I need to lower an audio signal by 20dB. *Schematic added*

[Bltzbrg]

Hi Magic,

So now I just added a 10kOhm 100mW resistor in the signal line before my output trs.

Oh, yes I just shortened the two signals of the mp3 player together and it seemed to work. These websites say with some resistors I could "merge" them:
https://www.instructables.com/Simple-Way-to-Convert-Stereo-to-Mono/
https://forum.arduino.cc/t/audio-input-switching-turning-stereo-to-mono-etc/242588/3

Could I do it like that? Which resistors do I need? The mp3 player is supposed to have equal left and right channels but I am not sure if it really is like that and there is no public information on that.

The cable solution I had before was like:
V-Adapter TRS Stereo to 2 TRS Stereo
reverse V-Adapter Left TRS and Right TRS to TRS "Stereo" (https://a.co/d/8TKS2M9)

Split the mp3 player signal with v-adapter1 to go into headphones and the other one goes into the reverse v-adapter e.g. left channel into the GoPro Adapter.
The mic goes directly into the reverse v-adapter e.g. right channel.

For simplicity I could obviously just use one channel but I don't want to risk it to loose sound that was just sent on the other mp3-player channel. Thats why I ask how I can convert the stereo signal to a mono signal.

We are almost there

Thanks again!

[magic]

They are not equal if there are different signals on L and R.
They also aren't equal if they have different DC offsets and there are no series capacitors on the outputs.

The resistors thing should work, but with a few dB loss of volume.

[Bltzbrg]

I just edited my previous post to avoid double posting. Sorry!

[Bltzbrg]

I would really appreciate an answer to my edited post 
Then I could continue working on the project.
Thank you

[magic]

Not sure what the question is at this point?

Yes, you should be able to mix the outputs of a stereo MP3 player to mono by means of those 33Ω resistors. As I said, it will be a few dB quieter than driving the headphones directly. Or you can simply connect the headphones in stereo and only send mono to the recording device, not sure what's the problem with that?

[Bltzbrg]

So 33 Ohm resistors are good for my usecase? How many dB would that be?
If a song plays "left, right, left, right" I want to hear that in the gopro. Not just "left,...,left,..."
I wired the headphones directly to the mp3-player and I guess I am going to use the two 33 Ohm resistors to connect the L and R channel together to get a mono signal.
You said that simply shorting the channels together would damage the mp3-player and drain the battery? Why do these small resistors prevent that?

Sorry, if I am being annoying but you are my only helpful source

[magic]

By Ohm's law.
Current flowing from left to right output (or vice versa) is voltage between these channels divided by total resistance (2x33Ω = 66Ω).

[Bltzbrg]

I bought some audio jacks and a resistor kit.

I connected the L and R channel of the mp3 player as following:
L-in->33Ohm---------->Mono-Out
R-in->33Ohm-----^
GND--------------------->

The signal is now about -30db quieter which is too much. I want to convert a stereo signal to a mono signal without making it quieter.

And I want (with a switch) to make the signal -10 or -15db quieter.

I also connected the 1k Ohm Resistor in just 1 channel to see how much quieter it makes the sound. Barely nothing. Sometimes maybe 3db.

Any tips or explanation why 1k Ohm does nothing but two 33Ohm do so much? If you are tired of explaining I get it. then just provide new Resistor sizes if you can

Thank you very much for your time

[magic]

This is the headphones mono adapter, correct?
Are you saying that the headphones are 30dB down now?

If not then what is connected to where and what are you measuring?

[Bltzbrg]

Oh ok, I thought I made it understandable, my bad.

First I tried the stereo signal to mono signal solution with the 2x 33Ohm Resistors. I connected everythin on a breadboard.
Mp3-player stereo signal TRS in -> 33 ohm resistor (1 per channel) -> connect both channels -> TRS out into my Gopro. Obviously I just had audio on the left channel (as intended) but it was too quiet (-30db). At this point I don't want to make it quieter. Just stereo to mono.


The second test I did, was the 1k Ohm Attenuator.
Mp3-player stereo signal TRS in -> on one channel 1k ohm -> TRS out into my gopro
I thought like that I could hear the about -15db difference between the channels. Or is that the issue that I connected the "louder" channel as well? In this scenario I almost had no attenuation. Maybe about -3db but not more.

I hope it is understandable now.

[magic]

Inserting 1kΩ between the source and a 2k2 input is expected to produce about -3dB, so nothing wrong with that. It's basic voltage divider math, use more resistance for more attenuation.

33Ω should produce much less attenuation, so something must have been wrong. Maybe used 33k instead?

[Bltzbrg]

The resistor-set I used was a cheap one from amazon. So that could be possible. Thank you for pointing that out. I found some useful calculators and I calculate it with 2.2 V.
I am a little bit embarassed. I made a mistake by taking 1k Ohm and not 10k Ohm and I didn't notice my mistake until now. I will try to find the correct 33 Ohm resistor tomorrow.
I feel like now I have all the missing numbers I need to use these online calculators
Do you have got a good source that I could learn why shorting the two channels with 2x 33Ohm is okay but not using the resistors is bad? I mean 33Ohm is not much, isn't it? Some other sources use two 1k Ohm Resistors. That would do an attenuation of about -2db. What would be the difference between 33Ohm and 1k Ohm in my use case?

Thank you!