Active Stereo Audio Mixer Circuit

Thread Starter

shirobon

Joined Nov 20, 2018
3
Hello all,

I'm designing an active audio mixer circuit so I can combine the outputs from my mp3 player and phone so I can stream news in my car while playing music in the background. These aren't for driving headphones, but powered speakers which take line input.

The idea is to mix the left channels of both streams and the right channels of both steams to create a stereo output.
Basically Output Left = (Left 1 + Left 2), and Output Right = (Right 1 + Right 2).

I'm going to power it with 5V from a USB cable so I'm using a single supply design. Basically it's just 2 separate summing amplifiers for the left and right of each input, with three 2-gang potentiometers for the 2 input channels, and master volume for the output.

Below is the circuit for just the left channel, but it will be duplicated for the right channel as well.
R2 and R7 are 50K Logarithmic pots so I can change the volume of each stream, R13 is a 100K Logarithmic pot for master volume, C3 is an optional capacitor which forms a low pass filter with fc around 30kHz depending on R13's resistance, and will be connected to a switch to disconnect/connect it if desired.



I've designed the circuit to work with a max 5Vpp input signal, using R2 and R7 values of at least 33K the opamp is guaranteed to never saturate, but that's in the worst case. The headphone output on my phone is max 1.4Vpp and mp3 player is max 2Vpp.

The above is just a functional design, it doesn't have anything for the physical implementation like decoupling caps, etc.


I've ran several tests in simulation, and everything seems alright. I get the desired output across the whole 20-20kHz range, and don't see any problems.

One possible concern is the slew rate of the opamp, it's only 0.4V/us, but in my calculation it can still handle a 22kHz 2Vpp output with room to spare, so I don't think it should be a problem. If it's a problem, what are recommendations for an op-amp that works well with a single supply and can get pretty close to the rails (at least 0.3V from Vdd)? Preferably one from Analog Devices/Linear.


I would just appreciate another set of eyes to look over it, and maybe to point out something that I'm missing or forgot to consider. Thanks!
 

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Last edited:

ericgibbs

Joined Jan 29, 2010
13,197
hi shirobon,
Welcome to AAC.
Please post your LTSpice asc file, there are many LTS user here, who could run the sim and report back to you.
E
 

Thread Starter

shirobon

Joined Nov 20, 2018
3
Hi,

Yes it is. For consumer electronics, peak amplitude of line level is 0.447V, so in this case it's about 3.5x below maximum level.
To increase the volume, you could either move the R13 potentiometer down towards 22K, so the output approaches the maximum value for line level, or you could keep the potentiometer at 100K and raise the volume on the stereo externally.
 
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