Simple Passive Stereo Mixer

[Kayleigh]

Hi all,

I am not the greatest electronics engineer in the world, but I know my way around a soldering iron and the basic principles of electronics.

I've adapted the tutorial on instructables for a passive 3 input stereo mixer in a die cast box, but I'm not sure about the level of resistance being applied.

I'm looking to mix my stereo out from my TV and the headphone out from my Echo Dot into one channel on my HT Amp.

I have wired up 5 inputs on my new mixer, for expansion.

I have two RCA Phono pairs and Three 3.5mm socket inputs.
Each input channel, left and right, is run through a 4.7k resistor before being soldered together and joining the main out channel on the "Line Out" 3.5mm connector.

So I have 10 resistors in total, with the grounds from each of the connectors joined together with the ground from the Line Out connector.


The problem is that the volume is now so low that I can't hear the TV even on max volume.

Could the same effect be gained from downgrading to 1k resistors?

 

[Audioguru]

Each input (resistor) on a passive mixer reduces the output volume. Also each input has the other inputs mixed with it. Also the volume changes if an input or more is disconnected. Changing the resistor value makes no difference.

If you use one opamp as an inverting mixer then the gain is the same as the original on each input or as much as you want and each input does not have any interference from the other inputs. The Instructable (some are written by kids who are only 10 years old) should have told you about these problems and solutions.

 

[BobK]

If you have nothing plugged in to the other inputs, you should get near full volume. If you don't, the amp's input impedance must be unusually low. If that is the case, you will need a gain stage.

Bob

 

[BobK]

Another thought. Are you sure they are 4.7K and not 47K or 470K resistors? What is the color code? It should be:

yellow violet red

Bob

 

[Kayleigh]

Another thought. Are you sure they are 4.7K and not 47K or 470K resistors? What is the color code? It should be:

yellow violet red

Bob

Thanks Bob,

I've checked the resistor code - They're 5 band, and they are Yellow Violet Black Brown Brown.

Kay

 

[lalika]

Thanks Bob,

I've checked the resistor code - They're 5 band, and they are Yellow Violet Black Brown Brown.

Kay

Hello, calculating from those color bands, we get a resistor with a 4k7 value.

 

[Audioguru]

Do you understand what an attenuator is?
You have five signal sources that are probably each the low output impedance of an opamp. One signal comes into one of your 4.7k resistors but it is loaded with the other four 4.7k resistors in parallel. Then the four resistors result in 4.7k/4= 1175 ohms. The resulting signal level of the passive mixer is 1175 ohms / (4.7k + 1175 ohms)= 0.2 which is a signal level that is 1/5th. An active mixer made with an inverting opamp can have some attenuation or no attenuation and can even have some gain.