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Old 22nd February 2007, 11:00 AM   #1
roender is offline roender  Romania
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Another small step ahead
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Old 22nd February 2007, 11:02 AM   #2
roender is offline roender  Romania
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Will be done at the end of this week !
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Old 22nd February 2007, 11:08 AM   #3
roender is offline roender  Romania
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I make a little modification in VBE thermal tracking stage.
2n5551 and 2n5401 mounted as diodes (BE) for tracking the drivers in CFP in series with 2nNJL diodes for finals
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Old 27th February 2007, 09:17 AM   #4
roender is offline roender  Romania
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Symasym HighPower, it's working and it is stable!
I had to make some modification in order to make it work:
1. I dropped the 2sk246 as cascode jfet in LTP because all my jfets are Y grade with only 200mV Vgs at 2ma LTP tail current. I put in that position 2n5551 and i added 12 volt zener diode
loaded by 1mA CCS in common base of cascode devices. All BJT's used as cascode has base stoppers in order to prevent possible HF oscillations.
2. I had to increase C8 and C9 in VAS from 100p to 220p and to decrease C15 in feedback network to 6p8, to stop some random oscillation.
3. I put two antiparalel 1n4148 diodes at the output of LTP for preventing heavy VAS saturation on clipping
4. Iq is set at 140mA (70mA on each output peer)

Some characteristics:
- very powerful compared with original symasym, ease to increase the number of output BJT's and rail voltages to gain more power.
- stable in occasional 2ohm load
- very stable bias current
- with input in short circuit, only -0.1mV Vout and this DC voltage is very stable with temperature
- no turn on/off thump
- very silent, even with input not shorted to ground
- if any fuse blow, the output will drop at near zero DC ( suitable for crowbar DC protection)

How it sound?
It is very pleasant surprise to be more dynamic than symasym and the soundstage is huge. It sound very uncompressed, with more clarity and volume in mid range and blade sharp but not
annoying trebles.
Bass is very very tight but not so deep, my first impression is that it is a little under symasym at this chapter, an "audiophile bass", well suitable for jazz and ambiental music.
In my opinion, it is a good amplifier and I will invite you my friends to build it.

ps. I will put later some pics and pcb EAGLE files
pps. to moderators ... maybe it's a good idea to make a separate thread?
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Old 27th February 2007, 09:39 AM   #5
roender is offline roender  Romania
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Schematic, final version
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Old 27th February 2007, 06:28 PM   #6
MikeB is offline MikeB  Germany
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Hi Roender, nice job! I like it.
Would you mind making a layout for normal MJLs? I have so many left...
Have you tried/evaluated the changes to each stage?
I'm quite sure that the 0.1mv DC is because of the modified current mirror. Do you have any experience about the effect of the cascodes?
I am happy to read that you were able to further improve the dynamics, i guess it's because of further improved psrr?

Good to see that you use a central power-gnd star. What color is the LED? IR?

Some will call that amp a complexity beast...

BTW, thanks for having done my job...

Mike
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Old 27th February 2007, 06:50 PM   #7
roender is offline roender  Romania
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Hi Mike.
Regarding cascoding VAS, in the first place I was convinced that this is not a good idea because it loose all 2nd harmonic ... until I added a CFP driver and decreaseD VAS load to ground. Sudennly, all 3,5 and 7 harmonics vanish 20dB below previous level, at the same level with 2nd
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Old 27th February 2007, 07:16 PM   #8
MiiB is online now MiiB  Denmark
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Just for fun i have simplified things and fitted a Lateral mos outputstage biased by a simple resistor.
simulationvise it maintains the nice distortion distribution.
I am quite curious of the drive capability of the small to 92 transistors, anyone care to try!!
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Old 27th February 2007, 07:23 PM   #9
roender is offline roender  Romania
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Quote:
Originally posted by MikeB

Would you mind making a layout for normal MJLs? I have so many left...
...
What color is the LED? IR?
I will send you all the files (EAGLE) to make any modification you want, but first I will add on PCB the positive LTP CCS (now is on a addon PCB)

The LED is a 3mm red one
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Old 27th February 2007, 07:37 PM   #10
roender is offline roender  Romania
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some pictures
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