• The conductors in a single signal channel.

• The left and right signal channels.

• The front and rear stereo channels.

• All of the above.

This difference noise is directly proportional to the physical layout of the cables with respect to the noise currents flowing to and from the car’s accessories. The best cable choice for low-frequency noise is small UTP (unshielded twisted pair). UTP cables pick up less low-frequency inductive noise than other cable designs because the conductors are equal in physical size and placed in close proximity with each other. The result is equal, but opposite, inductive noises in each conductor. When summed together, the equal noises in the small UTP cancel resulting in less noise.

But even when using the best possible low-noise cables, noise can enter via the UTP. The two main sources for this noise are Ground Loop Isolators (GLIs) and RCA “Barrel Connectors.”

Ground Loop Isolators
In car audio, we call coupling transformers Ground Loop Isolators (GLIs) because they break the DC (Direct Current) path between components. Installing a GLI on the signal path between two components prevents a DC connection while maintaining an AC path. This is the nature of a transformer. (See Figure A.)

The use of 1:1 coupling or isolation transformers is widespread in all realms of audio and car audio is no exception. Given the fact that the car’s chassis is used as a ground return for all of the car’s electrical accessories, ground currents are flowing everywhere as soon as the engine is started. The case of the deck is connected to the car’s chassis via the antenna, black ground wire, and a metal-to-metal bond. With a processor or amplifier in the rear of the car, it’s certainly possible for the processor to source current from the deck’s signal cables. Installing a GLI prevents DC from flowing on the signal cables by breaking the “loop in the DC ground circuit.”

President Garfield Dies
Back in 1881, an assassin shot President James A. Garfield. The bullet lodged in his chest and the dying President lingered for days while the doctors probed the wound searching for the bullet. After inventing the telephone, Alexander Graham Bell began working on a prototype metal detector. With X-rays 20 years in the future, the doctors decided to try the metal detector to find the bullet. Although instructions were given to keep Bell’s detector away from large metal objects, the technicians of the day forgot about the metal springs and frame in the President’s deathbed. So President Garfield died and Bell’s detector was relegated to the back burner.

What does Bell’s detector have to do with car audio? The detector’s pickup amounted to little more than a coil of insulated copper wire wound around an iron core. When the pickup coil is placed in the vicinity of a changing magnetic field, a copy of the changing field is induced in the coil. In Bell’s detector, a telephone receiver (handset) was connected to the pickup coil.

The Problem With GLIs
In car audio, the transformers inside the GLIs are electrically very similar to Bell’s pickup coil. When the transformers are placed in the near vicinity of the changing electromagnetic fields (read: noise) from the car’s chassis and other power wires, a copy of the noise gets induced into the coil. Then, at nearly the speed of light, the noise gets into the audio via the signal cables.

But there’s more to this problem. Stereo requires two channels, and this means that GLIs use two transformers. Separating the conductors increases the loop area between the left and right stereo pair. The result is more low-frequency inductive noise — even with UTP signal cables. By physically separating the transformers in the GLI, a different noise is induced in each cable pair.

The greater the separation between the transformers, the larger the difference noise will be. From the perspective of low-frequency inductive noise, the optimally-designed stereo GLI would have both transformers in the same exact place. Since this is not possible, the transformers should be as close together as possible. Although shielding is somewhat effective for the transformers, it’s not practical to shield the pigtail signal cables at either end of the GLI’s case. Low-frequency noise can be very strong, and even the most expensive shielding material cannot completely eliminate inductive noise. Also, the entrance and exit holes for the pigtails cannot be effectively shielded, so the noise gets into the transformers.

RCA “Barrel Connector” Coupler Problems
Figure C shows three generic dual female RCA couplers, commonly called “barrel connectors.” These connectors are typically used to increase the length of a signal cable. The problem is that even with the quietest of signal cables, the couplers increase the separation of the conductors in a single channel. Recall that the conductors were previously an identical twisted pair, but the barrel connector makes the conductors unequal in size and separates them by a greater distance. Furthermore, the L-R channel separation is increased with a stereo pair of barrel connectors. And in four-channel systems, the barrel connectors physically separate the F-R channels. The result can be a tremendous increase the inductive cable noise.

Measuring the Noise from GLIs and Barrel Connectors
To determine exactly how much noise the barrel connectors pick up, we use a fixed 1 kHz inductive noise generator to simulate alternator whine in the test lab. The cables are terminated into a dead short and a differential amplifier and measuring system is used to record the level of inductive noise picked up at the opposite end of the cables. The noise waveform approximates the higher frequency component of alternator whine. The test procedure calls for injecting low-frequency noise into a short section of stereo UTP. Next, the barrel connectors are subjected to the identical low-frequency noise.

The difference in inductive noise at the input of the differential amplifier is 32 dB. This is a huge amount of noise and can actually increase when the L-R channels are further separated. Splitting the cable pairs with a GLI generates even more noise. It’s not uncommon to measure increases in low-frequency inductive noise on the order to 40 dB.

Signal Routing for Minimum Noise
Adding barrel connectors or a GLI to extend a stereo signal cable can increase the amount of inductive low-frequency noise picked up by the cables. The exact amount of increased noise depends on the level of ground currents flowing on the car’s chassis and the physical proximity of the connectors or transformers with respect to the noise. Four-channel systems can be noisier than two-channel systems. Cars with trunk-installed extra batteries can be noisier than cars with a single battery in the engine compartment. Cars with a small amplifier under the front seat can be quieter than cars with multiple amps installed in the trunk.

The first step in using a GLI is to understand that the device consists of coupling transformers. Since a transformer is made of a coil of wire wound around an iron core, the transformer WILL pick up noise when placed in the near vicinity of a changing magnetic field — i.e., the chassis of a car. The amount of noise depends on the car’s electrical accessories, and the design of the car sound system.

The second step in using a stereo GLI is to realize that even with UTP signal cables, the transformers on the signal path destroy the integrity of the twisted pair. Not only are the conductors in a single pair separated as they enter and exit the transformer windings, but the Left and Right channels are split as the signals are routed to separate transformers. This separation will produce a different noise in each channel at the input of the following stage.

Take Care
If care is taken in the placement and orientation of the GLI and the “Barrel Connectors,” the amount of inductive noise can be limited. Always test the system for noise with a blank track (0-bit track) on the CD player before handing over the keys.