resistors and Bias
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wiseoldtech
diyAudio Member

Join Date: Dec 2018
Quote:
 Originally Posted by Ian Finch We've seen the service manual advice and the specified bias current measurement but it says nothing about the direction or setting for minimum or maximum bias. Steveu has analysed the circuit and determined the wiper should be in the R1 position for maximum bias. Perhaps this is a typo because there's no R1 in this section of the amplifier and he means the RH position. This is the logical direction but let's see if he has a comment to add.

I would rig up the amp on a dim bulb, then center the adjustment, then rotating it while measuring idle current.
That would clear up any questions, simple enough.

 Yesterday, 09:13 PM #32 dj_holmes   diyAudio Member   Join Date: Nov 2004 Location: london Thank you for all the contribution people but honestly I never thought it was going to be this difficult!!!! I mean the potentiometer varies something right? The reason I am so keen because its eating me alive! I presumed that if I can understand one I will understand more. I know someone will now post never presume anything!
Ylli
diyAudio Member

Join Date: Oct 2018
Location: Upper midwest, US
Shall we walk through a quick analysis?

Let's start out by making some assumptions:
1. The output, (C306 negative terminal), remains very near zero volts like a good little amplifier.
2. The drive from the connector pin 6 remains fixed. It won't but close enough for this analysis. (Just to put a number on it, say 2 volts.)
3. The bias feedback signal (Lbias) is working properly, but is of a small enough magnitude to ignore.

OK, let's continue:

1. Q305, Q306 and the associated resistors form a Sziklai pair configured as a Vbe multiplier. 2. VR302 adjusts the voltage seen at the positive terminal of C306.

3. If we increase the resistance of VR302, the voltage at the positive terminal of C306 will increase so as to provide sufficient current to turn on Q306. Note there is no indication in the schematic if maximum resistance of the pot is fully CW or fully CCW. If you can find the circuit board layout, you may be able to determine it one way or the other.

4. When the voltage on the C306 increases, that increase will be seen by R309 and the emitter of Q304 increasing the emitter and hence the collector current.

5. An increase of the collector current of Q304 increases the voltage dropped across R315. This will pull the gate of Q308 closer the ground, increasing it's drain current. (since the source is connected to the negative rail)

Conclusion:

*Increasing* the resistance of VR302 *increases* bias current. Again, note that depending on how the part is configured on the board, maximum resistance might well occur when the pot is set fully CW. But then again it might not.
Attached Images

Last edited by Ylli; Yesterday at 10:15 PM.

JMFahey
diyAudio Member

Join Date: Mar 2009
Location: Buenos Aires - Argentina
Quote:
 Originally Posted by dj_holmes Do I set the boas to maximum resistance or lowest then to get minimum bias?
For the 100th time: that question can not be answered until you post the proper schematic.

Whay is that simple truth so hard to understand for you?

Quote:
 I never thought it was going to be this difficult!!!!
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Design/make/service musical stuff in Buenos Aires, Argentina, since 1969.

Last edited by JMFahey; Yesterday at 11:09 PM.

 Yesterday, 11:24 PM #35 TonyTecson   diyAudio Moderator     Join Date: May 2003 Location: Maybunga, Pasig City a good way to start would be to know that: 1. tubes are normally conducting with zero grid bias, so that a resistor placed at the cathode ensures the grid is in negative voltage bias territory, a grid leak resistor to ground is mandatory... 2 bipolar transistors are normally off devices, you have to inject base current to get the collector current to conduct....jfets too... 3. enhancement mosfets are also normally non conducting...whereas depletion mosfets are like tubes in that they are normally conducting drain currents.... so with this you can start your own journey in the business of biasing... __________________ planet10 needs your help: Let's help Ruth and Dave...http://www.diyaudio.com/forums/plane...ml#post5010547[B
dj_holmes
diyAudio Member

Join Date: Nov 2004
Location: london
Quote:
 Originally Posted by Ylli Shall we walk through a quick analysis? Let's start out by making some assumptions: 1. The output, (C306 negative terminal), remains very near zero volts like a good little amplifier. 2. The drive from the connector pin 6 remains fixed. It won't but close enough for this analysis. (Just to put a number on it, say 2 volts.) 3. The bias feedback signal (Lbias) is working properly, but is of a small enough magnitude to ignore. OK, let's continue: 1. Q305, Q306 and the associated resistors form a Sziklai pair configured as a Vbe multiplier. 2. VR302 adjusts the voltage seen at the positive terminal of C306. 3. If we increase the resistance of VR302, the voltage at the positive terminal of C306 will increase so as to provide sufficient current to turn on Q306. Note there is no indication in the schematic if maximum resistance of the pot is fully CW or fully CCW. If you can find the circuit board layout, you may be able to determine it one way or the other. 4. When the voltage on the C306 increases, that increase will be seen by R309 and the emitter of Q304 increasing the emitter and hence the collector current. 5. An increase of the collector current of Q304 increases the voltage dropped across R315. This will pull the gate of Q308 closer the ground, increasing it's drain current. (since the source is connected to the negative rail) Conclusion: *Increasing* the resistance of VR302 *increases* bias current. Again, note that depending on how the part is configured on the board, maximum resistance might well occur when the pot is set fully CW. But then again it might not.

Thank you for this break down!

dj_holmes
diyAudio Member

Join Date: Nov 2004
Location: london
Quote:
 Originally Posted by JMFahey For the 100th time: that question can not be answered until you post the proper schematic. Whay is that simple truth so hard to understand for you? YOU are being incredibly stubborn and difficult about NOT doing your homework and instead begging/pushing/screaming for an impossible answer.
I cannot seem to attach the schematic but it can be downloaded here

NAD C300 - Manual - Stereo Integrated Amplifier - HiFi Engine

Ian Finch
diyAudio Member

Join Date: Apr 2010
Location: Coffs Harbour, NSW
Quote:
 Originally Posted by Ylli Shall we walk through a quick analysis?....................... 3. If we increase the resistance of VR302, the voltage at the positive terminal of C306 will increase so as to provide sufficient current to turn on Q306. Note there is no indication in the schematic if maximum resistance of the pot is fully CW or fully CCW. If you can find the circuit board layout, you may be able to determine it one way or the other................*Increasing* the resistance of VR302 *increases* bias current. Again, note that depending on how the part is configured on the board, maximum resistance might well occur when the pot is set fully CW. But then again it might not.
Just go to post #15 and look at the parts overlay for the PCB. That's where you see which end of the pot is common with the wiper and identifies which way the pot should be rotated to increase or decrease resistance. Is it so hard for everyone to scan the thread fully, click on the thumbnail and see that the OP already supplied the information?
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