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Old 13th October 2013, 12:33 PM   #21
Bensen is offline Bensen  Belgium
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Hi OS,

Could instead of connecting R25 and R26 to ground, two other resistors be placed between VAS base and collector (local feedback) or two resistors from VAS out to the LTP inverting input?

Why have choosen for this VAS loading?

Cheers
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Old 13th October 2013, 08:06 PM   #22
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Quote:
Originally Posted by Bensen View Post
Hi OS,

Could instead of connecting R25 and R26 to ground, two other resistors be placed between VAS base and collector (local feedback) or two resistors from VAS out to the LTP inverting input?

Why have choosen for this VAS loading?

Cheers
VAS loading -

Look at the comparison below:

Amp #1 - has no loading. LF gain is 78db. it's bandwidth (UG-unity gain) is 390Khz , and the lead
compensation (C6) had to be increased to 15pF to keep -103 degrees margin.
Less than 1db extra gain at 20K available for feedback. This combined with
the extra distortion of a larger C6 adds up to .01% THD20.

Amp #2 - Loaded by the 2- 100K resistors .LF gain is only 66db, the margin goes out to 618K
the gain at 20k is 32db! 32- the 26 of the amp leaves 6db extra.
Besides at much better .002% THD20 , C6 can be reduced to 10p to give the same -103 degree margin.
H3 also decreases with the loading.

Conclusion - loading sacrifices gain at lower frequencies for more gain at
the higher ones. 1/5th the THD at 20K is the result. TMC does the same
work in a typical "blameless" topology.
This is also why you see the THD plots of most amps " reach skyward"
at 25 to 30k - no extra gain above 20K !

PS-this is why Sansui used the loading resistor.

OS
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Old 13th October 2013, 08:25 PM   #23
Bensen is offline Bensen  Belgium
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This I understand OS.
Before I had read Cordell's book, loading the VAS with R's to ground is what I did also. "This is no the way a VAS stage should be treathed" according to Bob Cordell.

My current amp in progress is using high value R's between VAS output and base. It seems to me that this solution is minimum as good as VAS loading to ground.

Have you experience whit this?

Regards
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Old 13th October 2013, 10:16 PM   #24
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Quote:
Originally Posted by Bensen View Post
This I understand OS.
Before I had read Cordell's book, loading the VAS with R's to ground is what I did also. "This is no the way a VAS stage should be treathed" according to Bob Cordell.

My current amp in progress is using high value R's between VAS output and base. It seems to me that this solution is minimum as good as VAS loading to ground.

Have you experience whit this?

Regards
You mean C-B on the VAS devices. If that is what you mean....

(below) ONLY 54db LF gain (100k - 60db+ w/ 330k) , UG rises to 550+khz , Margin remains the same.
FFT remains the same (as with no loading). ?
Tried 330k as well ....
This seems to lower gain globally , instead of "shifting" it from LF to HF.
" no way to treat a VAS" ... ha ha ! It is treated far worse from the standard EF2 (load + capacitance).
OS
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Last edited by ostripper; 13th October 2013 at 10:20 PM.
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Old 13th October 2013, 11:25 PM   #25
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Originally Posted by lloop9 View Post
Wow, very nice map OS! The "Frankenstein" version is going to be my next project.

By the way, the name 'Survivor" reminds me of the 80s band that did the theme song "Eye of the Tiger" for Rocky III, a fitting name for a simple, rugged & quality amp.
Combined the 2 PCB's into one. Just print 1/2 the board for 70W , all of it for
"frankenstien" (400W).

76mm X 228mm , option to use TO-220 or TO-3p drivers (both pads).
You can mount the TO-3P on the main heatsink or use "bare" TO-220's above board (a little pcb heatsink ?)
Vbe will be a little TO-92 device face down under the board.

Break the board off (or don not print/transfer it) at the GLA70/GLA400 line
you have the original 70W Z5900 sansui.

OS
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Old 13th October 2013, 11:58 PM   #26
wahab is offline wahab  Algeria
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For reliability purpose it would be safe to put 1.5k-2.2k resistors in serial
with the regulating transistors wich should be BD139/140 or whatever cheap
TO126 complementary pair as the BC would have to endure too much
power for their rating , several hundreds mWs in that case.

Other than that i did a few sims to check the design and set apart
that the lead comp is mandatory the overall perfs are excellent ,
i ll post a few graphs once i have a better picture.
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Old 17th October 2013, 10:51 PM   #27
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Default I'm listening - I like...

The amp this project is reverse engineered from is indeed musical. The badger
has a little more bass ... but the GLA can have a bigger input cap.

As far as the circuit , a few improvements while preserving the "vintage" aspects.
#1 - level shifter has a added cascode (Q8)..can use KSA1845 safely here now.

#2 - C22 output "suckout" cap. The tiny switching distortion "pulse" is gone
from the stages.

#3 - C9 and C12 at 100uF. This brings PSRR (photo 1) to -82db , and even below
-90db at the high end (10Khz).
#4 - Reverse diode protection for the zener regulators (D2/3).
#5 - balanced the currents for Q1-4 , all are at 1.4mA (photo 2).

The final V1.2 schema (photo 3). .003% full power 20Khz at 2R !

OS
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File Type: jpg GLAv1.2-psrr.jpg (46.6 KB, 340 views)
File Type: jpg Q1-4.jpg (66.0 KB, 305 views)
File Type: jpg GLAv1.2.jpg (139.3 KB, 209 views)
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Last edited by ostripper; 17th October 2013 at 10:53 PM.
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Old 17th October 2013, 11:15 PM   #28
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I like the look of this little project ostripper, I will be keeping an eye on this one. Thanks for your efforts.
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Old 18th October 2013, 03:53 AM   #29
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Originally Posted by jkuetemann View Post
I like the look of this little project ostripper, I will be keeping an eye on this one. Thanks for your efforts.
A little variety is good !

Most have used the "blameless" , that design is overused.
Then there are the CFA amps (current feedback) , this amp CAN do
150+ V/uS slew ... and it is a VFA (voltage feedback) amp.


The boards will be done (below). This is a 200W/4R with only 2 pair 21193/4
50V rails .... perfect Sub amp .
The outputs will be driven by NJW0302/0281 (smaller outputs ! ).

This is the most stable triple OPS I've simulated ... PLUS , I have the real amp
for reference. I can't lose.

OS
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Last edited by ostripper; 18th October 2013 at 03:59 AM.
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Old 18th October 2013, 12:29 PM   #30
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Nice amp, this is my favorite old amp Pioneer A70.
All new Marantz amps are clone of old Pioneer amps, symetrical CFA are clone of Pioneer A400.
Regards
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File Type: jpg A70.JPG (121.2 KB, 218 views)
File Type: jpg A400.JPG (99.8 KB, 194 views)

Last edited by apexaudio; 18th October 2013 at 12:40 PM.
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