Pioneer PD9700 analogue stage question

[paulrerrie]

Hi. I was just curious to know, do R577 & C567 (see schematic) form a (12KHz) low pass filter? The stage is described as 'Buffer & de-emphasis' & I wondered if this was intended to actually filter the audio signal rather than decouple digital/RF noise.

Thanks, Paul

 

[SoNic_real_one]

Looks like that. If the 0.0056 is a 5.6nF capacitor...

 

[Mooly]

The whole opamp stage forms what's called a "multiple feedback" filter and is inverting at DC. The opamp is configured as an integrator (C565) and it's performance is hugely opamp dependant. That may be one reason why the stage uses FET's to boost output stage current delivery.

 

[paulrerrie]

Wikipedia calls it a 'multiple feedback topology' and from the corner frequency calculation, it only appears to be filtering at very high frequencies.

 

[SoNic_real_one]

The integrator made with C565 (680pF) is filtering very high. But the one at the input was in question... Is a simple low-pass with 2k4 resistor and 5n6 capacitor. That has -6dB at some 11.8 kHz.