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Old 11th December 2008, 08:31 PM   #1
ROLIKOV is offline ROLIKOV  Israel
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Default X-5 bias adjustments

I have replaced 4 mosfets and 2 resistors to the center channel which needed repair ( receive matched pairs from PASS LABS )
Look like the amp work fine now ,
Let me say that i am the second owner of this lovely unit , and i guess this was repaired before .
Then i went to re- adjust the bias voltage to each channel ,
When i measured the bias voltage at each of the 2 pins located at the sides of the amplifier ( for each channel ) - i measured around 50mV ... ?? i am sure , it is not a mistake .
However - when i looked at the service manual - it says that it should be adjusted to around 950mV . ( 0.95v at each 2 pins )
So i stated to adjust each channel to 0.95v , but after 2- 3 minutes , i felt that the heatsink gets very hot , and since i was afraid that it will damage the mosfets or other parts , i moved back the bias to around 55- 60mV .( as it was before to each channel )
Now i am not sure what is the correct adjustment . ( 0.95v bring the amp to be very hot ...
Can someone please suggest ?
Thanks
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Old 11th December 2008, 09:17 PM   #2
Zen Mod is offline Zen Mod  Serbia
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dunno exactly how these bias pins are arranged in your X5 but - if they're accordingly to Papa's X5 SM downloaded from his own ftp - these pins are across TWO adjacent source resistors

taking in account that you have 4 pairs of output in each X channel - 0V94 means ~ 1A per leg , so four times that , multiplying with 66V ....


grande drek . 4Ax66V = 264VA per channel ; 1320W of dissipation overall ;

OK- for start - divide that figure (940mV ) with 4 ; - so go for 220mV

set , then see - is it 1320W/4 = 330W of dissipation still too much for heatsink ;

one thing is certainly sure - Papa never goes above 55C (maybe 60C in summer ) on heatsinks .

thats 3-5 secs max with hand on heatsink .

I hope that Grand Joker will give you real answer .
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Old 11th December 2008, 09:27 PM   #3
Zen Mod is offline Zen Mod  Serbia
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Owner Manual from PL site sez that dissipation is arround 200W at idle

so - 200/5 = 40W per channel

so - 10 W per each leg

10W/66V = 150mA

150mA x 2 x 0R47 = 140mV across those pins

measure , just in case , with amp in OFF -- have you ~ 1R across these pins ;

if yes - set it to 140mV and enjoy ;

if heatsink temperature allows that ( hand burning test ) , increase it to real skin burning
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Old 11th December 2008, 09:49 PM   #4
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A 950 mV figure is clearly an error. Go by what you see on the working
channels.

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