Interference spikes in studio gears

[mattenylander]

but the cause is usually referred to by different terms (e.g. ground bounce, internal EMI, common impedance coupling).

Thanks for sorting that out!

Connectors. Look for info on "pin 1 problem,"

I actually looked in to the Behringer Mic 100, and when I did a continuity test and take one probe for continuity on the Pin 1 XLR jack and the other probe to the black cable to the power supply lug, it beeps. It also beeps at various places on the pcb. Is that a pin 1 problem?

The XLR is also soldered directly to the pcb…
I know it’s a super cheap preamp but I actually got it for free when I bought the warm bus comp so I see this more of a opportunity to learn how “not to do it” in circuits and hopefully learn something.

but these look like some decent sources to get started:

many thanks I’ll have a read!

 

[Newmarket]

The next step after than is filtering on signal connections, which could be as simple as a ferrite bead on signal connections instead of a straight wire connection. Sometimes clamp on ferrites which clamp around the exterior of the cable to increase common mode impedance (like often found on computer cables) can help. Those don't require modifying any internal connections so are an easy first step to try.

+1. Ferrite sleeves / clamps are the "first aid" measure of EMC lab work (well possibly after wrapping the kit in aluminium foil attached to "Ground").
If physically practicable loop a cable through it. There's a square law in action so having it pass twice gives x4 effect (well nearly as there's a small negative effect from the capacitance introduced). Beyond a single loop the capacitive effect rises but it all depends on the geometry.
And try different sizes and ferrite materials

 

[Newmarket]

Over here, the operator is legally bound to fix that RF interference. This kind of antenna can be tuned in software, so there shouldn't be a problem to turn the wave away from your apartment.

It usually takes a bit of effort to get them going, but if you don't try now, they'll argue there wasn't a problem in the future, so don't wait a few years to complain.

Hi. What is your location. I see Europe but in EU (or somewhere aligned with EU Directives wrt this ?

 

[mattenylander]

These are tuneable GSM antenna, but I don't recognise the brand. Depending on where you are, it could be interesting to check who's responsible for them.

Oh I didn’t know that! I live in Sweden, and check for some weeks ago and it was the biggest telecom company in Sweden who owns the antennas.
But I’ll give it a try to contact them and see if they can do something about it, didn’t know that might be an option

Because it seems that the interference is just passing exactly where I have my computer and all other studio gear, in other rooms it’s fine, or far far better.

 

[cyrano]

Well, I'm in Belgium, at the center of the EU

Brussels has some of the most restrictive emission rules in the world. But Sweden shouldn't be too far behind, I think

 

[mattenylander]

+1. Ferrite sleeves / clamps are the "first aid" measure of EMC lab work (well possibly after wrapping the kit in aluminium foil attached to "Ground").

Ok interesting I see, when you say attach to ground, do you mean a ground point in the circuit? or is it the ground lug on the wall outlet?

 

[Newmarket]

Thanks for sorting that out!

I actually looked in to the Behringer Mic 100, and when I did a continuity test and take one probe for continuity on the Pin 1 XLR jack and the other probe to the black cable to the power supply lug, it beeps. It also beeps at various places on the pcb. Is that a pin 1 problem?

The XLR is also soldered directly to the pcb…
I know it’s a super cheap preamp but I actually got it for free when I bought the warm bus comp so I see this more of a opportunity to learn how “not to do it” in circuits and hopefully learn something.

many thanks I’ll have a read!

A continuity buzz isn't informative here unfortunately. You'll normally get a buzz below some arbitrary level. But even taking a resistance reading won't really help as you are looking at very low level measurements.
You need to look at the connections on the PCB. If the pin 1 connection is returned eg by a wire to the chassis, and not connected to the Signal 0V on the pcb then that is good. Although not as good as a direct chassis connection.
If it connects to the Signal 0V the you can welcome the pin one problem But it's usually easy enough to sort of you're handy with a soldering iron etc.

 

[Newmarket]

Ok interesting I see, when you say attach to ground, do you mean a ground point in the circuit? or is it the ground lug on the wall outlet?

Hi. To be clear the "ground" point only applies wrt the aluminium (other metals are available but foil is very easy to use).
It would usually be the chassis of the kit. But connection to Earth on outlet would usually be okay too. Basically it just needs to be a low impedance node.
The ferrite clamp does not need to be "grounded". In many ways that's the beauty of it.

