Hi there, This voltage section makes me puzzled! so far I guess that the problem in the voltage section. Please provide a solution to this issue. I already posted about that and got some advice. according those suggestion I have made some changes. My understanding of electronics and amplifiers is quite limited. Would you guys kindly assist me?
diagrams, both the original and updated versions, are attached.
diagrams, both the original and updated versions, are attached.
Attachments
You're missing a load resistor on Q2 so you're creating an imbalance. That's probably why the circuit has high DC offset. Try adding a resistor in series with Q2 collector equal to R7.
Tom
Tom
Are you complaining about DC offset? The obvious problem is R4,R5, and D1. With a 40V supply and two 10K resistors, D1 will never reach 22V, so it does nothing. R4 needs to deliver enough current for Q1 and Q2, which should be 2x 0.65/R7. With a 12VAC*1.4V supply =~16V, the situation is worse. Normally D1 is a lower voltage Zener. R5 needs to be small enough so that there is surplus current for D1 to start working. Then, R4 should be V(D1)/(IC Q1+IC Q2).
A lesser problem is that R3 and R6 are not the same value. This creates a small offset as they drop a different voltage from equal base currents in Q1 and Q2.
Loading Q2 is useless and a bad idea. The load on Q1 is a the Q4 BE diode, ie essentially nothing.
A lesser problem is that R3 and R6 are not the same value. This creates a small offset as they drop a different voltage from equal base currents in Q1 and Q2.
Loading Q2 is useless and a bad idea. The load on Q1 is a the Q4 BE diode, ie essentially nothing.
Modern amps use a CCS (constant current source) instead of R4+R5+D1 so that the LTP current is defined for a wide range of supply voltage. This helps with start-up and shut-down thumps because the LTP keeps working as the supply voltage goes up and down. An even better idea is a current mirror in place of R7 because the LTP current remains balanced with a very small supply voltage. R7 demands a fixed current (0.65/R7) potentially leaving no current for Q2. Older simple amps deliberately run Q2 with a large surplus of the emitter current.
Unless C2 is a 25 V type in which case D1 protects C2.
R9, R10, and C9 are shorted out. That's probably not what you want.
Tom
Yes, but as is, it could be unstable, oscillate. You need either a ~10nF cap across Q7 CE or a series base resistor ~220 Ohms on Q7.
The purpose of D1 is to create a ~constant voltage and therefore constant current in R4. Normally D1 is a ~7 Volt Zener and R4 would be about 3k3 => ~2mA+. R5 = (20-7)/(2+2)mA = ~3k3
R9, R10 and C9 are drawn horizontally, not vertically as they should be, but they are connected correctly.
R9, R10 and C9 are drawn horizontally, not vertically as they should be, but they are connected correctly.
@tomchr @steveu @xXBrunoXx
Hi to all of you. Thank you for your helpful feedback. Based on what I've learned thus far, there are two possible solutions: either remove the diode and use a transistor as a voltage stabilizer, or just replace the resistors R5 (10K) to 560R and R4 (10K) to 2.2K. I've redrew the diagram; kindly review it and provide your insightful comments.
[I'll have to attempt to install another transistor even though the circuit board is too congested to function.]
Hi to all of you. Thank you for your helpful feedback. Based on what I've learned thus far, there are two possible solutions: either remove the diode and use a transistor as a voltage stabilizer, or just replace the resistors R5 (10K) to 560R and R4 (10K) to 2.2K. I've redrew the diagram; kindly review it and provide your insightful comments.
[I'll have to attempt to install another transistor even though the circuit board is too congested to function.]
Attachments
Ok in option #1, Ry should be half R7 so that the total CCS is twice the current required by R7, ie Ry=~330 Ohms. Both Ry and R7 will have one diode voltage drop and we want the same current in Q1 and Q2, ie 2x I(R7).
If you are going to stick with D1=22V Zener, and not something like 7V Zener, then (40-22)/4mA is 4500, so R5= 3900 Ohms. R4 is 22V/2mA=11k, so 10K is fine. (I(Q1)+I(Q2)+I(D1)~=4mA)
There are finer points wrt C7 and the LTP current for the required slew rate, but as long as you are struggling with basic bias issues, I'll save that.
If you are going to stick with D1=22V Zener, and not something like 7V Zener, then (40-22)/4mA is 4500, so R5= 3900 Ohms. R4 is 22V/2mA=11k, so 10K is fine. (I(Q1)+I(Q2)+I(D1)~=4mA)
There are finer points wrt C7 and the LTP current for the required slew rate, but as long as you are struggling with basic bias issues, I'll save that.
When you look at the photo of the board, it very much looks like you have an unsoldered pin on one of the 5401?
@steveu
Hello, I appreciate all of your suggestions. I worked on it this morning early and made the necessary changes. When I adjust the VR1 the DC offset affected and the VR2 also changes the bias as well. I had not check that thoroughly, I left that at home due to my office time has started. I will do the remaining tasks after office.
This Circuit board suffer me a lot, its come with some wrong resistor, they put 100k instead of 100R and the D699 soldered wrongly . any ways, the vendor is a good person he sent me a new board for free. that's why I'm experiment on it, before that I didn't know what is DC offset, what is Bias current etc! yesterday I have learned new term LTP and CCS. thank you guys to educate me.
I'm using 12/0/12 volt because of it is quite hard to find genuine transistor in market place. most of 5200/1943 are fake. I only had six pairs of NJW 0281/0302 that one dealer arranged for me, and I've already blown one pair.
Hello, I appreciate all of your suggestions. I worked on it this morning early and made the necessary changes. When I adjust the VR1 the DC offset affected and the VR2 also changes the bias as well. I had not check that thoroughly, I left that at home due to my office time has started. I will do the remaining tasks after office.
This Circuit board suffer me a lot, its come with some wrong resistor, they put 100k instead of 100R and the D699 soldered wrongly . any ways, the vendor is a good person he sent me a new board for free. that's why I'm experiment on it, before that I didn't know what is DC offset, what is Bias current etc! yesterday I have learned new term LTP and CCS. thank you guys to educate me.
I'm using 12/0/12 volt because of it is quite hard to find genuine transistor in market place. most of 5200/1943 are fake. I only had six pairs of NJW 0281/0302 that one dealer arranged for me, and I've already blown one pair.
Attachments
@steveu @xXBrunoXx
Hello to all of you. There is activity on the amp. Without your suggestions and comments, it would not be possible to run the amp properly. Many thanks.
For of this modification, I was quite hassle to couple Q1 and Q2 Transistor. Rather, I make a bridge out of copper wire to allow the two transistors to share heat. It's a dumb idea, but I have gave it my best shot.
Hello to all of you. There is activity on the amp. Without your suggestions and comments, it would not be possible to run the amp properly. Many thanks.
For of this modification, I was quite hassle to couple Q1 and Q2 Transistor. Rather, I make a bridge out of copper wire to allow the two transistors to share heat. It's a dumb idea, but I have gave it my best shot.
