Ferite Choke Beads on Audio Output

Hey peoples..

I noticed that a mic I own by Manley had a 6 hole ferrite choke after the transformer but before the mic out pins.

Is this really nessisary and wouldn't just twisting the two cables together create the same efi common mode rejection traits?

And second question, would the inductance value of the choke matter when used in this position?

Cheers

Jess

Wire-twisting is different - that "only" helps ensure that, whatever noise does get in, gets induced in similar amounts on both lines.

As i'm sure you can notice, each line goes through its own ferrite core, so there's not much (if any) coupling between the two. Those are basically just a couple of low(ish?) value inductors in series with each output line, acting as low-pass filters (in conjuction with whatever capacitance to ground there may be), to prevent or limit the ingress of RFI.

Khron said:

Wire-twisting is different - that "only" helps ensure that, whatever noise does get in, gets induced in similar amounts on both lines.

As i'm sure you can notice, each line goes through its own ferrite core, so there's not much (if any) coupling between the two. Those are basically just a couple of low(ish?) value inductors in series with each output line, acting as low-pass filters (in conjuction with whatever capacitance to ground there may be), to prevent or limit the ingress of RFI.

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Thanks mate,

Yeah your right each line goes through each ferrite independently of the other.
I think the values of them are 3300 could that be 3.3k ohms? theres no capacitance to ground. just a couple of 75ohm resistors between them and the xformer outputs.

Cheers

Jess

And where exactly did you read (or deduce) those values from, if i may ask?

Hint: inductors are measured in Henrys (and submultiples thereof)

Khron said:

And where exactly did you read (or deduce) those values from, if i may ask?

Hint: inductors are measured in Henrys (and submultiples thereof)

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they were kind enough to supply me with schematics for the alternative tube modification.

I was under the impression that ferrite chokes impedence was measured in ohms. RE: https://www.mouser.com/ProductDetail/Wurth-Electronics/74275223/?qs=sGAEpiMZZMtdyQheitOmRRCUY23OBnxUDmf3q274xV4yv7YkUhzyXQ%3d%3d

Cheers

Jess

Perhaps, but they usually also have the frequency mentioned (at least one), where the impedance is that value. 

That being said, as can be seen in your example, they're anything BUT precision devices (the +/-25% tolerance brings to mind the phrase "hot dog down a hallway").

Khron said:

Perhaps, but they usually also have the frequency mentioned (at least one), where the impedance is that value. 

That being said, as can be seen in your example, they're anything BUT precision devices (the +/-25% tolerance brings to mind the phrase "hot dog down a hallway").

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lol quite.

So I think we can conclude that this is pretty much why every other mic doesn't implement this.
Maybe they had issues with EMI when building but I can't see why or how.

Jess

It's probably to satisfied radio and TV compliance - it's why akg put inductors on the outputs of their solid state mics

Timjag said:

It's probably to satisfied radio and TV compliance - it's why akg put inductors on the outputs of their solid state mics

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Good point, maybe similar to Neumann filter caps on u67. I skipped these and had no issues in my build.

I'd take them out personally, the less crap in the signal path the better, but then, I don't work under a 30 megawatt transmitter. Often service techs say "...but they don't kick in till 40k" but the signal still passes through it.

I have a pair of Milab VM-44s and a pair of DC196's that both have an RF filter. Its an inductor based SMD daughter board where the transformer would have once sat before they went transformerless  it pisses me off seeing those SMD components in my signal path! I'm going to remove it when I get some time.

Timjag said:

Its an inductor based SMD daughter board where the transformer would have once sat before they went transformerless  it pisses me off seeing those SMD components in my signal path! I'm going to remove it when I get some time.

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So you'd rather have a xfmr in the signal path than a couple of inductors!

JessJackson said:

So I think we can conclude that this is pretty much why every other mic doesn't implement this.
Maybe they had issues with EMI when building but I can't see why or how.

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That's because broadcast users have stringent RFI/EMI requirements. Indeed, most users would not realize the benefits of it, because they have a rather clean electromagnetic environment.

JessJackson said:

And second question, would the inductance value of the choke matter when used in this position?

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Yes, somewhat. The big issue there is that these inductors are constituting a low-pass filter. Since RFI/EMI covers a very large spectrum, a simple LPF that would cut-off at about 500kHz should be all that's needed. However, the parasitic elements in ferrite inductors make them suitable for a limited frequency range, as reading the specs would show.
Some mic manufacturers use two sets of inductors and caps, in order to cover a wider range, but it increases the cost and bulk.
Most often, the inductor's value is the result of a compromise and estimation of the probable causes of interference.

Got you, thanks for that explanation.

Would they have any effect on cellphone RF?
For instance when holding phone next to mic to read lyrics.

Jesse

JessJackson said:

Would they have any effect on cellphone RF?
For instance when holding phone next to mic to read lyrics.

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Yes. In fact, requirements for RFI protection have changes over the years; in the 70's and 80's, the concern was broadcast transmitters and police/ambulances/firemen, so protection was centered at 100-600 MHz. Today, the center has shifted to GigaHertz bands.

but surely an LPF is an LPF regardless of how high the interference frequency is?

Jesse

JessJackson said:

but surely an LPF is an LPF regardless of how high the interference frequency is?

Jesse

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That would be the case if the ferrite beads were simple inductors; as I mentioned earlier,  parasitic elements (mainly the parallel capacitance) strongly interfere with the expected response. See how their impedance varies with frequency in databooks.

Abbey Road,

Yes I would rather have a transformer than an inductor!

Timjag said:

Yes I would rather have a transformer than an inductor!

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Doesn't make sense to me but you are entitled to your opinions.