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PostPosted: Mon Nov 20, 2017 12:00 pm 
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Cow

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As an improvement I tried to cascode the cascode. The sim said beautiful numbers.
But in reality I had to add a snubber in the cascode transistors midpoint to make it stable.

With this snubber it wasn't any better than the single transistor cascode.

What to try next?
I'm looking for about 10MOhm @ 10kHz impedance.
Attachment:
CCS_2.PNG


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PostPosted: Mon Nov 20, 2017 11:51 pm 
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Benjamin
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Kuei Yang Wang wrote:

So, there comes the reply:

The other place wrote:
This design is a good (and very clever) attempt to use a similar topology but it has one issue - MOSFETs when used as a common source in the VAS lack depth of imaging, and this is something that is only audible from listening.


So, there you go, any attempt to use any Mosfet, regardless of electrical parameters or application "lack depth of imaging". That's it. Great.

:rotfl: :rotfl: :rotfl: :rotfl: :rotfl: :rotfl: :rotfl: :rotfl: :rotfl: :rotfl:


"Lack depth of imaging"? That might be the most stupid thing uttered over there.

Quote:
Naturally, I have used this Mosfet as VAS in a number of applications and it is just dandy on imaging depth and being holographic. On listening.

And yes, it is better than using a modified Baxandall Pair (NPN Emitter Follower before a PNP VAS), which in turn is better than a simple temperature compensated BJT VAS (which is used in the original) again on listening.

Now there are some really good reasons I selected a Mosfet as VAS. If we look at the transconductance (gain) slope it is much different from a BJT, almost a pure straight line that will only produce H2. For more on this it is worth reading Samuel Groner's commentary on D. Selfs "Audio Amplifier Design hand book".

https://pdfs.semanticscholar.org/1375/ec0b3f02e765fd493437363e90abcf2c65bd.pdf



Interesting................especially the 7th harmonic, for the lowly 2N7000.

.......snip.................


Quote:
Now, how about we review some of the cuter tricks in the circuit, that got totally missed over the terrible, unforgivable, terminally stoopeed use of a Mosfet as VAS Stage and its "lack depth of imaging"...

.................snip.................

Oh if only I had not used this accursed, terrible, terminal Mosfet VAS, someone might have actually stopped to think and notice these subtle (actually, trust me, NOT SUBTLE AT ALL in listening) twists...

I already mentioned the boot-strapping of the differential pair's tail resistor, by using the resistive tail of the output follower, so no need to elaborate again.

Anyway, the reaction over there is really rather amusing. Reminds me why I'm outa there. Most of the self proclaimed Guru's holding court there have little more than prejudice and limited listening experience and an Ego that needs a 20ft Hanger door as workshop entrance. Good riddance.

Ciao T


Well, that is why it is best we are all out of there.

Look, I've said this before, and I will say it again................

T and I go at problems in much different ways. Somehow, we both get results that work. (At least for us.) We can debate why this way and not that way, and probably never agree, but that does not mean we don't respect the other guy, and say "You're full of crap!" Or "Anyone who isn't an idiot would do it my way."

No, that way doesn't cut it, at that other place. We are both outcasts, and in my case, downright dangerous.

I guess what really annoys me is that some of us pass on what we know, not as some proclamation from on high, but as a way to teach and inspire interested individuals to find what works best for them.

Instead, we have silly forums, based on hero worship, which is more like bowing down to false idols, but so be it.

But, when your hero is just some putz, who screams the loudest, then you deserve what you get.

[/rant]

Now, I have to think up something else to kvetch about.....................

Like IBS.................another one of life's "HA! Gotcha, ya old fart!" waiting for you, somewhere down the road..............


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PostPosted: Tue Nov 21, 2017 12:13 am 
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Benjamin
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hanski wrote:
As an improvement I tried to cascode the cascode. The sim said beautiful numbers.
But in reality I had to add a snubber in the cascode transistors midpoint to make it stable.

With this snubber it wasn't any better than the single transistor cascode.

What to try next?
I'm looking for about 10MOhm @ 10kHz impedance.


HUH??????

10 kHz....................10M....................

Unless my handy dandy Boonton reactance slide rule is broken, you can only have 1.56 pF.

Good luck!

The greatest invention of all time.......................surprised mine still works......................40+ years later...............

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PostPosted: Tue Nov 21, 2017 11:40 am 
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Muriel
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Hi,

hanski wrote:
Well, the sim didn't anticipate 50kHz resonance. It was worse there, but not horrible.


Gate stopper on lower FET, upper may not need it.

Ciao T

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PostPosted: Tue Nov 21, 2017 12:29 pm 
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Muriel
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Hi,

hanski wrote:
I'm looking for about 10MOhm @ 10kHz impedance.


Why?

A 5687WM at 10mA/125V (250V +B) will have around 3kOhm Anode impedance. With 300K you are probably past good & evil.

You could of course use a second half 5687 as cascode for the Depletion FET, I have done this many times with 10mA Idss selected 2SK170... This gives "only" 128kOhm Anode load in theory, but pretty much frequency independent and with a simple slope. I usually take the output from the upper valve cathode and use fixed grid bias on the lower valve...

