Noise on mic. output -  real or artifact? - Page 1
Forum section: electronics / beginners

author: aneevuser
author_rank: Frequent Contributor
timestamp: March 05, 2020, 03:23:20 pm
content: I have Behringer XM8500 mic. which has an XLR output. It needs no phantom power. The mic. is connected via a 1 m cable with the XLR connector at one end and a 3.5 mm jack at the other.I'm looking at the jack end hot and cold signals relative to ground, and their difference, on a DS1054Z. With only background audio noise, they look like so:Channel A and B are hot and cold (or vice versa) and the math channel is A-B. Math seems to be showing about 20 mV pp differential mode noise.When I hum loudly (well above the level of normal speech) about 1 cm from the mic, the signals look like so:I now seem to have about 40 mV pp signal on the math channel + the previous noise. If I speak at a normal volume at, say, 20 cm from the mic. I cannot visually detect a signal above the noise on the math channel.So the question is: does this look like an artifact of the A-B calculation on the scope, or is it more likely to be the realistic output of the mic? I currently have no preamp for this mic. so can't test it otherwise.I should point out that this mic. worked fine in the past, though it was last used about 4 years ago - it's been untouched in its packaging since then, in dry conditions.

author: magic
author_rank: Super Contributor
timestamp: March 05, 2020, 06:14:46 pm
content: Disconnect the mic, short the XLR and see what happens. Or short it with a resistor of equal value as the mic's impedance.This looks like some electric switching noise BTW.Or maybe it's a math artifact. IIRC math mode can be quite noisy. Try it with the probes directly shorted.If you have the probes in 10x mode, try at 1x.

author: rstofer
author_rank: Super Contributor
timestamp: March 05, 2020, 06:24:20 pm
content: The slower waveform is 50 Hz noise since it looks like 10 divisions per cycle at 2 ms/division.I think you said the microphone needed no power so why are you taking a differential measurement? Pick one side of the output as ground and the other as signal. Maybe I misunderstood what I read about no phantom power.Try putting some kind of load across the microphone output. Perhaps 1k Ohms or 10k Ohms. Something much less than the scope input impedance (1M at x1 and 10M at x10).Long wires make excellent antennas.

author: aneevuser
author_rank: Frequent Contributor
timestamp: March 05, 2020, 07:49:07 pm
content: Quote from: magic on March 05, 2020, 06:14:46 pmYou mean short it at the mic end, I guess? Or am I being dim?QuoteYes, but there seems to be lots of it, given the differential signal, to my mind.QuoteWhat difference would that make?

author: aneevuser
author_rank: Frequent Contributor
timestamp: March 05, 2020, 07:53:29 pm
content: Quote from: rstofer on March 05, 2020, 06:24:20 pmI don't follow - won't I just lose the benefit of the differential signal, and see all the common mode noise?QuoteCan you explain what that will do? Not sure I see what you're getting at here.

author: Brumby
author_rank: Unknown
timestamp: March 05, 2020, 08:56:05 pm
content: Quote from: rstofer on March 05, 2020, 06:24:20 pmDifferential signal transmission hasnothingto do with phantom power,per se. The benefits of using such a technique exist because of the circuit topology. Phantom power, however, needs it and simply makes use of this topology to insert some DC, without affecting the signal path.QuoteA cheap method of interfacing, which may not work very well depending on the connections within the microphone. If the microphone capsule is wired only to the hot and cold (as the balanced design expects), don't expect much of anything in the way of a useful signal wiring it this way.Sorry ... I just read your statement again and realised I got it ass-about. What you say is an absolutely functional method. Not the ideal balanced approach, but quite useful in everyday situations.QuoteThere are shortcuts used in some microphone arrangements that could lead to confusion. For many situations, the results are "acceptable".

author: StillTrying
author_rank: Super Contributor
timestamp: March 05, 2020, 09:40:40 pm
content: Google says it's just a 150R dynamic, so just connect the cold and hot to one scope probe. The 50Hz will be because both of the scope's inputs are just floating.If you have the probes in 10x mode, try at 1x.Yep.

