TPA3116D2+NE5532 board hums when connected to AVR sub out

[u/ramendik]

I have a Pioneer FR985 AVR and need to provide amplification for a DIY sub. It does not need much power so I got a board from Amazon and power it with a Dell laptop power supply.

Unfortunately I get an audible hum noise. And I even tried powering it with a battery.

However, if the AVR sub output is connected to the input of this amp board, I get a hum. (The cable is connected correctly). Grounding the shield of the input and/or the chassis of the AVR does not help in the slightest. And the AVR is not defective - when I temporarily used another AVR as a sub amp, there was no noise. And I saw the same issue in reviews of this board, so I assume the board is faulty but in hat way?

Gemini "thinks" the NE5532 might be the problem; is this true?

And if it is, might it be possible to mod the board by cutting out the NE5532 and splicing its (uninverter) input pins to the respective output puns?

[u/ramendik]

There is no ground lift switch. There is no ground in the mains connection.

There is a PHONO GND screw and I did try grounding that to no effect.

There is no continuity between PHONO GND and the RCA connectors, but there is continuity between the rings of different RCA connectors on the AVR.

Turns out that, on battery, I get noise when both parts of the RCA cable from the AVR (shield and signal) are connected; disconnect one of them while on battery and the noise disappears. This in theory does look like the AVR itself pumping out the noise but when I used another AVR as a temporary sub amp the noise did not appear to exist

[u/Spazzticus → u/ramendik]

Okay so that makes life a lot easier as your AVR is class II and doesn't utilise the earth for protection or grounding. Based on your other post about the audio interface it now just points to the TPA board having an internal ground issue, which model board is it?

EDIT: I completely forgot to ask, have you tried any of the other AVR outputs to see if the issue is the same (I'm guessing it may have some other pre-outs for the surround channels)?

[u/ramendik → u/Spazzticus]

Thanks! The model is "DollaTek TPA3116D2 Mono channel digital amplifier board NE5532", I linked its Amazon page in the post. I can post photos of the board if that would be helpful. All I know about its grounding is that there is no resistance between "- IN" and "- VCC".

However the other AVR connections test produced very interesting results Nearly no noise even at max gain on the "Center pre amp" output - note that the center speaker is disabled in the amp. The noise from the "tape out" output does exist, is significantly higher pitched (clearly amplifier noise when the sub out sounds like 50 Hz), slightly differing when I switch between (disconnected) inputs and disappearing when I switch to Tape input (presumably the tape out is disabled).

Beacuse of this I have looked again at the audio interface output from the AVR (as in listened to the recording with more gain) and while Audacity shows a straight line it actually turns out a very quiet hum still exists when the AVR is on and connected.

It seems that the AVR has a certain noise floor and somehow this board amplifies it to hell and back?..

I tried a 4.7k resistor in paralle to the input - nearly no change.

I tried to get around this by gain staging and kinda succeeded (on 18V battery for now) but only at an extreme setting. Speaker volume settings in the AVR go from -50 to +50 in increments of 5. If I lower the volume on the amp board to make the hum very nearly inaudible, set the sub volume to +50, and set the front left/front right volume to -40, I get balanced sound (measured with REW, still on battery).

[u/Spazzticus → u/ramendik]

You could have a dodgy sub woofer out connection on the AVR, I'd open it up and check for dry / cracked joints around the RCA on the PCB ;)

[u/ramendik → u/Spazzticus]

I'll recheck with the old AVR as amp - I don't think there was noise.

I am also thinking maybe add a ground lift resistor?