A good option for the XDR is to completely replace the stock audio preamp circuit with a new audio board based on a high-bandwidth opamp. This provides significant analog audio improvements:
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Measuring the XDR's FM Frequency Response from Antenna In to Line Out
Data suggests the XDR's analog audio circuit has drooping treble response. What if Sony did that on purpose to compensate for a corresponding treble boost in the digital domain? Seems unlikely, but not impossible.
To test the aggregate frequency response, I used my HP 8903B Audio Analyzer to generate a clean audio signal. This was fed into a Panasonic FM Signal Generator and used as the modulation source during testing. The Panasonic then generates an FM signal at the selected frequency on the FM band and modulates it with the HP's audio. Think of broadcasting, say, a 1KHz test tone on 97.3 MHz.
Analog FM frequency response is usually stated as being 30 Hz to 15 KHz so that's the range I measured.
Note About FM Pre-Emphasis: FM radio uses "pre-emphasis" at the broadcast side, and a corresponding "de-emphasis" at the receiver side. It's kind of like the RIAA curve used for vinyl records. In North America, the standard is 75 microsecond pre-emphasis. I used data found here to calculate the expected deviation from flat response, then measured the actual deviation. I used the data in their "Unlimited 75uS" column (that choice could be wrong, but there's little difference between Unlimited and Roofed in the audio band). Note that their reference frequency (the 0 dB level) seems to be 50 Hz. They don't show any data below 50 Hz but I took measurements at 30 Hz anyway.
The table & graph show the measured overall frequency response for Unmodified and Modified XDRs, from Antenna In to Line Out, with compensation for FM de-emphasis happening in the XDR. In the graph the 0.00 line is the reference, with deviation in +/- dB from there. The treble dropoff on the Unmodified XDR from Antenna In to Line Out matches up pretty closely with the same measurements when confined to the XDR's analog audio circuit.
Conclusion from this data: The XDR's treble droop is in the analog audio preamp circuit, not the tuner module or DSP section.
Improving XDR-F1HD Audio Quality
New custom audio board for the XDR-F1HD
Bench Testing New Analog Audio Board
Measuring % Distortion @ 1KHz, 0.35V Input Signal Level
Sony gave the XDR amazing reception capability, and its DSP section is very high quality. But its analog audio preamp can be made much better:
The audio analyzer plots below compare the unmodified XDR audio preamp to my custom audio upgrade board.
All voltage levels RMS