FA-1 Preamp Low Cut Switch Question

[bluelagoon]

I have a potential customer who has requested an adjustment to the traditional FA-1 Low cut switch, to accommodate shelving the low end from 200Hz to 65Hz
He is of the assumption that the Low Cut switch acts as a high pass filter- shelving low end. down to only 200Hz, and since he wishes to use it for a Bass guitar, plus 6 string with greater bass emphasis, he is wishing the low cut to take the Shelving filter as low as 65Hz.

 To me from what I could make of this switch's function is, it allows all the bass content through down to possibly 65Hz thereabouts already when not in the low cut position, and that when placed in the low cut position this then only has the result of raising the Bass cut off frequency response from down below 200Hz up to around 200Hz range, giving the impression of a loss of low end frequency thus called a Low Cut switch.

If what I assume is correct, then it likely wouldn't need any modification to reach the lower frequency of 65Hz as it is already attuned that way to start with, while in the non cut switch position.

Please correct me on my assumptions with some better understanding of just how low the frequency range on this pedal is while in the switched non bass cut position

And if there are modifications to the Resistor, Capacitors to get that greater bass low end in the non bass cut pos, then what would the new capacitor resistor array need be.
Thanks.

Have placed some images below depicting an explanation of the Low Cut switch given from Trabantland.

Hoping to make some better sense of it all.
Cheers.

[m4268588]

not verified

[bluelagoon]

Thanks for your response and calculations m4268588
I'm sure they are very meaningful, and I respect that your English isn't so good.
But could someone please decipher the calculations put forward by m4268588
that the layman Electronics enthusiasts might understand, and hold the Rocket Science please.
Thanks

[m4268588]

• A = 0.69813413087636 Hz
• B = 7.925325368994337 Hz
• C = 232.9788835670466 Hz

Corner frequency at "flat pos" is 20(20.52288112081178)Hz.

[bluelagoon]

Okay, So seems your formula calcs have garnered 3 different Shelf Frequency settings.
is this correct?
If so could you include the details of what C2, C3 and R4 values need be to attain these pre set frequencies.
And I see you have
A/ 0.69Hz
B/ 7.9Hz
C/ 232 Hz
Is there a way to achieve 65 Hz as the low end shelf Frequency when the Switch is in the closed position as in
Figure 2 -


 If so what value caps need installing to achieve 65Hz ?

[ElectricDruid]

I did a quick sim of what the switch does. Here's the results:





Out1 is "switch closed", Out2 is "switch open". With the switch closed, the cutoff (-3dB point) is at about 21Hz, which fits with m4268588's calcs.

[bluelagoon]

Thanks ED, I think I now have a better grasp on what my interested buyer is after.
Seems what he wants is the Low cut switch when selected in the Low cut position, switch open and R4 in the path
instead of the nominal Frequency of original circuit at 200Hz, I believe he is looking for the low cut to take it up to only 65Hz .
This way he still has the advantage of all the low end he is seeking without the chance of too much low end rumble at "Low Cut out" 20Hz blowing his speakers.

So the real question is what do the RC High pass filter components C2, C3 and R4 need be for when in low cut pos to allow for not such a dramatic cut in Bass but a more subtle cut of only up to 65Hz to get what the customer is looking for ?
Thanks, appreciate any further advice.
Cheers

[ElectricDruid]

The cutoff with the switch open is at about 240Hz, and that's set mainly by the 47n (since the effect of the 470n is limited by the big 470K resistor). 65Hz is roughly two octaves further down from 240Hz, so you'd need to bump up the 47n by a factor of four (x2, then x2 again). That gives roughly 220n for that value.
Since the 470+47n switch-closed position then also becomes 470n+220n, it would move a bit lower - but only to 15Hz or so.

Checking this in the sim gives what we expect, more or less:



You'll see from the graph the cutoff is slightly lower than the required 65Hz, more like 50Hz. You may decide this is close enough. Otherwise, you need to get the cap value slightly smaller to raise it a bit. 150n seems to give about 75Hz. You could also play with the gain resistors to fine tune the cutoff. As long as the ratio stays the same, the gain won't change, but the cutoff only depends on that input resistor, so you could tweak that, then alter the feedback resistor to match it back up.

HTH

[bluelagoon]

Thanks ED, gives me plenty to work with to fine tune the Low Cut frequency to suit.
Found an online calc for filter RC calculations, so will mess with it a bit to get the solution.

Low Pass/High Pass Filter Calculator

Cheers

[m4268588]

Query has already been resolved, so just for my pedantism.

In practical use, such an approximation is enough.

• f1 = 1/(2*pi*R1*C1)
• f2 = 1/(2*pi*R1*C2)
• f3 = 1/(2*pi*R2*C2)

Can be a different topology.