Audio Grounding, XLR - Page 1
Forum section: electronics / projects

author: gnif
author_rank: Unknown
timestamp: July 27, 2017, 10:37:34 pm
content: Hi All,I have been building an balanced mic pre-amp for my PC and I have come across an issue I can't resolve without isolating my circuit from earth ground (obviously not an acceptable option). I would assume the noise I get is a ground loop issue except the frequency is way too high, I am heading squelching and noise, peeking exactly at 1KHz. With some experimentation I have found that this noise is being radiated by the video card and DVI cables, if I place an antenna near any of the cables the noise is horrendous. What I do not understand is how disconnecting the earth ground removes this fault.I have been informed that using some audio isolation transformers will assist here, but I would like to avoid that if at all possible. I would also like to avoid switching to an external sound card/dac.The circuit I am building is heavily based on this schematic:http://apuckt.blogspot.com.au/2011/08/balanced-microphone-preamp.htmlThe main differences are, +-15V instead of 12V and I have skipped the phantom power segment as I do not require it, everything before R6 and R7 is omitted.

author: The Soulman
author_rank: Super Contributor
timestamp: July 27, 2017, 10:57:25 pm
content: Little bit more info is needed here.What microphone do you use, what cable (must be shielded) what housing does your pre-amp have, where is its power coming from, how do you connect to your computer?

author: gnif
author_rank: Unknown
timestamp: July 27, 2017, 11:03:48 pm
content: Quote from: The Soulman on July 27, 2017, 10:57:25 pmThe mic is some Jaycar cheap thing at the moment, it simply has a transducer in it, completely passive device.The pre-amp is in a full metal enclosure, grounded.Mic cable is XLR, shielded, etc.Cable shield is connected to the enclosure.Cable ground is connected to the 0V rail.All grounding is wired using a star configuration.The connection to the PC is RCA -> Stereo Jack, shielded of course.Also of note... when disconnected from the PC there is no noise at all, clearly the PC is the source.

author: The Soulman
author_rank: Super Contributor
timestamp: July 27, 2017, 11:28:42 pm
content: Quote from: gnif on July 27, 2017, 11:03:48 pmYou have a cable shield and a cable ground?The cable shield should go thru the xlr pin 1 to star ground.I've seen some ("onehunglow") cable mfgs connect pin 1 inside the xlr to the xlr housing, this is wrong and defeats the star ground.Do you have a similar power supply as in the schematic (well 7815 instead of 7812)?Is the current consumption below 10 mA per rail? (anything over could indicate oscillation).Don't omit R4 and R5.Add a ac coupling cap. (47uF or so) to the output, better save than sorry.

author: Someone
author_rank: Super Contributor
timestamp: July 27, 2017, 11:30:03 pm
content: Ignore above as its probably not the XLR or microphone that is at issue here as they sit on their own potential from the preamp. I'm guessing you actually have a problem connecting the signal from the preamp to the computer, confirms this by putting a dummy in place of the microphone input and checking that the noise still appears on the computer input. There are no quick or easy fixes as its a ground loop between the PC and the earthed power supply of the preamp.

author: HackedFridgeMagnet
author_rank: Super Contributor
timestamp: July 27, 2017, 11:32:03 pm
content: The noise is induced in a ground loop. and by breaking that loop you kill the noise.But you knew that, so my point is when you have a ground loop you don't have a star ground.Another important point is the "Protective Earth" and the "Gnd" in your circuit aren't necessarily connected and are not normally connected, because the moment you connect two devices you lose your star ground.To keep your equipment safe and legal (In AU but many other places too) you must make it as one of two Electrical classes.Single InsulatedAny exposed metal must be attached to protective earth. In which case your RCA sockets need to be isolated from the chassis, and double isolated from mains.orDouble insulated.Active and neutral must be double isolated from all exposed metal. In which case your metal chassis can directly mount the rca plugs.The proper definitions are somewhere if you want to look them up.I guess you should go double insulated into the transformer and don't ground the chassis. Properly rated Heatshrink is allowed as 1 layer of mains insulation.Hope this helps.

author: HackedFridgeMagnet
author_rank: Super Contributor
timestamp: July 27, 2017, 11:37:33 pm
content: One other thing, audio transformers aren't necessarily bad, I personally think they sound fine, much better than hum.But I learnt the hard way don't use a mic level transformer at line level. It will saturate, you would need a line level transformer.

author: The Soulman
author_rank: Super Contributor
timestamp: July 27, 2017, 11:42:32 pm
content: Quote from: gnif on July 27, 2017, 11:03:48 pmMeasured with a oscilloscope?Also get some capacitance on the output as a rf filter, some ceramics will do, don't go over a couple of nano Farads.

