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Can someone explain how this preamp section works?
Can someone explain how this preamp section works?
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Old 1st March 2018, 05:25 PM   #1
Hanatsu is offline Hanatsu  Sweden
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Default Can someone explain how this preamp section works?

Hi, I'm repairing a old Denon POA-2400 amp that has been dead for years. Both main boards were shot, with all all output transistors shorted B/C/E. Some work has been done to boards before and they obviously failed to repair them properly. A lot of traces and vias were damaged and to be fixed. Long story short, I've already repaired the right side board with both input Jfets dead and several diodes, resistors and all transistors in the output section's negative side replaced.

The left board is still acting up with horrible distortion coming from the preamp section, haven't troubleshooted it in detail yet but looking at the schematic I noticed something strange (to me) and I admit that I don't fully understand how the preamp section works here. The main question I got is why there are two more transistors in preamp section feeding PNP side of the output section than the other side? I'm referring to TR510 and TR516 (marked with red crosses).

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Old 1st March 2018, 06:19 PM   #2
aparatusonitus is offline aparatusonitus  Croatia
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Tr516 +D506 = current mirror load for 3rd ltp. Tr510 is a cascode (common base) in order to equalize voltage (disipation) over Tr512/514.
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Old 1st March 2018, 06:42 PM   #3
Hanatsu is offline Hanatsu  Sweden
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Ah, I see - thanks for the reply!
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