Logilink 7.1 DAC and 4x tpa3118 with hiss terrible noise

[papaarrugada]

After successless searching in this forum I want to ask for help. I have connected on a RasberryPi over USB a Logilink 7.1 DAC.

LogiLink USB Sound Box 7.1: Amazon.de: Computer & Zubehor

Every output is going to a pair of tpa3118 amp mono boards

DC 8-26V TPA3118 PBTL Mono Digital Amplifier Board AMP Module 1*60W for Ard E2P8 | eBay

For the RasberryPi I use a separate 5V powersupply and all the amps are connected to another laptop 9V powersupply.

If I connect only one DAC output to the 2 belonging amp inputs everthing is ok.

Now the problem:
When I connect only a second output from DAC with amps I have this hiss noise.
It can make disappear this noise when I disconnect the GND from the second amp powersupply.
--> Do I have this noise when audio output (gnd?) and amp powersupply gnd are connected?

Another test was successful. If I use a galvanic filter after the DAC audio output I can avoid this noise.

I found the suggestion in other threads that a connection between gnd of audio-input and gnd of amp powersupply can help, but I had no success.

Question to the experts:
Is there another way? Or what I am doing wrong?

 

[Khron]

I'm reasonably sure syncing the amplifiers together might not be a bad idea either.

http://www.ti.com/lit/ds/symlink/tpa3116d2-q1.pdf - page 11-12 ("7.3.1 Gain Setting and Master and Slave")

 

[papaarrugada]

thanks for the answer.
I hope I understand correctly.
If I want to use all outputs from the DAC going to the inputs of the amps, I need ac-coupling to the inputs. Right?
This could explain my successful test with a ground loop isolator in the input line.
I use the amps in mono mode and I have changed the gain to 26dB.
Should I use a capacitor in series to the inputs?

 

[Khron]

The inputs to the amps need to be AC-coupled anyway.

The point i was trying to make is that, especially if (or since) all the amplifiers are using the same power supply and/or are in the same box, you will benefit from synchronizing them with each other.

Combining class D in one audio application: beat tones

 

[ICG]

I got the same USB soundcard. In general, it sounds quite good. I've got some issues though, especally every now and then a clicking sound in the output. That might be a driver problem but that's not acceptable for me, even if it's rare (every 20-30 minutes).

I would test the amps separately first to check if it's comming from the amps or the soundcard. If it's the soundcard, try a different (pobably newr or even OLDER) driver. If that's no solution, try to change the USB port, give it a completely separate USB with no other USB devices on that port. And no, a HUB won't separate it properly.

If you fond the USB soundcard isn't to be blamed, try if every amp behaves the same.

 

[papaarrugada]

first of all, all amps are working in master mode with a changed gain of 26dB. I don't use master slave mode. And I have directly connected the DAC outputs to the amp inputs. If I only connect one output to the amps (2 for stereo) everything ist ok. The next connected output to the suitable amps (again 2) makes this hiss noise when additionally the GND of powersupply is tied.


Disconnecting the second DAC output or the GND of powersupply can avoid the noise. I have changed a lot, it's always the same problem.
If I use a ground loop separator in the DAC outputs the problem is gone too.


I come to the conclusion that in the DAC all gnd's of the outputs are connected and only one connection to the GND of powersupply makes that noise.


I will try to test the idea from ac-coupling the amp inputs, but I am not a expert for amplifiers.


I understood following:


1. step
ac-coupling the inputs of the amps for a gain of 26dB. In datasheet there is an example and I would choose a capacitor with 330n. Is this correct?
I would expect eliminating the noise.


2. step
syncing all amps together. Here it's necessary to go to the sync pin of the amp-moduls. I have to change the resistors from the belonging left 7 amp to 26dB slave mode and tie them with the one in master mode. Is that correct?

 

[theAnonymous1]

The amp boards are already AC coupled and there are probably caps on the output of the Logilink as well.

You have a ground loop problem. Try powering either the Pi or the amps from a battery supply. If the noise goes away then you need an isolated supply for one or the other.

 

[ICG]

If the noise goes away then you need an isolated supply for one or the other.

That could be solved then another way, with a galvanic separation, i.e. with the

DynaVox 207257 GLI 2.1
DynaVox 207257 GLI 2.1 Stereo Line Isolator schwarz: Amazon.de: Elektronik

Monacor FGA-22
Monacor FGA-22 Ubertrager 0401033 | eBay

You can build it yourself too with these
10X Audio Transformer 600:600 Ohm 1:1 EI14 Kupplung Isolation Transformator | eBay

 

[theAnonymous1]

That could be solved then another way, with a galvanic separation, i.e. with the...

Those tend to roll-off at low frequency. He said he already tried one and it worked, but he would need quite a few of them for all channels.

Another thing he could try is to disconnect the audio ground from all but one amp module.

 

[papaarrugada]

thanks for the ideas.
Yes I tried one, I connected one cable (2 amps) directly and the second cable over this:


Ground Loop Noise Isolator, HAVIT 3,5 mm Car Audio: Amazon.de: Elektronik


And it works.
I need at least 3 of them, that's not the point.
But I think I have a reduced soundquality.

 

[ICG]

How much impact it got on the SQ is depending mainly on the bandwidth/type of coils/transformers and the output/input impedance of the partners in play. While you can't exactly change the former (except buying different ones), the latter is pretty much up to you. If you change the power supply that could probably solve the matter but as soon as you introduce another piece of equipment or change the wiring that problem can resurface again. Why don't you measure the impact of the audio transformers? I mean, class D in hifi applications does not give you the esoteric ultrasonic which is soo sought after by the high enders anyway because of the needed output filters.

 

[papaarrugada]

Thank you all for the inputs.
I have checked different solutions.


- Working in Master/Slave mode is to much work for "ready to use" modules. I have 8 modules


- using the coils/transformers can be the "last" chance


- my choice after some tests is to optimize the GND wiring of the amps. A short amp to amp connection of the GND has the most benefit and I can reduce the terrible noise a lot. I want to use one power supply.