Rf protection in db25 connectors?

[JAY X]

Hi,

¿is there a way to implement RF protection to db25 input connectors wired as balanced tascam?
Besides tiying pin1 to chassis...is there any other help to stop Rf? ¿ferrite beads maybe?.. VK200...?

And what about inverting inputs in summing amps?... i saw schematics with ferrites at the inverting inputs...hmmm ¿do they help?

JAY X

 

[JAY X]

Hi,

¿no idea?... maybe it is not possible to implement RF protection in db25 conectors...apart from tying pin1 to chassis, an RF filter..¿is it possible?...

JAY X

 

[JAY X]

Hi,

I'm trying to know how to protect the inverting input of a summing amplifier from RFI, and apply RF protection schemes to audio connectors.

1. As i used DB25 conectors fo the inputs, i tied every pin1 to chassis, ok so far. but is there any other measure i can take to prevent RFI at the inverting inputs ?(mobile phone interference mainly) An inductor at inverting input? ferrites?

1B.I forgot to mention that the inputs of the balanced summing amplifier are 10k resistors. There are no IC receivers. DB25 conector---->Summing resistors---->inverting input.

2. Regarding the outputs, (of a DRV134 line driver ) i wired two XLR outputs together to reply the signal from one to other. ¿should i place an RF filter at both output connectors or only the first one?

Thank you for your help!
JAY X

 

[ruffrecords]

I seem to remember seeing somewhere some DB25 connectors with built in ferrite beads, I am afraid I cannot remember where.

Cheers

Ian

 

[JAY X]

Hi Ian,

i didn't found those db25 connectors.... but another option could be to place a ferrite bead in the inverting inputs...?? is it useful??

JAY X

 

[ruffrecords]

Hi Ian,

i didn't found those db25 connectors.... but another option could be to place a ferrite bead in the inverting inputs...?? is it useful??

JAY X

A quick search of Farnell revealed the following:

http://uk.farnell.com/harting/09-64-323-7802/plug-dsub-filtered-us-25way/dp/1779282

Why would you put them just on the inverting ( I presume you mean cold) inputs?

Cheers

Ian

 

[JAY X]

Hi,

Well, the problem is that i'm trying to stop RF entering the circuits. In the case of the summing amp, the inputs is in db25 cable format. And about the inverting inputs i saw some schematics where they place ferrites or inductors... but i don't know exactly why... or if they are useful to prevent RFI.

If the inputs of the summing amp were standard jack connectors, i would place T filters (470pf x2 + 47pf) near the connector.

jay x

 

[Speedskater]

Ferrite's are such simple little things, yet using them is anything but simple.  Jim Brown has several papers on using ferrite's.  A quick read and you will see that some thought is required when using ferrite's.

Jim Brown - Audio Systems Group, Inc. - Publications
http://www.audiosystemsgroup.com/publish.htm

 

[JAY X]

Hi,

Should an audio product totally block/ resist cell phone interference?... I will try T filters at the input/output connectors to hear what happens. I have seen the THAT design notes... they apply T filters to all their input/output IC products... so they should work if well designed...

In a recording studio, cell phones are banned or simply turned off...this is the most simple RFI protection...

Any comments wellcome!
JAY X

 

[Samuel Groner]

This is a large topic which is hopeless to cover in a few posts. It has as much to do with system design (case, connectors) and PCB layout (low-inductance routing, ground planes) as with circuit design (low-pass filters).

As a starter I suggest you show us more details (schematic, application context) of your project.

Samuel

 

[Samuel Groner]

In a recording studio, cell phones are banned or simply turned off... This is the most simple RFI protection...

Perhaps, but also a hint that proper RFI protection is not well implemented in many audio gear. A year back or so I re-designed an OEM power amplifier which suffered from serious cell phone sensitivity (full-scale demodulated output with the phone one or two meters away). In the redesign, you could place the phone right on top of the PCB (no case), with no audible demodulated output.

Samuel

 

[JAY X]

Hi Samuel!

I suspect that my monitors (KRK V6 year 2003), are not shielded against EMI/RFI, just video shield... they pick up phone frequencies when connected to the soundcard, which has unbalanced outputs and a plastic breakout box....bad for shielding... so all summed up leads to mobile phone interference!!...

JAY X

 

[Samuel Groner]

I don't pretend to being able to offer a simple fix for this issue, but a few thoughts:

* When connecting unbalanced outputs to balanced inputs it is advisable to use a simple adapter cable which allows the balanced input to sense the remote ground of the source, thus given at least some basic common-mode rejection. See an003.pdf, figure 2.1. This perhaps will also reduce the sensitivity to RFI.
* Keep in mind that both the input of the speaker as well as the soundcard output may demodulate RFI; so perhaps the culprit is at the sending end. You may try using both the speaker and soundcard with other equipment to see if the issue is not restricted to this particular combination. A helpful experiment is perhaps also to terminate the cable with a dummy resistor instead of connecting it to the soundcard.

Samuel

 

[Monte McGuire]

I think it's more simple than you'd think. What is RFI? Excessive input bandwidth. How would one remove that? Add shunt caps to ground and series inductors from each pin of the DB25 connector. It might be more clever to use lossy ferrite bead-type devices than pure inductors, but you get the point. The deal is to take the RF traveling on the input pin and make it "one" with ground so that it effectively disappears.