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Old 17th June 2017, 10:21 PM   #4151
maxlorenz is offline maxlorenz  Chile
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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, 695 views)
File Type: jpg CASCODO BJT COMPLETO.jpg (1,021.8 KB, 669 views)
File Type: jpg CASCODO BJT.jpg (1,011.7 KB, 636 views)
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Last edited by maxlorenz; 17th June 2017 at 10:28 PM.
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Old 17th June 2017, 10:34 PM   #4152
AndriyOL is offline AndriyOL  Ukraine
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Schematic?
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Old 18th June 2017, 01:00 PM   #4153
maxlorenz is offline maxlorenz  Chile
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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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File Type: png CASCODO BJT SCHEMA.png (17.9 KB, 561 views)
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Old 18th June 2017, 07:34 PM   #4154
AndriyOL is offline AndriyOL  Ukraine
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ussr teflon caps ft-3 are better than any of their polystyrene
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Old 23rd September 2017, 07:38 PM   #4155
AndriyOL is offline AndriyOL  Ukraine
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Quote:
Originally Posted by Lazy Cat View Post
Requested by many, SSA BIGBT HP, SSA CCS schematics and final product successfully made in June 2012
Hi LC.

Why the components on PCB differ from the schematic?
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Old 25th September 2017, 01:42 PM   #4156
maxlorenz is offline maxlorenz  Chile
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Hi,

I would also like to have a pair of SSA BIGBT PCB to build a cascoded version instead of an original version...

See, my cascoded BJT-VSSA (posted above) has now +/-6VDC only for the "cascoded transistor" (the rest for the "audio" transistor) and it sounds wonderful to me.

So, the next step is a high voltage all cascode version.

For the record, I also cascoded the outputs of my BLAME ST from Destroyer X and I got the same increase in transparency and dynamics.
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File Type: png CASCODO BJT SCHEMA.png (20.4 KB, 255 views)
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Old 27th September 2017, 05:29 PM   #4157
padamiecki is offline padamiecki  Poland
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Hi!
did you check stability/oscillations isuues?
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Old 27th September 2017, 07:25 PM   #4158
maxlorenz is offline maxlorenz  Chile
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Thanks Padamiecki for your interest.

I checked mainly for power headroom and a short test until 20KHz with the VSSA, which is not in a proper case yet. I plan to test it further and maybe add Zobel network when "casing". Both amps run cool with low bias and sound nice to me, without any strange noise.
Only the VFET amps have more transparent sound than these. Actually, I also want to build cascoded VFET amps...

Regards,
M.
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Old 28th September 2017, 10:25 AM   #4159
padamiecki is offline padamiecki  Poland
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Probably you do not have stability issues because you reported good sound...
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