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

  • Thread starter Thread starter NavinRJohnson
  • Start date Start date
  • Tags Tags
    Amplifier Power
AI Thread Summary
The discussion revolves around troubleshooting an older Adcom amplifier, focusing on its power section and associated components. The user is investigating the advantages of running the preamp section at a higher voltage compared to the power section, with suggestions that it may reduce noise due to lower gain. There are inquiries about the connection of power rails and their role in maintaining power to the input section during power down, as well as the function of specific transistors and zener diodes in overcurrent protection. Participants provide insights on saturation voltage, RC filtering, and potential errors in the schematic, indicating a need for further clarification on these topics. Overall, the conversation aims to enhance understanding of the amplifier's design and functionality.
NavinRJohnson
Messages
7
Reaction score
2
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.
 

Attachments

  • adcom_gfa-5400_sm.pdf
    adcom_gfa-5400_sm.pdf
    1.2 MB · Views: 2,080
  • schematic.jpg
    schematic.jpg
    50.4 KB · Views: 513
Engineering news on Phys.org
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
 
Last edited by a moderator:
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!
 
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.
 

Attachments

Hi all I have some confusion about piezoelectrical sensors combination. If i have three acoustic piezoelectrical sensors (with same receive sensitivity in dB ref V/1uPa) placed at specific distance, these sensors receive acoustic signal from a sound source placed at far field distance (Plane Wave) and from broadside. I receive output of these sensors through individual preamplifiers, add them through hardware like summer circuit adder or in software after digitization and in this way got an...
I have recently moved into a new (rather ancient) house and had a few trips of my Residual Current breaker. I dug out my old Socket tester which tell me the three pins are correct. But then the Red warning light tells me my socket(s) fail the loop test. I never had this before but my last house had an overhead supply with no Earth from the company. The tester said "get this checked" and the man said the (high but not ridiculous) earth resistance was acceptable. I stuck a new copper earth...
I am not an electrical engineering student, but a lowly apprentice electrician. I learn both on the job and also take classes for my apprenticeship. I recently wired my first transformer and I understand that the neutral and ground are bonded together in the transformer or in the service. What I don't understand is, if the neutral is a current carrying conductor, which is then bonded to the ground conductor, why does current only flow back to its source and not on the ground path...
Back
Top