View Full Version : How a "Sweet Switch" really works

05-09-2012, 04:59 PM
A 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 (http://www.datadelay.com/datasheets/1513.pdf), 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.

05-09-2012, 05:03 PM
Informative, thank you.

05-09-2012, 05:22 PM
Very cool and being a RF engineer for many years I can state that we certainly take the velocity factors into account when spec'ing the lines we use on Cell sites, although many are now fiber for both low loss reasons and the fact that thiefs kept stealing the copper ground and coax lines.

05-09-2012, 07:29 PM
All greek to me bruddah............................... But very cool at the same time :)

05-09-2012, 07:35 PM
Many EQs also make use of delay lines. That's one of the things that causes phase issues with some of them.

05-09-2012, 11:29 PM
Many EQs also make use of delay lines. That's one of the things that causes phase issues with some of them.

That's a tapped delay line. I have never seen a tapped delay line used on an audio equalizer, but I have seen it used to filter out multi-path distortion in receivers. When a signal travels over multiple paths between a transmitter and a receiver, it can produce echos due to propagation time differences between paths. Tapped delay line-based filters are often used to remove the echos (although echo cancellation is usually handled digitally today). A tapped delay line can also be used to produce echos. Bucket-brigade chips are basically tapped delay lines.

Now, normal RLC networks introduce phase shifts. Every inductor through which an AC signal passes introduces a phase shift between the voltage and the current, with voltage leading current by up to ninety degrees. Conversely, every capacitor through which an AC signal passes introduces a phase shift, with current leading voltage by up to ninety degrees. A good way to remember these relationships is with the phrase ELI the ICE man, that is, E (voltage) leads I (current) through an inductor and I (current) leads E (voltage) through a capacitor. The L in ELI is the symbol for inductance. The C in ICE is the symbol for capacitance.

The type of delay line that is used for the sweet switch is usually used to deal with race conditions in digital logic. A race condition occurs when two of more signals are racing to set a logical state. It is usually caused by a timing problem in sequential logic (the term "race condition" is also used in the software world to denote a problem that occurs when two or more concurrent threads of execution try to modify a unprotected shared variable at the same time).

In the end, the person who designed this circuit knew what he/she was doing. This component was not chosen at random. The phase shift is intentional, as a phase delay of 135ns matches the propagation delay imposed on a signal by 100-foot-long guitar cable.

Jack Gretz
05-10-2012, 08:54 AM
Pretty interesting description and explanation Thank you

05-10-2012, 09:59 AM
For those who are curious, a delay line is functionally a Cauer topology LC low-pass filter with a specified signal propagation delay.


Furthermore, the circuit does not mix the delayed signal with the original signal as would be seen in a modulation effect. The phase shift is just relative to the original signal. If we feed the delay line input and output signals to a dual-trace oscilloscope, we will see a 135ns time difference between the two signals. This time delta represents the rough equivalent of the propagation delay imposed on the signal by a 100-foot-long guitar cable. I do not understand why this approach was chosen over a simple low-pass circuit. I guess that Eric wanted to truly emulate a 100-foot-long guitar cable. It's definitely an example of "out of the box" thinking.

05-10-2012, 10:53 AM
This is all very interesting, but it does not answer the questions of why some sweet switches have washers and some do not, and how the washers affect tone

05-10-2012, 12:00 PM
This is all very interesting, but it does not answer the questions of why some sweet switches have washers and some do not, and how the washers affect tone