Hi!
My friend has copied my F5 builds (CLC input filter, dual mono).
I was over at his house helping him with the start up procedure, and everything was going well until we were starting to turn up the juice (adjusting the bias and offset).
I noticed that turning the variable resistors had little effect on the voltage reading across R7\R8. The offset (measuring voltage across speaker terminals) responds normal to adjustment of the resistors, but when I reached a reading of ca. 30 mV across R7\R8 the resistors had reached their end point.
We have tripple checked every resistor position and value. We have checked all positions and directions of transistors. We were getting tired at the end, but we must be missing something quite obvious?
He has built two amps. Both amplifier sides of both amps behave in the same way.....Help?
My friend has copied my F5 builds (CLC input filter, dual mono).
I was over at his house helping him with the start up procedure, and everything was going well until we were starting to turn up the juice (adjusting the bias and offset).
I noticed that turning the variable resistors had little effect on the voltage reading across R7\R8. The offset (measuring voltage across speaker terminals) responds normal to adjustment of the resistors, but when I reached a reading of ca. 30 mV across R7\R8 the resistors had reached their end point.
We have tripple checked every resistor position and value. We have checked all positions and directions of transistors. We were getting tired at the end, but we must be missing something quite obvious?
He has built two amps. Both amplifier sides of both amps behave in the same way.....Help?
Thank you for your answer, PMA is helping me out. I have changed the resistor (R3 and R4) in parallel with each pot from 2K2 to 4K7. Before the switch I reached 30 mV. I now get 180mV across the R7\R8 resistors before the variable resistors (P1 and P2) reached their end point.
What value should I choose for R3 and R4 to reach the final goal of 590mV?
What value should I choose for R3 and R4 to reach the final goal of 590mV?
should ........
I believe you have 9V battery at home and multimeter
all you need more is search on forum for JFet Idss testing
not actually needed to solve this puzzle ( keep increasing value of drain resistors 'till you have proper Iq) but handy to know for future
build and learn ..... not just build
I believe you have 9V battery at home and multimeter
all you need more is search on forum for JFet Idss testing
not actually needed to solve this puzzle ( keep increasing value of drain resistors 'till you have proper Iq) but handy to know for future
build and learn ..... not just build
He/she has a problem now.
They need to know how to proceed with a solution.
We can all guess and suggest a change of resistor value (just as Pass has !).
But to be useful, we can see the results of the Idss and the voltage measurements and see what is not working as predicted and then suggest the modification that is required to suit the test results.
Guessing does not help anyone, least of all any Member that comes up against a similar problem.
This particular problem has been posted a few times in the past.
Let's be helpful and stop guessing !
I have changed the resistor (R3 and R4) in parallel with each pot gradually to 2K2, 4K7, 6K8, 8K2, 10K, 15K, 22K and finally to 33K. I have gradually increased the voltage across the R7\R8 resistors before the variable resistors (P1 and P2) reached their end point. Using 2K2 I got 30mV I now get 590mV using a 33K resistor. Seems like I have to use 39K to have som room for adjustment around the target value of 590mV.
The amp is stable and the measurements are normal.
Is it OK/normal to use such a high value for R3 and R4?.
The amp is stable and the measurements are normal.
Is it OK/normal to use such a high value for R3 and R4?.
Idss of input Jfets is too low , so voltage sag across drain resistor groups is too small to reach proper voltage , needed for adequate Ugs of output mosfets
whatever , to grasp that , you need more reading/thinking ......
put 1K in series with trimpots ........ back them down to zero before that , re-bias
it would be best to use two (matched) in position of one , with so low Idss
so - up two (say piggybacked) 170s and down two (say piggybacked) 74s
in that case , go back with resistor values
whatever , to grasp that , you need more reading/thinking ......
put 1K in series with trimpots ........ back them down to zero before that , re-bias
it would be best to use two (matched) in position of one , with so low Idss
so - up two (say piggybacked) 170s and down two (say piggybacked) 74s
in that case , go back with resistor values
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