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TPA3251: TPA3251 + PCM3168a + PFFB

Part Number: TPA3251
Other Parts Discussed in Thread: PCM3168

Hey everyone, i got two questions about a combination of the PCM3168 and the TPA3251.

First one, using a direct connection only with a single set of DC-Blocking Caps should work as i think. The PCM has a dif-Output with 8Vpp, the TPA a gain of 10, therefore the PCM is able to drive the amplifier into clipping (assuming a +-36V output voltage over the speaker terminals), even so with 0,4V margin on the input.

--> The PCM is used without an additional second order lowpass, but only with a passive first order one. 

Should be ok.

The interesting part is no the use of the AC Coupling. Both, the PCM as well as the TPA do use a DC-Bias at their respective terminals, so using an electrolytic capacitor (+ to input of TPA, - to output of PCM), might be possible, the Cap is then biased with around 1,4V. My question which arises now:

The Capacitor is not biased enough for a full voltage swing, assuming an instantaneous fullscale output of the PCM, 5V is applied on the negative terminal, whereas only 3,9V is applied to the positive one, resulting in a negative voltage of 1,1V. This may not be fully applicable to audio applications, but im afraid using an electrolytic like this might increase the THD strongly in the "zero crossing". Does anyone have experience here? Or someone of the TI Staff who can help out with some hints here?

Second Question here, since the TPA is supplied up to 30V, the input voltage margin now increases to 1V, and i want to use the remaining margin to apply some PFFB with only a small influence on the input.

--> i will be using the suggested network (R, CRC) of the latest appnotes for this, but i do not use an additional OPA between the PCM and the TPA, therefore i cannot fully take over the app-note. Can this result in a stability problem or any other problems? I cannot use an additional OPA in my application due to size restrictions.

Thanks alot,

best regards,


Second one, 

  • Hi Lukas,

    Regarding the AC coupling cap selection, would you like to consider the MLCC? This is what we usually used for TPA series devices.

    I think as long as the DAC has enough driving capability, then it's OK to eliminate the OPAMP. However, such a small feedback depth may not bring much benefit, but introduce the mismatch between the resistor and capacitors. TPA device has good enough THD+N performance, I would recommend not to implement the PFFB here.




  • Hey Sam, thanks for the reply.

    I do not feel comfortable with ceramics in directly in the audio path, all boards i've had on had so far did underperform with MLCCs. Also, C0G/NP0 is not available for the required capacitance, thats why i'd like to either use film or electrolytic. 3µ3 Film caps. 3µ3 would result in a -22mdb dampening at 20hz, and like 4° Phase Change which would still be acceptable i think. Still, electrolytics would available with way higher capitance to reduce the influence on the audioband further, hence the question.

    Thanks for thoughts on the PFFB, i might reserve some footprints for the parts but will not populate them then i think.

    Best regards,


  • Hi Lukas,

    Understood. Thanks!