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PCM1774 line/headphone out

Wim Konings
Prodigy 200 points
Other Parts Discussed in Thread: PCM1774

Dear,

I want to use the PCM1774 with 1 output as a line out and the 2nd output as a headphone out.

According the datasheet, the impedance of the line out is 10K and headphone is 32 or 16 Ohm. I've configured register 74 as a stereo output but if I plug in a headphone, there audio signal is very weak while connecting a speaker results in a good signal quality.

Do I need to configure the DAC output line to select the matching impedance? Is there any hardware needed for a matching impedance?

Best regards

Wim

  • 0
    TI__Guru** 103725 points
    Hi Wim,

    Welcome to E2E and thank you for your interest in our products.

    What are your register settings?. Perhaps the Headphone Amplifier is powered down, you need to power up the Headphone Amplifier for the channel you want through Register 73 PHPR and/or PHPL bits (Register 73 information is on page 27 of PCM1774 Data Sheet).
    Hope this help you.

    Best regards,
    Iván Salazar
    Texas Instruments
  • 0 in reply to Ivan Salazar
    Prodigy 200 points

    Hello Ivan,

    First I set the register of the recommended Power-On Sequence (page 13 of the datasheet).

    Next I set the following registers (PHPR and PHPL are enbaled):

        vPCM1774_WriteControlReg(64, 0x0033);  // Mute disabled, 0 dB analog headphone volume for HPL
        vPCM1774_WriteControlReg(65, 0x0033);  // Mute disabled, 0 dB analog headphone volume for HPR
        vPCM1774_WriteControlReg(68, 0x003F);  // Disable digital soft mute for DAL and attenuation to 0 dB
        vPCM1774_WriteControlReg(69, 0x003F);  // Disable digital soft mute for DAR and attenuation to 0 dB
        vPCM1774_WriteControlReg(70, 0x0001);  // De-emphasis filter off, Audio interface for DAC to I2S, digital gain to 0 dB and oversampling enabled (recommended)
        vPCM1774_WriteControlReg(74, 0x0001);  // Stereo output and Vcom enabled
        vPCM1774_WriteControlReg(87, 0x0011);  // MUX 1 (AIN1L) and MUX 2 (AIN1R): enabled
    nd SW6 may be disabled since LOR is not connected)
        vPCM1774_WriteControlReg(88, 0x0037);  // SW1, SW3 and SW 4 enabled for analog input, SW2 and SW5 enabled for DAC output
        vPCM1774_WriteControlReg(89, 0x0057);  // 0 dB gain level control for PG5 and -6 dB for PG6 (Micro in)
        vPCM1774_WriteControlReg(90, 0x0002);  // 0 dB boost for PG1 and 20 dB for PG2
        vPCM1774_WriteControlReg(96, 0x0000);  // Digital stereo enabled
        vPCM1774_WriteControlReg(124, 0x0000); // 0 dB boost for PG1 and 0 dB for PG2 (used to get the 20 dB boost from reg 90)

    Are there any other settings that need to be done? I suppose that you can connect a headphone directly to the output.

    Best regards

    Wim

  • 0 in reply to Wim Konings
    TI__Guru** 103725 points
    Hi Wim,

    Your register settings seems to be fine. Could you share your hardware configuration?

    Regards,
    Iván Salazar
    Texas Instruments
  • 0 in reply to Ivan Salazar
    Prodigy 200 points

    Hello Ivan,

    Here is the schematics for the PCM1774. We connect a 200R headphone to CON1. According the datasheet it could drive a 16 or 32R load.

    Best regards

    Wim

    pcm1774.pdf

  • 0 in reply to Wim Konings
    TI__Guru** 103725 points
    Hi Wim,

    Perhaps changing your output hardware could help. Follow the recommended headphone output on page 42. Remove R9 and C8 from your schematic. Also you can change C7 to a 220uF capacitor for a lower cutoff frequency.

    Regards,
    Iván Salazar
    Texas Instruments
  • 0 in reply to Ivan Salazar
    Prodigy 200 points

    Hello Ivan,

    It was R9 that was causing an impedance mismatch when connecting a headphone. Many thanks for your support.

    Regards

    Wim