Question about cutoff point of 2-transistor high pass filter

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macf123

Joined Feb 5, 2023
2
I'm trying change the cutoff point of a vintage mixer's HPF from 80 Hz to 60 Hz

I have the service manual in front of me and located the high pass section of the schematic (attached below)

I found this excellent link explaining a vey similar configuration here: https://www.electronics-notes.com/a...ransistor/active-high-pass-filter-circuit.php

However the circuit in the mixer has an additional transistor, and the formula from the electronics notes article resulted in a f0 value fo 112, not 80 as it should be.

Could anyone let me know how I might go about calculating the correct value for the 2 transistor HPF, and ultimately adjusting its value from 80 Hz to 60Hz?


HPF MIXER.png
 

crutschow

Joined Mar 14, 2008
34,834
Below is the LTspice simulation of the circuit:
I experimentally tweaked the values of R6 and R7 to give a -3dB gain drop at 60Hz from the passband gain.
Those are not necessarily optimum values.

1675634492083.png
 
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