In terms of analog circuitry, electronics that are DC–coupled have their components connected directly together without any coupling capacitors. As capacitors tend to remove certain frequencies, coupling the circuitry without capacitors in-line allows the full spectrum of sound frequencies to pass through unfettered. This can be advantageous as many capacitors severely limit the passage of low frequencies, resulting in poor low-frequency response. Manufacturers such as Solid State Logic often use DC-coupled circuitry for this reason. As capacitors wear out over time, many devices will experience a decline in performance. By building equipment without capacitors in the circuit, a manufacturer ensures that will have the same frequency response and essentially sound the same years after its initial purchase. To illustrate this point, we’ll use a SSL DC-coupled XL 9000 K console as an example. Since no capacitors that can fail are in the signal path, channel 1 will sound the same as channel 32 (or any other channel, for that matter) year in and year out. DC-coupled equipment maintains a linear-phase relationship across all channels, further illustrating the advantage of this method of creating circuits. DC-coupled circuitry also tends to be utilized in audiophile and high-end recording equipment, as it tends to provide the “purest” sound.
Capacitors, of course, still play a large role in the construction of electronic equipment. In applications where DC voltage may damage the circuit, capacitors are placed in the signal path as a means of protecting transistors from being burned up by the DC current. Capacitors essentially condition the incoming signal by rejecting unwanted DC voltage. DC voltage is also typically undesirable in audio signals as it can cause distortion later in the signal path.