FA-1 Circuit Analysis: Low Cut Switch

Posted: 2009/06/23 in Boss FA-1 Clone, DIY Guitar Effect Pedal Projects

There has been some discussion questioning the value of the capacitor C2 which I thought was worth mentioning.  C2 is part of the “Low Cut” switch circuit fragment, which is shown in Figure…  The circuit fragment, consisting of C2, C3, R4, and R5, found on the inverting input of the first op amp, IC1, forms a high pass filter.  


Figure 1: Low Cut Switch Circuit

Essentially the switch moves resistor R4 in and out of the signal path, since when the switch is closed the  resistor is bypassed as shown below in Figure 2.  In this case the capacitors C2 and C3 add together  to about 0.52 uF as they are in parallel.  The -3dB cutoff frequency, given by f=1/(2*pi*R*(C2+C3)) is 20 Hz –  at the very bottom of the audible frequency range for the human ear, so the value of 0.47 uF makes total sense.  It is usually a good idea to filter out unnecessary (i.e. inaudible) frequencies.


Figure 2: Circuit with the Switch Closed

When the switch is open, R4 is back in the path, and given it’s resistance,  the signal path is essentially through C3,  C3 is ten times smaller than C2, so this raises the cutoff frequency.  In this case the -3dB point occurs at 226 Hz, which is perceived as a loss of some low end frequencies. Perhaps this is useful with humbucker or acoustic guitars or other situations where the tone has a little too much bass to one’s liking.  It’s certainly not a very important feature, but it will be included in the clone for authenticity’s sake.


Figure 3: Circuit with the Switch Open


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