# Increase gain reduction on op amp filter

The image below features the EQ section of a DJ mixer with -26db gain reduction on each frequency band (hi/mid/low). My goal is to modify this circuit so that the op amp filters are "full kill" (aka isolators) - meaning that when you turn the VR all the way down, it kills the frequency band completely. What can I do to accomplish that (or at least increase the gain reduction beyond -40db)?

• The true null only occurs at 1 frequency then the width at -3 and -26 dB depends on Q which cannot notch the entire width @ -26 dB as this would require more tuneable notches like a half octave EQ rather than a 3 to 4 octave EQ May 29 '19 at 16:03
• From which Yamaha gear this diagram is coming from? And the maximum/minimum gain will be equal around $\large \frac{R_{120}}{R_{116}} = +/- 28 V/V = +/-29dB$ or R112 and R108.
– G36
May 29 '19 at 16:23
• Thanks G36. This is from a Denon X400. I realize based on some of the answers that the db reduction rating is not for the entire band, and in this circuit theres a reduction curve across the frequencies of a given band. For my purposed I'm happy to roughly increase gain reduction per band without much concern for frequency precision (after all DJ EQing is crude). So it sounds like I can just increase R120 from 5.1K to 10K to change all three EQs from roughly -29db to roughly -50db? This should create a more audible "cut" within each given band in my understanding of this answer?
– wirl
May 29 '19 at 19:14
• But for 10K the gain is only 10k/130 = 77V/V which is only 37.7dB not a big improvement. And the R120 sets the "boost" gain. To change the "attenuation" gain only you need to change R106. But I do not recommend this way
– G36
May 29 '19 at 21:22
• Gotcha, 37.7dB for the mid range. I'm assuming the R106 your referring to is the resistor preceding the components in the screenshot i've uploaded (from the full schematic)? Just want to make sure that's not a typo. Also yes that's correct, I really just want to increase the attenuation gain, not the boost. Lastly - why wouldn't you recommend adjusting the attenuation gain with R106?
– wirl
May 30 '19 at 4:43

My goal is to modify this circuit so that the op amp filters are "full kill" (aka isolators) - meaning that when you turn the VR all the way down, it kills the frequency band completely.

Redesign using tunable notch filters as opposed to gyrator inductors and series capacitors. Gyrators are great for emulating inductance but it comes at a price of series resistance (R108, R112 and R116).

However, killing a signal completely is largely unfeasible - you might be able to get down to maybe 100 db reduction with careful circuit layout and decent components but you have to ask yourself if this is really what you want.

• Anything under 40db gain reduction would meet my needs. Ultimately these bands should be close to inaudible when turned down, which doesn't necessarily mean they need to be non-existant. To clarify - i'm modifying a prebuilt circuit in a DJ mixer. This doesn't allow much space for new components. Is there a way to simply change some of the R/C values in this circuit to increase gain reduction?
– wirl
May 29 '19 at 15:53
• No, there is no way to achieve what you want with minor or lightweight modifications. May 29 '19 at 16:46