Radio Killed The Studio Mix

[sr1200]

I recently started having a weird issue in the studio about a month ago. My speakers started picking up a radio station. (only during the day). Other devices in the studio (like my 6K video camera and some other recording devices) started picking things up as well. I figured I had some wiring issues or something since when i plugged my camera into an isolated / filtered outlet the problem seemed to go away. I narrowed the issue down with my monitors down to the RTA that i have in between my DAC and my amplifier. When I plug my DAC directly into the amp... no issues. So it seems the RTA is responsible...

I then began the task of trying to track down what station it was. I dont speak spanish, so... couldnt just listen for clues... but SHAZAM app to the rescue. At the start of a song i shazamed it. Found out the song, and went to my car stereo (only place i had an AM radio) and began flipping thru the stations until i found something similar sounding. Shazamed that and it was a match. I look up the radio station and find this:

In June 2023 WJDM moved from 1520 AM to 1530 AM and increased its power from 1000 to 10000 watts thereby providing a stronger signal especially over New York City and portions of northern New Jersey.

On top of that, i found out they moved from NJ to just a town over from me in NY. So im gettin the full brunt of that 10k watts. Also turns out that they are only allowed to broadcast until 6:30pm. So that answered the question why it was only during the day.

What can I do on my end to rectify this? I've read of ways to loop ferrite beads in places to get rid of issues with cables, but since the issue seems to be emanating from the RTA, not sure what I can exactly do. Part of this solution was to be able to identify the frequency that is present... so check that, its 1530am. Is there a filter i can attach to the outputs? (couple of caps or something?)

Appreciate the help!

 

[Brian Roth]

Just curious....what make/model RTA? Balanced I/O's through that signal path?

Bri

 

[sr1200]

Just curious....what make/model RTA? Balanced I/O's through that signal path?

Bri

Its an apogee DAC feeding into a dbx unit, (drive rack studio) everything in the studio is balanced except the guitars/amps (but they're not receiving lol.) If I unplug one output cable from the RTA (leaving one in) the signal is LOUD. Ive tried using different length cables. If i disconnect the DAC to the RTA The signal persists (so i assume the issue isnt BEFORE the RTA), and as i mentioned, if i go from DAC to AMP... no issues.

 

[zamproject]

I assumed there would be a video to kill that...





Sorry, can't help much for this issue nor can't help myself for the stupid joke
Hope you'll find out

Cheers
Zam

 

[scott2000]

May be something here..or not...
Kinda remember disconnecting shield at receiving end but terminating like a 100pf cap to ground on that end...not sure....

https://groupdiy.com/threads/xlr-grounding.84011/#post-1089514

 

[Brian Roth]

What happens if you "concoct" a pair of XLR male plugs with pin 2 tied to pin 3 and plug them into the dbx input jacks?

Bri

 

[sr1200]

I will try that and report back. Thank you! @Brian Roth

@scott2000 I've seen things like that along with ferrite beads, but vaguely remember you needed a certain value of something depending on the offensive frequency... i could be completely off base with that though. I'll look into that as well The DBX has a ground lift that the signal gets buzzy if i engage the lift.

 

[pucho812]

Have you tried quad cabling here?

 

[rogs]

Finding which part of the signal path is actually doing the rectification can be a real nightmare....
The attached page from the 1980 National Semiconductor Audio Handbook has some useful suggestions --- although the last bullet point in the left hand column does suggest that even the 'pros' can find sorting this type of problem difficult...

 

[ccaudle]

I narrowed the issue down with my monitors down to the RTA

Have you tried the ground lift switch on the DriveRack?
First thing I would check is pin 1 problems on input and output connectors. I would assume PCB mount connectors on that style of chassis, so might be hard to fix without doing some tricky surgery on the I/O PCB.
Neutrik EMC XLR connectors on the cables may help, you could try that first if the connector surgery looks to difficult.

 

[Brian Roth]

Pin 1 problems also crossed my mind as well. Just because it's dbx/Harman doesn't mean they designed it correctly <g>.

Bri

 

[sr1200]

Have you tried the ground lift switch on the DriveRack?
First thing I would check is pin 1 problems on input and output connectors. I would assume PCB mount connectors on that style of chassis, so might be hard to fix without doing some tricky surgery on the I/O PCB.
Neutrik EMC XLR connectors on the cables may help, you could try that first if the connector surgery looks to difficult.

Yes first thing i checked since there is a ground lift switch on the inputs. Just adds some hum to the signal.

 

[mappee]

Contact the engineer at station 1530 and explain problem. He should be able to suggest a trap in your
system to tame the freq.

