Electronics > Projects, Designs, and Technical Stuff

LPF (6th order) with TL072C

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JimWest:

--- Quote from: Jay_Diddy_B on November 28, 2019, 12:25:24 pm ---Do you have a working one that you can measure?

--- End quote ---

No, I only have a picture of the board.

Based on the picture I have, I was able to identify two capacitors. One is marked A3 (1nF), the other A2(100pF).
The one with A2 is used in the output LPF of the sound IC. The other A3 in the input LPF as C5 in the above circuit.
If I interpret the capacitor values only on their optical appearance, the resulting cut-off frequency is 21685Hz.

I was looking around the internet and found a service manual from Sony. They used the same IC for Dolby Surround.
The cut-off frequency of the Sony circuit is something about 9773Hz.

Yansi:
Those 10kHz cutoff is probably right.  20 is not.

You can't simulate the filter without the first input RC stage. This way you are being missguided about the result.  The sallen-key filter is always designed for a zero output impedance source at the input of the filter. Hence  omitting the first RC lowpass filter pole is a mistake (I mean R3 C5 on your original schematic).

Also, resulting filter Q shall not be much above 0.7, the filter is not likely to be designed resonant (peaking on high frequencies).

Y.

iMo:
Antialiasing low pass for the DSP?

Yansi:
it is an antialias filter.

But unlikely at 20kHZ with a 23kHz sampling rate, even though the ADC is 2x oversampled.

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