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https://electricdruid.net/boss-mt-2-metal-zone-pedal-analysis/

https://sound-au.com/articles/gyrator-filters.htm#s9

I have been using these websites to try and work out how the eq works, but I've had a bit of a problem when it comes to the gyrator used for the low tone control. I can't quite seem to work out the relationship between the frequency response and the components.

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  • \$\begingroup\$ Welcome to EE.SE! I would suggest a free SPICE simulator. Either download LTspice or use the built in one on this page by pressing edit, click on the schematics symbol and draw your schematic. From there you can tinker with all values and find out what does what. \$\endgroup\$ – winny Nov 19 '18 at 9:53
  • \$\begingroup\$ It's interesting that the time constant associated with R012 and C008 corresponds to a frequency of 328.8 Hz, which correlates closely with the lower inflection point on your graph. Similarly, the combination of R061 and C044 has a cutoff frequency of 7234 Hz. \$\endgroup\$ – Dave Tweed Nov 19 '18 at 10:55
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According to the Wikipedia article, the simulated inductance has a value of

$$L = R_{012} R_{013} C_{009} = 10.34\text{ H}$$

This inductance is in series with a resistance equal to R012, giving an L/R time constant of 4.7 ms, which corresponds to a frequency of 33.86 Hz. This is the lower frequency limit for the bass control's effect.

Similarly, the series combination of C008 and R012 has a time constant that corresponds to a frequency of 328.8 Hz, and this defines the upper limit of the bass control's effect, and corresponds directly to the inflection point shown on your graph.

Note also that the series combination of C008, R012 and the simulated inductor is also in series with a minimum source resistance of 22 kΩ (either R014 or R015), which insures that the circuit is overdamped (no resonance). If this source impedance were reduced, you would start to see a resonant peak at around 105 Hz.

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here's what I see, we have a op-amp in the middle with a simple 22k feedback and 10pF to stop oscillations.

then we have the input signal going to the non-inverting input through a 22k resistor.

between these two nodes is a potentiometer with the center tab to a gyrator. a gyrator acts like an inductor with one end grounded, it absorbs low frequency currents.

Now if you turn the potentiometer one way and the gyrator soaks up the input signal and sees less of the feedback signal - so the low frequencies are "killed" both by the gyrator sucking in the input signal and by the gyrator not attenuating the feedback signal.

If you turn it the other way the gyrator is close to the feedback signal and distant from the input signal, so it soaks up the low frequencies in the feedback signal, but not from the input signal allowing a larger amplitude to be produced.

C008 R012 form a high pass filter stopping low frequencies from going into the gyrator, causing the base control to roll off below 300Hz.

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