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I'm a novice in this and planning to build a small inexpensive audio interface for myself.

It will have:
1. One AUX input, that can be connected with a 3.5 mm jack.
2. One output, that can be connected to a speaker with a 3.5 mm jack, and
3. One knob to set a decibel level.

The idea is to always have the set decibel level as output.

For e.g., if I connect an MP3 player as a source to the device and a regular external speaker as output, and the decibel level in the device is set to say 70db. No matter what the volume is set in the MP3 player, the output from the device's speaker should always be 70db.

If the MP3 player's volume is low, the device will amplify it to 70db. If it is high, it will reduce it to 70db.

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I've been searching for such mechanism but all I could find were examples of building Amps.

EDIT:
MP3 is just an example. It can be any source like a TV also. To simply put my idea, if you are watching TV, the commercials are louder and also some channels are louder or much lower in loudness.

I just wanted to know if we can build a device/circuit, that will give a constant sound output (volume/loudness whatever the actual tech. term should be)

I hope you got the idea.

What I'm trying to achieve, is this even possible. If yes, please help me with any pointers. Also, are there any such devices already available.

I found this question here which talks about a similar case but it is done using software.

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    I'm not going to provide an answer, but will say, in the gentlest possible way, that there is so much wrong with this that there's probably not going to be room in the comments section to explain it all. Suffice to say that every 'mp3' player with a volume control will output a different level when set to '20'. There's no standard for this. Also, there's no such thing as a 'Db'. There is a "Decibel" which is a measure of relative level, so you have to have something you are measuring it against. Speakers don't output Db's but you measure sound pressure level at a set distance from a device.
    – Mark
    Jun 28, 2019 at 6:37
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    No. A decibel is not a measure of anything. It can only be used in conjunction with a reference, or as an indication of level change. i.e. 3dB higher, or 6dB lower. You indicate measurements of Sound Pressure Level in dBSPL - more specifically dBSPL/C or dBSPL/A. The premise of what you are trying to do is fundamentally flawed, however I think I can see what you are trying to achieve. You should research "Loudness" measurement and consider that your 'device' needs a way of measuring the SPL output of the speakers at the listening point in order to standardise the output level of itself.
    – Mark
    Jun 28, 2019 at 10:12
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    Additionally, when aligning speaker monitoring levels, you generally will use pink noise so this will add another layer of complexity to the overall solution.
    – Mark
    Jun 29, 2019 at 10:42
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    Responding to the edit: What you're seeking is a constant loudness device. Unfortunately the biggest issue here would be latency. By the time a system has worked out the actual loudness of the programme material you are listening to, the loudness of the input would have changed. If you react too quickly to changes in the programme material, the device will constantly be changing level to keep up. I fear this is the wrong rabbit-hole sir.
    – Mark
    Jul 1, 2019 at 12:34
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    At a very basic level, Qwerty could look at compressors. They will sort of do what you ask, but not really.
    – Rory Alsop ♦
    Jul 3, 2019 at 19:26

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