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Old 20th September 2019, 07:49 AM   #131
spookydd is offline spookydd  France
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I suppose this wasn't mentioned in the previous edition of the book. I can't afford any more books right now, so I can't check on this.
Maybe if you could be more specific and provide examples to try out..
I did try inserting the fet css between the tails of those 2 current sources, but it doesn't bring anything, and maybe it's actually worse with it, so that's no improvement.
Perhaps the led based css might be better than the dual tranny one in regard to the thermal performance... Something to compare.
Otherwise we can also look at not using mirrors for ltp loads and instead going for cascodes there...
Enhancing the vas also might be good, maybe..
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Old 20th September 2019, 09:02 AM   #132
chalky is offline chalky
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Here are the techniques that I mentioned. In order they are Cordell, Kleinschmiidt, Sansui, and Stochino. The authors may not all be the originators of the ideas but they are the first references that I could find.
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File Type: png Cordell.png (18.7 KB, 138 views)
File Type: png Kleinscmidt.png (14.6 KB, 139 views)
File Type: png Sansui.png (29.0 KB, 139 views)
File Type: png Stochino.png (79.6 KB, 135 views)
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Old 20th September 2019, 09:57 AM   #133
spookydd is offline spookydd  France
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I see what you mean now.

Turns out I was already experimenting with the 1st option.

So that's what Cordell was proposing to fix that bias issue then.

Adding that resistor increases thd a little, but works.

I don't really understand how those 2nd and 3rd options work, but I really am wary of that 4th, with all those diodes and clamping... I'm thinking that can't be very linear..

I lowered the gain a tiny bit, closer to 6db and this improves noise performance a bit too. The input impedance is a little low, but if we use higher values to increase that, then thd goes up and so does noise.

Difficult compromises...

I also aimed at keeping that class A dissipation down a bit in the outputs, to reduce the need for a heatsink. The vas trannies still would run warm as well.

I'll have to experiment with cascoding instead of the mirrors...
Attached Images
File Type: png Screen Shot 2019-09-20 at 11.50.05 AM.png (531.8 KB, 134 views)
File Type: png Screen Shot 2019-09-20 at 11.50.00 AM.png (91.1 KB, 33 views)
File Type: png Screen Shot 2019-09-20 at 11.50.12 AM.png (73.2 KB, 16 views)
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Old 20th September 2019, 10:50 AM   #134
chalky is offline chalky
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Incidentally I was talking utter bo----ks when I said that you needed either a npn/npn or pnp/pnp darlington pair for the Cordell method to work - of course you don't, your configuration is fine. Perils of posting late at night! In fact your npn/pnp darlington configuration has a hidden advantage in that the helper transistor is no longer strictly essential, because you don't need the extra voltage at the mirror collectors to overcome the 2x Vbe at the darlinton input. You just need the resistors R25 and R26 in your simulation. Incidentally you will probably have to reduce them a bit from 100k in real life. You might get away with 10k.
The second of my options works by directly impressing a voltage on the ( high impedance ) mirror collector. I've seen this method used a couple of times in working amplifiers. More parts than Cordell but you don't need the differential load resistor so you get higher open loop gain and lower distortion. So conceptually similar to Cordell with lower distortion but less elegance.
The Sansui method works by cascoding the vas and then making the cascode one leg of a current mirror. The other leg sets the vas current. They took out a patent on this - I can find you a reference if you're interested.
The Stochino method simply makes the vas one leg of a current mirror with the vas current set by the other leg. This is a snip from a working amplifier with 300V/uS slew rate and vanishingly small distortion. The anti-saturation ( Baker clamp ) diodes are everywhere because this is a very fast amplifier and without the diodes clipping would not be very pretty to say the least.
Mid morning so maybe I'm talking less bo----ks now.
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Old 20th September 2019, 11:07 AM   #135
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The sim of post #129 plus the differential load resistors and maybe replace R25 with a two terminal current source looks good to me.
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Old 20th September 2019, 05:52 PM   #136
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Spookydd what are the values of R5 and R7 and the voltage at the other end of R26 in your last sim?
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Old 20th September 2019, 06:14 PM   #137
spookydd is offline spookydd  France
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Voltages added to sch (attached).

The resistor values are shown in the variables.

When I may need to change values that affect more than one part at a time, it's easier to put that in a variable, and when that value needs to be stepped, it's unavoidable to do it that way.

I'm about to tweak a version (of the doa) for use on my own cloned 28b, and I will compare using a single tranny+led and the dual tranny ccs.

Using a led may not be quite as good thd wise, but the thermal characteristics might be an advantage... The led and its companion tranny can be thermally bonded for increased thermal stability.

That should be done if possible anyway for the ltp pairs...
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Old 21st September 2019, 09:01 AM   #138
spookydd is offline spookydd  France
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I'm trying out a few things I saw bryston is supposed to be using now in their "cubed" design, from their patent filed.

I have yet to see any schematics for any of their cubed amps, so I don't know for sure if they actually have used some or all of their patented ideas in the newer models.

But from what I found in their patent, apparently they ditched the concept of "keeping it simple", by no longer using only plain resistor loads in their ltps, switching to mirrors. Plus they add cfp to the ltp, as well as ccs for the ltp tails.

And for the vas, they use a cascoded cfp instead of the plain single.

That does increase the part count significantly and totally departs from their previous approach of "purity" by keeping it spartan.

I will also try out that idea of using the led based ccs instead of the dual tranny, which might perform a bit less well on thd but would likely be more thermally stable.
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Old 21st September 2019, 11:06 AM   #139
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You talking about the "Amplifier and Signal Filter" patent? Seems to give them rights over just about every power amplifier ever designed or built! I guess they still use the same output stage.
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Old 21st September 2019, 11:14 AM   #140
spookydd is offline spookydd  France
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Yes, that's the patent I was looking into. Trying to figure out exactly what they actually used in their new cubed amps. Since we can't find any schematics for any of those models, we can only speculate what they actually did.

And yes, I think the patent system is broken. They do grant patents to just about anything nowadays and that's been happening for years. Causes too much litigation, over stuff that's public domain really.
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