What's the deal with the power section in my Adcom amp?

[NavinRJohnson]

It's been over 20 years since I received an electronics degree I never utilized. Hopefully you guys can help jog my memory a bit as I really want to start getting back up to speed.
I've now taken on a more aggressive project, other than soldering pre-made boards, to help lubricate my old wheels. It's an older Adcom amp that had bad caps on the PSU that were repaired and still has some slight hiss issues that I'm working through. I'm sure I still have a few caps to replace and I need to clean up the rat nest wiring.

In the meantime I had a few questions about the power section and a couple transistors
1: The transformer has 4 output taps and is running the input (preamp?) section at 65Vdc and the power section mosfets at 55Vdc. This seems common among some of the "higher end" amplifiers. What's the advantage of running the front end at a higher voltage? Just lower gain so noise is less of an issue?

2:The +55V and +65V rails are connected just before R905 in the PSU section. A friend of mine suggested suggested this was to keep power to the mosfets on the input section during power down because there is more capacitance on the power section PSU board. That sounded like it fit the bill but, wanted to make sure I wasn't missing something else. Sound correct to you folks?

3:Q633 and Q635 right before the power section followed by the zener diodes. I honestly can't make heads or tails of what's going on there. Any ideas on the purpose of these transistors?

The full res schematic is at the end of the attached PDF if you can make out the image below.

 

[Jony130]

Q1 - To reduce the output stage saturation voltage. Thanks to this the front end is able to fully drive the output stage.
Don't forget that the output stage work as a common drain amplifier. And this means Vsaturation > Vgs(th).
Also notice that front end use cascode amplifier witch also have high Vsat.
http://www.apexanalog.com/wp-content/uploads/2012/10/AN48U_4.pdf

Q2 - I would say simple RC filter.

Q3 - Zener diodes are overvoltage protection diodes. Because MOSFET's has low Vgs_max 20V or so.
Q633 and Q635 are simple over current protection circuit.
http://users.ece.gatech.edu/mleach/lowtim/prot.html
http://www.thatraymond.com/downloads/solidstate_guitar_amplifiers_teemu_kyttala_v1.0.pdf

 

[NavinRJohnson]

Thanks Jony! I really appreciate the links and input.

Q1: I vaguely remember saturation voltage and I'll need to read up to understand this better. Thanks for pointing me in the right direction.

Q2: Not sure I understand how connecting the positive rails would help filtering in this circuit. Can you expound on this?

Q3: Something is wrong, I understand this with your links!

 

[Jony130]

Q2: Not sure I understand how connecting the positive rails would help filtering in this circuit. Can you expound on this?

I don't understand what you are trying to say?
And maybe R905 is also a inrush current limiting resistor when power up.

Q3: Something is wrong, I understand this with your links!

What is wrong ? It seems that they made a error on the diagram. And you need to swap the emitter with collector for Q635.

 

[alphy]

I can say that the power section in your Adcom amp is a crucial component that helps to regulate and amplify the electrical signals in your amplifier. It is responsible for providing the necessary power to drive the speakers and produce sound.

Regarding your first question, the advantage of running the front end at a higher voltage is to allow for lower gain and thus less noise in the system. This is because higher voltages can provide a stronger signal and require less amplification.

In response to your second question, your friend's suggestion seems plausible. The connection between the +55V and +65V rails may be to ensure that the power to the mosfets is not interrupted during power down. This is to prevent any potential damage to the amplifier.

As for your third question, Q633 and Q635 are most likely used for voltage regulation. The zener diodes are also used for voltage regulation, as they have a specific breakdown voltage that can be used to regulate the voltage in the circuit.

Overall, it seems like you are on the right track with your project and are taking the necessary steps to troubleshoot and improve your amplifier. Keep up the good work and don't hesitate to reach out for further assistance or clarification. Best of luck!