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Old 15th April 2017, 05:20 PM   #1
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Default CFP Amplifier - Help Please!

Hello guys.

I've made this amplifier and after a change of bias resistors on VAS transistors I forgot to connect GND on front end part of the board when powered it on for some seconds. The components on front end have stressed 80 VDC without central point connected.
When I powered it again correctly several faults appeared: DC offset before was 10-20mV, after fault became negative 100-200mV; voltage drop on 15V Zeners only 4.8V like on 5V Zeners, however they are not damaged, I replaced them; voltage drop on 1.5K feedback resistors - 0.475 on positive side, 0.925V on negative side. On the entering points of front end voltage - 40V on both sides.
I measured all front end transistors and haven't found any significant variations in voltage readings.
Could someone help me to find what's wrong with the amplifier?
From the schematic I use only 3 output pairs and 40/32 VDC PSUs.
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Last edited by AndriyOL; 8th May 2017 at 11:15 PM.
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Old 16th April 2017, 11:42 AM   #2
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Perhaps some of the input transistors aren't working correctly?
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Old 16th April 2017, 02:53 PM   #3
Nrik is offline Nrik  Denmark
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There is something wrong with that schematic.
How does the input section get power?
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Old 16th April 2017, 04:06 PM   #4
arpagon is offline arpagon  Bulgaria
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When speaking about BJTs I wouldn't rely on voltage measurements in order to check if they are defective or not. Just unsolder all 8 transistors of both inputs and measure them old fashion way (or measuring Hfe with multimeter).
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Old 16th April 2017, 04:53 PM   #5
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Quote:
Originally Posted by Nrik View Post
There is something wrong with that schematic.
How does the input section get power?
What's wrong with this schematic for 3 output pairs? It gets power from separate PSU.
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Old 17th April 2017, 09:16 AM   #6
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Originally Posted by arpagon View Post
When speaking about BJTs I wouldn't rely on voltage measurements in order to check if they are defective or not. Just unsolder all 8 transistors of both inputs and measure them old fashion way (or measuring Hfe with multimeter).
Do you think the problem in 8 center transistors? I thought it could be a top or bottom pair of transistors
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Old 17th April 2017, 10:19 AM   #7
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Quote:
Originally Posted by Nrik View Post
There is something wrong with that schematic.
How does the input section get power?
The drawing might be confusing. Look at the 15V zener. From here it is a current source for the input emitter which is 0.65V.
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Old 17th April 2017, 10:49 AM   #8
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Do you mean one of the first input transistors damaged? Btw, voltage drop on emitter resistors haven't changed after the fault.
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Last edited by AndriyOL; 17th April 2017 at 10:51 AM.
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Old 17th April 2017, 08:54 PM   #9
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I desoldered all 8 transistors and they appeared to be fine
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Old 18th April 2017, 01:51 AM   #10
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You may want to check the discussion on SSA thread or better yet send a pm query to member Lazyy Cat [Andrej Lakner]. He designed this amp..
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