Would looks something like this:

Attachment:
5687WB_LINE.png


Long standing personal fav line stage, with the actual attenuator AFTER this stage and if needed the input voltages padded to match... SNR is a trifle above 100dB re. 2V RMS Output.

We can trade SNR for THD by raising input levels...

Ciao T


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PostPosted: Tue Nov 21, 2017 1:30 pm 
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Cow

Joined: Fri May 30, 2014 6:24 am
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Kuei Yang Wang wrote:
Hi,

hanski wrote:
I'm looking for about 10MOhm @ 10kHz impedance.


Why?

A 5687WM at 10mA/125V (250V +B) will have around 3kOhm Anode impedance. With 300K you are probably past good & evil.
Ciao T

Perhaps some day I'll buy some vintage triode with high rp (plate resistance). :D
Overkill in diy circuits? Diy freedom?

This one I got over 10MOhm @ 10kHz. At higher frequencies it was about the same as single transistor cascode due to the snubber shunting the current.
Attachment:
CCS.PNG


The life of the middle transistor is not easy in this cascode. It has all three legs on slippery ice.
It's legs see only tiny varying capacitances to the outside world.
Upper and lower transistor have one leg in firm ground so their life is easier.

Would anyone have an alternative suggestion for the cascode fet?
It would need to have a few volts gate voltage with 10mA drain current.


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PostPosted: Wed Nov 22, 2017 1:26 am 
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Muriel
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Hi,

hanski wrote:
Would anyone have an alternative suggestion for the cascode fet?
It would need to have a few volts gate voltage with 10mA drain current.


Is anything wrong with using a second BSP129?

Ciao T

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PostPosted: Wed Nov 22, 2017 2:21 am 
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Benjamin
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hanski wrote:
This one I got over 10MOhm @ 10kHz. At higher frequencies it was about the same as single transistor cascode due to the snubber shunting the current.


Bet you didn't actually measure it, did you?


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PostPosted: Wed Nov 22, 2017 7:47 am 
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Cow

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Jocko Homo wrote:
hanski wrote:
This one I got over 10MOhm @ 10kHz. At higher frequencies it was about the same as single transistor cascode due to the snubber shunting the current.


Bet you didn't actually measure it, did you?

Me and a soldering iron. That's dirty.
Attachment:
Meassing.jpg

Signal generator feeds about 4Vp-p across 10R resistor. It's in series with a 100Vdc power supply. Then the EUT and then 330R resistor of which voltage is measured with oscilloscope 1mV/div.
(I don't own the stuff).

The setup shows nicely that a off the shelf 1M 1/4W through hole resistor is only 1M at audio frequencies (due to it's parallel capacitance)


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PostPosted: Wed Nov 22, 2017 10:54 am 
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Cow

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Hi!
I measured a few different combinations.
I don’t think there’s not that much real differencies with them in this use case.
The cascoded cascode seems to need a snubber always to be stable.

Some results with 10mA current (first transistor is CCS and second is the cascode):
BSS169 + BSS159 = 1MOhm @ 270kHz (since both are SOT-23 case, it's tested with lower voltage)
BSS169 + BSP149 = 1MOhm @ 50kHz
BSP129 + IXCP10M90S = 1MOhm @ 40kHz
BSS169 + IXCP10M90S = 1MOhm @ 40kHz

BSS169 + BSS159 + IXCP10M90S + 470p/100R snubber = 1MOhm @ 220kHz


I’ll think I’ll settle for the simple BSP129 + IXCP10M90S.
It’s simple and it’s current can be scaled easily.


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PostPosted: Fri Nov 24, 2017 4:47 pm 
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Cow

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Kuei Yang Wang wrote:
Hi,

hanski wrote:
Would anyone have an alternative suggestion for the cascode fet?
It would need to have a few volts gate voltage with 10mA drain current.


Is anything wrong with using a second BSP129?

Ciao T

I was slightly worried because it produces only 1V across the bottom FET with 10mA current.
But now I measured BSS169 + BSP129 and as BSP129 is lower capacitance device than e.g. BSP149 it gives good results.
I got 1MOhm @ 140kHz.
So it's over 10MOhm @10kHz!

Just got to remember that it's max. 340V part and the cooling needs to be arranged really well as it's SOT-223 case..


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PostPosted: Sat Nov 25, 2017 12:28 pm 
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Cow
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BSP129 is 240V part.
If you want higher voltage and depletion part, try DN2540. TO220 package available.

And if u are emotional attached to Infineon, all their portfolio:


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PostPosted: Sat Nov 25, 2017 4:23 pm 
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Goat

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DN2540 is a great part but is easy to damage. I suppose it has a very thin oxide layer. Employ anti-static measures when handling and soldering.


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PostPosted: Mon Nov 27, 2017 2:32 pm 
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God

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employ anti-stat when handling anything today.


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PostPosted: Mon Dec 04, 2017 2:45 am 
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Muriel
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Hi,

hanski wrote:
I got 1MOhm @ 140kHz.
So it's over 10MOhm @10kHz!

Just got to remember that it's max. 340V part and the cooling needs to be arranged really well as it's SOT-223 case..


Well, cascode it again with a higher power, higher voltage device?

Ciao T

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