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Radio interference only trough cable (budget studio monitors)

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#1
(dislaimer: audio noob with no electronics knowledge)

I'm a bit confused about my experience comparing my KRK Rokit 5 G3s and M-AUDIO BX5 D2 monitors.

Setup: desktop computer -> (a) M-AUDIO MTRACK 2x2 USB with balanced TRS cables to monitors, or (b) MiniDSP DDRC-24 (USB input) with RCA cables to monitors

When both are connected to the audio interface's balanced outputs, both are faint with a slight hiss (the Rokit hisses more than the M-AUDIO, but is still fine for listening at 1m away). But when both are connected to my MiniDSP's RCA outputs (by using either RCA-RCA or RCA-TRS cables), though, the BX5 is as silent as with the balanced setup, but the Rokits start playing radio and buzzing. The issue alleviates if I pick the RCA cables and lift them higher with my hand. Cables are 1.5m long.

So, I'm not sure if this is expected behavior in my setup/location, but I'm sure the BX5's have something better in them to prevent them from picking interference from the unbalanced input. Of course, when I compare them I use the exact same setup, including cables and power source (everything connected to the same power strip).

The interference the Rokits are picking only comes through the unbalanced cables (again, none of the monitors pick interference when running balanced). Am I right to assume that it is externally shielded, but has bad "filters" inside What is probably happening here?

Thank you,
 

Vini darko

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#2
That's wierd looking forward to hearing what answers come up... I got nothing
 

solderdude

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#3
It could be the RCA cables are not really well shielded.
It could be the input stage of the Rokit is more sensitive due to the way it was made.
It could be the not connected part of the balanced input is not properly) grounded (TRS used instead of TS).
It could be caused by a groundloop, in which case you could try a unbalanced/balanced converter (with a small transformer inside)
It could be RF getting into the input which, in some cases, can be solved by placing a clamp ferrite close to the input with the cable run through it 3 to 5 times (when possible). The RF signal only needs to be lowered enough so that the input circuit doesn't act as an AM detector. The ferrite could just do the trick.
 
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OP
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Thread Starter #4
It could be the RCA cables are not really well shielded.
It could be the input stage of the Rokit is more sensitive due to the way it was made.
It could be the not connected part of the balanced input is not properly) grounded (TRS used instead of TS).
It could be caused by a groundloop, in which case you could try a unbalanced/balanced converter (with a small transformer inside)
It could be RF getting into the input which, in some cases, can be solved by placing a clamp ferrite close to the input with the cable run through it 3 to 5 times (when possible). The RF signal only needs to be lowered enough so that the input circuit doesn't act as an AM detector. The ferrite could just do the trick.
Thanks!
I'm not sure, but issues 1 (bad rca cables) and 4 (ground loop) should cause interference on both the Rokits and the BX5s, right?
 

wwenze

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#5
Summarizable into a single phrase: Poor common-mode rejection of that amplifier

If I remember right the Rokit secondary ground is not connected to mains earth (through capacitor or resistor or otherwise... that or the impedance is still too high). Somebody had the same issue and I advised him to earth the ground somehow (eventually settled for the wire to socket earth route) and it worked.

Computer should be relatively earthed and so would USB devices connected to it, while your Rokit isn't, so you get a floating end and the RCA cable becomes your antenna. A coaxial cable means the outer conductor eats up all the EM while the inner conductor is shielded, hence resulting in a voltage difference. A twisted pair probably would work better. That said, cable may not be the bottleneck if it's the entire ground on the equipment that is raised.
 
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Speedskater

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#6
Radio Frequency Interference problems are black magic types of things. The output stage, the cable construction, length & placement and the input stage all enter the picture.
There are three parts to the problem: 1} an interference source 2] an antenna 3] a circuit that is susceptible interference.
 
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solderdude

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#7
Thanks!
I'm not sure, but issues 1 (bad rca cables) and 4 (ground loop) should cause interference on both the Rokits and the BX5s, right?
Not necesarilly, also depends on ground layout and input circuit. These could both be fine balanced but differ when connected to SE..

