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New complementary V-FET amp: Best from the past ?
New complementary V-FET amp: Best from the past ?
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Old 1st May 2010, 10:12 PM   #1
mos57 is offline mos57  Italy
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Default New complementary V-FET amp: Best from the past ?

Hi everybody,
in the last months i have developed a 50 watt class AB power amp with the employment of complementary Sony V-Fet 2SK82 2SJ28. The input and driver circuit was thought to employ also low voltage power supply. Another my will was to configure the output stage to have also voltage gain, so the result was a topology similar to trans-nova.
Input stage is a folded cascode, driver stage is configured as a simply bjt follower to add up the complementary signals for the power stage. The little caps in the circuit not are much necessary, but in audio test with my friends, seems better with it in.
In reality i have prepared 2 main driver circuit: one with all j fet cascode and another with jfet-bjt cascode.
Distortion measurement tell me that the best was version B (all jfet), but in audio test my friends prefer version A (jfet-bjt). This last has a faster and cheerful sound with more mid-bass and the sound seems more out of speakers.
Version B is preferred if it's necessary to put in evidence especially the high or if you prefer a more relaxed sound.

Francesco.
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File Type: bmp V-fet complementary 2SK82_2SJ28_Ver_A.bmp (99.8 KB, 819 views)
File Type: bmp V-fet complementary 2SK82_2SJ28_Ver_B.bmp (99.8 KB, 488 views)
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Old 2nd May 2010, 06:17 PM   #2
housing is offline housing  Hong Kong
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How do they sound? Do you have any pictures showing the amps? I'm curious.
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Old 2nd May 2010, 07:44 PM   #3
mos57 is offline mos57  Italy
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Quote:
Originally Posted by housing View Post
How do they sound? Do you have any pictures showing the amps? I'm curious.


Hi, at this time i have realized only a prototype.
Here the photos. Sorry, Data of photo machine isn't current time.

Francesco.
Attached Images
File Type: jpg SANY3778.jpg (127.3 KB, 1521 views)
File Type: jpg SANY3779.jpg (150.3 KB, 1501 views)
File Type: jpg SANY3780.jpg (180.2 KB, 1465 views)
File Type: jpg SANY3781.jpg (159.2 KB, 1414 views)
File Type: jpg SANY3782.jpg (108.8 KB, 1380 views)
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Old 19th May 2010, 08:30 PM   #4
mos57 is offline mos57  Italy
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Hi people.

I have released a new other version of this amp.

You can see the figure also in my website:AMPLIMOS one stage amplifiers, amplificatori audio monostadio

There are others good schematics.

Francesco
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File Type: bmp V-fet complementary2SK82_2SJ28_Ver_C.bmp (127.7 KB, 463 views)
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Old 20th May 2010, 09:33 AM   #5
housing is offline housing  Hong Kong
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It looks great! Is there a substantial amp built or just a single channel prototype?
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Old 20th May 2010, 03:36 PM   #6
mos57 is offline mos57  Italy
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It is currently only a prototype channel.
In the past, prompt I have made the PCB, but now I devote more time to optimize the schema and components employed, helping with listening sessions, before making the printed circuit boards.

If there is sufficient demand (GB?), I can achieve and provide the amplifers PCB (ver. A, B or C) with all components and also completed with power Vfets.

Francesco
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Old 30th November 2012, 12:00 PM   #7
indra1 is offline indra1  Indonesia
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Hi Francesco,

Last week I found 8 pcs of k79 and 4 pairs of K70 and J20 sitting nicely in a friend's electronic parts store. I am contemplating how to build an amp that brings out the sound characteristics of the parts. Very happy to have just discovered this thread.

However, I still could not understand how you get a voltage gain on the output stage vfet with source follower configuration as per your schema. Do you think that voltage gain on the output vfet is mandatory to realize the vfet sound characteristic?

Thanks in advance
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Old 30th November 2012, 09:45 PM   #8
mos57 is offline mos57  Italy
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Quote:
Originally Posted by indra1 View Post
Hi Francesco,

However, I still could not understand how you get a voltage gain on the output stage vfet with source follower configuration as per your schema. Thanks in advance
This output stage isn't follower configuration. See where the GND is applied: it is on the two emitters, so the output stage transistors are in common emitter configuration. Therefore on its Drains there is voltage gain. The amplified signal is then applied to output across the two big caps. This is "transnova" topology, and i have just choose this because it allow:
1) very simple driver circuit for Vfet or Sit final stage
2) voltage gain of the o/p stage
3) low voltage rails for input and driver stages. You can choose a better variety of active devices. Generally low voltage transistor have better performances than high V.

Quote:
Originally Posted by indra1 View Post
Do you think that voltage gain on the output vfet is mandatory to realize the vfet sound characteristic?


No at all. In the past Vfet are employed in source configuration (except in the Yamaha B1) so you can obtain a classical sound characters of ancient vfet amps.
But in my opinion the true sound of vfet are different.
This because when output stage is a follower, the swing amplitude of the signal is generated by first stages. So if for example we have a 3 stage amp with 1 and 2 stage realized with Bjt and the output stage with follower Vfets, the voltage gain is realized by first 2 stages. In that conditions the shape of the output signal is mainly due to the distorsion of these first stages. Vfet follower limits itself not to adds more distorsion.
But if you can choose from which devices you can obtain the need amplification, you sure choice the more linear component.
So i prefer Voltage gain realized from Vfet also, because they are more linear.
Vfet amplifier with voltage gain output stage moving on the reference compared to the old amplifiers known

Francesco
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Old 1st December 2012, 01:51 AM   #9
indra1 is offline indra1  Indonesia
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Ok, I get it now, thanks for the explanation.
I'm planning an amp with the K79 vfet input followed with a 2SA929 and bootstrapped 2SC1845 bjt follower to drive the transnova output stage.
Due to my rusty skill and a very busy schedule, development will be very slow. However, I will post progress and schematic along the way.
I value any suggestion that you may have.
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Old 19th June 2013, 08:37 AM   #10
Mavwong is offline Mavwong
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Mos, thanks for the circuit. I would like to try some SIT as well. Is ur circuit input single ended and the bulk of the front end is doing inverting?

If I already have a balance output from my preamp, how would I change the circuit?
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