author: aneevuser
author_rank: Frequent Contributor
timestamp: March 06, 2020, 09:18:54 am
content: Quote from: rstofer on March 05, 2020, 06:24:20 pmOK having slept my mind is now clearer - I misread what you wrote last night - I thought you were asking me to measure between hot and ground - I'll give your suggestion a go later todayQuoteI also now understand the point of measuring at x1 rather than x10 (signal attenuation vs unattenuated noise) and I'll try that later also.However, I'm not clear about the "load across mic" idea:1) Where should the load go, given I've got differential inputs - hot to cold, hot and cold to ground, or what?2) It's still not clear to me what this will do - regardless of the load, the noise induced on the cable won't change, so I should still see the same signal at the jack end, no? - what am I missing here?

author: aneevuser
author_rank: Frequent Contributor
timestamp: March 06, 2020, 09:29:34 am
content: Quote from: StillTrying on March 05, 2020, 09:40:40 pmOK, maybe I'm misunderstanding something fundamental here - doesn't the dynamic mic drive the XLR output via a transformer? If so, isn't there a dead DC short at the mic. end, as far as the scope probe is concerned?[Edit: I've tested the mic. and hot, cold, and ground seem indeed to be open connections regardless of which combo I try - OK, I'm now pretty confused - I imagined that the the XLR signal was generated from a centre trapped transformer with the centre going to the ground pin, but this doesn't seem to be the case - can anyone tell me WTF is going on inside this mic? How are they generating the differential o/p?]

author: Zero999
author_rank: Super Contributor
timestamp: March 06, 2020, 10:51:57 am
content: Quote from: aneevuser on March 06, 2020, 09:29:34 amAll voltage signals are differential. I doubt the mic contains a transformer. Either side of the transducer will be connected to a shielded twisted pair, with the ground isolated from the signals.Regarding the original question: there does appear to be some aliasing/quantisation noise, which will be from the oscilloscope. Modern DSOs don't have differential inputs, but perform subtract one channel from the other, after digitisation, which can create artifacts. Have you tried connecting the mic to a suitable transformer and looking at the secondary voltage on the oscilloscope? The screen should also be connected to the oscilloscope's ground connection, along with the any transformer's shielding.

author: aneevuser
author_rank: Frequent Contributor
timestamp: March 06, 2020, 02:25:39 pm
content: [Quote from: StillTrying on March 05, 2020, 09:40:40 pmI'm not sure what you're trying to say there unless it's the observation that we measure voltage differences between two points - just to be absolutely clear, by "differential" in the context of XLR, I'm referring to this:https://en.wikipedia.org/wiki/Differential_signalingQuoteSadly I don't have a suitable transformer, so that's out.

author: StillTrying
author_rank: Super Contributor
timestamp: March 06, 2020, 02:35:58 pm
content: Quote from: aneevuser on March 06, 2020, 09:29:34 amSo am I, I don't know why you don't see 150R and many mV of real signal between cold and hot.Wiring fault ?New ?Old ?Been modified in some way ?https://www.manualslib.com/manual/592564/Behringer-Ultravoice-Xm8500.htmlhttp://www.aviom.com/blog/balanced-vs-unbalanced

author: rstofer
author_rank: Super Contributor
timestamp: March 06, 2020, 03:20:31 pm
content: Quote from: aneevuser on March 06, 2020, 09:18:54 amMake it simple, put the resistor across the probe ground and probe tip and then probe the circuit.There's a problem measuring high impedance circuits, they are easily influenced by outside interference. Adding a bit of load will sometimes damp this out. That 50 Hz hum is coming from a high impedance source (through the air) and if you load it down at all (with the resistor), the voltage divider (and that's what it is with the added resistor) will reduce the noise.Even at X1, a scope's input impedance is still 1M Ohm and that's very high.On the 3 pin 3.5mm connector, the contacts are named "Tip", "Ring" and "Sleeve". I would expect the signal to be on "Tip" and "Ring" while I would expect the shield to be connected to "Sleeve" and earth grounded at the amplifier end. Assuming a mains powered amplifier, the exposed metallic surfaces would have to be earth grounded for safety (and that's why the BNC connectors on your scope are earth grounded) and, by convention, this is carried on the "Sleeve".About 80% of the way down this page, there is a connections tablehttps://en.wikipedia.org/wiki/Phone_connector_(audio)#Balanced_audio

author: Cerebus
author_rank: Super Contributor
timestamp: March 06, 2020, 03:44:32 pm
content: Quote from: rstofer on March 06, 2020, 03:20:31 pmExcept this isn't a high impedance circuit. This is a dynamic microphone with a150 ohmsource impedance. That's the pukka impedance of the voice coil in the microphone, so there's no way that this isn't physical. The only high impedance node is the 'scope input and that's being bridged by the mic's voice coil. It's quite possible there's an output transformer on the mic, they're pretty much universal on all but the cheapest mics, but that doesn't alter the argument with respect to impedance at the mic's output.What's the bandwidth of that scope and has the OP thought to engage the scope's bandwidth limit?This is an audio device, so 20MHz scope bandwidth is no problem; whereas leaving it at 100/200/350MHz or whatever you're going to see a lot of broadband noise that has nothing to do with practical noise from the source.Is the OP actually grounding the shield from the mic?If there is an output transformer then there is no guarantee that cold is connected to the shield, and thus all the shielding on the mic or cable is going to waste.

