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AM radio and switching frequencies


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Hi All,

 

just wanted to check you with people who know a lot more than me.

 

I have recently put together a Class D amp that has a switching freq. 300-900khz depending on load. 

 

ever since i got this amp AM radio has become very noisy even though i have 4 out of 5 bars in quality

 

Do You think the amp is interfering? if so is there a easy fix?

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On 14/01/2021 at 8:09 AM, misternavi said:

Hi All,

 

just wanted to check you with people who know a lot more than me.

 

I have recently put together a Class D amp that has a switching freq. 300-900khz depending on load. 

 

ever since i got this amp AM radio has become very noisy even though i have 4 out of 5 bars in quality

 

Do You think the amp is interfering? if so is there a easy fix?

 

I suspect it would be the culprit.

AM Radio band is 526.5 – 1605.5 kHz, so the higher fundamentals of the switching amp and/or 2nd & 3rd harmonics will certainly interfere.

Your signal strength meter only indicates overall RF signal level, not the quality of the modulation.

 

Can easily check by turning the amp off at the power point while listening to the radio.

 

What type of case is the amp in?

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1 minute ago, surprisetech said:

 

I suspect it would be the culprit.

AM Radio band is 526.5 – 1605.5 kHz, so the higher fundamentals of the switching amp and/or 2nd & 3rd harmonics will certainly interfere.

Your signal strength meter only indicates overall RF signal level, not the quality of the modulation.

 

Can easily check by turning the amp off at the power point while listening to the radio.

 

What type of case is the amp in?

Yep, headphones with amp off sounds clear.
Amp is in a aluminum or steel chassis

 

 

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2 minutes ago, misternavi said:

Yep, headphones with amp off sounds clear.
Amp is in a aluminum or steel chassis

 

 

 

The metal chassis is a good start and should help.  Might need better RF filtering on the speaker outputs or other I/O.

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On 14/01/2021 at 10:09 AM, misternavi said:

Hi All,

 

just wanted to check you with people who know a lot more than me.

 

I have recently put together a Class D amp that has a switching freq. 300-900khz depending on load. 

 

ever since i got this amp AM radio has become very noisy even though i have 4 out of 5 bars in quality

 

Do You think the amp is interfering? if so is there a easy fix?

I would suggest using a dedicated antenna for your AM radio like this one,  your Class D amps really should not radiate frequency beyond the confines of your chassis. To test either turn your amp off and see if the interference cancels  or the other way bring a portable radio closer to the amp chassis to assess if it is radiating into the AM band.   

https://www.tecsunradios.com.au/store/product/tecsun-100-loop-antenna/

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4 hours ago, stereo coffee said:

I would suggest using a dedicated antenna for your AM radio like this one,  your Class D amps really should not radiate frequency beyond the confines of your chassis. To test either turn your amp off and see if the interference cancels  or the other way bring a portable radio closer to the amp chassis to assess if it is radiating into the AM band.   

https://www.tecsunradios.com.au/store/product/tecsun-100-loop-antenna/

I will try a portable radio/another radio.

Noise does go way when I turn off the power amp and use headphones on  the headphone amp/pre amp.

AM radio station isn't the only thing that get interference - ABC Tv also stops working. (I get the same thing when I use the coffee grinder)

I don' think a better antenna will help with local interference? 
 

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19 hours ago, misternavi said:

I will try a portable radio/another radio.

Noise does go way when I turn off the power amp and use headphones on  the headphone amp/pre amp.

AM radio station isn't the only thing that get interference - ABC Tv also stops working. (I get the same thing when I use the coffee grinder)

I don' think a better antenna will help with local interference? 
 

Think of the antenna narrowing reception to just to your needed reception, rather than spurious signal which is interference. 

What style of AM antenna do you use presently ?

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2 hours ago, stereo coffee said:

Think of the antenna narrowing reception to just to your needed reception, rather than spurious signal which is interference. 

What style of AM antenna do you use presently ?

those plastic black square ones that come with tuners.

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13 minutes ago, misternavi said:

those plastic black square ones that come with tuners.

Hmmm...  IMO not selective enough to receive AM radio and at the same time reject spurious emissions, rather expect both as you are presently experiencing.  AM radio transmissions are vertical polarization, so require antenna that similarly are arranged vertically, therein a big advantage for you, as the loop you are using at the moment, is broadly both.  

 

I would try disconnecting the loop as it appears to be working against you for receiving AM radio properly.  Try instead a simple piece of wire of about 4 metres length and have it heading skywards.  If that works well then job done, but another if you are still getting interference  is to use coax to make the antenna part distanced from being near the interference.   

 

Although unconventional arranging instead a piece of RG58 coax of about 10 metres length and spend 15 mins or so, stripping back the ground wire at the receiving end  leaving about 4 metres of the inner to work as your actual antenna. 

 

At this point you might be up in the roof,  taking all precautions with height,  but all for good cause as you are distancing proper AM radio reception that you need from the source of interference.  arrange that 4 metre section vertically, then at your tuner end prepare the coax ground to the ground symbol and the positive of the coax to the AM antenna input.  The purpose of the coax is to shield and we could say harness the actual AM radio reception.  

 

That should be a few hours to set up and about $30.00 in parts but the wire even cheaper, just have it going vertical.  

 

 

 

 

 

 

 

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12 hours ago, stereo coffee said:

Although unconventional arranging instead a piece of RG58 coax of about 10 metres length and spend 15 mins or so, stripping back the ground wire at the receiving end  leaving about 4 metres of the inner to work as your actual antenna. 

I can't run the cable from inside to outside  without drilling through -It's a rental

 

Could run it out the window to test it.......

Edited by misternavi
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Unfortunately the medium wave AM broadcast band it prone to interference especially in the modern world of switch mode power supplies (SMPS) which have infiltrated almost every appliance.   Most LED lighting uses a SMPS instead of transformers the halogen incandescent bulbs we used to use.  Cheap and nasty ones or faulty units can cause a lot of interference.

 

SMPS generate a lot of unwanted RF which is usually well suppressed in good designs, but if one of the EMI filter parts fails the power supply can radiate considerable levels of RF pollution causing havoc with other devices like your AM tuner.  It's possible to track down the culprit by switching off all appliances in your home and switching them back on one at a time whilst listening to your AM tuner.  This may help ID the source of the interference.  Be aware that the interference may be coming from a neighbouring property.  In which case tracing the source might be a problem and even if you do, convincing the neighbour to rectify the problem is another thing because often the offending faulty appliance which is the source of the interference is working just fine.  Just unfortunately generating a lot of interference.

 

Back when ADSL was the most popular broadband internet connection I spent many hours investigating interference issues for Telstra.  I can assure you most of the interference problems came from appliances with a faulty SMPS.  If the source was within the same premises it was easy to prove to the customer which appliance was the culprit and encourage them to have it repaired.  Doing the same with a neighbour who is not experiencing the problem is another issue.

 

Good luck with identifying the source of the interference.  I hope you get it resolved.

 

Cheers,

Alan R.

 

 

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