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Old 17th June 2017, 10:21 PM   #4151
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Default Epiphany's night.

I did not found VSSA's thread so I post here.

Dear LC, guys,

What if?
Most good things begin with this question.

What if if we combine one the best simple amps available (thanks to LC's generosity), the VSSA, and one the best amplifiers ever made, Sony TA-N7?
VSSA is "fast" and TA-N7 uses VFET in cascoded output, and both are "fast ".
I bought some NEC VFETs and want to try the cascoded output.
First I modded a BJT VSSA to a cascoded output with very old 2N5883/5885 transistors (gift) and NOS 2SK1173/2SJ473 low power transistors, to experiment and learn.

Please study cascoded ouptut theory from the net. I am not good enough to teach you. Briefly, there are many excellent improvements to be expected and, as any good society, both transistors mutually "protect" each other. Plus, when using VFET, apparently we can have a sort of automatic biasing, apart the protection, in which the low power device "sacrifices" itself in case of trouble.

The results of cascoding the VSSA output are astonishing to the point of absurdity. That was clear to me from first notes yesterday. The most open amp I ever heard...see: no matter how good is the CFA amp, the output devices will always limit the transparency. I imagined the way to go was this cascode-VFET solution, but the BJT-cascode is not bad at all!

Though I am still experimenting best bias, best connections etc. I cannot imagine how good it will sound with VFET, the superior transistor.

I will now "cascode" all my outputs from my different amps. I recommend that you experiment with this mod.
Attached Images
File Type: jpg CASCODO BJT2.jpg (802.7 KB, 217 views)
File Type: jpg CASCODO BJT COMPLETO.jpg (1,021.8 KB, 213 views)
File Type: jpg CASCODO BJT.jpg (1,011.7 KB, 205 views)

Last edited by maxlorenz; 17th June 2017 at 10:28 PM.
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Old 17th June 2017, 10:34 PM   #4152
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Schematic?
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Old 18th June 2017, 01:00 PM   #4153
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If I recall well...

Apparently you can connect Vbias for the cascode (common base BJT) to ground OR to the output node, but I did not tried this yet.
Besides, it is supposed to be "AC ground", with a cap (didn't try it yet), but it works like it is now and most schematics available for free do not include a cap, so I assume this provides "AC grounding"... I've researched for the theory behind "AC ground", unsuccessfully, so I would appreciate enlightenment from the gurus, on that subject, or in relation to any other aspect of the problem for that matter...

Apart from this mod, this cascoded output VSSA differs only in one aspect compared to my regular BJT-VSSA: one extra // 0u25 Russian polystyrene cap on the FB path. This caps improve things wherever I use it.

Cheers,
M.
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Old 18th June 2017, 07:34 PM   #4154
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ussr teflon caps ft-3 are better than any of their polystyrene
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