to make the load line almost horizontal.
OT's on the other hand don't do this. Gyrators are highly superior to OT's with regards to linearity. Just draw the load lines and compare them.
Unless somebody wants to assert (this has happened before several times, to my suprise) that load lines are not very relevant to sound reproduction quality.
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Correct. The designation 'power' to the last tube is not without reason; to fully utilize the available B+ voltage, you need a low plate resistance tube. It's a special role.
I've done experiments with high mu high rp tubes driving a follower final stage, and you never get that much of the B+ voltage used in the signal swing. Therefore it's completely logical and practical to have the final tube be low rp.
Also with regards to gain structure; everybody's system is of course different, but I find that it's easier to get to the "gain sweet spot" (I don't like to have extra gain that much) with a medium or high µ tube driving a low µ tube.
Okay. So what about it?
I find these word games about 'hybrids' and such simply silly and unnecessary. If somebody wants to have a hobby that's about making "tube amps with a technology cut off at 1955", then by all means, do that, have fun. But if somebody else has a hobby that's about "searching for fidelity and/or sound reproduction that my ears think is fidelity", then that's another thing.
What is a hybrid? Is it a hybrid if you use SS diodes? They weren't widely used in 1955. What about if you use a russian special high gm triode that wasn't available in 1960? Is it too modern, above the cut off?
What about if you use film caps made with technology not available in 1970? Is is a hybrid then, because it is mixing old technology with new?
MOSFETs are just components. If they had them in 1936 they would've used them. And we'd call them classic circuits now, and people would be reproducing them down to the last dot today.
It's about the tubes. The FETs are simply helping them do their thing to the best. Change the tubes to another type with worse curves - you'll hear it. Change the FETs a million times - you won't hear it.
It's a silly word play.
I've been pondering those kinds of things for a while.
The thing is that tubes are high impedance high voltage devices. My headphones and speakers are low impedance devices.
I have not to this day found low voltage low impedance solutions to get voltage gain that I find to be as good as properly set up tubes.
I tried out a bunch of different cap output OTL things, and I was sorely underwhelmed with the performance of all the caps I tried. Only small value caps perform really well enough. So if you can find a way to have under 10µF caps as cap output to my 118 ohm heaphones or 6 ohm speakers, I'd be interested.
I know Wavebourn has made a monster amp that is directly coupled to the load, and I've looked into that a bit, but so far that project is out of my scope on many levels.
OTs perform really, really well if driven properly. They are NOT the source of the tube sound when driven properly.
When taking into account all posts I've tried to explain my design philosophy and reasoning, I find this statement to be simply in bad faith.
It's the constant current to voltage transfer (delta I over delta U) that makes a triode linear.
For some triodes that might be best achieved with a horizontal load line, depending on operating point.
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By 'rationale' I meant 'reason(s)'. Listening might well be a reason to choose or not to choose a component or particular circuit toplogy.
Looking for a better way (within an SE topology) to provide more fidelity (more listening pleasure). This is achieved with lowering distortion, and providing more ideal conditions for the OT to operate.
In short, better results.
On the path of more listening pleasure, I've had a look at some of the technical parameters of the ST MOSFET you've chosen for your gyrator. I have a hunch that you may well get more satisfaction from going to a lower capacitance part. One I've come across so far is Fairchild FQP2N90 but I'll continue looking.
abraxalito: I actually use IRF820 and IRF830 most often in the under 500V applications. All the FETs sim exactly the same (looking at the output FFTs) so I sim with whatever. I've never used that specific FET that's in the spice schematic.
FET capacitance is not an issue in the follower role. They perform identically.
FET capacitance is not an issue in the follower role. They perform identically.
What sim do you use and what models within that sim? I've been using LTspice for some considerable time and the models for the MOSFETs in that program are really developed for SMPSU designers. As such the models don't look to be too hot for doing linear stuff.
<afterthought> In my experience (that's been in trying to correlate SQ with DAC I/V stage simulations) FFT has a poorish-to-very-poor correlation with SQ. Quite recently I've found that PSRR is a much more promising avenue of exploration towards understanding why things sound the way they do.
<afterthought> In my experience (that's been in trying to correlate SQ with DAC I/V stage simulations) FFT has a poorish-to-very-poor correlation with SQ. Quite recently I've found that PSRR is a much more promising avenue of exploration towards understanding why things sound the way they do.
