Figure 2 - Simplified dual supply operational amplifier output circuit.

Many systems use a standard op amp in an inverting or non-inverting format to take the unbalanced output of a DAC or SoC to the outside world. This, by far, is the simplest way to create a 5.6-Vpp swing output, biased around GND.

Figure 3 - Ideal ground biased output from a dual supply output stage.

Using this method, the designer can fine tune the sound they are looking for, cherry pick the op amp they want, and even insert some filtering in the output path.

However, many consumer audio systems aren't running voltage supplies higher than +5 V, and the hope of having a negative supply rail adds to COB. Some systems on the market generate the rails for audio line-level amplifiers, however, normally at the cost of two additional low dropout regulators (LDOs), as well as the passives needed to set up the device.

Conclusions:

• Benefits: Nice to have, if you can afford it in your system.
• Concerns: Cost, PCB space, form factor.

Solution #2: Single-supply op amp with DC bias of VDD/2 and a DC-blocking cap.

Figure 4 - Single-supply line driver op amp.

Here's another old favorite. In the past, a low-cost method used in many systems was to run a single-supply op amp from a high-voltage rail of 8 V+. However, many systems don't like receiving a signal with a DC bias on it - designers normally add DC-blocking capacitors to ensure only AC signals leave their product.

Figure 5 - The output from a traditional single supply opamp.

In such a system, the capacitor charges up with DC as the receiver completes a path to a lower potential. During normal operation this wouldn't be much of a problem. However, as soon as the amplifier changes state (i.e., is switched on, switched off, or moves to low-power mode), that DC is going to discharge, causing audible pops and clicks.

Additionally, those who have played with analog filter designs will see what happens when you put a capacitor directly in the audio chain. The nature of a capacitor's impedance is that at low frequencies, it displays a higher resistance to the signal. In other words, they don't just block DC. DC-blocking capacitors introduce a high-pass filter effect to the output of your system. One of the few ways of moving the 3-dB point of the filter is by getting a higher value capacitor. The following equation calculates the 3-dB filter point for a RC filter:

f_3db = 1 / 2πR_lC_o

Where R_l is the load of the next device in the chain and C_o is the output capacitance. Large value capacitors typically cost more and take more PCB space.

Conclusions:

• Advantages: Low cost.
• Disadvantages: May need a dedicated VCC, potential pops and clicks, poor bass response.