Ferrite bead between input and circuit ground

[Signaling]

So I took this clock radio apart to see what makes it tick and noticed that there's a ferrite bead in series with the 3.5mm input jack ground and amplifier circuit ground.

Does anyone know why they did that and has anyone here tried it on their own designs? I've seen this done on the signal lines before but never between the grounds on an input jack.

The first picture is of the circuit board itself, ferrite bead marked in red whilst the second image is a schematic drawing to clear any confusion.

Thanks. 😎

 

[gofar99]

Hi, Possibly to prevent it from picking up EMI.

 

[JonSnell Electronic]

To reduce RF interference, both ways.

 

[Signaling]

hmm so would it be worth putting some there on my homemade stuff? I have a bunch of them laying around.

Has anyone tried using them there before on amps they have made?

 

[scottjoplin]

Not aware of a downside. Usually put on inputs

 

[Speedskater]

Normally both the Send and Return wires go thru the ferrite.

The go-to EMI/RFI/ferrite expert is Jim Brown.
Several of his 50 papers & Power Points have ferrite sections.
Audio Systems Group, Inc. Publications

 

[Ken Newton]

I would presume it's there to help suppress common-mode noise current flow (noise between the input signal's return wire and the clock's local ground) by inserting an impedance between the two. Ferrites only function at high frequency, so, only high frequency noise current will be impeded. Audio band noise will not be suppressed. Even so, should either the input signal source or the clock generate undesired high frequency noise, this can still be an effective low cost treatment.

 

[Osvaldo de Banfield]

Ferrite beads are for two reasons:

1) The maintain the headphone cable live for RF signals, because it is the FM antenna;

2) It prevents harmonics irradiated from the cable in the AM bands, which in turn can cause completely oscillating system (When detecting AM signals, lots of harmonics are generated and can be amplified by the audio sector and irradiated by the able, re entering in the AM ferrite antenna or loop.).

There is a lot of information about this topic in the audio book from National Semiconductor.

 

[kasey197]

So I took this clock radio apart to see what makes it tick ...

Thanks. 😎

and don't, for one moment, think that your pun went unnoticed 😀😀😀

 

[AndrewT]

and don't, for one moment, think that your pun went unnoticed 😀😀😀

I'm slow, I didn't get it until your spelled it out for me.

 

[Signaling]

Thanks everyone! I guess it makes sense to add some RF loss when the ground paths are exported from the board since the power supply return wire + audio cable return can add up to several feet of continuous conductor perfect for picking up radio signals.

Plus many laptops being used as an audio source have the negative side of their power supply's connected to mains earth in the power supply so even more potential for RF pickup especially with a UK ring main.

and don't, for one moment, think that your pun went unnoticed 😀😀😀

The funny thing is that this clock doesn't even have a ticking mechanism

Another pic for those interested, at the heart of it is the TDA2822M running on 6v. It is stereo but I managed to bust one of the speaker connectors.

 

[sgrossklass]

TDA2822M running at full gain, eh? I'd guess it's more than a bit hissy. Assuming maximum achievable volume is more than ample, you could hack it to include a few hundred ohms in series with the caps on pins 5 and 8 (negative feedback ground), they'll usually take that fine. Though with these Chinese knockoffs, you never really know.

I'm guessing the speakers in this thing are rather tiny? The output coupling caps don't look very big at all, maybe 220 µF?

 

[nintendo torkorotno]

once i have tried a ferrite core ie transformer with 1:1 turns ratio, to isolate input signal , just like line signals connect. i wondered as it eliminaes the noise pick by very long ""isolated wire and with a noisy source lacking enough decoupling and more of that some resistors is in te path as well
that core was tiny

 

[john65b]

I know this thread is quite dead, but if you ever have an amp that buzzes like a buzzsaw, but when you put your finger on the input, all the noise suddenly goes away, well, therein lies your perfect application for a Ferrite Bead. Works a charm for me!!