pucho812

ferrit beads in the line input path?
« on: September 14, 2020, 01:21:35 AM »
what is the purpose of a ferrit bead on the line input of a piece of gear? I have seen it a few times usually after the connection and before the differential opamp.

You tell me whar a man gits his corn pone, en I'll tell you what his 'pinions is.


john12ax7

Re: ferrit beads in the line input path?
« Reply #1 on: September 14, 2020, 03:04:54 AM »
Suppresses high frequency crud.

Re: ferrit beads in the line input path?
« Reply #2 on: September 14, 2020, 04:05:25 AM »
what is the purpose of a ferrit bead on the line input of a piece of gear? I have seen it a few times usually after the connection and before the differential opamp.

There are mixed feelings about ferrite beads, on one side they help suppress RF noise, on the other side they tend to saturate and distort, pick your poison.

abbey road d enfer

Re: ferrit beads in the line input path?
« Reply #3 on: September 14, 2020, 06:31:46 AM »
There are mixed feelings about ferrite beads, on one side they help suppress RF noise, on the other side they tend to saturate and distort, pick your poison.
I've never seen a ferrite bead saturate at audio frequncies in mic or line level applications. Anyway, their reactance at audio frequencies is so small even huge variations would have no consequences on distortion.
Ferrite inductors do introduce distortion when used as a filter element operating in the audio band, though.
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.

trobbins

Re: ferrit beads in the line input path?
« Reply #4 on: September 14, 2020, 08:03:47 AM »
Just adding slip on beads is pretty much an increasing resistance, with a peak resistance depending on the ferrite - effectively no inductance to saturate.

JohnRoberts

Re: ferrit beads in the line input path?
« Reply #5 on: September 14, 2020, 10:08:50 AM »
"ferrite" beads act like a magnetic element to form a crude one turn inductor from the input lead wire.

Ferrite beads are old school through hole technology, nowadays we use SMD inductors. (I use a couple SMD on the output of my class D audio amp chip inside my drum tuner.)

Generally you should see a capacitor shunt after the inductors to form a proper LC low pass filter... Ferrite beads have been used as a band aid in field repairs, service calls, to mitigate RF issues in established designs. 

 JR
Don't only half-ass tune your drums. Visit https://circularscience.com to hear what properly "cleared" drums sound like.

Newmarket

Re: ferrit beads in the line input path?
« Reply #6 on: September 14, 2020, 11:55:42 AM »
I've never seen a ferrite bead saturate at audio frequncies in mic or line level applications.
Same here. But if concerned about it useful to note that placing a ferrite around current balanced conductors cancels out the effect on the differential signal. The likely common mode rfi does see the impedance.
Also note that the impedance is complex and has both inductive and resistive components.
At higher frequencies (talking 100s of MHz here) the resistance tends to dominate.
Additionally the ferrite material itself can act as a dielectric and form the capacitor in an filter if ferrite is against eg the chassis that acts as rf 'ground'.

pucho812

Re: ferrit beads in the line input path?
« Reply #7 on: September 14, 2020, 02:55:04 PM »
which brings up the follow up questions  as to how necessary they are and if they can be left out?
You tell me whar a man gits his corn pone, en I'll tell you what his 'pinions is.

squarewave

Re: ferrit beads in the line input path?
« Reply #8 on: September 14, 2020, 03:30:06 PM »
If you look at pretty much any vaguely new schematic, they have chokes and caps on inputs. For differential inputs, frequently they're common mode. So apparently more than one engineer thought they were important. If you want to keep RF out of the enclosure, it's necessary. It might help known down a static shock as well.

I would definitely not use ferrite beads though. I use SMD fixed inductors mostly because just about all of the off-the-shelf parts are SMD. But can find a bigger package so that it's easier to solder and work with. For example, a 0805 (2012 metric) package at 100uH with a typical DC resistance of 4 ohms is ~0.20 USD. With a 100nF cap, that will get well under 100kHz and relatively low Q.

Also, they need to be very close to the entry point. The length of wire connecting the pin to the inductor determines the wavelength of RF that will be radiated within the enclosure. I make little circular boards for PC mount XLR connects that have pads for the RF filter directly on it.

JohnRoberts

Re: ferrit beads in the line input path?
« Reply #9 on: September 14, 2020, 05:02:59 PM »
which brings up the follow up questions  as to how necessary they are and if they can be left out?
It depends on the RF environment the SKU is exposed to, and the design of the input stage. It is possible to design an input stage that harmlessly LPF RF input but many older designs will slew limit and rectify the RF.

These days there are more RF sources everywhere than back when I was in the trenches. 

JR

PS: I recall some specialized input common mode filters, that became popular to fix RF problems in a high volume mixer designed by a competitor  :o. IIRC it was something like a transformer that passed differential audio but rejected common mode noise. I don't recall more details because my consoles didn't need the bandaids.   8)
Don't only half-ass tune your drums. Visit https://circularscience.com to hear what properly "cleared" drums sound like.


abbey road d enfer

Re: ferrit beads in the line input path?
« Reply #10 on: September 14, 2020, 05:34:22 PM »
It depends on the RF environment the SKU is exposed to, and the design of the input stage. It is possible to design an input stage  I recall some specialized input common mode filters, that became popular to fix RF problems in a high volume mixer designed by a competitor  :o. IIRC it was something like a transformer that passed differential audio but rejected common mode noise.
Common-mode chokes. They used to be expensive when they started to appear, at the time manufacturers were made conscious of EMI/RFI regulations.
Now they cost pennies.
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.

Rusan

Re: ferrit beads in the line input path?
« Reply #11 on: September 14, 2020, 05:42:27 PM »
"These days there are more RF sources everywhere than back when I was in the trenches." 

Wow, I never considered the "RF soup" that the entire world is swamped in today.  It's been well over 30 years since I tinkered with the electronics hobby, and have just gotten back into it this year.  I've been fiddling with tubes a lot, mostly 12A_7 types, EF86 and 6922, but also the 5842 and 6C45.  On those frisky little mega-Gm guys, in addition to the requisite grid stoppers I use ferrite beads on the heater leads to thwart any chance of cathode-coupled VHF funny business.  But, in consideration of the abundance of RF sources in today's world, I guess it would be a good idea to include applicable RF suppression any and everywhere you even think a device might have a chance of RF oscillation/detection. 

Yup, everything was better back in the good old days... :(

Rusan
“I’m sure the universe is full of intelligent life. It’s just been too intelligent to come here.” - Arthur C. Clarke


 

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