XLR audio problems; advice solicited

In My Humble Opinion
1

This is really a GQ kind of question but will involve more opinions about the cause and its fixes, so IMHO it goes.

Short form: I have a digital audio recorder that when used with the AC power supply and external XLR cables and mikes puts a pronounced 60Hz hum in the recording.

Longer form: I recently became the volunteer audio tech for a community performing group. They use a small digital recorder (desktop, large-book sized) to record their performances for later review. I take the recordings and turn them into MP3s and CDs for sponsor gifts and fund-raising sales.

The setup is the recorder (which I’d rather not identify by brand or model here), its AC power supply (separate brick with input and output cables; output is 15 VDC without a ground), two 15-foot XLR cables and two active cardioid XLR mikes. All the gear is good but not great, very low-end professional stuff.

I have tested every combination and permutation and it boils down to this: on the AC power supply, with exernal mikes, it hums. On batteries: no hum. On internal mikes: no hum. Plugged in and externals: mmmmmmmmmmmmm.

I have been notch-filtering the noise away and am prepared to add rechargeable batteries to the kit for no-AC recording, both of which are workarounds but not fixes.

I have been in touch with the manufacturer, who has been VERY helpful and responsive (which is why don’t want to say anything negative here about the brand or model). They sent me a new AC supply; hum. They sent me a tested-good replacement recorder; hum.

Today I did extensive A-B testing with the cables and mikes, tested the cable continuity and resistance, etc. Still all the same old hum job. However, I discovered something interesting, and I don’t have enough background with enough pro audio setups to judge whether it’s significant.

The Question: The unit has two “superjacks” that accept either XLR or 1/4 inch. I discovered that touching the XLR release button (metal, goes into the jack mechanism) produces a strong hum. Strong when no cable is inserted, about the same when a cable with no mike is inserted, a little bit attenuated when a mic is attached, whether it’s switched on or not.

Now, in my general experience, audio gear should:

[ol]
[li]Have a fully chassis-grounded AC supply. This 2-lead DC supply is not providing that chassis/rack common ground.[/li][li]Have a chassis grounding lug, if not #1. It doesn’t.[/li][li]Not pick up noise from the jack release button, even if the input is active and the jack is empty.[/li][/ol]
Am I off-base on any of these three assumptions? I’d like some input from more experience audio techs about XLR behavior in general before I get back to the company. I don’t have easy access to any other set of cables or mikes for a swap test, but the behavior with no cable plugged in concerns me.

Also, the end shells on these cables do not appear to be connected. Should XLR cables have a sheath ground between shells as well as the in-line common?
Barring some huge “whoa” from a knowledgable audioDoper, my next step is to find another power supply and/or rebuild this one with much stronger filtering components. (If my cases of electronics gear weren’t buried in front of a car I can’t get out because of the snow, I’d have done this by now.)
Thanks for any advice/opin on this.

2

What kind of mics are they? If they are condenser mics - is phantom power (+48VDC) engaged.

XLR pinout is 231 hot cold ground.

Stupid question, but have you checked the system plugged into a different power circuit entirely?

Have you checked continuity of chassis ground?

Feel free to PM with more specific questions or info if that’s easier.

picker

  • pro audio engineer/college level instructor of same.
3

picker, that is not a stupid question by any means.

AB, don’t assume the church electrical is wired properly. Sadly, there could be screw-ups everywhere “in house.” Check as much as you can before altering any equipment.

4

picker’s post is great and as a former professional live & recording audio engineer (now a lighting guy), the only thing I would add is that neaerly every problem I ever had that was similar was the result of poor grounding.

5

You have a ground loop, a quick and dirty way to eliminate this is to “lift” the ground wire between all devices with their own power supply.

6

On an xlr, short cold to ground to lift. And your ground doesn’t necessarily have to connect to the barrel sleeve, but it won’t hurt anything to try to connect them.

But I would try these three steps in order:

[ol]
[li]check rig on a different circuit[/li][li]check polarity and accuracy of pinout on terminal xlr end (where it connects to the mic)[/li][li]try a ground lift on the xlr[/li][/ol]

7

Thanks for all replies so far. I pretty much suspect it’s a grounding/ground loop problem, but I’m not sure how to fix it.

The recorder has no grounding lug or pin of any kind. The power supply is two-wire AC to the (lightweight; probably switching) power supply, then a coax lead to a coax DC plug. No ground anywhere.

I was overjoyed to find, on first look, that the schools (not church) were using an extension cord on which some dimwit had busted off the ground pin. Easy solution… except that five more minutes showed it had nothing to do with it. The power supply is ungrounded anyway, and the hum persists at three different venues and two different locations in my (modern-wiring) house.

I also tried (after consulting with the manufacturer’s tech) a short from the negative power barrel to the XLR sleeve. It had no effect. I can’t think of a simple way to ground to XLR pin 1 - suggestions?

To answer picker’s question specifically - They are cardioid mics with internal AA power. Switching phantom power in and out has made no difference.

As good as the company and tech rep have been, I am beginning to think this is a substandard design issue. The unit really should have better grounding design and a chassis-grounding lug, and it doesn’t.

I am going to try building a more heavily-filtered or stable power supply, and if that doesn’t work, I will just have to modify the use sheet to indicate battery power should be used (and making sure there is a case of recharged NiMHs before each performance).

Any further thoughts appreciated.

8

Common problem. Easy fix. Buy a line conditioner.

Sweetwater sells music gear and this nifty $80 line conditioner.
http://www.sweetwater.com/store/detail/HumX

I bought one and it removed the hum from my guitar amp.

you can easily spend hundreds on a line conditioner that allows many items to plug in. (Sweetwater sells the expensive ones too) But this little guy is great for just one amp or recorder.

9

I’d try a couple more things. Do you have a dynamic mic available? Dynamic mics such as the shure sm57 or 58 require no external power to operate. I’d be surprised to find a church with a pa to not have a couple of those lying around.

If the dynamic works then we can eliminate all phantom power issues off the bat.

Cardioid refers to the polar pattern of the mic, not the type. But the fact that it has batteries means it’s almost definitely a condenser.

To lift the ground easily: you can either make an in-line ground lifter with a male to female xlr barrel connector or actually mod the cable. Both are simple.

To mod the cable pop the rubber plug from back of the connector and remove the single screw midway down the barrel. The connector pin assembly will easily pull out the barrel to the length of the available leads. You can slide the barrel back off as well for more space to work. Then solder a jumper from pin 3 and pin 1 on the back side of the pin assembly.

It’s exactly the same for the in line barrel connector but you can put the jumper at either the male or female end. You can use a barrel with a switch in it for a more elegant solution.

Hope that helps.

10

Have you tried reversing the “polarity” of the AC mains connection? I’ve seen some power supplies internally connect the DC “ground” to the AC line’s grounded conductor through a high-ohms resistor. If everything’s plugged in and wired right, the DC ground has a reference to earth ground.

11

“line conditioners” are worthless. the problem Amateur Barbarian is experiencing is most certainly a ground loop.

I would agree. especially if the hum is only when using external power. sounds to me like the external power supply is an incredible piece of junk.