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If you want a more horizontal load line, use a higher primary inductance OPT.
Ok, you lose some output power, but power efficiency of the gyrator drive is !%&*$ too.
FET capacitance does have a noticeable influence on the frequency response (if using high impedance low current drive). Rds,on is also different between parts, having an impact on freuqency response and drive impedance as well. Sims clearly show this.
IRF820 is however quite good, I doubt there are noticeably better mosfets for this application. So far we agree.
Using different families of technology working together towards the same goal in one assembly. Here it's tubes and solid state parts for power (voltage & current) amplification. That's hybrid.
You were talking about the superiority of your approach compared to other tube approaches. I thought it's just fair to give the whole picture, and that is that your approach is not purely tube. It'a a hybrid of tubes and SS. Nothing less, nothing more.
In your proposed topology, the FETs are not just 'helping' or supporting. The FET plays a vital role, providing the current drive and determining the output impedance. The low output impedance of the FET also impacts the frequency response of the OPT. The same results would just not be attainable using tubes only.
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Let's make it a challenge. Provide a sim with FFT from output; output secondary is loaded with 8 ohm resistor and there's a sine voltage of 3 VPP across it. Tube is 2A3, and you're free to select op point and plate load as you wish. B+ is limited to 600V.
Yes, output stages I present have horrible efficiency. People interested in efficiency should definitely look for other solutions.
For SE, it would have to be 50 to 100 kohm primary. That is too low efficiency even for me. Why bother when you can just use a gyrator and a normal and very cheap Hammond 10k.
These things simply are not relevant.
It doesn't matter if the plate load presents a 250k load or a 700k load. It doesn't really matter if the gyrator has a 10 ohm output impedance to OT, or a 50 ohm.
It would matter if the gyrator would have a 500 ohm output impedance, or 1k. Or that the load would be only 50k.
I've used mostly those FETs I mentioned, but occasionally I get a good deal on other types, and all types I've used provide my amps with freq response to the bat territory at least, so I'm really not convinced I need to care about FET properties in this particular use.
In balanced amps, it's important to have the gyrator plate loads be the same type, and preferably same production batch. But in SE: simply not important.
The common FETs are pretty much all able to overpower the other components (tube, OT, caps) in the current domain. Use what's cheap and that's ok.
Why bother with a gyrator at all, when you can have - for the same total power consumption - the same power output with comparable distortions + additional headroom, by just going to a bigger output tube?
(This is a serious question - no joke.)
The whole picture is clearly and easily visible in the schematics I provide. Anyone who DIYs tube amps sees what's there. There is no need to talk silly semantics. I'm not hiding anything, no unfairness going on.
I find the need to point out "hybrid status" very strange (this is a common theme). Am I performing blasphemy that needs to be pointed out, so that it's fair? What's unfair?
Let me clarify why I think it's silly word playing:
If you have a parafeed tube output stage with choke plate load, does the choke not play a "vital role"? Surely the amp wouldn't operate at all without that choke - do you now have a "tube / choke hybrid"?
Or, if you have RC filters in your PSU, surely it would not operate without the caps (possibly made with technology not available in 1960). Do you now have a "tube / cap hybrid"?
Nobody calls a design that uses gas regs or voltage references a "vacuum tube / gas reg hybrid", even though they are very different types of components. Is it because they look the same, or that they are from the same time period?
Sure, it's a tube + MOSFET hybrid, but is this simple categorization really a meaningful thing worth mentioning or pointing out in the name of fairness?
The same results would not be attainable using tubes only. Regarding all other components a particular tube schematic happens to have.
MOSFETs are just components. My proposed circuit would not operate without them; it would also not operate without resistors or caps or transformers.
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Comparing apples with oranges. It's not fair. It's pointless.
One could make the same arguments as you do on why a given pure SS topology performs better than your hybrid circuit. But what's the point?
Your further semantics on caps chokes tubes and all are equally pointless and silly. These are totally different parts for different purposes. Tubes and FETs are fundamentally the same class of parts, serving the same purposes. That's why a tube / FET circuit is hybrid, and a tube / resistor / cap circuit not. What's so difficult about that?
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Where in my circuit presented in this thread do tubes and FETs serve the same purpose? Where is there FET doing voltage gain? Where is there tube handling current to load?
You yourself said, one cannot do the same things with tubes. So, how can they be for the same purposes?
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