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Old 15th February 2019, 01:03 PM   #1
gile73 is offline gile73  Serbia
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Default Crescendo ME bias problem

Hi All,

I've just finished my late project, building Crescendo Millennium amp and I run to a problem with Bias. Namely, I cant reduce bias setting to the desired value. Bias is twice higher than it suppose to be. Between TP3 and TP2 I measure 0.1V instead of 0.05V. I assume that one of the fet transistors is oscillating. Can anybody advice on this? Thank you a lot!
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Old 15th February 2019, 02:35 PM   #2
Bensen is offline Bensen  Belgium
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Hello,

Did you use the original elektor pcb or layout?
If not, put the gate stoppers as close as possible to the K1530 and J201 gate.

You could lower R28 from R270 to R100, to increase bias spreader voltage.
Regard
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Old 15th February 2019, 09:47 PM   #3
gile73 is offline gile73  Serbia
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Hi Bensen,

Yes, I have used PCBs from Elektor and all components are premium grade all values are as prescribed.
Could you please elaborate further on these stoppers since I am novice in building fet amp. Thank you in advance.
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Old 16th February 2019, 11:45 AM   #4
Bensen is offline Bensen  Belgium
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I've build the Crescendo ME about 8years ago and used self made PCB and had connected the output devices with 4inch wires to the pcb. placing the gate stoppers R32 +R33 directly to the gate of the FET stopped the oscillation.



I've noticed I've giving you wrong advise regarding the bias spreader output voltage! I read your bias current is to high (0.1V/0.22R = 0.45A). You have to lower the bias spreader output voltage!
You have to increase R28, try 560R. There is almost no risc involved, the higher R28, the lower the bias current throught the output fets.


Search for bais spreader of vbe multiplier (Vgs multiplier if Fet) for more details.
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Old 16th February 2019, 06:11 PM   #5
gile73 is offline gile73  Serbia
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Thanks. I've just tried advised resistor increase, no success. Now I got 0.9V. Any other idea? Now i started to believe that 2sk537 is not original...
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Old 16th February 2019, 06:29 PM   #6
nikosokey is offline nikosokey  Greece
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Hi, maybe your mosfet are not original....
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