Anybody willing to explain the Oxide (Ibanez Mostortion) 3-band EQ circuit to me?

[ctelecaster]

Hi,

Would like to learn more about the 3 band EQ on the Oxide (Ibanez Mostortion) - I can't seem to wrap my head around how the circuit works. Interested specifically in learning more about which components determine the frequency range of each band of the passive EQ, as I'd like to play around with tuning the response to my tastes.

If anyone smarter than I would be willing to give me a quick rundown, I'd be super grateful. Thanks! Attached is the schematic for reference.

 

[eh là bas ma]

Unfortunately i won't be able to explain how it works, i will follow this interesting thread and learn as much as i can.

I also built an Oxide distortion, all i can say is that the EQ is among the most effective EQ i ever tried.

I am surprised that you are not fully satisfied with it.

If i may, i will quote Mr Hammer, a famous diy wizard, about this circuit :

"I've built a few Mostortions, with 3260 and 3240 chips. There is an audible difference between the two, but I can't say I like one better than the other. They both sound great. The real secret to this little beast is the use of a 2+2 diode complement (just like a Timmy) and the terrific 3-band EQ. Personally, I don't feel it benefits from mods. I say "Enjoy it as is" and leave the diode switching to someone else. "

https://www.diystompboxes.com/smfforum/index.php?msg=1256606

I wanted to try a change of clipping diode, just by curiosity. I did it despite this advice, and i have to admit that it's not really useful after all.

Here is a tiny bit of information i could find on Aion's website :

"The 3-band passive tonestack is similar to Marshall and Fender tonestacks, and nearly identical to Peavey solid state guitar amp tonestacks. It’s a passive tonestack, contrasted with the typical Tube Screamer tone control which is driven by an op amp. In the process of shaping the tone, the overall signal level is cut, so it needs to be followed by a gain recovery stage in order to get back up to a reasonable volume level. The second half of the CA3260 op amp provides a gain of 15 dB, with the R-C filter having a corner frequency of 15 Hz—in other words, a full range boost."

 

[skillzmcgavern]

Figured I'd post in this related thread, to help consolidate information/advice on Mostortion/Oxide mods.

I'm trying to compare to the Aion FX Quantum schematic, which looks quite similar. Kevin from Aion mentions that lowering the Quantum's C5 value will cut bass, making it easier to get high-gain tones without "flub". Is it the same on this PedalPCB Oxide schematic? Meaning, lower C5 to 150n-100n territory for better high-gain results that doesn't cut as much bass as a Tube Screamer?

 

[Feral Feline]

C5 on both schematics is the same — so whatever Kevin told you would work for Aion's C5 will do the same for the PPCB C5.

The differences between the two schematics is so very slight:

- Aion adds some clipping options (easy enough to do so on the Oxide)
- different buffer transistors used
- output cap on Aion Quantum is 1µ, whereas PPCB's output cap is 10µ.

 

[thomasbe86]

C5 at 220n (with R8 = 2.7k) cuts the bass before clipping at 267.9376 Hz - that is the stock mostortion
C5 at 47n (with R8 = 4.7k) cuts the bass before clipping at 720.4841 Hz - that is the stock tube screamer

Play around with an RC high pass filter calculator (online example) to find what you would like to have :
C5 at 100n (with R8 = 2.7k) = 589.4628 Hz
C5 at 120n (with R8 = 2.7k) = 491.2190 Hz
C5 at 150n (with R8 = 2.7k) = 392.9752 Hz
...

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Additionally, you can have a look at this thread about high pass filters in the feedback loop:
Bass response of your opamp gain stage and why is the timmy awesome
You can see in there the difference in f response of the two (TS in red and MT10 in green):

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Explained a bit here for the tube screamer (1.4.3 High Pass Filter in the Feedback Loop.):
tube-screamer-analysis

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Out of subject bur regarding the tone stack of the mostortion; you can play around with this toy and see how it affects the frequency response by recreating the tone stack here, using the similar "crate" eq, I adjusted the values here to reflect the MT-10 (ignore and leave as is, Rin, RL, R2 and R3):
MT-10 tone stack

 

[skillzmcgavern]

Great info and explanations, thanks folks.

 

[andrewsrea]

Lots of good answers. Mine is simplified: it is a passive 3-band EQ which takes the full-frequency signal and passes specific frequencies to ground, via the filter cap on the Vref circuit.