 

[ccaudle]

when I did a continuity test and take one probe for continuity on the Pin 1 XLR jack and the other probe to the black cable to the power supply lug, it beeps. It also beeps at various places on the pcb. Is that a pin 1 problem?

Not necessarily, because an ohmmeter is testing at DC, and the connection should eventually meet up again.
The cable shield pins should be connected to chassis as directly as possible (assuming conductive metal chassis), but the circuit common/reference/"ground" conductor should also be referenced to the chassis. You will still detect a low resistance connection, but if you follow the current path which any interference on a cable shield would take to get to other cable shields or the power supply, the path should not be forced to go across the circuit board.

The XLR is also soldered directly to the pcb…

Depending on how inexperienced the designer was, the easy way to connect the XLR pin 1 is directly to the circuit common (what is usually called circuit "ground") which is the definition of the "pin 1 problem." Confused thinking about the purpose of various different things all sometimes called "ground" is what leads to the problem, any current flow through a conductor will result in a voltage, described by Ohm's Law. If that current flows through the circuit reference, the voltage created now is the reference voltage for the circuitry, which means it will show up directly on the outputs. You want the current from cable shield to be able to flow on the chassis to return to the source via other cable shields, or power supply lines (through filter or parasitic capacitance), or the safety earth conductor. If you are thinking wouldn't the voltage still show up on the outputs because the circuit reference is still connected to the chassis? The answer is yes, but the entire reference will be at that voltage, there will be no difference at different locations of the circuit, so it becomes "common mode" noise, i.e. the same on both legs of a balanced connection. So balanced connections are required for lowest noise, since an unbalanced connection has no way to reject the common mode noise.

There is a way to create a PCB layout without pin 1 problems with PCB mounted connectors, but it requires having a copper pour around the connectors with only pin 1 connections, and a strip of metal connecting that copper pour to the chassis (a wide but short length strip rather than wire so the inductance of the connection is very low), and the rest of the circuit ground only connecting to that shield pin connection at one spot. If the XLR connectors are the kind with four pins, a separate pin for the shell, then just connecting the fourth pin of all the XLR connectors to that copper pour may be enough.
If the chassis is plastic, or anodized aluminum, or some other non-conductive construction that may be all you can do.

But that is more a design issue than a modification suggestion, I don't think it would be very practical to try to make modifications on an existing design with PCB mounted connectors. Probably in that case using the Neutrik EMC enhanced connectors would be the best thing to try.

 

[KA-Electonics.com]

This - as ccaudle points out - appears to be rectification of an RF carrier envelope.

This post shows how an LME49720 responds to a DECT cordless phone several feet away by rectifying its carrier. LME49720 DECT Cordless Phone EMI Susceptibility - Pro Audio Design Forum

I've found DECT phones produce a 100 Hz pattern due to the base station polling handsets. My 2.4/5 GHz WiFi access point plants 30 Hz-spaced spurs.

Whenever I see 1 kHz intervals I first look at USB whose polling rate (at least for USB 2) is 1 ms. This appears to be RF but I would look for USB noise as well which is also a rectification issue.

I'll go out on a limb here and suffer the flame wars but the LME49720 and its ilk are rectifying pigs. I'm not the only one that's documented this. Not saying your problem is an LME49720 but I am saying it's an issue. Use one as a sniffer to find your RF ingress...

 

[mattenylander]

A continuity buzz isn't informative here unfortunately. You'll normally get a buzz below some arbitrary level.

Ah Ok I see I’ll check the connections in the PCB instead!

 

[mattenylander]

It would usually be the chassis of the kit. But connection to Earth on outlet would usually be okay too

Great,I’ll try to wrap my Behringer Mic 100 in aluminum and ground it and see what happens to the RFI.

 

[mattenylander]

If you are thinking wouldn't the voltage still show up on the outputs because the circuit reference is still connected to the chassis? The answer is yes, but the entire reference will be at that voltage, there will be no difference at different locations of the circuit, so it becomes "common mode" noise,

Ok, I’ve scratched my head a lot trying to fully understand this, so here I go, do you mean that all the ground connections in the circuit will meet up to one single circuit ground connection and that single circuit ground connection is then connected to the chassi ground and that makes the circuit ground, referenced, to the chassi ground, which then makes the chassi noise to common noise?

