Now, anyone without a background in electronics or physics is probably wondering why the designer of this circuit chose a 135 nanosecond delay line as the filter for the sweet switch circuit. Well, the answer is quite simple after one performs a few calculations. An electromagnetic wave travels at the speed of light through a vacuum, which is 300,000,000 meters per second; therefore, an electromagnetic wave can travel 300,000,000 / 1,000,000,000 x 135 = 40.5 meters in 135 nanoseconds through a vacuum. However, an electrical wave does not travel at the speed of light through a coaxial cable because all coaxial cables have what is known as a velocity factor. A velocity factor is a fraction of the speed of light, which means that a coaxial cable has different electrical and physical lengths. The velocity factor of a guitar cable is roughly around 78%, which means that an electrical signal can travel 300,000,000 / 1,000,000,000 x 135 x 0.78 = 31.59 meters in 135 nanoseconds. That number is important because it roughly translates to 31.59 x 39.37 / 12 ~= 104 feet of guitar cable. In effect, the delay line simulates the frequency attenuation and the phase shift imposed on a guitar signal by a 100-foot-long guitar cable.
Okay, I took some measurements and did some math.
First off, Dirk's assertion that 1 ft of cable roughly equals 100pF of capacitance is way off.
Easy to miss, I did a double-take when I read it as well."Modern guitar cables have an average capacitance of approximately 100 pF per meter, which is very low and allows long cable runs without audible degenerations."
"Modern guitar cables have an average capacitance of approximately 100 pF per meter, which is very low and allows long cable runs without audible degenerations."
Easy to miss, I did a double-take when I read it as well.
100pf per meter is around 30pf per foot.
As a little guideline to calculate the best additional capacitance, you can use this chart:
10 ft. cable (approx. 3 meters): 1 nF
15 ft. cable (approx. 4.5 meters): 1.5 nF
20 ft. cable (approx. 6 meters): 2.2 nF
30 ft. cable (approx. 9 meters): 3.3 nF