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Old 8th January 2019, 10:59 AM   #21
vzaichenko is offline vzaichenko  Russian Federation
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Originally Posted by 35101 View Post
UgsQ5=Ur27+Ud3
That's right but that's ok. Otherwise, clipping begins a bit earlier for the bottom half-wave in quasi configuration. By the way clipping at the bottom is not as good and clean as at the top - this is something to think about in the future

P.S. Clipping with complementary OPS is very nice and clean.
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Old 8th January 2019, 02:18 PM   #22
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Quote:
Originally Posted by vzaichenko View Post
That's right but that's ok. Otherwise, clipping begins a bit earlier for the bottom half-wave in quasi configuration. By the way clipping at the bottom is not as good and clean as at the top - this is something to think about in the future

P.S. Clipping with complementary OPS is very nice and clean.
All rubbish UD3 increase by Q5.
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Old 8th January 2019, 05:29 PM   #23
vzaichenko is offline vzaichenko  Russian Federation
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All rubbish UD3 increase by Q5.
Well, the voltage gain of Q5 is close to 1 - what will amplify the rubbish are the output mosfets. However, the amount of rubbish with the diode and without the diode is practically the same - whatever happens in the negative rail, adds to Vbe of Q5.
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Old 8th January 2019, 07:27 PM   #24
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Should resistors R17/19/21/23 be on mosfet Q5/7/9/11 sources instead on drains in quasi output stage? Top mosfets operate as common drain (gain less then 1), but lower ones operate as common source (with the gain more then 1) as they see speaker +drain resistors as load? Also, would putting resistors in sources instead of drains in lower half quasi OPS will give better current sharing between mosfets and better temperature compensation?
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Old 8th January 2019, 10:02 PM   #25
vzaichenko is offline vzaichenko  Russian Federation
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Originally Posted by aparatusonitus View Post
Should resistors R17/19/21/23 be on mosfet Q5/7/9/11 sources instead on drains in quasi output stage? Top mosfets operate as common drain (gain less then 1), but lower ones operate as common source (with the gain more then 1) as they see speaker +drain resistors as load? Also, would putting resistors in sources instead of drains in lower half quasi OPS will give better current sharing between mosfets and better temperature compensation?
Gosh! Of course, they have to be on sources. Thank you for pointing this out.
I have them on sources in the initial design and simulations, but then re-drawing the schematic in Diptrace with some copy/paste always leads to some stupid mistakes
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Old 9th January 2019, 08:40 AM   #26
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Originally Posted by vzaichenko View Post
Well, the voltage gain of Q5 is close to 1 - what will amplify the rubbish are the output mosfets. However, the amount of rubbish with the diode and without the diode is practically the same - whatever happens in the negative rail, adds to Vbe of Q5.
Ud3=U(j6)-Uc2. U(j6) do not constant.
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Old 9th January 2019, 09:09 AM   #27
vzaichenko is offline vzaichenko  Russian Federation
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Originally Posted by 35101 View Post
Ud3=U(j6)-Uc2. U(j6) do not constant.
Uc2 is not constant either. We can consider a forward-biased diode being a voltage reference (a level shifter) in a wide range of currents.
As long as the "garbage" amplitude at j6 is not too high, it will be replicated at C2. Only if its amplitude will make D3 close at some positive peaks - then we have a certain difference of potentials at D3's ends.

In overall, you're right - it' will be better to move the diodes further "to the left" in this quasi design - putting them between the drivers and the VAS stage.

After all, unless I find a good solution for clipping in the negative half of the output signal, I consider the quasi option as an experimental one.

Cheers,
Valery
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Old 9th January 2019, 12:31 PM   #28
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Could you try in sim whether adding "Baxandall diode" parallel to R26 in quasi output stage brings anything useful...the idea is to mimic the same paired V drop and impedance characteristic in the lower output half of the output stage as the paired base-emitter junctions occurring already in the top half.
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Old 9th January 2019, 12:37 PM   #29
vzaichenko is offline vzaichenko  Russian Federation
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Originally Posted by aparatusonitus View Post
Could you try in sim whether adding "Baxandall diode" parallel to R26 in quasi output stage brings anything useful...the idea is to mimic the same paired V drop and impedance characteristic in the lower output half of the output stage as the paired base-emitter junctions occurring already in the top half.
I have already performed some experiments - because of the HexFETs' rather high Vgs, I have to use something higher-voltage than a diode, but the idea is pretty much the same and it works well, also eliminating the "blip" when clipping. Please see the schematic in the next post.
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Old 9th January 2019, 12:45 PM   #30
vzaichenko is offline vzaichenko  Russian Federation
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Arrow Updated Quazi OPS with lower distortion and good clipping

OK, now this one is good.

The network with zener at the driver's emitter leads to slightly lower distortion and much better clipping behavior. The bottom side clips a few volts earlier than the top one, but that's not an issue, considering there's no rail sticking or any other artifacts - clipping is flat now.

I will update the layout accordingly soon.

P.S. The OPS runs well at the quiescent current = 100mA per output pair. It can be raised up to, say, 300mA per pair for widening the class A region - just make sure to have some good heatsinks in this case - overall dissipation at 1.2A of the total idle current is not a joke anymore With +/-55V rails it comes to 130W per channel.
Attached Images
File Type: jpg 03 Referenz Output Quazi Sch 1.2.jpg (295.9 KB, 273 views)
Attached Files
File Type: pdf 03 Referenz Output Quazi @Sch 1.2.pdf (845.7 KB, 30 views)
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Last edited by vzaichenko; 9th January 2019 at 12:52 PM.
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