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Satgroen 13th January 2021 07:38 PM

Design of AC-DC Converter
Hi all,

A few years ago I got the trusted FA960 Philips Amp from my dad.

After some more years of good service, it has given up last week after a short death bed.

However, when opening to see if operation is possible I found that the amplifier board itself has given up hope and the other parts are still functioning properly.

Instead of buying a new one I want to try to make a retro build of it. I am still fairly new to the audio world and the only project I have done so far is updating my old wooden boxes to bluetooth speakers. To repair the Amp I want to update the amplifier board for a new model. I found a new board that might suit me (I am still new to the audio world):

AA-AB32221 2x150W TAS5613 Class-D Amplifier Board - SoundImports

However, updating the part results in a minor issue. The trafo used has an output of 38.7 or 24.3AC while all the new boards work with DC.

I have two options to solve this:
  1. add a regulating voltage supply parallel to the mains
  2. Use the trafo to create a DC signal
The first option has a drawback that I have limited size (the place where now the board is for full solution implemantion) and I was told this can decrease the quality of the audio output.

Therefore I am looking into the second solution.

After some research I found a proper schedule. to create a fitting solution. However, I am having issues with some of the parts I could use. I plan to use the 38AC signal so I can have a stable 24V signal since the diode bridge will decrease the other AC signal below 24V and then the LDO reduces it even further.

The issue I have is that the LDO's have a maximum load of 2A. My first question now is, wouldnt that decrease the sound output by a lot? since the max load of the amplifier board is 9A. I know the LDO will burn out if I incease the load to much, but I assume this load is based on the volume setting? Or can I just use a LDO with max 2A at 24V without an issue?
In the meantime I thought about a different solution. If the voltage ripple due to the full bridge rectifier is low enough (what is low enough) it should be stable enough as well.

However, this creates another issue, (which also still has to be solved for the other system) namely the capacitance of the smoothing capacitor.

I found this calculation to detemine the voltage ripple. However to finish that calculation I need to know the resistance of the full system. I wanna use this capacitor (highest capacity I could find) to check if I the voltage ripple is neglible and no LDO is needed. How can I check the resistance without having to buy the amp board? I am still a student so if possible I want to be as sure as possible before buying the parts.

I know I can easily buy a new AMP kit, to resolve the issues. But this amp also has some emotional value to me and if I can salvage it i want to try it.

Thank you for the help in advance. Keep in mind my studies are not in electrical engineering but I try my best to learn and research needed topics but some parts are over my head and help is welcome.

PRR 14th January 2021 03:48 AM

This transformer is a very bad fit for other amplifiers. It wants to make two rails of 53V DC, which is more than most modern amps can eat. Regulating would make sense for small loads, but rarely makes sense for a BIG load like two 100W-150W channels.

The Philips power amplifier can probably be fixed quicker and cheaper than trying to wrestle-together these mis-matched parts.

Satgroen 14th January 2021 07:05 PM

Alright thank you

How could I locate the issue?

The issue is that a high pitched sound comes out of the boxes when I play music. It changes frequencie pretty often and sometimes stays as a stable hum and after a 10-30 seconds it goes back to altering high pitches.

Lowering the volume does not change the volume of the pitch in any way. I checked the manual and the volume control is the last step in the signal processing, giving me the assumption something happens to the signal afterwards, so in the amplifier board.

russc 15th January 2021 05:47 PM

One channel or both channels?

rayma 15th January 2021 05:53 PM

Likely to be some bad electrolytic capacitors, probably the two large 6800uF.
The rest of the circuit may very well still be functional.

You can measure with a DVM the various other regulated DC supply voltages,
to see if they are still ok.

Satgroen 19th January 2021 07:03 PM

Thanks for the help, I will check it if they are ok. All 4 channels have the same issue

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