G7 noise troubleshooting session. [Solved]

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Tergo247

Active member
Joined
Mar 20, 2010
Messages
31
I've just finished my G7 and am having awful noise issues. I've been reading through the forums about it and the only thing I can't be certain of is the conductivity of the copper tube mic body. I've grounded the heater to the HT, I have reasonably fine mesh (two layers even), I've triple checked my XLR connections (both), grounded to the PSU chassis through the XLR7 lug, and my heater and HT voltages are 6.3V and 164V respectively. I'm afraid the conductivity of cooper with the frame and grill isn't sufficient and was wondering if anyone else had dealt with it.

Thanks in advance!!!
 
My copper body is working excellent, so it is defitately doable. Hope you find the problem!
 
Is it somekind of plated copper or why would you question the conductivity of copper?

What kind of noise, is the signal otherwise healthy?
 
Hi! There is no inherent problem with copper - look elsewhere.

As Sredna says, check that the grill electrically connected to the body, and is the body soldered or otherwise securely connected to earth?

A good trick is to take a croc-clip lead and try connecting the mic body to the PSU housing etc.

Is the noise hum? Or some other (hiss, popping) noise which would suggest the capsule, hi-Z area, or tube?

Stewart
 
Thanks for the replies guys! The noise that I hear sounds a great deal like pink noise. I would imagine that its the same noise that you guys hear when you take your capsule/electronics out of the grounded body (the noise I'm dealing with gets far worse when I do so). No buzz that I can detect.

I have continuity between the body, the grill, pins 1 (HT ground) and 7 (heater ground) of the XLR7, pin one of the XLR3, the PSU chassis, and the ground of the IEC. I'm using brass mesh and haven't been able to solder it to the pipe however (not for lack of trying), so it's just pressure fit ATM.  I've tried connecting a wire directly to the mesh and a couple of different ground locations without any luck.

The signal is a little anemic but about the same level as the noise; on the front capsule anyway. The switchable side of the capsule isn't functioning properly, and I suspect this is due to a bad capsule lead. Could that be it? I assumed that the two sides were independent of each other...

Another symptom is that what signal I have sounds way better with the phase switch flipped on my pre, despite Out+ having continuity all the way to pin 2 of XLR3.
 
That doesn't sound like a simple case of poor shielding.

Check the voltages around the microphone itself and report back.

As a guide, there are some voltage measurements from one of my mics here...

http://www.xaudia.com/xaudia/Documents/Entries/2008/3/5_G7_observations.html
 
Doesn't sound like a grounding or shielding problem to me neither. Because of you're pattern control not working I would double check that your capsule leads are connected to right places. IIRC I had the front membrane connected to the wrong side of the 1n in my G7 inpired mic and the sound came through but it was noisy and pattern switch did weird things. And even if it solves the problem it's good to do what zebra suggested.
 
Hi,

Another one to consider, is the tube it self, 90% of noise is induced by the active element in this case the ef86, if you can swap it with a other one it is worth to try.

DJN
 
Picking up interference, other than hum and low frequency buzz is locational sensitive.  Go to a friends place and see if you still have noise.
 
G7 using a PCB or point to point etc?
Read the G7 and other microphone metas and threads when you have time pay, attention to things like the following
What I would do first is disconnect the capsule and wire a 50pf to 100pf cap  grid to ground leave the grid to ground resistor.
Close the microphone up and what kind of noise do you hear?
If the same noise check/change the tube and make sure the circuit is very clean around the grid section.  You can have one or more noisy tubes just because you tried one or two or three or four etc means little with tube microphones.
What type capsule mount are you using homemade or bought?
What capsule brand and type (edge or center terminated) and is it clean?

There are reasons for the above questions

We should make a copy paste section to copy paste a standard list of things for questions to a thread.
 
Gus said:
We should make a copy paste section to copy paste a standard list of things for questions to a thread.

Good idea. It could also be like a FAQ for each type of project - i.e. don't ask questions until you have tried the following...
 
I did not mean it like an insult.  I meant it as a helpful thing.  People who are new to building a microphone might not know enough to know what to look for.  A summary of things to look for might be useful.  I did list a number of important things to check.  Most if not all have threads or posts about the issues.
 
I think that's totally a good idea. I read all the posts I could find on noise, the things I tried in my first post was the result I came to before posting. An amended "what to check if..." page would be a big hit I think.

To answer questions:
I'm using a PCB from Chef, a Peluso capsule with his mount (edge terminated and a little dusty now that you mention it), and a Telefunken EF86 tube. I'll definitely try the cap substitute idea when I get some time, and check around the mic for voltages.

To update:
A techie friend of mine took a look at my PCB and told me that he thought my solder joints looked cold. I turned up my iron a bit from where it was and hit them all again. Noise at startup is greatly reduced. As the tube heats up however, it reaches a point where it gets noisy again. This time it sounds alot like fire (with spurts of cutting out) with occasional hissing. I'm going to buy a couple more tubes to try today, and let this one run for a while to see if it helps.

I still don't have polar pattern adjust... I've wired the two center-tapped capsule leads to the center solder point, the lead that was attached to the front ring of the capsule to front solder point, and the back to the back. If there's not more to it than that, then I don't think it's the problem. I'm showing my ignorance here: I wasn't aware these poly caps had a polarity. My electrolytic cap is definitely correct.
 
Tergo247 said:
I still don't have polar pattern adjust... I've wired the two center-tapped capsule leads to the center solder point, the lead that was attached to the front ring of the capsule to front solder point, and the back to the back. If there's not more to it than that, then I don't think it's the problem. I'm showing my ignorance here: I wasn't aware these poly caps had a polarity. My electrolytic cap is definitely correct.

They don't have polarity, I meant i had the lead connected to the wrong leg of a cap in my point to point circuit.

How many leads there is coming from the capsule? Is it a double bacplate capsule with four solder points (front and back membrane and two backplates). I don't understand the bolded sentence, both membranes connected to same point? That would be wrong. Backplates should be connected together if there's two of them.
 
There are four leads coming off of the capsule, two blue jacketed leads are attached to two different backplates, and one red lead is attached to either membrane for a total of four leads. So my backplates are connected together at the PCB.
 
Before you go spending money on tubes, do take the time to check the voltages in the mic and report back. Then troubleshoot with capacitors in place of the capsule, like Gus says.

Another symptom is that what signal I have sounds way better with the phase switch flipped on my pre, despite Out+ having continuity all the way to pin 2 of XLR3.

Are you listening through headphones?  Then it's normal to have some phase cancellation in your head. Worry about polarity when everything else is done, by checking agains a known mic.

 
Interesting about phase cancellation in my head... I definitely have that problem (if it is one) on the back burner. My voltages under load with the capsule connected referenced to ground unless otherwise noted are:

164V @ pin 2
0V @ pin 3&4
82V @ pin 5 (switched to cardioid)
6.2V @ pin 6
-7.8mV @ pin 7
164mV across 220ohm resistor
1.34V across 220ohm and 1.6kohm resistor
80.8V across 2.2uF cap
155.9V across pin 2 10Kohm resistor
76.3V across 440ohm resistor
82.2V across 100nF cap connected to 33Mohm

I've gone through my cap stock and can't find any extras to temp that test. I'll have to get some when I can. In the mean time, how does one go about cleaning a capsule?
 
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