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Old 3rd December 2015, 10:04 AM   #1
Rokman is offline Rokman  Finland
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Question Cambridge Audio 840a Biasing

Hello.

I have a 840a v1 and i'm suspecting false bias. There is nothing mentioned in service manual about bias adjustment/voltage or current.

Could anyone help me with this? Please.

I quess the right resistors are r21 and r22 (both 0,1ohm), and xd should be "OFF", but i get only 2,7 mV reading ..
I assume this is too low ?? (chematics of 840W does mention these same resistor numbers and bias voltage 14 mV, but for 840W those resistors are out of question.. misprint ??, perhaps those values should be for 840A, but could someone please confirm this, or any other proper values!?

Last edited by Rokman; 3rd December 2015 at 11:02 AM. Reason: additional info
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Old 3rd December 2015, 12:10 PM   #2
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Quote:
Originally Posted by Rokman View Post
Hello.

I have a 840a v1 and i'm suspecting false bias. There is nothing mentioned in service manual about bias adjustment/voltage or current.

Could anyone help me with this? Please.

I quess the right resistors are r21 and r22 (both 0,1ohm), and xd should be "OFF", but i get only 2,7 mV reading ..
I assume this is too low ?? (chematics of 840W does mention these same resistor numbers and bias voltage 14 mV, but for 840W those resistors are out of question.. misprint ??, perhaps those values should be for 840A, but could someone please confirm this, or any other proper values!?
Hi!
I have the service manual of the 840A which is extremely detailed, but it doesn't say a single word about bias current values and adjustment.
However the 2.7 mV on the emitter resistors (0.1R) means 27 mA bias current per transistor, which is about the industry "standard" value for those kind of amplifiers.
With the +/-48V power supply rails and 2 pairs of power transistors it means cca. 5W idle dissipation per channel, again quite common in that power class.
Personally I wouldn't bother readjusting them, likely the designers were setting it to that value with a reason. Higher value can yield lower crossover distortion, but at the price of increased idle dissipation, and possible thermal instability
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Old 3rd December 2015, 06:46 PM   #3
Rokman is offline Rokman  Finland
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Quote:
Originally Posted by dragonweed View Post
Hi!
I have the service manual of the 840A which is extremely detailed, but it doesn't say a single word about bias current values and adjustment.
However the 2.7 mV on the emitter resistors (0.1R) means 27 mA bias current per transistor, which is about the industry "standard" value for those kind of amplifiers.
With the +/-48V power supply rails and 2 pairs of power transistors it means cca. 5W idle dissipation per channel, again quite common in that power class.
Personally I wouldn't bother readjusting them, likely the designers were setting it to that value with a reason. Higher value can yield lower crossover distortion, but at the price of increased idle dissipation, and possible thermal instability
Hi Dragonweed.
Sorry, I put it wrong, 2,8 mV is a sum over both resistors, actually one resistor is ~1,4mV/0,1ohm= 14mA. Am I right, and if so, isn't it too low value??
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Old 3rd December 2015, 06:58 PM   #4
Rokman is offline Rokman  Finland
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Hi again, of course should be mentioned this is a secondhand amp, previous owner did some preamp psu cap swapping, because of several shutdowns, what else, one doesn't know, so bias isn't perhaps what it was at factory ??
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Old 3rd December 2015, 08:07 PM   #5
Mooly is offline Mooly  United Kingdom
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2.8 mv as the sum across both 0.1 ohms sounds just about spot on as this amp uses a CFP type output stage. Increasing (or decreasing) that current will only worsen the distortion.
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Old 4th December 2015, 04:38 AM   #6
Rokman is offline Rokman  Finland
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Thank You guys for fast replies. The bias should be OK then..
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Old 4th December 2015, 07:47 AM   #7
Mooly is offline Mooly  United Kingdom
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Originally Posted by Rokman View Post
The bias should be OK then..
... yes.
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