Create an Active or Passive Audio Mixer

Thread Starter

JustMe234

Joined Feb 25, 2017
50
Hello i am trying to build an audio mixer active or passive. I have the 3 inputs PC, Phone, MP3 player. And the output will be on 1 headphones. I want to make an audio switching box to switch between audios. I try the passive mixer with 3 potentiometer but the volume was low.
Then i use the schematic below it works great but the audio is distorted when i all up and the volume is low not as the input. Insted of bc109 i use 2N222.
Now i want to make an audio mixer to have an output volume at the same level of inputs and with minimun distortion.
What you suggest?
Thank you
 

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AnalogKid

Joined Aug 1, 2013
8,106
If you search for 'audio mixer schematic' you will get hundreds of images. This page has several workable circuits:
https://www.electroschematics.com/tag/audio-mixer-circuits/

The most simple next-step-up from your one transistor circuit is a one opamp circuit. I'm surprised I didn't see more schematics for it. Here is the general idea. In this circuit, I think I would change all of the 100K fixed resistors to 10K.



ak
 

Thread Starter

JustMe234

Joined Feb 25, 2017
50
If you search for 'audio mixer schematic' you will get hundreds of images. This page has several workable circuits:
https://www.electroschematics.com/tag/audio-mixer-circuits/

The most simple next-step-up from your one transistor circuit is a one opamp circuit. I'm surprised I didn't see more schematics for it. Here is the general idea. In this circuit, I think I would change all of the 100K fixed resistors to 10K.



ak
Thanks for that design. Yeah i have found on google hundred schematic for audio mixer but they use different opamp or different transistors. Which i don't have available now. I have to order online.
In this case i was searching for a complete circuits who really works and then order all the parts.
I will give a try to your schematic. Thanks for your suggestions.
Should i insert an 1N4148 diode to prevent interfering signals?
 

Audioguru

Joined Dec 20, 2007
11,251
I was taught to NEVER bias a transistor like you did since a transistor part number has a range of the amount of current gain.
I simulated your circuit with a high and with a low current gain 2N2222 transistor. The result is extreme distortion at the top or bottom of the waveform.

You have a 22k input resistor feeding the input impedance of the transistor that is about 2k ohms. Then you have a voltage divider that reduces the signal to 8.3%.

You have the 2.2k output impedance of the transistor feeding the 32 ohms impedance of an earphone. Then you have a voltage divider that reduces the signal to 5.7%.
 

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Audioguru

Joined Dec 20, 2007
11,251
Since an opamp has a high open-loop voltage gain and since it is inverting then its input impedance is very low then the signals do not interfere with each other. A diode is not wanted, it is used with DC, not with an AC signal.
 

sghioto

Joined Dec 31, 2017
1,039
A
I think I would change all of the 100K fixed resistors to 10K.
Well not all. It's important for the resistors coming off the volume pots to be at least 10X the value of the pot. You don't want the volume control settings affecting the gain of the circuit.
SG
 

AnalogKid

Joined Aug 1, 2013
8,106
Well not all. It's important for the resistors coming off the volume pots to be at least 10X the value of the pot. You don't want the volume control settings affecting the gain of the circuit.
Not automatically. If the pot has a linear taper, one way to give it a pseudo-audio-taper is with a non-infinite load impedance on the wiper. Old trick.

ak
 

Thread Starter

JustMe234

Joined Feb 25, 2017
50
I was taught to NEVER bias a transistor like you did since a transistor part number has a range of the amount of current gain.
I simulated your circuit with a high and with a low current gain 2N2222 transistor. The result is extreme distortion at the top or bottom of the waveform.

You have a 22k input resistor feeding the input impedance of the transistor that is about 2k ohms. Then you have a voltage divider that reduces the signal to 8.3%.

You have the 2.2k output impedance of the transistor feeding the 32 ohms impedance of an earphone. Then you have a voltage divider that reduces the signal to 5.7%.
What's are your suggestions how to improve
 

Audioguru

Joined Dec 20, 2007
11,251
Most of us are or were electronic engineers and we like discussing circuits here, not teaching the basics that we learned a while ago.
 

Thread Starter

JustMe234

Joined Feb 25, 2017
50
If you search for 'audio mixer schematic' you will get hundreds of images. This page has several workable circuits:
https://www.electroschematics.com/tag/audio-mixer-circuits/

The most simple next-step-up from your one transistor circuit is a one opamp circuit. I'm surprised I didn't see more schematics for it. Here is the general idea. In this circuit, I think I would change all of the 100K fixed resistors to 10K.



ak
I am following your schematic and now i want to add a switch on inputs to cut the audio when is not in use. An DPDT switch will be great for stereo inputs but i want to add an small spy led to indicate when the audio from that channel is active or not.
How can i add a switch to turn ON/OFF audio and the led?
Thanks
 

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