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Old 5th November 2005, 07:27 AM   #1
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Default bias adjustments

hello,

I would like to know what is the reason that bias settings on amplifiers go wrong sometimes. I built an amplifier and sometimes the bias setting goes to high and makes the output transistors overheat(luckily they dont get destroyed because i turned off the power before that could happen). Is it because of an instable house power supply or something else?
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Old 5th November 2005, 08:19 AM   #2
AndrewT is offline AndrewT  Scotland
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Hi,
what type of drivers and output devices do you have?
What system of thermal equilibrium (feedback) are you using?
If you use BJT or vertical FETs then you must use some kind of thermal compensator to control the bias.

There may be oscillation that causes the system to iffset and overheat due to excess current flow and due to one half dissipating more power.
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Old 9th November 2005, 02:14 PM   #3
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Default Bias woes...

Good Day,

My amplifier uses bipolar junction transistors in both the driver and output stages. The driver stage consists of 2SD669 and 2SB649 transistors, while the output stage consists of Motorola's MJ15003 and MJ15004. There is another 2SD669 transistor connected across the bases of the drivers. This transistor is the thermal compensation transistor and it will track the temperature of the output devices(mounted on the same heatsink as the output devices), adjusting the bias accordingly if the temperature gets excessive. I have emailed the schematic of my amplifier to you, Mr. Andrew because the thing is too large to be posted here. The thermal compensation transistor is Q9 on the schematic. The bias potentimeter is P1 and it is adjusted so that there is 47mV between the emitters or Q16 and Q17(the output devices). By the way, my amplifier has two of these built on seperate PCB's. Thank you for your help.

However Mr. Andrew, I need your email address please because I do not know how to resize the image of the schematic, it is about 2.42 megabytes. Please give your email address and I wil promptly send the schematic over. Thank you
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Old 9th November 2005, 02:36 PM   #4
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or it could just be a cheap pot that doesn't hold it's value. I saw a number of them during my calibration days.
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Old 9th November 2005, 03:19 PM   #5
AndrewT is offline AndrewT  Scotland
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Hi,
do not even think about sending a mega bit email to me!!!!!!
Find a way to post something on the forum so that all can help.
You have confirmed the drivers and output devices and the existence of a thermal compensator.

Following on from Retro try fitting a multiturn cermet pot with wiper connected to one end so that open wiper does not destroy your ouput stage. Is the pot in the base emitter side of the multplier?

I think it unlikely that house wiring could cause this.
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Old 10th November 2005, 10:51 AM   #6
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Good Day,

What is a "multiturn cermet pot"? Thank you for your help.
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Old 10th November 2005, 11:47 AM   #7
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Good Day

Here is the schematic diagram. managed to shrink it, and i hope it will help. Thank you.
Attached Files
File Type: zip amplifier project schematic diagram 5.zip (90.1 KB, 72 views)
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Old 10th November 2005, 05:32 PM   #8
AndrewT is offline AndrewT  Scotland
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Hi,
the bias adjust pot is wired in the simple way to save components.
This method is more likely to give problems.
It would be better if the upper leg of the multiplier was a resistor and the lower leg was the pot, preferably with a resistor in series.
It is possible this could be the problem you are experiencing.

What value of r11 is fitted?
It is there to prevent the VAS blowing up if the protection triggers!!!!!!

A multi turn cermet is a high reliability pot that need many turns to adjust it. Most are sealed against the environment.
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Old 11th November 2005, 12:17 PM   #9
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Good Day,

Thank you for that conclusion Mr. Andrew. R11 is 33 ohms. How big is this 'multiturn cermet pot'?
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