Problems with dual rectifier circuit for an audio amplifier.

Thread Starter

nullrage

Joined Dec 15, 2021
7
Sorry if it is posted in the wrong thread, also sorry if my English is bad.
I have purchased this amplifier board from aliexpress: Amplifier PCB Link

I have purchased it knowigly, that i have dual secondary tranformer with no CT.
I have found a thread here (thanks to- @ericgibbs ) on how tomake dual rectifier circuit in order to have +v 0 -v (i cannot bridge the 2 secondarys, because voltage goes way wayy out of spec for the pcb itself. )

My problem is: when i connect a 3.5mm to a laptop it works fine and sounds good, but when i connect it to a Desktop PC the signal wire (R GND L) starts smoking and then combusts.

I suspect it may be a Grounding problem, since i dont have any.

Can anyone help me out on how to properly ground this amplifier in a Dual Rectifier setup?

Sorry, i am only getting into electronics and power supplys and etc.
 

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wayneh

Joined Sep 9, 2010
17,164
You have a ground loop problem. That is, your two power supplies use a different "ground" and thus allow damaging current to flow from one to the other. You should NEVER connect two wall-powered supplies without first checking for this situation. I use a 1KΩ resistor to connect the grounds, look for any voltage across it (or heat) and then proceed with caution.

Your laptop survived because it has an isolated power supply - the battery. If you had your charger attached, you might see the same problem appear.
 

Thread Starter

nullrage

Joined Dec 15, 2021
7
You have a ground loop problem. That is, your two power supplies use a different "ground" and thus allow damaging current to flow from one to the other. You should NEVER connect two wall-powered supplies without first checking for this situation. I use a 1KΩ resistor to connect the grounds, look for any voltage across it (or heat) and then proceed with caution.

Your laptop survived because it has an isolated power supply - the battery. If you had your charger attached, you might see the same problem appear.
Can you please visualise for me what i have to do, because already i had few short circuits. Sorry i am really new to this.
 

BobTPH

Joined Jun 5, 2013
4,742
How is the audio amp grounded? You show an isolated supply. If that is the full schematic, there should not be a problem.

Bob
 

Marley

Joined Apr 4, 2016
468
Your rectifier circuit will work but you can do the same thing with 1 bridge rectifier.
Just connect your secondary windings in series. That effectively gives you a center-tapped single winding at X2 voltage.
The centre-tap to 0VDC. Single bridge rectifier for the +ve and -ve DC outputs.
I would definitely recommend an audio input transformer when outputting audio from a laptop or a mobile phone headphone socket. Saves a lot of grief in case of a fault! Also no ground-loop hum and noise problems.
Good quality 600Ω 1:1 transformers can be fairly expensive but look at an old modem card or fax machine for second-hand parts.
 

Thread Starter

nullrage

Joined Dec 15, 2021
7
I have tested with known working isolation transformer, tested it on a working amplifier with no ground loop problems, works fine. When I attach it to the amplifier i built myself there is a extremely loud humming noise. (I have double checked that the isolator transformer is far away from main power supply) - still the same loud humming noise. When I remove all input wires and leave it on, its silent. Can anyone assist me with explaining how i can properly ground this amplifier?
 

Thread Starter

nullrage

Joined Dec 15, 2021
7
I will be able to test with isolating the amp earliest after New Years, will let you know how it went. Thanks for the info!
 
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