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Rotel RCD971 PCM63 output filter question
Rotel RCD971 PCM63 output filter question
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Old 2nd February 2018, 02:04 AM   #11
Ken Newton is offline Ken Newton  United States
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Op-amp U702 doesn't appear to be connected as an I/V transimpedance amplifier, it instead appears to be connected as an simple inverting voltage amplifier having a gain of (3.9k / 100R = 39). The DAC's current output feeds the 100R op-amp input resistor on one side, and has a virtual ground node on the opposite side. So, the 18.2nF of capacitance from the DAC output to ground is effectively in parallel with the 100R input resistor, and therefore forms a low-pass filter with it. The cut-off frequency looks to be around 87.5kHz.

The output current of the DAC will develop a voltage across the 100R input resistor, which then would be multiplied by the x39 gain of the amplifier.

Last edited by Ken Newton; 2nd February 2018 at 02:07 AM.
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Old 2nd February 2018, 07:38 AM   #12
Mooly is offline Mooly  United Kingdom
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Rotel RCD971 PCM63 output filter question
I felt that the numbers didn't really sound right for the stage to be an inverting amp tbh although the configuration could be either with a value as low as 100 ohm.

One way to tell would be to deliberately increase the 100 ohm. If the DAC was giving a voltage output then the output from the opamp would change as expected. If it was a current output then changes in the 100 ohm (within reason) would not show a change in output voltage at the opamp output, but would instead show a voltage change at the DAC output pin as the current developed a voltage across the resistor. The final output voltage would not change though for the 'current' theory.
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