R.F. Noise Issue?

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OffGridKindaGuy

OffGridKindaGuy

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Joined 2013
#1
Scenario..

I've been building some low power audio projects, using the PAM8403 chip amp boards. They require 5 volts to operate. I've been using a battery powered supply (8-"AA" batteries in series.. 12 volts) and regulating the voltage down to 5 volts, using LM7805 regulators.. (I have my reasons for doing this)

Problem..

When I try to use a battery powered FM receiver for an audio source, all I get is a bunch of noise from the receiver, regardless where I tune! I can put headphones on the receiver and it works fine. If I plug into one of my projects, noise wipes out the receiver. I can use XM as a source and all is good. CD player works fine..

Is the regulator generating noise (RF) that kills the receiver? It doesn't degrade the sound of the amp as far as I can tell..

Comments? Maybe a fix?

Thanks in Advance!
 
#4
@ peufeu..

The amp is a micro board that was ordered on Ebay from China. I don't have a schematic but very basic, I'm sure..

@ DigitalJunkie..

No caps on the regulator. Just a basic connection. (In-Common-Out) One of them I connected the Common-Out pins directly to the amp board, using the thru hole connection points..

I've seen reference to ferrite chokes for Class D amps on the speaker output leads but always assumed that it was to keep RFI from getting IN to the amp. Did I learn something today? I'm a complete newbee to audio experimentation. I know enough to get me into trouble..
 
#5
A class-D amplifier connected to long speaker leads without proper output filtering makes an excellent wideband radio jammer. Your FM tuner also seems susceptible to conducted EMI through the audio cable. The class-D amp could generate lots of EMI if not properly designed.
 
#8
If it is an oldskool 7805 regulator, don't forget those were born before ceramic caps and can become unstable if used with cpacitors which have too low ESR. I have not specifically measured LM7805, but I have seen LM317 go unstable when used with ceramics.

"Small value bypass caps" (I'm assuming it means capacitors, like SMT ceramics, not inductors like red wima cubes) should be placed at the load, not at the regulator.
 
#9
(Putting my hand in the air for another question)

I have some ferrites laying around but I have no idea on what they are. A couple are snap on chokes that fit the audio wire between the receiver and the amp. A couple are the snap together type. (The square ones) I used these ones on the speaker leads near the amp..

I installed these in various areas in my wiring but I still have issues. (Leading up to my question..)

I find ferrites of different properties @ 100 mhz. (Beads, Rings, ect..) Some are low impedance, some are higher. What do I need to consider when choosing ferrites? Should I go high or low? My instinct tells me low..

@ peufeu..

The 5 volt regulators I'm using are newskool, I believe. Some from RatShack, some from Mouser..

Thanks in advance!
 
#10
Dont forget the capacitors as others have mentioned. Linear regulators NEED capacitors otherwise they can quickly go into oscillation. In their most basic form they are just an op-amp in differential configuration, a voltage reference and a FET. As you can see, this can very easily go from a voltage regulator to an oscillator in no time without some dampening. This is even more true for LDO regulators.
 
#12
Everything and more that anyone would want to know about ferrites.

"A Ham's Guide to RFI, Ferrites, Baluns, and Audio Interfacing"
Revision 5a 5 Jun 2010

by Jim Brown K9YC
Audio Systems Group, Inc.

The basis of this tutorial is a combination of my engineering education, 55 years in ham radio, my
work as vice-chair of the AES Standards Committee working group on EMC, and extensive research
on RFI in the pro audio world where I’ve made my living. That work is documented in technical
papers and tutorials that can be downloaded from the publications section of my website.
http://www.audiosystemsgroup.com/RFI-Ham.pdf

More Jim Brown papers on audio, video and/or Ham radio.
Audio Systems Group, Inc. Home Page
 
#15
The shield of the audio cable is the antenna for the rcvr. (Sony Walkman Type) I agree that it might be a hard nut to break, being there's a solid path between the rcvr. and the amp..

Perhaps an isolation transformer to break the d.c. path and some chokes beyond the isolation?
 
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