Yamaha @PET RP-U100 personal receiver Measurements
Most of the Yamaha's digital measurements were taken by driving one of its optical S/PDIF inputs from the Audio Precision System One's digital data output, with the DSP effects bypassed and the volume control set at "25." This is the unity-gain setting for analog sources; with a 1kHz tone at 0dBFS, it resulted in a maximum output at the speaker terminals of 2.506V, or 785mW into 8 ohms. Although the volume control will go up to a maximum setting of "64," a 0dBFS digital input clipped the amplifier's output stage at "41."
The digital input was noninverting. Fig.1 shows the RP-U100's frequency response with de-emphasis (bottom traces) and without (top). Points to note are the bass rolloff (-3dB at 12.5Hz), the slight (0.3dB) peaking in the top octave, and the 0.25dB channel imbalance. The channel separation via the digital inputs is shown in fig.2. Quite respectable at almost 70dB in either direction at 1kHz, this degrades at 6dB/octave below 300Hz due to power-supply limitations, and above this frequency due to capacitive coupling between the channels.
Fig.1 Yamaha RP-U100, frequency response via digital input at 0dBFS (top) and response at -12dBFS with de-emphasis (bottom) at 44.1kHz sample rate. (Right channel dashed, 0.5dB/vertical div.)
Fig.2 Yamaha RP-U100, channel separation vs frequency via digital input, from top to bottom: L-R, R-L (10dB/vertical div.).
Fig.3 shows a 1/3-octave spectral analysis of the Yamaha's output while it decoded 16-bit data representing a dithered 1kHz tone at -90dBFS. Some power-supply components can be seen at 60Hz, 120Hz, and 240Hz, but these are all below -100dBFS. Other than a peculiar wrinkle at 10kHz in the left channel, the traces are free from HF spuriae. This is good performance, but increasing the input word length to 24 bits produced no lowering of the noise floor, implying that the RP-U100 has a 16-bit data path.
Fig.3 Yamaha RP-U100, spectrum of dithered 1kHz tone at -90dBFS, with noise and spuriae, 16-bit data (right channel dashed).