Troubleshooting - Filtering FM Frequencies - diyAudio
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Old 27th November 2014, 04:43 PM   #1
shlshh is offline shlshh  United States
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Default Troubleshooting - Filtering FM Frequencies

Hi folks,

I made a chip amp based on the LM4752 and two 8 ohm speakers (FaitalPRO 3FE25).

I followed the circuit diagram from the LMF4752 manual from TI (Figure 1).

The problem was, when I connected an input cable, I would start to hear a lot of a local FM radio station in the background of whatever I played. This was only when I connected a stereo cable and persisted when I plugged connected the other end to my iPod or phone.

I tried trouble shooting a couple ways
1. bought a ferrite choke (reduced but didn't eliminate the noise)
2. bought a shielded cable (little affect)
3. added a low pass filter at the input

With as a low pass filter, I just used an RC combination of 39kohm and 102pF for each channel, right before the 0.1uF capacitors Ci in the circuit diagram. The ends of the capacitors are connected to each other, then grounded. The noise was reduced, but one of my inputs no longer works, and the chip is running very hot (I don't have a heatsink yet). I wonder if one of my problems is that I am grounding the inputs via the capacitors, whereas in the original circuit they seem to be floating and grounded after the bias.

Maybe you can tell, I'm not fluent in electronics but I'm trying to learn. I have two questions. First, is my approach of making two low pass filters at the input theoretically OK, or is my connection method incorrect? Further, could this RC circuit be the cause of the chip over-heating, or is it more likely that I just have a bad connection now?

I appreciate any replies!

Shail

Last edited by shlshh; 27th November 2014 at 04:58 PM.
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Old 27th November 2014, 04:56 PM   #2
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The capacitors should be grounded to the noise ground.

Chip amps are very particular about layout, and grounding. Power and noise grounds need to be separated on the board.
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Old 27th November 2014, 05:01 PM   #3
peufeu is offline peufeu  France
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Please show a photo of your amp...
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Old 27th November 2014, 05:23 PM   #4
shlshh is offline shlshh  United States
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I am uploading a photo of my amp ... I believe it will be very hard to follow as it is on a small RadioShack PCB and I have had to redo some connections, so it is not very clean.

I also think I understand your comment about grounding. I sketched my grounding layout on Paint, I think this is what you meant. Is this layout OK?

Thanks for the quick replies!
Attached Images
File Type: jpg 20141127_131031[1].jpg (718.9 KB, 109 views)
File Type: png ModifiedCircuit.png (68.2 KB, 106 views)

Last edited by shlshh; 27th November 2014 at 05:26 PM.
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Old 28th November 2014, 04:31 AM   #5
tomchr is offline tomchr  United States
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I would add 100 nF from pin 3 to pin 4 on the IC. Solder it directly to the IC pins. That way you ensure that the IC is properly bypassed. I'm a bit concerned by the lead length from the IC to the perf board. This probably won't solve your problem, but would be a good practice regardless.

Your main problem is that when you hook an antenna (= cable) to your input connector, RF gets into the circuit and gets demodulated and amplified. You need to keep that RF out of the circuit. Your idea of an RC filter sounds good and your values would result in a 40 kHz cutoff (fc = 1/(2*pi*R*C)). Personally, I would have aimed for a 150-200 kHz cutoff to keep any rolloff and phase change from the filter out of the audio band. Something like 10kΩ+100pF followed by another RC of, say, 2.2kΩ+100pF. This would create a filter with two poles - one at 160 kHz and the other at about 700 kHz. These two combined should attenuate RF rather well.

In order for your filter to work well, you need to mount it on the RCA socket directly. If that doesn't do the trick, try adding 100 pF directly from the center pin to ground on the RCA connector.

~Tom
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Old 28th November 2014, 03:16 PM   #6
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Are you sure that it's an FM radio station? Chip-amps are not very good at demodulating FM signals. But then if you are very near the broadcast transmitter, there is a small AM component.

This is a long Jim Brown paper on troubleshooting RFI problems.
"RFI, Ferrites, and Common Mode Chokes For Hams"
http://www.audiosystemsgroup.com/RFI-Ham.pdf

Many other Jim Brown papers and Power points.
Audio Systems Group, Inc. Publications
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Old 29th November 2014, 06:22 AM   #7
JMFahey is offline JMFahey  Argentina
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Add a 100pF ceramic capacitor between pins 2 and 5 , straight under the PCB, for the shortest path possible, and another between pins 6 and 5 .

What you really need is them between + and - input pins (I'm talking signal input, not supply power) at each amp but the - input is buried inside, not available to you.

Remove the RF input filter you already added, it's too far away from chip inputs, what I suggest is minimum path.

If not effective, mount the amp inside a grounded (to chipamp pin 4) metal box and also ground Cb negative leg straight to pin 4 .

Consider it like closing all the windows so flies don't get in.
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Old 29th November 2014, 10:11 AM   #8
AndrewT is online now AndrewT  Scotland
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All our audio equipment should have RF filtering on the inputs.

If you care to dismantle a redundant video product, before you bin it, you will fill that it has dozens and maybe hundreds of filters to protect the inputs from unintended signals.
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Old 30th November 2014, 06:00 AM   #9
tomchr is offline tomchr  United States
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Quote:
Originally Posted by AndrewT View Post
All our audio equipment should have RF filtering on the inputs.
+1

~Tom
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Old 30th November 2014, 07:12 AM   #10
JMFahey is offline JMFahey  Argentina
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I make Musical Instrument amplifiers and all of them have 100pF across + and - inputs and 100 to 470pF to ground after with 10k to 47k input resistors, both preamps and power amps.

Sold thousands that way and have not heard any radio through any of them in the last 20 years (or more).

So much so that I had almost forgotten that such thing happens sometimes.

Started doing it in the early 80's, when "Free Radio" FM stations started popping on every city block and their poorly adjusted (ofted homemade) transmitters splashed all over the place.
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