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How to use an old multimedia mobile phone as an music player – The signal integrity issues

November 17, 2014 Leave a comment

It is quite easy to convert a mobile phone into a music player, just connect your amplifier to the audio connector on the phone. Just that simple. However, problems start creeping up, when you want to power up both the phone and the amplifier out of the same supply. That means the phone doesn’t have a battery and is getting power form a 4.2V(Assuming original battery was Li-ion) regulator that you have built, and connected to the battery terminals. The problem is that in this case there is a lot of noise in the audio compared to the case with battery.

The Problem Statement

The reason is that the phone’s audio ground is no longer same as the amplifier ground. There are two cases. Case 1 is that the output stage of the phone has a bridge configuration. Case 2 is that the battery circuit uses low side current measurement. Case 1 is less probable because both the Left and Right channels share the common connector on the jack. For case 2, I have no clue why so many engineers prefer low side current measurement for the battery (connecting a resistor in series with the phone ground and battery negative terminal).

Phone audio electrical ground difference

In the figure, based on activity in the ‘Noisy Digital Circuit’ I1, the current through R1 changes and hence the voltage across it. This causes a varying voltage between the amplifier ground and the phone ground. The no-load voltage between Out+ and amplifier ground shall be V(R1) + V1. V(R1) is the noise introduced.

My case was only case 2. I could hear a lot of noise from the voltage converters on the phone. Because, based on their current consumption my phone’s ground was changing, and as a result the audio signal was also changing. The problem is illustrated in the figure above. The problem below shows the equivalent circuit used for simulation (Along with the solution circuit).

The LT-Spice circuit used for simulation of ground noise.

The LT-Spice circuit used for simulation of ground noise.

Here is a simulation output form the circuit above

Red is the op-amp's output voltage and green is the voltage at Out+

Red is the op-amp’s output voltage and green is the voltage at Out+

What is the solution?

The tough part is understanding the problem. Solution is simple – just use a difference amplifier with high common mode rejection ratio(CMRR).

Solution is to use a difference amplifier. Here is the one I used.

Solution is to use a difference amplifier. Here is the one I used.

The high CMRR is a catch here. As you can see from the problem statement diagram, the impedance on the two lines (Out+ and Out-) are in most cases, not matched. R2 shall add-up with R5 of the difference amplifier. R2 is typically a low resistance 10-33 ohms to protect the audio generator circuit in case of a short circuit, at the audio jack. A high CMRR can be achieved by increasing R5 and all the other resistors accordingly. 150k for R5 was a reasonable choice between CMRR and the op-amp bias current.

Still I have a bit of noise coupled from power supply remains. Already have the phone and amplifier grounds in a ‘star ground’ configuration. Hence very unlikely that noise is coupled through ground. Probably I have to add a voltage regulator for the amplifier.

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