author: rstofer
author_rank: Super Contributor
timestamp: March 06, 2020, 06:01:52 pm
content: I agree, the mic impedance should be very low but that doesn't appear to be the case. It would be extremely difficult for 50 Hz noise to affect such a low impedance. Yet, here we are, a display clearly showing the 50 Hz noise to be somewhat high. As I recall, the trace represented about 20 mV - maybe that's not high, just a low scale.There's something wrong: Either the mic is defective or the measurement technique is faulty. A simple check with a DMM on Ohms scale should be able to read the resistance, particularly if there is a transformer, and there should be some low value. I would bet the measurement should be taken between the "tip" and "ring" of the 3.5mm connector. I would think that the shield is connected to any metallic surfaces of the microphone itself.Maybe try the resistance test at the XLR connector. It is not unheard of for cables to be defective.The big 50 Hz voltage display won't be affected by the 20 MHz bandwidth limit but it wouldn't hurt to give it a try.

author: magic
author_rank: Super Contributor
timestamp: March 06, 2020, 06:24:55 pm
content: Quote from: rstofer on March 06, 2020, 06:01:52 pmThe 50Hz noise affects a 5MΩ impedance. Common mode of the mic is connected between two Hi-Z probes. Connect a meter of cable to the tip of a scope probe (or just touch it with your finger) and guess what will happenSeriously,There is nothing to debate here, only to test. If the OP is curious, disconnect parts of the signal chain one by one and see when the noise goes away. Nothing is wrong with the mic at any rate. And Cerebus is right, you haven't even grounded its shield (I presume).

author: drknob
author_rank: Contributor
timestamp: March 06, 2020, 07:36:19 pm
content: For best results, pin 1 of the xlr connector should be connected to o-scope ground. Typical mic preamp input impedances are between 1k2 and 2k5 Ohms. Connect something similar across pins 2 & 3 of the xlr to properly load the mic output transformer. Although you can measure differentially, it's not necessary. Ground pin 3 and measure signal on pin 2. That is all.

author: aneevuser
author_rank: Frequent Contributor
timestamp: March 07, 2020, 11:23:32 am
content: Quote from: magic on March 06, 2020, 06:24:55 pmThanks for all the replies - I'll look at this in detail when I have time - however, the traces I put up were measured off the jack end of the XLR cable, with both probe grounds connected to sleeve, and channel 1 to tip, and channel 2 to ring - that should ground the shield, no?

author: aneevuser
author_rank: Frequent Contributor
timestamp: March 07, 2020, 11:31:05 am
content: Quote from: StillTrying on March 06, 2020, 02:35:58 pmOK, yes, there was a wiring fault - but it was in the crappy multimeter clip that I was using. Measured it just now with other probes and I get 232R hot to cold.

author: aneevuser
author_rank: Frequent Contributor
timestamp: March 07, 2020, 12:07:36 pm
content: OK folks, I think I now understand why I've effed up - I'm measuring hot to ground, cold to ground - both of those are open at the mic. end so I've essentially got floating probes, haven't I?

author: Brumby
author_rank: Unknown
timestamp: March 07, 2020, 12:57:26 pm
content: Yep. That'll do it.The hot and the cold are connected across the microphone element - and in the case of a dynamic microphone (where no phantom power is involved) the ground/shield has NO connection to either in any way, shape or form. So, yes, you had a floating input for your scope measurements in both cases.Guaranteed to pick up environmental hum and EMI.

