Audio output levels on Headphone L and R

Hi all,

On my PCB I have 2 separate audio amplifiers, one for Left and one for Right.
Everything on the PCB runs basically on the same GND.

When I look at the output level on the headphone signals (unloaded) I see around 100mV pk-pk. Why is this so low?
There are pins on the module called headphone GND and Mic GND, what should I do with these?

Hello Ecoi_oc,

can you tell me which SOM you are using and share your schematic. I will take a look at it.
Also which amp are you using ?

Best Regards,
Matthias
Toradex

I am using the LM4954 audio amplifier and the module is an imx7D-EMMC.

I have pins 15 and 17 headphone L and R connected capacatively to the amplifiers. I still would like to know how I should connect pin 13 headphone GND and MIC GND pin 3. My Mic IN signal comes from a MEMS microphone on my carrier board

ok can you share your schematic?

alt text

There you go

What colibri modul you are useing? keep in mind that you can set the output volume of the SOM. what are you setting on the output volume ?

The module I am using is the imx7, it is already stated above. The only way I can get decent volume out is by putting the PCM setting in alsamixer to above 90, you still haven’t answered my question about the headphone_gnd and mic_gnd, what should I do with these?

Thanks for the SGTL5000 explanation, but you still keep ignoring my question about headphone_gnd and mic_gnd, what should I do with these?

Hello Eoin_OC,

We at toradex try to answer fast so that you can cantinue to develop. If a ticket contains multiple question it might be. That we can answer one question faster then the other. So to not let you wait to long we might sometimes give you a partial answer to your question.

We will answer the GND also soon.

Hello Eoin_OC,
Well we have some information in our colibri carrier board design guide. https://docs.toradex.com/102491-colibri-arm-carrier-board-design-guide.pdf page 30/31

The pseudo ground should not be used when you are driving amplifiers only if you use headphones.
I would create one separate GND for all the analog audio part and couple this one with the module GND via a cap and an ferrite. Like shown in our reference in the design guide. Also check our iris ref. Design.
In some cases it can be better to not populate the caps, but it is good to keep it in the schematic. Also

Hello eoin_oc,

The SGTL5000 has two gain stages like show in the picture below. A digital DAC gain stage and an analog headphone amp stage.

So you can control both gain settings. it is alway good to start with the highes gain first. otherwise you just amplify ti noise floor with the second stage. but this is also stated in the Datasheet of the SGTL5000
“HeadphoneStereo headphone outputs are provided which can be used to drive a headphone load or a line level output. The headphone output has its own independent analog volume control with a volume range of -52dB to +12dB in 0.5dB step sizes. This volume control can be used in addition to the DAC volume control. For best performance the DAC volume control should be left at 0dB until the headphone is brought to its lowest setting of -52dB. The register CHIP_ANA_HP_CTRL is used to control the headphone volume.”

This is how you can change both settings: keep in mind that you can also change the gain of you Speaker Amplifier.
HeadPhone gain:
Simple mixer control ‘Headphone’,0

DAC gain:
Simple mixer control ‘PCM’,0

To change Headphone playback volume:
amixer set Headphone 100
(0…127)
To change DAC playback volume:
amixer set PCM 100
(0…192) (edited)