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Old 16th December 2014, 10:59 AM   #1
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Arrow "Compact" VFA

Hi All,

I was thinking about slowing down with new developments for a while and focus on building some earlier designs and refining some older PCBs.
However... a friend of mine asked for something rather compact yet high quality, utilizing the old slow MJL21193/21194 (I have also got some of these in stock).

Here is what I came up with. Nothing "revolutionary", mostly proven arrangements. Interesting for me (I've tried push-pull and single-ended with CCS load VAS topologies initially), this kind of balanced VAS with bootstrapped load showed an excellent simulated performance.

It's not live-tested yet (PCB design is in progress). Trying to squeeze the whole thing onto a 3"x5.5" inch board.
Power supply is +/-50V DC, quiescent current if 90mA per output pair.

It will also work with modern output pairs, like NJW3281/1302 of less powerful NJW0281/NJW0302, or more powerful Sanken MT-200 2SC2922/2SA1216 (although these ones will require certain PCB rearrangement due to their wider cases).
If somebody would like to experiment with alternative PCB designs, etc. - you are welcome.

Cheers,
Valery
Attached Images
File Type: jpg @Compact-VFA-01.jpg (495.0 KB, 751 views)
File Type: jpg Compact-VFA-PCB-in-progress-01.JPG (220.4 KB, 698 views)
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Old 16th December 2014, 06:53 PM   #2
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Nice simple amp.
I do not why but I like a lot simple design's.
The fine tune resistors ratio R20 and R29 in lower part of the VAS can give nice sound resaults.
Waiting for the pcb.

The input j-fets can be substytuted with BC546 ??


Regards
Peter
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Old 16th December 2014, 06:57 PM   #3
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Same as Borys, thread subscribed.

Marc
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Old 16th December 2014, 07:35 PM   #4
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Quote:
Originally Posted by vzaichenko View Post
Hi All,

I was thinking about slowing down with new developments for a while and focus on building some earlier designs and refining some older PCBs.
However... a friend of mine asked for something rather compact yet high quality, utilizing the old slow MJL21193/21194 (I have also got some of these in stock).

Here is what I came up with. Nothing "revolutionary", mostly proven arrangements. Interesting for me (I've tried push-pull and single-ended with CCS load VAS topologies initially), this kind of balanced VAS with bootstrapped load showed an excellent simulated performance.

It's not live-tested yet (PCB design is in progress). Trying to squeeze the whole thing onto a 3"x5.5" inch board.
Power supply is +/-50V DC, quiescent current if 90mA per output pair.

It will also work with modern output pairs, like NJW3281/1302 of less powerful NJW0281/NJW0302, or more powerful Sanken MT-200 2SC2922/2SA1216 (although these ones will require certain PCB rearrangement due to their wider cases).
If somebody would like to experiment with alternative PCB designs, etc. - you are welcome.

Cheers,
Valery
I have "live tested" on my last modular "supersym" (EF2).
Slight issues = Thump on startup ,
"un symmetric" performance - I used 2 diodes to drop the bootstrapped side rail voltage down as the bootstrap will swing higher than the active side , and the B-S resistors are rail dependent - I used about 10K between the 2 for 70V with 7ma VAS.
Had to drop to 6K to bench test at 40V.

Otherwise , the bootstrap actually sounded better than the CCS'ed version.
I like your CFP input /differential VAS scheme.

PS - did you read my mind ? .... A bootstrap was the next IPS I was planning.

OS
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Old 16th December 2014, 07:42 PM   #5
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Quote:
Originally Posted by vzaichenko View Post
Hi All,

I was thinking about slowing down with new developments for a while and focus on building some earlier designs and refining some older PCBs.
However... a friend of mine asked for something rather compact yet high quality, utilizing the old slow MJL21193/21194 (I have also got some of these in stock).

Here is what I came up with. Nothing "revolutionary", mostly proven arrangements. Interesting for me (I've tried push-pull and single-ended with CCS load VAS topologies initially), this kind of balanced VAS with bootstrapped load showed an excellent simulated performance.

It's not live-tested yet (PCB design is in progress). Trying to squeeze the whole thing onto a 3"x5.5" inch board.
Power supply is +/-50V DC, quiescent current if 90mA per output pair.

It will also work with modern output pairs, like NJW3281/1302 of less powerful NJW0281/NJW0302, or more powerful Sanken MT-200 2SC2922/2SA1216 (although these ones will require certain PCB rearrangement due to their wider cases).
If somebody would like to experiment with alternative PCB designs, etc. - you are welcome.

Cheers,
Valery
Nice! I've got a couple boxes of MJ21195/6 steel can TO-3s waiting for a home. Looks like a good candidate.
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Old 16th December 2014, 09:42 PM   #6
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Quote:
Originally Posted by ostripper View Post
I have "live tested" on my last modular "supersym" (EF2).
Slight issues = Thump on startup ,
"un symmetric" performance - I used 2 diodes to drop the bootstrapped side rail voltage down as the bootstrap will swing higher than the active side , and the B-S resistors are rail dependent - I used about 10K between the 2 for 70V with 7ma VAS.
Had to drop to 6K to bench test at 40V.

Otherwise , the bootstrap actually sounded better than the CCS'ed version.
I like your CFP input /differential VAS scheme.

PS - did you read my mind ? .... A bootstrap was the next IPS I was planning.

OS
Hi OS,

I see slightly asymmetric clipping here - exactly, as you say, bottom half swings a bit higher - however, clipping is rather soft, so I don't consider it as a big issue.

We think pretty close sometimes

Cheers,
Valery
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Old 16th December 2014, 09:53 PM   #7
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Arrow PCB

Done!

Who wants gerbers? Please send me your e-mails.
Jeff - I know yours

Q1, Q9 are expected to have a local heatsink.
Q15, Q16, Q17 go to the main heatsink.
Small output transistor cases are shown - MJL21193/21194 have got the bigger ones - anyway, the pinouts are the same, only the holes for them will go a bit wider...

Cheers,
Valery
Attached Images
File Type: jpg Compact-VFA-PCB-01.JPG (246.5 KB, 509 views)
File Type: jpg @Compact-VFA-PCB-01-Silk.jpg (208.8 KB, 467 views)
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Old 16th December 2014, 09:56 PM   #8
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I'm in! Would you mind sharing the DipTrace file as well? I'd like to experiment with the TO-3s.
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Old 16th December 2014, 09:59 PM   #9
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Quote:
Originally Posted by borys View Post
Nice simple amp.
I do not why but I like a lot simple design's.
The fine tune resistors ratio R20 and R29 in lower part of the VAS can give nice sound resaults.
Waiting for the pcb.

The input j-fets can be substytuted with BC546 ??


Regards
Peter
Hi Peter,

jfets are much more preferable here - with their high input impedance they allow good balancing of the circuit, also this IPS topology gives better linearity and sounds good. So... I would recommend to leave it as is
Unless you like to experiment

Cheers,
Valery
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Old 16th December 2014, 10:00 PM   #10
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Quote:
Originally Posted by jwilhelm View Post
I'm in! Would you mind sharing the DipTrace file as well? I'd like to experiment with the TO-3s.
This is what I was going to do
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