PRS et PRS-like User Club

lot of people think that the PRS sweet switch circuit is a simple low-pass filter. However, the sweet switch circuit is in fact a very interesting use of an electronic component that is generally not found in music electronics. If one opens up the cavity of a sweet switch-equipped guitar, one will find an odd looking component wired to the back of the switch that bears the number 1513-135Y. Expecting a simple resistor/capacitor network and finding this device is the kind of thing that makes one scratch one's head.

Looking up the part number, one discovers that 1513-135Y is the device number for an electronic component known as a delay line. Furthermore, it is a delay line that imposes a 135 nanosecond (135 billionths of a second) delay on the signal. The device's characteristic impedance is 75 Ohms (most of the common types of coaxial cable have characteristic impedances that fall into the 50 to 75 Ohm range). The characteristic impedance of the device at audio frequencies is only important because a higher characteristic impedance cable usually has a lower capacitance per foot rating, and a lower capacitance per foot rating translates into less high frequency attenuation per foot of cable.

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 (cables of this length were popular before the guitar world went wireless).

If one researches the history of the sweet switch circuit, one discovers that it was created for Carlos Santana. Carlos was known for using long guitar cables before he switched to using a wireless system. The sweet switch was PRS Guitars’ answer to the increase in brightness that a guitarist experienced when the capacitive loading effects of a long cable were removed from the signal path. There is little doubt in my mind that Eric Pritchard was behind this circuit.