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Old 11th April 2009, 06:23 PM   #761
roender is offline roender  Romania
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Andrew and Sheldon,

What power BJTs did you used, NJL3281/1302 or NJL0281/0302?
There is a big difference in Cob between them, 600pF vs 400pF, but I'm not sure if this could be a problem ...
Maybe a little more lag compensation may solve the problem?

M
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Old 11th April 2009, 07:31 PM   #762
pooge is offline pooge  United States
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Quote:
Originally posted by roender
Andrew and Sheldon,

What power BJTs did you used, NJL3281/1302 or NJL0281/0302?
There is a big difference in Cob between them, 600pF vs 400pF, but I'm not sure if this could be a problem ...
Maybe a little more lag compensation may solve the problem?

M
has anyone sim'd this amp? Andy_C has posted some models of the 3281/1302, which is a lot easier to obtain than the 0281/0302s.
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Old 11th April 2009, 07:58 PM   #763
AndrewT is offline AndrewT  Scotland
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I used 3pair NJL3281/1302.
I have a temporary PSU. +-30mF @ the rectifier and +-4700uF on the PCB.
I have built everything else as the schematic.
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regards Andrew T.
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Old 11th April 2009, 10:40 PM   #764
Sheldon is offline Sheldon  United States
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Quote:
Originally posted by AndrewT
I used 3pair NJL3281/1302.
I have a temporary PSU. +-30mF @ the rectifier and +-4700uF on the PCB.
I have built everything else as the schematic.
Same transistors as AndrewT. Everything else exactly as spec'd. I have no problem powering up the amp. It's only probing the front end, or connecting something to the input.

Sheldon
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Old 13th April 2009, 06:10 AM   #765
roender is offline roender  Romania
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Guys,

Did you put the heat sink to GND?
If the problem still persist then try to increase the input capacitor to 220/330pF ... maybe your environment is full of EMI
I have just checked my amplifier with the input unconnected, taping with an 1m unisolated coper wire in the input RCAs. No problems here

Hope this help,
M
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Old 13th April 2009, 01:41 PM   #766
Sheldon is offline Sheldon  United States
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Yes, heatsink grounded. Will try increasing input comp cap later today.

Sheldon
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Old 13th April 2009, 07:35 PM   #767
pooge is offline pooge  United States
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OK. The difference in output transistors appears fundamental here. Other than a change in lag cap, can there be any other issue or fix. The change in R6/R41 was mentioned. Is this change common to all three here?

Also, can this output be sensitive to the capacitance of the transistor insulator pads? If so, which pads were used?
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Old 13th April 2009, 07:54 PM   #768
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What about increasing the VAS current?
As far as I remember it was 2 or 3 mA?
Try 10mA.
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Old 13th April 2009, 08:06 PM   #769
Sheldon is offline Sheldon  United States
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Added 100pf in parallel with the 100pf in place. Square wave shows no overshoot at all. Clean, but as expected more rounded leading and trailing edge, with the transition now around 36kHz.

However, if I plug and unplug the output from the signal generator, fuses go poof.

The insulators are some material I got at a local electronics surplus place as a sheet, so I don't know exactly what it is. By construction, it's a grey polymer reinforced with what looks like fine square woven glass, and with a self stick adhesive on one side. It's about 0.2mm thick.

Sheldon
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Old 13th April 2009, 08:14 PM   #770
roender is offline roender  Romania
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Quote:
Originally posted by stinius
What about increasing the VAS current?
As far as I remember it was 2 or 3 mA?
Try 10mA.
What is your reason for that?
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