Bryston 4B SST clone

Jump to Latest
F

Fetamp

Member
Joined 2017
chalky said:
Hmm thats odd. So Q1 and Q2 are cold but Q3 and Q4 are hot - is that right? If so, that suggests that there is very unequal current sharing in the Q1/2 and Q3/4 differential pairs. Are D1 and D2 inserted the right way around?
Click to expand...

Sorry Chalky as you highlighted I rechecked keeping few minutes all transistors became hot except Q9,12...Q9 and 12 little hot.D1,2 are in correct way.when disconnect output of input board from AMP they are cold.
 
C

chalky

Member
Joined 2003
Can you measure the voltage at the output of the input boards when they are disconnected and then connected to the power amp. It may be that your input boards are oscillating when connected to the power amp. Do the power amp output transistors get any hotter after you've connected the input board.
 
F

Fetamp

Member
Joined 2017
chalky said:
Can you measure the voltage at the output of the input boards when they are disconnected and then connected to the power amp. It may be that your input boards are oscillating when connected to the power amp. Do the power amp output transistors get any hotter after you've connected the input board.
Click to expand...

Thanks chalky....Output transistors are cold all the time.no significant difference in DC voltages of output of input board when disconnecting and after connecting .it’s around 3.5mv
 
F

Fetamp

Member
Joined 2017
amplidude said:
Sounds a lot like oscillations, can you check with scope, but still strange, the pairs should share current better., maybe signal ground on/off creates ground loop.
Try break one off signal ground.
Click to expand...

Problem solved....I changed R13 and 14(150R) to 1.5k as compared to Same resistor in power amp board it’s too low.But I did it blindly so I kindly request from Chalky and Amplidude to give me exact theoretical value or to redesign it for me.
Thanks
 
C

chalky

Member
Joined 2003
Hmmm. Not really a solution as you've reduced the VAS current from about 12mA to 1mA - too low. The opamp circuit in the power amp operates under very different dc conditions to that in the input stage so you can't go swapping component values from one to the other. I suspect that you need to insert C16 and C17. These are not usually necessary but if you have very high gain BC546C/BC556C in the VAS position the circuit may oscillate. On my input board there is provision for C16 & C17 but nor sure if the numbers are silkscreened on. Anyway see if you can find the positions and start with 100pF COG capacitors. If this solves the problem try lower values, like 33 or 47pF. Stick with the lowest value which solves the problem.
 
F

Fetamp

Member
Joined 2017
chalky said:
Hmmm. Not really a solution as you've reduced the VAS current from about 12mA to 1mA - too low. The opamp circuit in the power amp operates under very different dc conditions to that in the input stage so you can't go swapping component values from one to the other. I suspect that you need to insert C16 and C17. These are not usually necessary but if you have very high gain BC546C/BC556C in the VAS position the circuit may oscillate. On my input board there is provision for C16 & C17 but nor sure if the numbers are silkscreened on. Anyway see if you can find the positions and start with 100pF COG capacitors. If this solves the problem try lower values, like 33 or 47pF. Stick with the lowest value which solves the problem.
Click to expand...

Thanks a lot Chalky......highly appreciate for your input as I’m not an expert in electronics like you.if you can kindly give me answers for following questions.
1) why my input and output board’s same section behave in different way (hot transistors)

2) did you have same experience in your input board....if not please give your transistor numbers that you have used

3) please give another substitute for BC546/556 which can resolve my issue.

4)There are places to C16/17 in my board but I have no COG capacitors....may I try ceramics?
Thanks
 
C

chalky

Member
Joined 2003
Thet behave differently because they have different vas ransistors; PN100/200 vs 2N5401/5551 in the original, and BC546C/556C vs 2N5401/2N5551 in your version. You can try the 2N5401/2N5551 pair in place of Q6,Q5 but leave the others unchanged. Don't forget to put the 2N5401/2N5551 in backwards as the pinout runs in reverse order from the BC546C/556C. You could also try the BC546B/556B. In both cases the lower gain might stop the oscillation, if that is what is happening. I'd try C16/17 first.
 
F

Fetamp

Member
Joined 2017
chalky said:
Thet behave differently because they have different vas ransistors; PN100/200 vs 2N5401/5551 in the original, and BC546C/556C vs 2N5401/2N5551 in your version. You can try the 2N5401/2N5551 pair in place of Q6,Q5 but leave the others unchanged. Don't forget to put the 2N5401/2N5551 in backwards as the pinout runs in reverse order from the BC546C/556C. You could also try the BC546B/556B. In both cases the lower gain might stop the oscillation, if that is what is happening. I'd try C16/17 first.
Click to expand...

Thanks chalky.....will precede with your advice and will update you
 
F

Fetamp

Member
Joined 2017
chalky said:
Thet behave differently because they have different vas ransistors; PN100/200 vs 2N5401/5551 in the original, and BC546C/556C vs 2N5401/2N5551 in your version. You can try the 2N5401/2N5551 pair in place of Q6,Q5 but leave the others unchanged. Don't forget to put the 2N5401/2N5551 in backwards as the pinout runs in reverse order from the BC546C/556C. You could also try the BC546B/556B. In both cases the lower gain might stop the oscillation, if that is what is happening. I'd try C16/17 first.
Click to expand...

Hi chalky....
1)tried adding c16/17 no positive outcome

2)Q6,5 changed to 2N numbers ...Q1,2,3,4 are became cold :):):)but Q6,5,7,8 same as previous
Thanks
 
You must log in or register to reply here.
Top Bottom