author: gnif
author_rank: Unknown
timestamp: July 27, 2017, 11:51:24 pm
content: Quote from: The Soulman on July 27, 2017, 11:42:32 pmYes, measured with a scope.Noise exists even without a mic attached, it is not the source.I can reproduce the noise simply by connecting audio ground to the PC chassis.Quote from: HackedFridgeMagnet on July 27, 2017, 11:37:33 pmThanks, the reason I am avoiding them is because simply I don't have any... this circuit was built just "because"... out of what I have on hand, I don't really want to invest any money into it.I think I will go the route of double insulation... Thanks for confirming that this is still a ground loop issue, I wanted to be sure before I went down the wrong rabbit hole

author: The Soulman
author_rank: Super Contributor
timestamp: July 27, 2017, 11:58:19 pm
content: Quote from: Someone on July 27, 2017, 11:30:03 pmOver here it's not common (pun intended) to connect the "center tab" of the output of such a psu directly (inside the psu)to the mains ground, if it is than yes that is likely to be the cause of the noise.And you're correct the microphone and its xlr cable can't cause ground-loops on it own, but when the pin-out is wrong it could easily become a antenna and without band-with limiting (as per schematic) in the pre-amp, all sorts of crap can result.

author: The Soulman
author_rank: Super Contributor
timestamp: July 28, 2017, 12:03:47 am
content: Quote from: gnif on July 27, 2017, 11:51:24 pmStill don't now what you are powering it now from.

author: HackedFridgeMagnet
author_rank: Super Contributor
timestamp: July 28, 2017, 12:12:21 am
content: One other thing if you are double insulated and your chassis is attached to circuit ground, then you will need rubber feet as any metal object or equipment that comes into contact with your chassis can cause a ground loop.

author: gnif
author_rank: Unknown
timestamp: July 28, 2017, 12:58:59 am
content: Quote from: The Soulman on July 28, 2017, 12:03:47 amThe circuit is in the schematic... lower 1/2

author: gnif
author_rank: Unknown
timestamp: July 28, 2017, 10:21:36 am
content: I have made all the suggested changes, the noise from the PC is gone from the signal!Now I have another issue... as I raise the gain a 50Hz signal appears on the output, even with the input disconnected and shorted.I would assume power supply but it seems clean, I can't for the life of me figure out where this is coming from. Again I call on the communities wisdomI am not sure either if that is an acceptable noise level for this kind of a device.If I probe before R10 it doesn't show on the scope, either the signal is too low to see it, or it's not there. After R10 it is barely perceivable.

author: capt bullshot
author_rank: Super Contributor
timestamp: July 28, 2017, 11:25:38 am
content: OK, you'll have to avoid safety ground in the preamp to kill the noise.Do not connect the case and / or GND of the amplifier to protective ground.Otherwise you get a ground loop through the audio connecting cable between the amp and the PC. Part the noise from the PC diverts through the cable shield and protective ground. Since there's no ideal conductor in the world, this causes a voltage drop (noise) over the shield which in turn results in the noise that you experience.Since you don't want to isolate this path, your only option is to isolate the amplifier circuit from protective ground.This is safety-wise not a big deal, a metal enclosure doesn't have to be protective grounded if you take care of the line voltage isolation.

author: gnif
author_rank: Unknown
timestamp: July 28, 2017, 11:38:44 am
content: That was with nothing connected, no PC, input shorted, no output... just on the bench on the scope. I have already isolated earth ground from the chassis, and signal ground.

author: capt bullshot
author_rank: Super Contributor
timestamp: July 28, 2017, 11:52:25 am
content: Missed your last post stating you've done that before what I saidYour signal looks like 100Hz rectified line hum.Most common: ripple on the power supply, you ruled that out. BTW did you check the +17V for ripple?Other cause: bad GND wiring, coupling C4 / C5 charge current from xformer into the audio path. Sounds plausible, since you don't see it at the output of the first amp stage but on the second stage and this OPAMPs + input is tied to GND.So check your layout / wiring. It is quite important that all amplifier related GND comes from the GND junction of C4 / C5 and the connection from the xformer goes there, but no common piece of conductor for both paths exists.

author: gnif
author_rank: Unknown
timestamp: July 28, 2017, 03:02:33 pm
content: I figured it out! It was a power issue in the endI am not sure why the scope wouldn't show it up though, I couldn't see any drops in voltages on either of the rails, but after swapping out the two main filter caps for fresh ones the noise was gone. It was the +15V rail losing regulation, so the opamp was become biased to -15V