TDA7850 Car engien whine, buzzing noises

[KevinDB]

Hello,

would someone confirm my mistakes on this TDA7850 schematic??
There is a ground loop isolator BA3121 on inputs and softstart mosfet to limit inrush current.
Ignition/remote ON provides a buzzing noise, as soon as engine or cabin heater/fan starts you can hear the whine amplified trough speakers….
Any tip is welcome :)
 

 

[Karl Yamashita]

The good old BA3121 is a life saver. 

FL- and FR- don’t seem to be going anywhere? If i remember correctly, you should feed the FL- and FR- to the VM1 and VM2. The BA3121 will convert the differential audio signal to single ended audio for the TDA7850 

[AScha.3]

...but little problem:

 

[Karl Yamashita]

The OP didn’t ask where to purchase the BA3121 but instead asked for help with his schematic and ground loop issue. 

[AScha.3]

afair When i had problems with noise on ground in cars, i used the cheap isolation transformers, like this:

 

or

 

[KevinDB]

Hi Asha,

Not useable for me cause the inputs on the amplifier, are directly from the headunit speaker outputs BTL.

 

 

[AScha.3]

So you want connect a small power amp to the BTL output of another power amp ?

Whats the deeper sense of this ??

[Karl Yamashita]

​@KevinDB 

I used to work for PAC Audio and Crux Interfacing years ago so I’ve been through all the design phase of making audio interfaces. Some interfaces were straight forward while some were head scratchers when dealing with engine noise.

As stupid as it sounds, I used this trusty old portable Archer mini amplifier with 1/8 jack to help trouble shoot ground loops if the interface had any. It has a 10:1 voltage divider (heat shrink).

If you can get your hands on something similar, you can test at what stage you’re getting the engine noise. Before the BA3121, after the BA3121, before or after the capacitors. If it’s clean audio then you’ll know it’s your last stage, the TDA7850. 

 

Also experiment with the capacitors. I had designed a product for a Chrysler/Dodge vehicles to add an aftermarket amplifier to the stock audio system. There was even BT audio that was differential audio that I needed to interface with. That audio went through some op-amps to create a single ended audio. From the op-amps the audio went through some caps before the 3-band audio IC chip i used. Even though the audio should have gone through capacitors, I ended up using some 10k resistors in place of the caps to fix the engine noise. Of course, YMMV.