Help on C12 Build...

Hello to the forum! I'd like to inquire about a hum issue I've been having with a mic built by Oliver Archut, which unfortunately I can no longer ask him about.

I have used the mic successfully, with no hum issues, at many places - all of which are places which have a U.S three prong power system, with third prong ground. It sounds amazing, no hum or interference at all. The mic is a "C12CG", with a brass CK12. It uses the same kind of 7 prong connector that he typically did, as opposed to the one that was on a vintage C12.

However…

When I use the mic at my home, there is what is either a ground or RF issue. My home is an older place, that was wired with a two prong hot/neutral system.

The symptom is a hum around 50~60 hz. When I touch the body, the hum will go away. If I get my hand very close to the grill, the hum gets worse…but if I touch the grill, it goes away. If I touch the grill or body of the mic to the XLR coming from the power supply, it seems as though the "loop" closes, with the hum going away.

I have other tube mics with similar design power supplies - UM17's, which don't exhibit any kind of similar ground hum with my power that I can tell. The attachments and wiring in the C12 power supply appear to be the same as the UM17's.

I'm going to attempt to just put in a grounded receptacle at my home, but I can't help but wonder why the setup with this mic won't work with my two prong system… and that's why I'm telling my story here! I'm wondering if anyone has any insight into how the AMI power supplies are typically setup and/or why I may be encountering this issue with the mic.

Hi George,

i read about similar problems with the CS-1 or CS-4  working flawlessly in the US vs humming in the EU.
The thread is still there::
http://lucasmicrophone.lefora.com/topic/4214675/Hum-Noise-and-other-repair?page=1#.VPZgxeH9kqM

hope that helps,

Max

Open the PSU and check if PIN 1 of the 3-pin XLR is connected to anything.

Even better, post a picture of the inside of the PSU with connections showing.

Thanks to both poster for the thoughts…

Yes, pin 1 has a kind of bridge thing going on…this is the same as the power supplies for my UM17 mics basically.





When I originally got it, the green ground wire that attaches by the transformer in the power supply had come loose. The washer/nut on the backside of it had come loose in transit. I talked to Oliver and he told me to go ahead and reattach it.  It is snugly screwed down.

Is the mic power supply plugged into the same outlet as the mic preamp and monitor system you're listening with? Please describe the test conditions exactly.

Good question.

So, the problem scenario is two prong power with three-ish items for the most simple test:

-macbook pro
-presonus 10x10 linked to macbook pro via firewire
-c12CG and C12 power supply

All items have three prong power chords. They go to a multi plug with a few three prong inputs (I've tested more than one). The ground lift happens at the end of the multi plug right before it goes into the wall.

In this scenario, the chain creates the hum, with that side effect of it stopping when touching the mic (or even the power supply). The macbook pro being plugged in or not makes no difference with the hum. I make this point because their power supplies can generate noise in some other cases I've run into.

When all the same items are plugged into a three prong type power system (three different locations that I've used it at), none of the hum effect happens at all.

My UM17 mics appear to have a similarly wired power supply as far as I can see, with just a different value in R6, and maybe different trim adjustments (didn't look too closely at that). No touch sensitive hum. But the reason I think this is of some importance to my scenario, is that it shows that a similar PSU and a tube mic can definitely not make the similar hum effect with the power at my home.

I have an acquaintance who has put together an AMI kit or two that made a remark like "oh no, the PSU really wants grounded power". Which I can kind of accept, but seems weird considering the fact I have had no issues with the UM17's with the similar PSU's…and also, since a C12 must have originally been able to work on a two prong type system.

Also, I did turn over the two prong connector attached to the multi strip and plug it into the wall receptacle the opposite way to see if it made less noise, but no dice.

I also happened to look in the mic for the first time ever today… nothing is obviously visually awry. I mean this in the sense that nothing is hanging loosely or anything crazy, all set screws appear tight. I kind of wanted to reconfirm the solidness of the connection between grill and body more than anything. Which looks good.

Make sure that the outer metal sheath on all cable ends is also connected to ground.

The soldering on the (added?) bare XLR case ground wire does not look that good in the pictures

Is a B+ cap leaking? look at the middle of the PCB picture by the diodes and cap

Is the power supply missing a fuse? The PCB section does not have one installed.
OR is the supply fuse in the power IEC connector?

I don't know what the added blue board is is it some kind of resistor board for pattern control?

Looks like a standard China built power supply with the stock caps

I would trace the ground circuit used at the power supply, cable and microphone

Is this microphone fixed bias or cathode biased?  I ask because the heater supply looks  stock from the pictures

I doubt this has anything to do with three versus two prongs.  Neutral is the same as 'ground' at the junction box.

The stock wiring in these Chinese supplies is a bit strange, although it makes sense from a cost perspective.  Circuit ground and chassis ground is freely intermixed along the chain, to the point where if you aren't careful, shield currents can flow through the head amp up in the microphone.  In general, using the large conductors is good practice to minimize impedances, however when you mix in 100M+ impedances things can go awry.

