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AM Reception Problems
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.
More
about the N-777
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
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