PCM1796: feeding TPA3255 with balanced signal

Hi Tomasz,

Figure 35 has a good example, as you do not need the final amplifier stage as shown in figure 36. We have some great amps that can be used for this application, I recommend the OPA1678. You should favor thin film surface mount resistors and NP0/C0G type capacitors for the discrete components. You will also need a DC blocking capacitor, as the output of the amps will have a negative bias voltage, while the input of the TPA will have a positive bias. A large electrolytic cap would serve this purpose. At the very least a passive RC low pass filter should be implemented as well. This can be on the output of the OPA1612, though note you may need to account for the resistance in the gain calculation.

Thanks!
Paul

Hi Paul,

Thank you for your replay but I didn't get it... Did you mention the circuit:

The circuit shows unbalanced implementation - I need balanced/symmetrical signal on the output to feed balanced input of the TPA3255

Here is the TPA3255 stereo BTL implementation example:

The INPUT_A and INPUT_B control two separate amplifiers working in bridge mode (BTL):

To optimally use the amplifier I need symmetrical/balanced input audio signal... The PCM179x DACs have balanced current output but typical application like in datasheet converts the balanced signal to unbalanced using operational amplifier. The amplifier provides balanced-to-unbalanced conversion and filter functionality combined. Have you got any example hot to implement the output filter without converting signal to unbalanced?

Hi Tomasz,

I was referring to figure 35.

In this case you would not implement the differential to single ended conversion, and instead connect it to your class D amplifier.  This would provide the I/V stage while still being differential.

You could still have a RC low pass filter right at the input:

Thanks!

Paul

Hi Tomasz,

There is actually two low pass filters in the design.  The first is in the feedback of the amp, and the other would be an RC on the input.  The most aggressive what to remove OoB noise is to introduce another multiple feedback LPF.  I have created a TINA model for you as well.

Thanks!

Paul

PCM1796 Diff-out.TSC

Hi Tomasz,

Your design would also work well.  As far the resistors are concerned, this will be a balance between power and noise.  I think you should be fine with lower values.

Thanks!

Paul