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Question About Boss GE7 Schematic

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eev_carl:
Hi,

IC1, IC2, and IC3 in this schematic look like Sallen-Key High Pass Filters.  I had expected to see band pass filters since the schematic is for a 7-band eq.  Is this the correct analysis (boosting / cutting HPFs rather than band pass filters) or is there an LPF part that I'm missing?

Thanks,
Carl

https://www.hobby-hour.com/electronics/s/ge7-equalizer.php

RyanT:
Near the bottom of the page, they're listed as TL022CP dual op-amps, and the schematic looks to me like the standard arrangement with 2 channels per IC, for channels 1&2, 3&4, and 5&6.

eev_carl:

--- Quote from: RyanT on October 27, 2018, 09:39:25 pm ---Near the bottom of the page, they're listed as TL022CP dual op-amps
--- End quote ---


I'm asking about the Sallen-Key high pass filters (which includes the op amps).  I thought each potentiometer would cut or boost a band (band pass filter) but I don't see an LPF paired with each frequency midpoint.

RyanT:

--- Quote from: eev_carl on October 27, 2018, 09:44:05 pm ---I'm asking about the Sallen-Key high pass filters (which includes the op amps).  I thought each potentiometer would cut or boost a band (band pass filter) but I don't see an LPF paired with each frequency midpoint.

--- End quote ---
Sorry, I misunderstood what you were asking. You are right that S-K can be configured as a band pass filter, but your schematic looks more like a multiple feedback BPF, although it has been a decade or two since I've done this kind of filtering, so I'm sure someone will set me straight if I'm wrong here.

Andy Watson:
IC1-3 are gyrators - essentially negative/reverse impedance converter that make capacitors C3, C6, C8, C13, C14 and C15 look like inductors - in the sense that their impedance appears to increase with increase in frequency.

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