hader-churve

AM Reception Problems0

Dear David,

I’m very new to car audio and I have just installed an aftermarket Pioneer amplifier using speaker level inputs on my VW OEM deck. Since then my OEM speakers are driven better and sound better. However, when switching to AM, especially at night (I commute around 5:00 AM), I get a whistling sound on some AM stations and it’s really aggravating. There is no noise on the FM or the CD changer. The VW has an amplified antenna. The noise is noticeably worse at night though some does happen during the day. Can this poor reception be fixed?
Jim

Hello Jim,
Humm.. well the “whistling sound on some AM stations” probably has something to do with the PWM (Pulse Width Modulated) power supply in your aftermarket power amplifier. The aftermarket power amplifier must output more than the car’s nominal 12 Volts DC, so the power supply in the amp “steps-up” the voltage by chopping the DC on and off very fast… The exact speed depends on the power amplifier brand and the model, but typically we’re looking at something like 50 kHz to 100 kHz. The problem is that the sharp cut-off of the voltage in the power supply creates Radio Frequency Interference (RFI) that can certainly wreak havoc with AM reception (550 kHz to 1.7 mHz or so). Harmonics are produced and contain enough energy compete with the weaker AM stations. At night, AM stations must reduce their power to avoid conflicting with other AM stations.

So how does car audio handle this problem? Well most folks who add aftermarket power amplifiers to their vehicles are not particularly interested in AM reception. (Personally I only listen to AM talk radio = 90%, some PBS on FM = 5% and certain Satellite Radio broadcasts on XM or Sirius = 5%). This means that degradation of AM reception is NOT in the design parameters of some aftermarket power amplifiers. “Repairing” the power supply by gently slewing off the sharp square wave edges only weakens the power output and ruins the performance of the power amp. So repairs are not recommended.

As you may be aware, my company has been testing and reviewing audio components for publications and manufacturers since 1990. In our reviews of power amplifiers and other components with switching power supplies, we always include a report on the effects of AM reception. I do this by simply placing a small transistor radio on the test bench a few feet away from a loaded power amp under test. If my favorite San Francisco AM radio station reception changes, I note it in the report. Sometimes it makes it into publication.

So the industry would like to sell you something to improve your situation. Traditionally, it would be a new aftermarket radio or possibly a Satellite radio receiver. But the real solution lies in working with your existing aftermarket power amplifier.

First of all, let’s make sure it’s the amp. To handle this task, temporarily connect one speaker directly to the output of your OEM deck. Remove the fuse from your power amplifier. What happens to the reception? If it improves, then it’s the power amp. If not, we’ll look elsewhere.

Next, we’ll add a stage of isolation between the speaker output of your OEM deck and the input of your aftermarket power amplifier. We use transformers to couple the audio and avoid a direct connection between the OEM deck and the aftermarket power amplifier. We have products such as the Navone Engineering, Inc. N-7V or N-777 designed to maintain isolation between the OEM and aftermarket components. Perhaps you can borrow one from a local shop.

If the isolation doesn’t help, then I’d recommend selling your power amp on eBay and buying another brand/model.

But guess what we’re going to do just prior to your purchase? We’re going to install the amp, power it up under load, and carefully test the AM reception. Get it?

Thanks for the good question.

Dave