I wanted to build a DIY amplifier for my speaker. I just assembled it on a perfboard for testing, and when I tried adjusting the bias, nothing happened. When I checked, there was no bias voltage, so I started checking from the long-tailed pair and found weird voltages. I tried replacing the current mirror transistors (Q4 and Q3), and there was no difference, so I tried replacing Q5 and Q1, but nothing. Looking at the board, I don’t really see missing connections. I tried disconnecting the VAS, and nothing really changed. Where should I look?
Also to mention i still havent added the output transistors and i was meassuring for bias between R19. In here some meassurments are different because on the first one i disconected the VAS because i thought thats the problem but i dont think it is really that. Here the VAS is connected.
When meassuring from ground there was about 0,45V on both sides of R19. Between R8 and R17 is 12,64V.
When meassuring from ground there was about 0,45V on both sides of R19. Between R8 and R17 is 12,64V.
There's way too much DC voltage drop across the bias spreader Q10, almost the entire supply.
Try adjusting the base resistor "pot".
Try adjusting the base resistor "pot".
I have the divider but the input connects do nothing since i havent added the output transistors.
Im not sure exactly what i did but now i get 11V from the collector of Q5, 1,5V on the collector of Q1, 0,6V on the collector of Q2 and 10,3V on the base.
Seems much better. Maybe you had a loose connection.
Hard to know without the exact schematic and voltage readings.
Remember all the usual "grounds" should be at 10V instead of 0V, due to the single supply.
Hard to know without the exact schematic and voltage readings.
Remember all the usual "grounds" should be at 10V instead of 0V, due to the single supply.
So i looked at it one more time and the collector of Q11 wasnt connected to anything. I connected it and now the bias adjustment works fine but im still getting weird voltages on the collectors of the ltp? on Q1 im getting 1.7V and Q2 0.55V. I checked the connections and everything is right.
If you don’t have output devices installed, there’s no bias path to base of Q2. I’m guessing you have 8 ohm load connected at output, so the Q2 base is probably held near 0V. The base must be about mid-supply for proper bias.
Given the large voltage you reported across C5 earlier, I surmise none of the four outputs are present. Assuming this is still the case, try lifting the top lead of R34 and temporarily connecting it to collector of Q7. Q7 collector should bias to mid-supply.
im getting about 9,6V on the base, but honestly im probably giving up on this since the bias spreader keeps breaking and i tried everything, i checked all connections and i dont see loose ones, i changed the pot and looked at for about 20 mins and couldnt get it to work and now randomly out of nowhere it started workimh :/