Mic capsule picking up HF noise from cables - Page 1
Forum section: electronics / projects

author: gnif
author_rank: Unknown
timestamp: August 01, 2017, 01:57:05 am
content: I have been building (per my other thread) some balanced audio stuff just for learning and fun. I have acheap microphoneI am using that was from a party years ago.... so just what I have on hand.The pre-amp I built works a treat, silent, noise rejection is excellent... but when I have the mic attached, if I move it near any source of RF it induces noise into the circuit. The source is the HDMI/DVI cables to my monitors on the PC, strongest is at exactly 1KHz and all the multiples up the spectrum are picked up.Is this just simply due to the quality and lack of RF shielding in the microphone? The mic's enclosure is connected to cable shield, and in the XLR socket it has shield connected to signal ground, which from what I understand is correct.I am certain the noise is coming from the mic capsule, if I turn the switch off on the mic which shorts the balanced pair the noise is completely gone.

author: RJFreeman
author_rank: Regular Contributor
timestamp: August 01, 2017, 02:13:45 am
content: check continuity between pin 1 on the XLR and the metal case of the microphone.a Dynamic microphone capsule will often be wired between pins 2 and 3 of the XLR connector, while pin 1 is used for shielding.Also I have not browsed your other thread yet, but you will need to ensure your Mic Pre-amp is well grounded as a general rule.

author: RJFreeman
author_rank: Regular Contributor
timestamp: August 01, 2017, 02:17:21 am
content: As you are a prolific poster (and Admin) I cannot immediately find your other thread....

author: gnif
author_rank: Unknown
timestamp: August 01, 2017, 02:22:02 am
content: Quote from: RJFreeman on August 01, 2017, 02:13:45 amI have already confirmed pin 1 and chassis is connected on the mic. As for grounding, to isolate from ground loop issues the circuit has been designed as follows.1) Earth ground connected to transformer, and isolated from the chassis (metal)2) Mic shield and ground connected to preamp chassis at point of entry.3) Chassis connected to signal groundBefore performing the above steps the noise in the circuit was terrible, after the above changes there is no noise unless I move the mic near an noise source. If I disconnect the mic and move the cable in proximity to the noise source there is nothing.

author: gnif
author_rank: Unknown
timestamp: August 01, 2017, 02:24:37 am
content: Quote from: RJFreeman on August 01, 2017, 02:17:21 amSorry, the thread is here:https://www.eevblog.com/forum/projects/audio-grounding-xlr/

author: RJFreeman
author_rank: Regular Contributor
timestamp: August 01, 2017, 02:40:34 am
content: Ah yes, an Op-amp input will tend to be rather noisy as a Mic pre-amp, and I am not sure why they went for 17V phantom power, but that aside, try a big fat (by non-electrolytic standards) maybe 1uF or so capacitor between mains ground and chassis.Bear in mind CMRR for an active circuit drops as frequency increases, although fortunately Shielding does become more effective as frequency increases (although the converse is true, that shielding becomes less effective as frequency decreases). So you want to provide a good shield for RF.Although the rub here, is a good balanced system should be impervious to hum, but I gather you are using this to connect to a PC (you haven't said what sort though, laptop? desktop?) and they are not renowned for being Audio friendly....

author: gnif
author_rank: Unknown
timestamp: August 01, 2017, 02:43:30 am
content: Quote from: RJFreeman on August 01, 2017, 02:40:34 amEventually the idea is to connect to a PC (desktop), however this noise is induced when feeding the amplifier directly, the PC is out of the loop. I have omitted the 17V phantom power, I didn't believe this was a requirement of the mic I am using.

author: RJFreeman
author_rank: Regular Contributor
timestamp: August 01, 2017, 03:07:04 am
content: Quote from: gnif on August 01, 2017, 02:22:02 amhmmm, pondering the above, the cable doesn't pick up noise, so I am wondering if the mic is unbalanced? or the capsule is inadequately shielded?confirm there is no continuity between either pin 2 and Mic case or pin 3 and Mic case (that would be a problem) although I am still thinking you want an RF path to ground. you may also want small capacitors between pin 2 and ground and pin 3 and ground.A Desktop should have decent grounding, and the metal case usually provides adequate shielding, and you are correct, for a dynamic mic you will not need phantom power

author: Habropoda
author_rank: Regular Contributor
timestamp: August 01, 2017, 04:58:41 am
content: I'm not an expert but you could try adding feedback capacitors in parallel with R8 and POT1 to roll off the frequency response of the op amps above the audio range. Otherwise the op amps will amplify higher frequencies, too, so maybe that is screwing with it.You would need to simulate or calculate but maybe in the range of 22pF to 100pF might do for a test. Ordinary ceramics probably won't work well.

author: gnif
author_rank: Unknown
timestamp: August 01, 2017, 08:59:20 am
content: Upon further investigation the noise is coming from one of my cheaper monitors, switching it off stops the noise. It has a 12V external PSU which isn't flooding the area with anything but bring the mic close to the screen and it becomes extremely loud. I think that this thing is spewing out a ton of noise as I am seeing it elsewhere too now I am looking for it (grounded scope probe). It must be the back light driver, any suggestions on how to prevent this?Edit: Setting the monitor to full brightness stops the noise, it is certainly coming from the back light driver's PWM circuit. Seems a tear down is in order to see if I can shield this thing.

author: RJFreeman
author_rank: Regular Contributor
timestamp: August 02, 2017, 02:55:00 am
content: Ah, ok so it sounds like the monitor is probably injecting noise current into the ground return on your HDMI/DVI cables, this would explain why your ground loop noise ( in the other thread) was described as a 1khz noise, rather than the usual 50Hz hum or Buzz.