DIY Jfet Preamp: Low pass filter and signal attenuation
Back to our super fun schematic analysis of the jfet high impedance preamp!
Step 3: The signal's being pushed and pulled from all directions!
So we have this modulated signal coming through Capacitor1, and it encounters a number of components that it can be affected by - C2, C3, and R3. C2 and C3 are mediated by Switch2 - a low pass filter cutoff selector. From what we know about low pass filters, the cutoff frequency can be manipulated by changing the values of specific components - such as the capacitor.
C2 is 100nf and C3 is 330nf. From what we've done before with calculations, we've found that as you increase capacitance (while keeping resistance uniform), the cutoff frequency decreases - thereby cutting off more high frequencies. So if Switch2 were on C2, you'd hear more frequencies than if Switch2 were on C3.
So the higher the capacitance, the lower the frequency cutoff, and therefore the more frequencies that are cut out of the output signal. Vice versa, the lower the capacitance, the higher the frequency cutoff, and fewer frequencies that are cut out of the output signal. Mull that over for a bit :p.
Step 4: Volume attenuation!
So again, using the same diagram, the signal's been affected by a low pass filter. Through Switch3 (labeled output attenuation selector), you can either make the signal go straight through to the output signal OR you can make it go through resistor 3.
As you know, resistors are voltage dividers - thus an easy method of weakening or attenuating a signal. Put it through the resistor, you decrease the volume/voltage of the output signal. You could theoretically place a potentiometer, or variable resistor here, to be more detailed in your attenuation controls. But it seems that this schematic chose to do it with a simple resistor.
Lovely!
@atdiy/@tymkrs