Please send questions to: Car Audio and Electronics, c/o Tech Panel2400 E. Katella, 1100, Anaheim, CA 92806e-mail: techpanel@mcmullenargus.com
Q I hope I sent this to the right place. My name is Lee and I would just like to say, "Great magazine". Thanks to you I got interested in car audio and now work in this great business myself. I recently went to USAC Finals and had a blast. I got to meet some of the greats such as Mark Eldridge, Gary Biggs, Rob Rice, Alma Gates, Chad Baker, and Frank Rougeu to name a few, and to them I would like to say thanks for signing my shirt and giving me a little of their time. It is people like them that keep people interested and give this sport a good name.
Anyway, on to my question: I was recently working on a 1984 GMC and it had a popping sound coming through all the speakers whenever you went to defrost or turned on/off the heater control. The deck is an old school Pioneer tape player w/changer going to a Pioneer 6500 EQ. Then on to two new Pioneer amps, one being a 50x2, powering two 8" Pioneers, and a 50x4 powering two Pioneer 3-1/2's and two Pioneer 4x10's. I took a deck that was powered by a small 12-volt battery and hooked it directly to one of the amps and still got the popping sound through both amps. So basically I got rid of the deck, EQ, RCA's, and I think a ground loop idea. I never replaced the amps because I don't think two brand new amps would both be shot. If you could help it would be greatly appreciated.Lee Mocko Sidney MT
It's great to hear that Car Audio & Electronics encouraged you to get into the industry. Isn't it great to get paid for your hobby?
I want to commend you on your troubleshooting. I can tell you are experienced as an installer by the methods you used. You obviously suspected the power line as the entry method of the noise, so using an outboard power source would prove the possibility. Then you ran into the roadblock that messes up many professionals.
The noise you are dealing with is called "pulse noise" and is caused either by an inductive spike produced by the coil of any relay, or it is a wave of energy produced by a high current contact making or breaking connection. The vehicle feature indicates the type of noise. For instance, if you discovered that the fog lights or horn caused the "snap" to be heard through the speakers, I would attribute the cause to the fog light relay or horn relay. The switch itself is not the problem because the switch is passing a very small current to activate the relay. Contact snaps can only be produced by high current "arc" discharges.
Two parts of the relay can produce the noise: the coil will produce what sounds like a dull thud, while the switch contacts will produce a sharper snap. In either case the noise is hard on tweeters and other system components. In the case of a defroster switch, you have two possibilities: if the pop occurs only when the switch is moved from any other position (especially the "OFF" position) to defrost, then the likely cause is an arc at the switch contacts. If, however, the pop occurs about every 15 to 30 seconds, the cause is the electric clutch (which is a giant electromagnetic coil) on the compressor of a vehicle equipped with air conditioning.
The cure is simple and quite effective. Install a 0.5 microfarad capacitor across the switched (defroster) wire and ground if the truck has no air conditioner. A second capacitor may be required on the clutch wire at the compressor.
Q I currently have an Alpine CDA-7875 CD receiver, 4-channel MTX Thunder 304 (Polk DX9 & Polk 365), 2-channel Alpine MRV-757, and a pair of JL Audio 10W3 subs on a Pro Wedge Box. Both amps are mounted behind the back seat. I have a 1999 Acura Integra GS Sedan.