A Multichannel Mess

Learn what steps to take to effectively troubleshoot a multichannel system.

Installing video displays and multi-channel components can require more time and utilize more resources than normal two-channel systems. The costs are higher, but experiencing a DVD movie with a car full of exciting sounds can be rewarding — except when unwanted noise is a factor. Let’s examine the case of a DVD system installed into a late model Honda Accord LX. A phone call from a shop in a nearby city explained that after the components were installed, pesky alternator whine could be heard in all speakers except the woofer. The shop manager said, “We’ve tried everything and the whine is still there!”

The system contained an in-dash CD/DVD player/controller with a foldout color LCD monitor. The “brain” for the system was snuggly mounted under the front right passenger seat. Amplification for this system included a couple of two-channel amps and a large four-channel amp. One of the two-channel amps was bridged for the subwoofer and the rest of the amp channels were fed to various HF component speakers mounted throughout the Honda. The factory speaker wires were replaced with the standard aftermarket “zip cord,” and the multichannel signal outputs of the “brain” were connected to the amps via four-channel and two-channel UTP (Unshielded Twisted Pair) signal cables.

The amplifiers were grounded on the left side of the Honda’s trunk, and the main power cable was routed on the left side up to a 100-amp fuse and the car’s positive battery post. Serving as the main power buss for this system were the platinum terminal bars on top a pair of 1-Farad Stiffening Caps. Turn-on voltage for the amps was supplied via an automotive relay that was activated by the HU’s remote turn-on output. The signal cables were routed on the right side of the car, through the fold-down rear seat, and into the amp rack. Power for the head unit came from the car’s stock wiring harness

Since the shop had followed the Autosound 2000 Troubleshooting guidelines for many years, I really wanted to examine this car. The day it arrived, we verified that the alternator whine was most noticeable on quiet passages, changed in pitch with the rpm of the engine, and increased in level when the Honda’s headlights were activated. So far, nothing appeared to be overtly wrong with the install, except for the shop manager’s comments about “trying everything” to eliminate the whine — there’s always a solution.

Step One
At Autosound 2000, the first step in debugging a system is to mute the amp(s). A muting plug is simply a dead short at the input of the component. This means that a signal level of zero volts, with a source impedance of zero ohms, is fed into the amp channels. Any noise here would indicate a problem with the amplifiers, speaker leads, passive crossover elements, and/or the speakers. With muting plugs inserted into all the inputs, we increased the amp sensitivity to the max, started the engine, and activated the headlights. There was no noise in any channel.

Step Two
In logical troubleshooting, the next step is to connect the deck to the amp. For most systems, this step bypasses any equalizers, electronic crossovers, DSPs, etc., and connects the signal from the deck directly into the input of the amp. However, with the DVD player in the dash and the “brain” under the passenger seat, it’s not possible to intercept the analog signal at the deck. We were forced to use the “brain” as our source and alternator noise was the result.

Step Three
The third and final step is the most hated, and most avoided, in all car audio. This is the “move the deck” step, which is designed to temporarily eliminate the signal cables from the system. Any noise with the deck temporarily resting directly on top of the amplifier would indicate a problem with the deck. And if the noise goes away when the deck is on top of the amp, then we’d take a serious look at the signal cables.

With the DVD system in this Honda, moving the deck amounts to temporarily relocating the “brain” from under the front passenger seat to the trunk. This was no small task. We used male-male gender changers to connect the pigtails from the “brain” to a couple of the HF inputs of the four-channel amp. When the engine was started, the result was no change in the alternator whine.

The situation here is very unusual and in 99 percent of the installations, moving the deck to the amp will completely eliminate the alternator whine. Then we concentrate our efforts on the signal cables and get the car out of the install bay. But, when there’s noise with the deck mounted on top of the amplifier, then we know that the system never had a chance with the deck installed in the dash cavity. We first have to get the deck quiet at the amp.

Step Four
If the noise was not entering via the signal cables, that leaves few options. To find out exactly what’s going on here, the next step is to run the deck from an isolated power supply. We typically use a small 12-volt DC motorcycle battery or a 120-volt AC to 13.8-volt DC bench power supply. The maximum current requirement is only a few amps, so any power supply or 12-volt battery will work.

It’s important for the deck to be placed on an insulating towel or non-conducting board during this test because we don’t want the deck to be making contact with the car’s chassis. The deck’s 12-volt constant and the 12-volt accessory leads were connected together and wired to the isolated power supply. With the deck’s ground lead tied into the power supply, we activated the stereo system and started the car’s engine. The result was no noise — the system was completely quiet.

It appears that the noise was entering over the power supply. Before ending our troubleshooting session, we used an alligator clip to connect the ground (-) terminal of the isolated power supply to the Honda’s chassis. The system remained quiet, so we knew that the deck could be replaced into the dash cavity, the radio antenna could be re-connected, and the “brain” could be re-installed. Again the result was no noise — provided the deck was powered from the remote power supply. Notice that I no longer called it an isolated supply. This is because the isolation was lost as soon as the ground (-) terminal of the power supply was connected to the car’s chassis.

Down the Road
Our troubleshooting determined that the noise was entering over the deck’s +12-volt power wires. We could have implemented a permanent 12-volt remote power supply, but this would have cost a few hundred dollars due to the current requirement of the motorized display. So, I selected 10-amp electronic noise filters for both the +12-volt constant and +12-volt accessory leads. The result was no noise.

All it took to solve this highly unusual noise problem was a couple of relatively inexpensive filters. Usually the deck’s internal filtering is adequate, but, for some reason, this deck had a problem. Before adjourning our troubleshooting session, we discussed the troubleshooting procedure and how we arrived at our solution. On their next noise problem, some installers tend to choose the remedy that cured their last noise problem.

This is not the professional way to handle problems. Stick with the debugging steps and you’ll always arrive at a solution.