PCM2903C: Schematic Review: RCA Output Not Detected/Quite Low

By the way, I've verified that the amps work with other RCA outputs and looping the audio from the RCA out on our device back into the RCA in works.

Hi Tom,

What's the input impedance of the amps you are using? The PCM2903C requires the load of the line outputs to be at least 10kOhms.

Best regards,
Jeff McPherson

I'm not sure, I'll have to check with the manufacturers. But if the loads are less than 10kOhms, what might happen?

I've tried hooking up a 1KOhm resistor directly to my RCA out and saw only a 0.11 V drop (consistent with a ~70 Ohm output impedance). I wouldn't expect this to cause a significant degradation in volume.

BTW, this does seem to be vary from amplifier OEMs because we haven't had any issue with our RCA out to Sonos Connect, Wiim, or Marantz.

If the loads are less, then the divider formed by the output impedance is at risk of lowering the volume seen by the amps. Also depending on what kind of current the amp pulls you might see the drop that you already measured. However since it's pretty low even at 1k that's probably not the issue. However it would good to confirm that the input impedance of the amps is at least 10k because that's the lowest impedance that TI can guarantee the performance of the device at scale.

If it varies between amps, then the crux of the issue is probably on the amp side rather than the codec. The TI op amp in your circuit belongs to a different team than myself, but before I send you to them I want to confirm that you're measuring the expected line voltage out of the PCM2903C?

Can you elaborate more on your loopback test that you mentioned in your follow up post? I'm having trouble visualizing what you're describing. 

Thanks,
Jeff McPherson

With the design that we've implemented the Vpp that I'm seeing is roughly 1.9 V peak to peak or ~0.67 V rms. This is with a 0dB 1kHz signal with open load.

The loopback test I mentioned in my initial response is that I connected the RCA out of the PCM2903C back into the RCA in of the same PCM2903C chip on our product.

Hi Tom,

Gotcha. The typical output voltage is 0.6Vccc = 1.98Vpp so I think the codec is behaving exactly as it should be, also given that your loopback test is working; so there's some kind of compatibility with interfacing. I'd keep in mind the 10kOhm load requirement as you research the amps as a precaution.

I'm going to have our op amp team (OPA4353 experts) take over and to review your second schematic and see if anything there might be interfering with the amps.

Best regards,
Jeff McPherson

Hi Tom, 

Have you tested with the PCM2903EVM-U with the devices pre-populated? 

Just from the schematic I do not see anything obvious, but could you clarify what your intended input and output signals are and what your actual measured values were? 

Regards,
Ashley

Sorry, could you clarify what you mean by 'devices pre-populated?'

We didn't really design with an intended line level in mind for the RCA out or in. What we noticed is that we begin clipping and hitting the limits on the RCA in when we reach past 0.7 Vrms and we're only able to output at most 0.6 Vrms.

So after testing, I think I may have some clues. When I am plugging my RCA output halfway into the RCA in of the Monoprice Unity amp, I am able to get audio out. But once the ground part of the RCA connector touches the female connector the audio signal drop to nothing. I'm not sure if it's something we're doing wrong or Monoprice but it's strange. Adding a ground loop isolator in seris solves the problem entirely. 

I also want to note that we're feeding the Vcom line from our design to our RCA out. I wonder if a difference in the ground potential could pull Vcom up and cause the op amp to saturate? Is that possible?

Hi Tom, 

I was referring if you have tested the Monoprice unity amp with the PCM2903EVM-U board: https://www.ti.com/tool/PCM2903EVM-U

Are you connecting your RCA outputs directly to the connector? And are they these outputs circled below? 

Regards,
Ashley

Oh, I have not done that but I certainly can. I'll let you know how the testing goes. With regards to your question, yes we are connecting the RCA outputs directly to the connector. Each connector gets VCOM and RCA_OUT_LEFT/RCA_OUT_RIGHT.

Hi Tom, 

Thank you for the confirmation. In addition if it is possible, could you probe the IN-, IN+ and OUT for channel A & B when the connector plugged in halfway and then when the connector is fully plugged in? And then with the ground isolator? 

It would be helpful to see how the op amp behavior is changing based on these scenarios. 

Regards,
Ashley

Hi Ashley, so here's what my testing shows

Prior to inserting all the way:

VCOM: 1.6V

RCA_OUT_LEFT: 1.6V +- signal

RCA_OUT_RIGHT: 1.6V +- signal

After inserting all the way:

VCOM: 0V

RCA_OUT_LEFT: 0V

RCA_OUT_RIGHT: 0V

With ground loop isolator inserted all the way:

VCOM: 1.6V

RCA_OUT_LEFT: 1.6V +- signal

RCA_OUT_RIGHT: 1.6V +- signal

---

VCOM: 0V

RCA_IN_LEFT: 0V +- signal

RCA_IN_RIGHT: 0V +- signal

It seems like the Monoprice amp is bringing Vcom down to ground. Could this be a design flaw on their end?

Hi Tom, 

Thank you for the additional testing details. It does seem odd that the Vcom is being pulled to ground. Could you help to provide a schematic after RCA_OUT_LEFT, RCA_OUT_RIGHT, and VCOM? I am trying to understand how the connector is electrically connected to the Monoprice amp and where each of the connections are going. 

Regards,
Ashley

The RCA_OUT_LEFT, RCA_OUT_RIGHT and VCOM go directly to the RCA physical connector. See below:

Here is the link to the connector that we're using

Hi Tom, 

Thanks for the additional schematic. What may be happening is that the connector pin 1 is expecting a GND connection and with VCOM connected to it (originally 1.6V), VCOM will then be pulled to GND when the connector is fully inserted all the way. 

One way to test this theory would be to disconnect connector Pin 1 (remove the VCOM connection) and solder a jumper wire to GND. Then insert the connector all the way and see if the issue is fixed. 

Please let me know if you have any questions.
Thank you!

Regards,
Ashley

Is this the typical configuration for RCA? It's my understanding that Vcom provides the reference voltage for the RCA left and right signal. Otherwise, the left and right signals are going to have a DC bias of 1.6V.

I confirm this theory by only inserting the RCA jack halfway in and the audio plays fine on the Monoprice amplifier. I don't have the same issue with other amps where I have to plug in halfway to hear audio.