For example, you see that little wire between the two XLR connectors?  That is tying pin 1 of the XLR (e.g. shield common) to the circuit ground.  There is a little wire on the 7-pin XLR connector from the large brass tab (shield common for the 7-pin cable) to circuit ground (one of the pins).  This also happens up in the microphone, where the brass shell gets tied to circuit ground through action of how the 7-pin XLR gets mounted in the mike body.

Touching it adds your body as a shield, and gives another path for these currents to flow away from the circuit through the body.  It's 50/60Hz because it's un-rectified electrostatic interference, likely radiating from light fixtures in the room, etc.  You probably don't hear it elsewhere because the fields in those rooms are different.

Here's where I would start:
1) Measure the ground resistance by seeing how many ohms there are between the PSU ground and the internal circuitry up near the head amp up in the mike.  There should be 0.1 ohms or less.
2) Make sure that the ground connection up in the microphone is solid by making sure the 7-pin XLR insert in the microphone is tight

What TC said.
Verify there is continuity from mic body/grill to power supply chassis.

@Tim - I have used a few different cables - I just checked the neutrik mic cable I've used most often…

It appears that the ground is connected to the connector…which when the Neutrik metal sheath is screwed on makes contact with that metal. Does that sound correct?

@Gus - no that cap isn't leaking, it's a spec of dust/dirt that I'm guessing came from some finger oil or something else being there when it was built, and some dirt accumulated there. When I look closely it is definitely not emanating from the cap, it doesn't come from the base. I can't say I'm sure how the mic is biased. The blue board is for pattern.

@Matador…

I notice the part that there is a wire that comes from power ground and attaches to the chassis, on one of the screws where the transformer is. Will that work right on a two prong system? I ask this because of your mention of neutral resolving to ground in the two prong system. I have wondered if this setup gives it a potential to resolve to ground or not. (However, I do have other supplies for the 17's that work with this arrangement for sure.)

I was really excited for a second last night, when I noticed that the screw on top of the 7 pin inside the power supply was a bit loose - right where that bridge from 3 pin XLR to the 7 happens. However, screwing it down tighter made no difference.

When I look at the mic, I'm not sure how to get in where the binder connection is to check the ground attachment. I'll take another look! I have in fact had an issue exactly once with the ground in one of Oliver's mics being loose and not attached to the bell, which I sent to him and he fixed. So the issue rings familiar in that regard, but it could be a coincidence too.

Thanks for the input everyone, I really appreciate it greatly.

Use a continuity tester to check that there really is a connection. I recently had a U47 and C12 come in with this same problem. Both had newly made cables and both were missing this connection.

Make no mistake that there is no safety earth when using two-prong plugs.  But I doubt that is the source of your noise problems.

Bill Whitlock's paper is a good read about this: http://web.mit.edu/jhawk/www/tmp/p/EST016_Ground_Loops_handout.pdf

Start by making sure you have good audio 0V continuity (e.g. circuit ground, for lack of a better term):  wiggle the shell, tighten the screws, etc.  The 7-pin XLR connector binds into the microphone via a small screw in the end bell:  typically these must be *loosened* (e.g. turned fully anti-clockwise) in order to be *tightened*.

Is the sleeve of the 7 pin supposed to literally be connected to the ground, or is it just supposed to make contact when it the outer ring is screwed down?

Let me ask a more fundamental question:  what tools do you have access to?  A DVM?

I don't know what a DVM is! Digital voltage/ohm meter? I don't have one handy…though I think I have a basic ohm meter around here somewhere and I'll try to track it down.

The hum does stop when the grill is touching the XLR coming out of the power supply, for whatever that's worth.

I don't really have any tools to do anything except if it's patently obvious, and can be done by screwing something down/unscrewing something, swapping a connector, lifting a ground, etc….

If I wind up having to have a tech take care of the problem, I wouldn't mind having a better idea what the problem really is.

I have looked at the internal connectors, all wires… everything *looks* like it's solid.

I admit I was almost hoping there would be a simple workaround!

The hum does stop when the grill is touching the XLR coming out of the power supply, for whatever that's worth.

Click to expand...

I think you have found your problem. In many  modern XLR's the only way to make this connection is to take part of the braided sheild and crimp it under the retaining clamp inside the XLR, there is no direct way to solder. It sounds like you have lost the connection between the mic body and the power supply ground.

This is the case with multiple different XLR cables that all appear to be wired correctly.

So I should look at taking part of the shielding and crimp it under the retaining clamp in the XLR? Which part of the XLR is the retaining clamp?

Also, I mean specifically when the grill is touching the connector, while the mic is on a stand…not necessarily the XLR cable itself, that causes the hum to appear to stop.

Thank you all for your help!

George, I am talking about the multipin cable running between the mic and power supply, not the XLR cable running to the mic pre.