Like wwenze said, it could also be a lack of proper grounding (3 prong mains cable in 2 prong socket).
Maybe even wrapping some alufoil around the cable and connecting that to the chassis/ground at the speaker side could even help.
 

AnalogSteph

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#9
The interesting part here isn't so much that the Rokits are having issues - ground loop problems are 100% expected when using an unbalanced connection between two IEC Class I (i.e. earthed) devices. BTDT. (Get e.g. a Behringer HD400 and - if necessary - another pair of TRS --> XLR cables, and peace and quiet should return.)

Rather, it is is that the M-Audios don't. This makes me suspect that possibly they may be floating their entire audio circuitry, with input jacks being insulated from the back panel, and only the back panel itself being connected to mains protective earth (PE).

I have seen some indications for even the G3 Rokits being afflicted with the old Pin 1 Problem, but you said things were quiet with balanced connections on both, so that's not it. (The Pin 1 Problem shows up as ground loop issues occurring on IEC Class I devices in spite of balanced connections being used. It can be eliminated by following AES48-2005 guidelines for construction, though that's obviously more easily said than done at times.)

Do you have a multimeter, even a basic one? (Nobody should be forced to go broke from owning one of these. If you have access to one but don't know how to use it, look up a tutorial on their basics.) If so, please conduct a continuity (/ resistance) test between
  • IEC power connector earth (middle) pin and
  • TRS input sleeve (to be accessed via naked plug or cables - check cable connections via continuity test)
  • and possibly the back panel or a back panel screw
on each speaker type. That should tell us whether my theory is holding water in any way.
 
OP
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Thread Starter #10
The interesting part here isn't so much that the Rokits are having issues - ground loop problems are 100% expected when using an unbalanced connection between two IEC Class I (i.e. earthed) devices. BTDT. (Get e.g. a Behringer HD400 and - if necessary - another pair of TRS --> XLR cables, and peace and quiet should return.)

Rather, it is is that the M-Audios don't. This makes me suspect that possibly they may be floating their entire audio circuitry, with input jacks being insulated from the back panel, and only the back panel itself being connected to mains protective earth (PE).

I have seen some indications for even the G3 Rokits being afflicted with the old Pin 1 Problem, but you said things were quiet with balanced connections on both, so that's not it. (The Pin 1 Problem shows up as ground loop issues occurring on IEC Class I devices in spite of balanced connections being used. It can be eliminated by following AES48-2005 guidelines for construction, though that's obviously more easily said than done at times.)

Do you have a multimeter, even a basic one? (Nobody should be forced to go broke from owning one of these. If you have access to one but don't know how to use it, look up a tutorial on their basics.) If so, please conduct a continuity (/ resistance) test between
  • IEC power connector earth (middle) pin and
  • TRS input sleeve (to be accessed via naked plug or cables - check cable connections via continuity test)
  • and possibly the back panel or a back panel screw
on each speaker type. That should tell us whether my theory is holding water in any way.
I'll see if I can grab a multimeter to do these tests.
Meanwhile, let me ask you something: if it was a ground loop issue, shouldn't I be hearing Humms instead of buzz and/or radio signals? Also, if it was ground loop, why does it alleviate (almost goes entirely away) when I lift the RCA cable about 3ft higher?
Flavio
 

solderdude

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#11
Also, if it was ground loop, why does it alleviate (almost goes entirely away) when I lift the RCA cable about 3ft higher?
Because the capacitance to ground changes and thus radio 'reception' is tuned differently as the cable acts as a 'tuned' antenna.
Also handling it changes capacitance.
HF groundloops do not need to travel via mains (hum and some other nasties often do) but can also be directly to ground as it acts as a capacitor.

Groundloops can be low frequency (audible) and couple through interlinks, mains or via other devices
High frequency (capacitance coupled) groundloops can also take capacitive paths which are more difficult to spot.
 
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