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Old 22nd January 2013, 06:32 AM   #11
hm4nine is offline hm4nine  Europe
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Originally Posted by Gyuri View Post
hm4nine, you are ahead of me!
Yes, I know, I am a quick guy!
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Old 22nd January 2013, 07:49 AM   #12
AAAudio is offline AAAudio  United States
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I use a 1ohm RC filter which makes it possible for me to drop over 3v in each rail.

My voltage is 91/92vpp during operation

My caps are new.

It is curious. The amp is stable and stayed in idle for 16 hours straight with no problem. Bias is stable.

What could be the reason for the current imbalance? I will replace the mosfet but I want to understand. Thermal contact problem?

Thanks
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Old 22nd January 2013, 09:51 AM   #13
hm4nine is offline hm4nine  Europe
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Originally Posted by AAAudio View Post
I use a 1ohm RC filter which makes it possible for me to drop over 3v in each rail.

My voltage is 91/92vpp during operation

What could be the reason for the current imbalance? I will replace the mosfet but I want to understand. Thermal contact problem?

Thanks
Your measurements are in contrast to the specs of your tranny and elcaps you wrote in your opening post and let me assume that your tranny has secondaries with 2 x 33 VAC.
Besides this I don't think you have a thermal problem with this MosFet. I think that its matching was not optimal. Hope for you that you can get a better matched substitute.
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Old 22nd January 2013, 10:58 AM   #14
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Originally Posted by AAAudio View Post
Thank you.... what would be the audible effects of:

1) blown mosfet

2) mosfet not in shortcircuit but not working?

Thanks
i had a blown output on one of my aleph2's (not shorted, just dead) weeks went by before i discovered it. the only way i discovered it was that the heatsink seamed cold right in that output area. so i checked the voltage over that Source resistor. and sure, it was 0.00mV. the other 5 transistors was at a little under 600mV at that time. wich is a raise of about 100mV pr fet *5 fets. its easy to calculate that the total current flow was exatly the same as With 6 outputs
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Old 22nd January 2013, 01:03 PM   #15
AAAudio is offline AAAudio  United States
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So it might be that the 800mV is failing and is overloading. it then allows the other mosfets to pass less current in order to meet the current draw of the gain devices?
Failure can.be gradual no? it is not only opencircuit/shortcircuit
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Old 22nd January 2013, 09:18 PM   #16
AAAudio is offline AAAudio  United States
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I replaced the mosfet. The values are now correct. 540mV on each resistor. There is still some variance but I think it is because of crap resistors (will order 1% soon)

The boards and circuit is here:

Pass Labs ALEPH-2 DIY amplifier PCB

another questions. I used a pot in place of the 56,2k resistor to change bias. I turned it all the way so that there is maximum resistance (maximum bias). The strange thing is that initially when the amp is turned on the bias levels are low: 2.7v on the RC 1ohm resistors so 2.7A then it varies for some time. I haven't had time to allow the amp to stabilize but is it normal such behaviour?

what is the bias behaviour before the amp fully stabilizes? Does it vary a lot?

Thanks!
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Old 22nd January 2013, 09:26 PM   #17
Zen Mod is offline Zen Mod  Serbia
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why don't you ask all those Q a guy who sold you pcbs ?



do not worry about initial offset (up to 350mV cold ) and bias ;

what's important are values you have/set when amp reach temperature equilibrium
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Old 22nd January 2013, 09:30 PM   #18
AAAudio is offline AAAudio  United States
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I will

So just to be sure: largely varrying values are ok when cold. Offset can be high at frst (good to hear). Bias can be low in the beginning and then increase with time.
Correct?

I am afraid equilibrium is an issue. My heatsinks are 60x60x10cm and weigh 30kg each. I am afraid I migh have exagerated!
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Old 23rd January 2013, 07:36 AM   #19
AAAudio is offline AAAudio  United States
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Could I just ask confirmation that a 200k resistor in place of the 56.2k resistor will increase bias to the limits imposed by the circuitry?

Thank you
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Old 23rd January 2013, 09:44 AM   #20
Zen Mod is offline Zen Mod  Serbia
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yes
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