 

[ccaudle]

first thing i checked since there is a ground lift switch on the inputs. Just adds some hum to the signal.

That might be a good thing. If the wiring is bringing in the RFI into the chassis, the presence of the ground lift switch likely makes the routing unnecessarily long. Since you verified ground lift doesn't help in your system, you can potentially cut that stuff out if you need to wire the pin 1 connections properly.

 

[Mr. Moscode]

Sometimes rerouting cables can do a lot to attenuate the problem. Sometimes you need heavy braid to tie chassis grounds together or ground at one end, which may/may not solve the problem. Putting heavy shield braid over each or both cables in one shield could work. Then you need to experiment with where to ground it - sender, receiver, both, separate ground? Maybe a good earth ground if you're close enough to one - radiator, Cu pipe, a rod from home depot into the ground.

The length of the cables could be resonating at the carrier frequency or a harmonic. Try a different length or a different brand. Float the chassis to mains ground on one of the boxes with 2>3 pin AC adapter from the hardware store.

Wishing you good luck. It can be infuriating.

 

[sr1200]

Finding which part of the signal path is actually doing the rectification can be a real nightmare....
The attached page from the 1980 National Semiconductor Audio Handbook has some useful suggestions --- although the last bullet point in the left hand column does suggest that even the 'pros' can find sorting this type of problem difficult...

lol that last point...
The only wires in the unit are ribbon cables connecting the encoder to the front panel, the ribbon cable to the LCD display and the 2 transformers that are in the power section. Everything else is on the board, SMD city. Jacks are all soldered directly to the board as well. Trying to find a point to be able to implement any of that will probably prove pretty difficult. Thanks for the reference though. I wonder if putting a ferrite clamp thing on the in or output cables would help at all

 

[sr1200]

Contact the engineer at station 1530 and explain problem. He should be able to suggest a trap in your
system to tame the freq.

No Habla Engles.

 

[Ike Zimbel]

lol that last point...
The only wires in the unit are ribbon cables connecting the encoder to the front panel, the ribbon cable to the LCD display and the 2 transformers that are in the power section. Everything else is on the board, SMD city. Jacks are all soldered directly to the board as well. Trying to find a point to be able to implement any of that will probably prove pretty difficult. Thanks for the reference though. I wonder if putting a ferrite clamp thing on the in or output cables would help at all

Make sure the screws that connect the XLR shells to the chassis are tight and make good metal-to-metal contact. Most "modern" XLR's have a chassis ground connection via the mounting screws, assuming yours do, you want to make sure that's a solid connection. ALL screws loosen up after assembly.

 

[ccaudle]

I wonder if putting a ferrite clamp thing on the in or output cables would help at all

Yes it can, although AM radio is so low frequency you may need a large core, and multiple loops of cable through the core.
Jim Brown has written a lot of good papers on the subject, both formal (journal of the AES), and informal. Look for some of the ham radio focused papers at his web site, hams don't run 10kW, but their equipment is usually right beside the transmitter so relative levels are relevant to the problems you are seeing.
Jim Brown published papers
I would recommend specifically these to get started (in this order):
RFI and Ham Radio
RFI in Audio Systems
Understanding How Ferrites Can Prevent RFI
Ham's Guide to RFI, Ferrites, and Audio Interfacing

Short version: fixing pin 1 connection is the best option, if you use ferrite then get a big #31 (formulation) ferrite core and wind and many turns as you can through the core.

The length of the cables could be resonating at the carrier frequency or a harmonic

Commercial radio broadcast is pretty well controlled spectrum, there isn't much harmonic content to deal with. The wavelength (in air) of 1530kHz is over 195m, so 48m cable lengths and multiples would cause the most problems. That would be pretty long for the control room connections in most studio (mic lines or PA system feeds in auditoriums could be that long).

Float the chassis to mains ground on one of the boxes with 2>3 pin AC adapter from the hardware store.

That is an irresponsible suggestion for class I equipment. Presumably if the device has a power line earth connection it is not double insulated, so any power fault would put power line voltage on the chassis and audio connector shells.

Jacks are all soldered directly to the board as well

Can you follow the routing of the pin 1 connections for the output and input connectors? It is possible you could do relatively minor surgery on the pcb, or on the connectors themselves, and get rid of the problem. Or could see if a parallel connection from pin 1 back to shell (e.g. solder some copper braid or copper tape from the pin back to the connector shell) without fully removing the incorrect pin 1 problem can drop the interference to a level you can live with.

First step would be to try to isolate whether the input or output connectors are the primary problem (could be both).

Did you ever try Brian's earlier suggestion?

What happens if you "concoct" a pair of XLR male plugs with pin 2 tied to pin 3 and plug them into the dbx input jacks?

