Question About Boss GE7 Schematic - Page 1
Forum section: electronics / beginners

author: eev_carl
author_rank: Regular Contributor
timestamp: October 27, 2018, 09:27:31 pm
content: 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,Carlhttps://www.hobby-hour.com/electronics/s/ge7-equalizer.php

author: RyanT
author_rank: Contributor
timestamp: October 27, 2018, 09:39:25 pm
content: Near the bottom of the page, they're listed asTL022CPdual 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.

author: eev_carl
author_rank: Regular Contributor
timestamp: October 27, 2018, 09:44:05 pm
content: Quote from: RyanT on October 27, 2018, 09:39:25 pmI'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.

author: RyanT
author_rank: Contributor
timestamp: October 27, 2018, 10:38:14 pm
content: Quote from: eev_carl on October 27, 2018, 09:44:05 pmSorry, I misunderstood what you were asking. You are right thatS-K can be configured as a band pass filter, but your schematic looks more like amultiple 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.

author: Andy Watson
author_rank: Super Contributor
timestamp: October 27, 2018, 11:11:45 pm
content: 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.

author: eev_carl
author_rank: Regular Contributor
timestamp: October 28, 2018, 12:19:01 am
content: Quote from: Andy Watson on October 27, 2018, 11:11:45 pmThey look like gyrators to me also. However, I was wondering about the frequency response. It seems like if it's a gyrator, it is a notch filter as in the attached image from sound.whsites.net.I thought that an EQ like this would be in a band pass configuration (parallel C). Is this 7 notches?

author: eev_carl
author_rank: Regular Contributor
timestamp: October 28, 2018, 12:25:32 am
content: Quote from: RyanT on October 27, 2018, 10:38:14 pmIt doesn't look like that to me. My schematic has two capacitors in series going into the positive terminal. Yours has two capacitors in parallel going into the negative terminal. The negative feedback resistor is wired differently too.I'll check both out with LT Spice tomorrow and post the Bode plots.

author: Andy Watson
author_rank: Super Contributor
timestamp: October 28, 2018, 12:46:00 am
content: Quote from: eev_carl on October 28, 2018, 12:19:01 amI think (you might want to check this) that the gyrators are configured to appear as a series L-C element - i.e. their impedance is low at the desired frequency. This low impedacne is coupled into the feedback path of IC4 (pins 1, 2, 3) to cause either a boost or a cut - depending on the setting of the potentiometer - at the desired frequencyQuote6 low-impedance notches - the 7th is not a notch but a low-impedance at high frequencies.

author: eev_carl
author_rank: Regular Contributor
timestamp: October 28, 2018, 01:29:03 pm
content: It seems like the frequency response are different. The band pass filter on the left (not in the GE7) looks like other graphic eq schematics I've seen.

author: eev_carl
author_rank: Regular Contributor
timestamp: October 28, 2018, 01:31:44 pm
content: Quote from: Andy Watson on October 28, 2018, 12:46:00 amIs IC4 used as a Negative Impedance Converter that turns the notch filters 1-7 into -- functionally speaking -- band pass filters? Does it invert the frequency response?

author: eev_carl
author_rank: Regular Contributor
timestamp: December 13, 2018, 01:02:11 pm
content: I didn't follow some of the replies to my original post at the time, but I was reading Self "Small Signal Audio Design" and found that this Boss schematic is a very standard RLC circuit that uses gyrators instead of inductors. The screenshot shows a similar 7-band eq, the notional RLC circuit, and the gyrator part.