Boss GE-7 Changing the frequencies

[ntuncer]

Hello all,
I have a question about changing the frequency of 6.4K. According to the schematic the other frequencies could be calculated as bandpass EQ but I couldn't understand 6.4k as there is no IC for it. Could you please explain how to achieve to 4K instead of 6.4k with the logic?

 

[vladimirsky]

I think it functions as a high pass, as it’s the end of the line

 

[OldManAP]

I’m pretty sure R10 isn’t involved, I think it is the CLR for the indicator LED.

When a filter is in this configuration of wiper to cap to resistor, what sort of filter is it, and how is the frequency determined?

 

[vladimirsky]

isn't it still a high pass, since the pot is acting as a voltage divider, not a rheostat? assuming the usual filter formula, the cutoff is around 4.2k?

 

[OldManAP]

isn't it still a high pass, since the pot is acting as a voltage divider, not a rheostat? assuming the usual filter formula, the cutoff is around 4.2k?

That’s what I got as well. So where’s the 6.4K come from?

 

[vladimirsky]

i think it's an issue of the corner of the stop band vs pass band. since 4.2k is the -3db, there's a ways to go until you reach the frequencies that aren't cut off at all. cut off is around 70% or so. so that being the case, 4200/.7 is about 6k. considering the pretty large gaps between notes in that range(200-300hz) , i'm assuming that being ultra accurate isn't that big of an issue. then again, not an engineer, so i may be way off base.

 

[OldManAP]

Based on @vladimirsky ‘s reply above, replacing C1 with 68n seems like it would probably be pretty close. Does that seem correct?

 

[OldManAP]

I just checked against the highest band on the MXR 10-band, which is labeled 16k. It uses the exact same type of filter but with different values. And the values they used come out to a cutoff frequency of 15.3K. A lot closer to the labeled 16K than the 4.13K and 6.4K of the Boss. So I kinda think Boss just lied about the 6.4K and hoped no one would notice.

 

[OldManAP]

Ok, the values for the highest band on the MXR 6-band work out to just south of 2K, but they labeled it 3.2K. And 2K is pretty close to 70% of 3.2K. Do with that info what you will.

 

[ntuncer]

Thanks for your answers, it is more clear now but I still couldn't understand how to calculate the target frequency to do some modifications on it. By the way on the schematic of GE-7B for bass guitars, the last band represents to 4k. That means, if I use the same values, can I also reach 4k on GE-7?

GE-7B schematic:

 

[OldManAP]

See, now it gets more confusing to me. Because the GE-7 values are labeled 6.4K, but actually yield a cutoff frequency around 4K. The GE-7B values (which are different values than the GE-7) are labeled 4K, and yield a slightly different cutoff frequency which is also right around 4K. I’m still wondering how much of this is actual well-thought-out engineering, and how much is “eh, close enough”.