PCM1808 ADC Module Modification

The PCM1808 module, as shipped, includes RC low pass filters on both inputs. The resistor value is 10kohms, with a capacitor value of 330pF. This creates a 3dB cut-off frequency of about 48kHz.

This problem was discussed early in the history of the T41:

https://groups.io/g/SoftwareControlledHamRadio/message/18876

I had forgotten about this problem! Thank you to Julia G5MAI for reminding me that the 10kohm resistors should be removed.

This low pass filter action creates a visible roll-off which can be seen in the RF spectrum display. It is visible in 2X zoom as a dip in the spectrum about 10kHz above the low end of the displayed spectrum.

The solution is to remove the 10kohm resistors and replace them with 0ohm resistors. The correct resistor is the same as the resistors used to make the Teensy Audio Hat modification:

https://www.digikey.com/en/products/detail/panasonic-industry/ERJ-3GEY0R00V/135656

I used the small nozzle on my hot air soldering station to remove the original 10kohm resistors. I removed the remaining solder on the pads with solder wick, applied a small amount of solder paste to each pad, placed the resistors, and then resoldered using the same small hot air nozzle.

Please note: I don’t recommend doing this modification until after you have tested the module. In the early days of the T41, the PCM1808 modules were observed to have a high failure rate.

Here is a photo of the revised board:

Install the module in the Main board and the modification is complete.

73 Greg KF5N