RCA line noise has been a major issue for car audio installers from the beginning of car audio.
The RCA signal from a head unit is a very low level signal, and it is fairly easy for a noise source like an ECM or other component of the cars electrical system to enter the signal path through the RCA cables. This is even more problematic with today's very low output head units. To make matters worse, different equipment manufacturers use different methods of referencing the signal ground to the main electrical ground of the car. If these methods do not match well they create a "ground loop" causing noise (whine) in the signal.
The car stereo installers can solve the noise problem by tracing all the ground paths to eliminate the ground loop and sometimes they may need to make many changes in the grounds of the different pieces of equipment to find the best solution. Unfortunately this can be a very time consuming process, often taking more time to solve the noise than it took to install the system. So what can be the solution to the problem?
Solution 1977 to 2106: The ubiquitous Ground Loop Isolator has been to "easy" answer since the mid 70's. The good news - the Ground Loop Isolator does indeed work to eliminate the ground loop noise. The bad news - the Ground Loop Isolator destroys the signal by acting as a high-pass filter and eliminating bass frequencies. It also adds multiple layers of distortion to the signal. Why does the old Ground Loop Isolator damage the signal? Because it was designed ONLY to stop the noise with no thought to signal quality. Low cost, low efficiency components were combined to provide a low cost "quick fix" to the noise problem. So for all these years installers have used the old Ground Loop Isolator as the last resort when they don't have the time (or possibly the ability, depending on the equipment) to solve the grounding problems. They have been forced to sacrifice signal quality to stop the noise.
Solution 2017: Zapco introduces the ASP-L2T signal noise eliminator. The ASP-L2T utilizes custom designed and custom wound high efficiency transformers and other high SQ components to remove the direct connection between the different reference ground systems and eliminate noise, without affecting the bass content of the signal and without adding distortion into the signal.
Performance: Signal to Noise is over 130dB, and THD+Noise is less than 0.1%. Signal level and impedance of the output are exactly the same as the input, and the frequency response of the ASP-L2T will even exceed the response of the input signal from the head unit. In fact the ASP-L2T can even be considered a "conditioner" to improve the signal from the head unit before it arrives at the amplifier. Maximum performance upgrades occur between 500mV and 1 Volt.