I take that to mean male plugs with no cables attached, just pin 2 tied to pin 3 to short the input.
If the noise still exists that highly suggests that the output connectors are conducting the noise back in to the circuitry (the input connectors may as well, but it is more difficult to listen to the effect of just the input connections with nothing connected to the output connectors).
You can also do things like make up cables with the signal pair cut back, and pins 2 and 3 tied together at the male connector, to check the effect of just shield currents flowing between different pieces of equipment without signal flow. It could be coming in on the signal pins, but odds are on pin 1 problems (especially since the dbx brochure claims that both input and output connectors are "RF filtered").

You could also test if it really is a pin 1 problem by using either bare twisted pairs as cables (i.e. not shielded, so nothing to connect to pin 1), or make some cables with the shields not connected to pin 1 on the dbx end. Or cables with only a wire connecting pin 1 to pin 1 (you would need at least one twisted pair connecting an output to your amp, or to a headphone amp or some kind of monitor device). That assumes that the DAC and amp also use earthed power connections so that the common mode voltage on the signal lines is constrained to a low value. If one of the devices is a two pin line power entry you will have to make provision for that.
For best audio quality you would always want shield connected at the transmit end in a cable with shield connected only at one end, but in this specific case of tracking down where the problem occurs in your DriveRack you could make a cable with just the receive end shield connection.

 

[sr1200]

Yes it can, although AM radio is so low frequency you may need a large core, and multiple loops of cable through the core.
Jim Brown has written a lot of good papers on the subject, both formal (journal of the AES), and informal. Look for some of the ham radio focused papers at his web site, hams don't run 10kW, but their equipment is usually right beside the transmitter so relative levels are relevant to the problems you are seeing.
Jim Brown published papers
I would recommend specifically these to get started (in this order):
RFI and Ham Radio
RFI in Audio Systems
Understanding How Ferrites Can Prevent RFI
Ham's Guide to RFI, Ferrites, and Audio Interfacing

Short version: fixing pin 1 connection is the best option, if you use ferrite then get a big #31 (formulation) ferrite core and wind and many turns as you can through the core.



Commercial radio broadcast is pretty well controlled spectrum, there isn't much harmonic content to deal with. The wavelength (in air) of 1530kHz is over 195m, so 48m cable lengths and multiples would cause the most problems. That would be pretty long for the control room connections in most studio (mic lines or PA system feeds in auditoriums could be that long).



That is an irresponsible suggestion for class I equipment. Presumably if the device has a power line earth connection it is not double insulated, so any power fault would put power line voltage on the chassis and audio connector shells.



Can you follow the routing of the pin 1 connections for the output and input connectors? It is possible you could do relatively minor surgery on the pcb, or on the connectors themselves, and get rid of the problem. Or could see if a parallel connection from pin 1 back to shell (e.g. solder some copper braid or copper tape from the pin back to the connector shell) without fully removing the incorrect pin 1 problem can drop the interference to a level you can live with.

First step would be to try to isolate whether the input or output connectors are the primary problem (could be both).

Did you ever try Brian's earlier suggestion?


I take that to mean male plugs with no cables attached, just pin 2 tied to pin 3 to short the input.
If the noise still exists that highly suggests that the output connectors are conducting the noise back in to the circuitry (the input connectors may as well, but it is more difficult to listen to the effect of just the input connections with nothing connected to the output connectors).
You can also do things like make up cables with the signal pair cut back, and pins 2 and 3 tied together at the male connector, to check the effect of just shield currents flowing between different pieces of equipment without signal flow. It could be coming in on the signal pins, but odds are on pin 1 problems (especially since the dbx brochure claims that both input and output connectors are "RF filtered").

You could also test if it really is a pin 1 problem by using either bare twisted pairs as cables (i.e. not shielded, so nothing to connect to pin 1), or make some cables with the shields not connected to pin 1 on the dbx end. Or cables with only a wire connecting pin 1 to pin 1 (you would need at least one twisted pair connecting an output to your amp, or to a headphone amp or some kind of monitor device). That assumes that the DAC and amp also use earthed power connections so that the common mode voltage on the signal lines is constrained to a low value. If one of the devices is a two pin line power entry you will have to make provision for that.
For best audio quality you would always want shield connected at the transmit end in a cable with shield connected only at one end, but in this specific case of tracking down where the problem occurs in your DriveRack you could make a cable with just the receive end shield connection.

I plan on trying a whole bunch of the suggestions this weekend when I have some time off. I ordered some ferrite clip on things that were specifically designed for 1-30mhz. (cheap enough to just try, if they dont work). I have a bunch of cables i can mess with.