But if one would connect the say pin 1 directly to the the circuit ground instead, that would create a potential difference between the chassi ground and the circuit ground and that will present itself as ground noise? Or in my case the RFI spikes?

 

[mattenylander]

Whenever I see 1 kHz intervals I first look at USB whose polling rate (at least for USB 2) is 1 ms. This appears to be RF but I would look for USB noise as well which is also a rectification issue.

Ok, are there any ways to reduce this polling rate issue on a USB interface?
And do the the thunderbolt interface or a usb C interface suffer from this issue as well?

 

[Speedskater]

Retired AES interference expert has a good tutorial on this type of problem:

A Ham's Guide to RFI, Ferrites, Baluns, and Audio Interfacing
http://www.audiosystemsgroup.com/RFI-Ham.pdf
The first part of the paper is about the problems, only the last part is about Ham radio solutions.

 

[ccaudle]

do you mean that all the ground connections in the circuit will meet up to one single circuit ground connection

Yes. Essentially this has to happen in all typical audio circuits, the circuit design necessitates that all of the circuitry use a common reference as the 0 point. Manufactured PCB's commonly use an entire layer as a solid board-wide ground connection (aka ground plane). That is nearly required for modern digital electronics, but you can get away without it for audio circuitry, so depending on the circuit and how it is laid out that may be explicit, or may be implicit because all the circuitry references a solid layer used as ground.

that single circuit ground connection is then connected to the chassis ground

Yes

if one would connect the say pin 1 directly to the the circuit ground instead, that would create a potential difference between the chassis ground and the circuit ground and that will present itself as ground noise?

To understand the problem that causes first remember that voltage is proportional to current flow and impedance, and that every connection has resistance and inductance. When pin 1 (shield pin) is connected directly to circuit reference (aka "ground"), the current which had been flowing on the shield now flows across the circuit to wherever will complete the current loop, often to the safety earth, or to shield connection on another cable, or back through the power supply connections, and the voltage drop at any particular point will depend on how far along the path that point is located. That means that different parts of the audio circuitry see different voltages as that parts reference for 0V. If that particular part of the circuit has high gain that voltage difference can be amplified considerably, but even low gain or unity circuits will have their output "riding" on that voltage level, which can be audible at surprisingly small voltages.

The idea of connecting all the shields directly to chassis is that the current now develops a voltage across the resistance of the chassis metal, and since you connect the circuit reference/gnd to the chassis at only one point, no matter what that chassis voltage may be, the entire circuit is referenced to that voltage with no differences on the circuit board between different parts of the circuit. The entire circuit will be "riding" on that voltage, which is why you need balanced connections so that the receiving device differential receiver can ignore that power line related fluctuation of the output voltage, but since it will be the same on the hot and cold legs, the balanced receiver can remove that noise and just recover the signal as the difference voltage between hot and cold legs.

 

[mattenylander]

Retired AES interference expert has a good tutorial on this type of problem

Than you! I’ll make myself a cup of tea and have a read!

 

[mattenylander]

That means that different parts of the audio circuitry see different voltages as that parts reference for 0V.

Many thanks for for the explanation! It definitely makes more sense now

 

[Big Valley Audio]

This has been a great thread that I think will affect more and more people as time goes on.
I wanted to add a video clip I took from a client of mine that was having an issue. Hopefully it will be helpful.
The studio has had a cell tower located beside it for over 15 years and has never had an issue before a couple years ago when the cell company started using the 600MHz and 700MHz band with its 5G service.
In the video you can hear a high pitched whine that is being picked up through condenser mics in the live room and then I compare that to the demodulated audio that is coming from a spectrum analyzer demod output.
The spectrum analyzer was using only the small antenna that you see plugged into it and when we demodded a frequency we knew belonged to the cell company (734MHz in this case) you could hear the same whine.
We even got the cell company to shut down the service at that location and then everything was absolutely silent.
The Neutrik XLR connecters mentioned earlier in this thread helped.

 

[Speedskater]

Retired AES interference expert has a good tutorial on this type of problem:

Somehow I didn't type in his name:
Jim Brown

About 50 more of his great papers and Power Points:

http://www.audiosystemsgroup.com/publish.htm