Sasmit,
I've tried to replace all of the transistors (driver and power) but now the problem won't go away. I've even tried to replace the LME49810 but still same problem.
I can post my new layout if you want to look at it.
Regards,
Simon H.A.
I've tried to replace all of the transistors (driver and power) but now the problem won't go away. I've even tried to replace the LME49810 but still same problem.
I can post my new layout if you want to look at it.
Regards,
Simon H.A.
I think your bias might be a little less for the 3 pairs of output transistors you have, the crunchy sound might just be crossover distortion . Can you measure the Vce ( one probe at collector and other at emitter ) of Q9.
VBIAS = VBE(1+RB2/RB1) (V) ( page 15 , national data sheet )
if you have Rb2 = 680 and Rb1 ( 500 pot + 300), your bias comes to around 1.11 . If you adjust your pot to 0 ohms you'll get around 1.9 volts which still not enough for 3 pairs. I think if you change the 300 ohms to 150ohms you might get the correct bias for 3 pairs which will be around 3.0 volts after adjusting the pot to 0 ohms.
Do not adjust the pot to 0 ohms at one go, slowly go on reducing till the crunchiness disappears. After every adjustment allow some stabilization time and keep measuring the drop across your output resistor ( one of your multimeters should be permanently monitoring the output resistors )
BTW you can look at my thread "miller cap" also, I was too dumb not to go through the data sheet carefully. Andrew pointed out some obvious mistakes in my input resistor values and gain setting was also wrong as pointed out by Godfrey.
My topology is a bit different though, it's slightly more complex than yours.Since I am testing only with one pair of output transistors ( although I have provision of 2 ), I had too much bias which I tried to reduce.
My topology is a bit different though, it's slightly more complex than yours.Since I am testing only with one pair of output transistors ( although I have provision of 2 ), I had too much bias which I tried to reduce.
Do not change to 150 ohms keep the 300 ohms and try adjusting the pot first . Since the 3 pairs of output are driven by only one pair of drivers , I think your bias should only consider the drops across the two drivers ( 0.6 + 0.6 ) ~ 1.2 at a minimum(
This is the Vce voltage of Q9 ). This should be increased to a larger value to increase current as the pairs of output devices increase. You have 3 so might require a bit more. My earlier calculation of requirement of around 3V might be too much. I added up the junction voltages of all your output pairs and thought it would require 0.6*6 but I think we should only consider your driver forward voltages.
Last edited:
I found that three pairs of output devices is prone to oscillation. I would suggest you to try one pair, two pairs before moving to three pairs.
You schematic shown in post #7 has the wiper open circuit. You layout doesn't. However, we better connect the wiper to one end of the pot to avoid wiper contact problem.
You schematic shown in post #7 has the wiper open circuit. You layout doesn't. However, we better connect the wiper to one end of the pot to avoid wiper contact problem.
- Status
- Not open for further replies.