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Old 1st March 2010, 10:41 AM   #7211
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it will be close to zero ohms.

to check the pots are still working measure Ohms from the two legs joined to the leg by itself.. if the pot is melted replace it.

are you aware you need to adjust both pots to get the correct bias and dc offset?

for example if you wind both pots up a turn at a time you may get (example) 0.4 V and 100mV dc offset you then need to increase the othe pot which brings down the dc offset and then re measure the bias which will have probably dropped. then increase the other pot and repeat until you get the correct bias on both resistors and also a low dc offset.

how i did it was:

1 turn on P1, one turn on P2 (bias increases) say approx 0.05 V
1 turn on P1, one turn on P2 (bias increases) say approx 0.15 V
1 turn on P1, one turn on P2 (bias increases) say approx 0.25 V
1 turn on P1, one turn on P2 (bias increases) say approx 0.35 V
1 turn on P1, one turn on P2 (bias increases) say approx 0.45 V
1 turn on P1, one turn on P2 (bias increases) say approx 0.55 V
Wait about ten mins for temp to stabilize

Add a small amount to P1 , add a small amount to P2 increase or decrease P1 and P2 slightly to achieve <10mV DC offset and 0.59 or 0.6V across R11 and R12

Does that make sense to you ?

what i am trying to get across is you will need to increase the resistance on both pots slowly until you reach the correct bias level and then adjust P1 and P2 either up or down to get the correct values.

the amp is quite stable and predictable when biasing.


-Dan
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Old 1st March 2010, 11:09 AM   #7212
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Thanks for your instructions, I almost did the same thing!

But as I said I made the mistake to fire up the amplifier (for a minute) with the pot not in zero but in full resistance (my mistake).

so now the problem is:

the working channel : when I get 0.59v(stable) along R11-R12 the output gives 0.35v (stable) is there also a problem? (i left this channnel for 30 minutes but there isn't any changes in dc, also the thermistor are not very closed to mofsets body, only very closed to mofset legs)


the problematic :as I said R12=0.20v and R11=0v when I setup R12=0.59v then R11=0.15v (I didn't count the output dc offset) when I put R11= 0.45v then R12=1.2v.... In addition the feedback resistor get really hot in a second!!!

The first thing I am going to do is to check the trimpots
What else I have to pay attention or measure?
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Old 1st March 2010, 11:24 AM   #7213
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Maybe I lost both channels!!!!!!!
Now I thinking, maximun 0.05v dc offset and I have around 0.35v........
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Old 1st March 2010, 11:26 AM   #7214
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when you are adjusting the bias you need to adjust one at a time to balance the bias on the resistors aswell.
when i was biasing mine i went turn for turn on the pots and the Volts across R11 and R12 was within 0.1 V the whole way.
after you get the bias on both resistors to be the same then check the DC offset. they should not be far apart though.

any how try this before you start desoldering things i think it still may be an adjustment to get it right.

-Dan
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Old 1st March 2010, 11:33 AM   #7215
sangram is offline sangram  India
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don't set full bias on one resistor and then check the other. Bring both up slowly.

I actually never measure the bias on both resistors. I keep the meter connected across only one resistor (R11 or R12) and another meter plugged into the output. Fiddling with the pots back and forth still increases the overall bias (even if I'm adjusting the 'half' that the measured voltage is not part of). This way I might end up with slightly unequal bias across the two halves due to device non-matching but the offset is very stable.

I use the offset to set the amp. Keeping the offset within a 100mV range and adjusting back and forth. As the bias for the -ve channel goes up, offset moves negative. Once it reaches -100mV, I go and bring up the +ve bias and the offset also goes toward +100mV. Once I hit 450mV bias, I narrow the offset range to +/-10mV and do the final adjustment. Usually takes about five minutes to get stable offset and bias this way.
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Old 1st March 2010, 11:35 AM   #7216
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That is a good method also sangram (also easier to understand !!!!)

-Dan
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Old 1st March 2010, 01:28 PM   #7217
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As I said in the working channel I have stable bias in both resistors 0.59v (though It' correct biased), but the dc offset is stable at 0.37v(too much) not even 0.05v!

The channel that has problem there is no way to make stable the bias between the two resistors! In addition when the pots are to zero position the bias to the one resistor is 0.25v and zero to the other!



If the pots are to zero ohn then the bias to the resistor should be also zero ohm?
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Old 1st March 2010, 01:38 PM   #7218
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by prooptiki View Post
As I said in the working channel I have stable bias in both resistors 0.59v (though It' correct biased), but the dc offset is stable at 0.37v(too much) not even 0.05v!
this tells you that the 0r47 resistors are a different value.
You could have selected them for better matching.

Now ignore one of the Vrs and instead monitor the output offset and the other Vrs. Get the offset down to spec value.
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Old 1st March 2010, 01:40 PM   #7219
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by prooptiki View Post
The channel that has problem there is no way to make stable the bias between the two resistors! In addition when the pots are to zero position the bias to the one resistor is 0.25v and zero to the other!
Hi,
set both the variable resistors to zero ohm. Measure them using resistance range.
Power up.
Measure the voltage across the variable resistors. It should be zero volts.
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Old 1st March 2010, 03:19 PM   #7220
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Quote:
Originally Posted by AndrewT View Post
Hi,
set both the variable resistors to zero ohm. Measure them using resistance range.
Power up.
Measure the voltage across the variable resistors. It should be zero volts.
I use 2% resistors
Mayde when I burnt them they lost their value?
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