author: aneevuser
author_rank: Frequent Contributor
timestamp: March 07, 2020, 01:36:12 pm
content: Quote from: Brumby on March 07, 2020, 12:57:26 pmYep, I now understand everything a lot better - however, if my original assumption re: the centre tapped transformer at mic. end had been correct, then my setup would have been fine - is that right?Anyway, this has been a right comedy of errors - I had earlier been measuring the mic impedance with a broken multimeter clip, I have just discovered that the XLR cable has an intermittent fault at the mic. end (loses signal completely) and to top it all off, there is 100% genuine 10 mV pp switching noise on the XLR cable that it picks up from the monitor that I'm using - switch that off and it's clean as a whistle. So even if I sort out a decent preamp for this mic. I probably can't use it with the monitor switched on. This probably deserves my first

author: Cerebus
author_rank: Super Contributor
timestamp: March 07, 2020, 02:01:21 pm
content: Quote from: aneevuser on March 07, 2020, 01:36:12 pmNot really. Were the microphone transformer centre tapped that wouldn't be connected to ground. Phantom power, when it is used, shows up across the xformer centre tap and ground (or the shield if you prefer, which should be grounded to stop electrocuting performers - even if there are some you'd gladly electrocute). Even though a dynamic mic has no use for phantom power you don't want it shorting the phantom supply if someone has forgotten to turn it off for that input.QuoteNope. That's the whole point of having (1) shielding which will kill some of the interference, and (2) differential signalling which will kill much of the rest of the interference if it's done properly. Moreover, much of that switching noise will be ultrasonic and there ought to be filtering in the mic preamp that starts rolling off the response above the audio band, so that will also kill the interference further. A well designed and properly executed mic + cable + preamp combination will deliver a clean signal in a surprising electrically noisy environment.

author: aneevuser
author_rank: Frequent Contributor
timestamp: March 07, 2020, 02:54:45 pm
content: Quote from: Cerebus on March 07, 2020, 02:01:21 pmOK. But in my fevered imagination (and I'm not quite sure why I had this idea) the arrangement was: centre tap to ground (and therefore to the jack sleeve), hot at top of transformer, cold at bottom - then when I measure hot to ground, cold to ground neither scope would have been floating AFAICS, and I could (in my imaginary setup) make a differential measurement just fine. Am I confused?Quote1. OK, I'm not sure why I'm seeing so much switching noise. I'm now grounding the sleeve and measuring with one probe across hot to cold - that should give my any benefit of the differential signal, no? i.e. it should eliminate pretty much all the switching noise? I'm seeing about 10 mV pp noise or more and the mic. signal only seems to be about 5 mV pp for speech - I seem to have more switching noise than signal. Does that sound right? I'll put up a screenshot later.2. Good point, well made re: ultrasonic switching noise and possibility of filtering. However, isn't switching noise typically around 50-60kHz? Then you'd need fairly steep roll-off to attenuate that, given that it's only 2.5 octaves above the 20 kHz preamp bandwidth, and given the noise level that I'm seeing.

author: rstofer
author_rank: Super Contributor
timestamp: March 07, 2020, 03:56:38 pm
content: Are you still testing without putting a load across tip and ring? It has been suggested that you put a 1k to 2.2k resistor across the signal pins. Maybe loading up those 'antenna' leads will reduce the noise.The sleeve should be tied to earth ground, the scope ground lead will be good enough. Then measure across tip and ring by putting the scope probe ground on either contact and the probe tip on the other contact (I believe you are doing this).Dynamic mics are considered 'low output' microphones. That is, their output voltage is around 2.5 mV according to this article:https://www.sweetwater.com/insync/understanding-signal-levels-audio-gear/That you read 5 mV suggests the very upper end of output voltage.Short answer: Take care of the shielding, the microphone output is probably just fine and higher frequencies will need to be filtered out.

author: aneevuser
author_rank: Frequent Contributor
timestamp: March 08, 2020, 04:49:18 pm
content: Quote from: rstofer on March 07, 2020, 03:56:38 pmYes, I've got a 1K resistor across hot and cold.QuoteYes, I am. I've been fixing my decaying roof today so no time to do any more with this, but I'll put up a scope trace at some point showing the switching noise.QuoteThe 5 mV may have been with mouth close to mic and making lots of noise so possibly unrepresentative - I'll remeasure at some point. Yes, I now think that the mic. (but not XLR cable) is fine but I'm still concerned about the switching noise.

author: magic
author_rank: Super Contributor
timestamp: March 08, 2020, 06:51:37 pm
content: Short both probes together and connect to any random wire in the cable. If you still get this junk, it's likely an artifact of math mode.Short both probes together and connect to ground. If you still get junk but both signals are perfectly flat, it certainly is an artifact of math mode. If one channel shows this junk but the other is flat, you have your answer right there.