G7 electrical "hum" problem

Hi guys, I've got my G7 built but having problems with a humming sound. I am getting sound (i.e. picking up my voice) on all 3 polar patterns as required but all 3 patterns are also giving me the same amount of this hum. I've checked all earthing/grounding and have even given the mic to a friend of a friend who has a lot of electronic experience who has tried a couple of different things with routing and is getting the correct voltages from each point (as illustrated in a diagram posted on the forum) but still no cure for the hum.

Is it possible that a faulty component could cause this noise or is it still most likely to be a grounding issue? Sorry if my description of the problem is a bit vague, if any other info is needed just ask and I can hopefully get the required info from my friend!

Thanks in advance!!!

Hum can come from a PSU with insufficient filtering, a ground loop, poor shielding, or a bad tube.
Have you eliminated anything?

One test is to turn off the psu while listening to the mic - the tube will continue to be powered for a few seconds after the mains is killed. The hum will probably go away. Then other tests... turn off only the heater power, only B+, etc...

Make sure to connect both HT ground (which is also audio ground) and heater ground to microphone chassis.

I'm pretty positive I've eliminated a grounding problem. When I turn off the PSU the hum continues for a few seconds until the caps empty.

I have already ordered a new tube and a new 7-pin XLR as the one I have came with an Aurycle DIY mic kit and so i'm not sure of the quality as the kit price was very cheap!

When I turn off the PSU the hum continues for a few seconds until the caps empty.

Click to expand...

Interesting... probably not filtering or a ground loop.
I'd suspect shielding (if not a defective tube) - if you turn/rotate the mic does the hum change? What mic body are you using?

When I turn off the PSU the hum continues for a few seconds until the caps empty.

Click to expand...

This means that the hum you hear is picked-up hum - not generated by the supplies. Whish also means that you must have insufficient shielding.

Try shoveling it into a fully-closed metal box and connect box metal to 0V/Gnd/mic-chassis.

Hum now goes away, and proves that you somehow skipped on the shielding (ALL parts of the mic must be grounded - including protection mesh and such)

Jakob E.

Thanks to both of you for your help. The PSU is in a fully closed metal box with all grounding attached to the chassis. The mic is currently with the friend of a friend but if I recall correctly the mic PCB has a connection between the solder points for XLR pin 1 (ground) and pin 7 (heater ground). Pin 7 then had a grounding to the mic chassis within the connector block at the base of the mic.

Within the PSU, pin 7 (heater ground) was definitely grounded to the PSU chassis and pin 1 (ground) was grounded through its connection to pin 1 on the 3 pin XLR.

I'll certainly check all this again but if it is indeed grounded like this would this then suggest the problem may indeed be the valve or, more liekly, the 7 pin XLR cable?

Try a new tube. Worst case scenario, you will have a spare tube for when the one in the mic fails.

..on the other hand, a tube error resulting in symptoms like this (requires short from heater to cathode) is quite rare..

I'll certainly check all this again but if it is indeed grounded like this would this then suggest the problem may indeed be the valve or, more liekly, the 7 pin XLR cable?

Click to expand...

Maybe post a picture of your mic body. The mic body & headbasket shields the electronics and the capsule. Check for continuity for the entire mic body (i.e. headbasket to body to connector...)
If the connections are poor you can have problems like this.

Ok, I'll get a photo over the weekend when I get a chance to look at it with the guy who's helping me out. He's very experienced with electronics so that's why I thought it probably wouldn't be a problem with any of his wiring/connections but I'll show him this thread and eliminate possibilities.

Still having the same problems! Have rebuilt the whole mic using a new PCB from Gustav. The guy who's been helping me is completely beat by it! We've checked and double checked all grounding, and all voltages are correct.

Since we're getting sound through too it seems like everything is correctly wired. But here's a few things I've noted while building it which one of you may notice to be my problem. The problem is exactly the same as the original PCB build so it made no difference!

1. I left out the 100u cap and the trimmer from the mic (as instructed) but I also notice that the 10n and 1n caps are marked with an asterisk too, should these be left out too? I built it with both of these caps included.
2. When the guy helping me built the second mic PCB, he noted that the 2u2/250 (the one at the top of the schematic attached to XLR pin 2) and 1/63 caps in the schematic are not marked on the PCB and so built without them. He later attached a 2u2/250 cap to see if it made any difference but it didn't.
He also noted that the 100/35 cap on the schematic is marked on the PCB as a 220/25 cap and looked like it was back-to-front on the PCB and he attached it the opposite way round. I had it attached according to the PCB on my original PCB build and this fix made no difference.

I know plenty of you have built on Gustav's PCBs and so I very much doubt this is the pROblem at all, but just wanted to check these points anyway, to help with my understanding of the project.

The fact that the second build has the same problem suggests that the problem lies with the PSU, but again we can find no problem with it.

pictures

Check integrity of mic housing's shielding effect:

gyraf said:

This means that the hum you hear is picked-up hum - not generated by the supplies. Which also means that you must have insufficient shielding.

Try shoveling it into a fully-closed metal box and connect box metal to 0V/Gnd/mic-chassis.

Hum now goes away, and proves that you somehow skipped on the shielding (ALL parts of the mic must be grounded - including protection mesh and such)

Jakob E.

Click to expand...

Thanks Jakob and DMP, I ended up giving the mic and PSU to a mic repair guy this morning to see if he can find the problem. I always prefer to work things out myself to increase my understanding, but since this build is for a college project (which is dangerously close to the deadline and I still need to test and record with the mic) I had to bite the bullet and get further outside help!

At the moment I only have pics of the mic and not the PSU, so probably won't be of much help! But here they are

The metal of the mic body should be connected to ground - either in the mic or back through the cable and at the PSU (you can try both and see which is quieter).
I don't see a connection of the mic body to ground.

Apologies for the photo not showing the full mic body, there's a ground link at the bottom of the mic (out of shot) connecting to XLR pin 7 (ground)

make sure you have the high impedence parts (capsule connections ) super clean. Otherwise current leaks in and you get noise.

Great, thanks guys. As I say, it's now in the hands of the repair guy but this is definitely all good info for me to help with my understanding!

Just an update on the mic! The repair guy found a solder bridge, which I managed to miss so many times when looking over it for problems!! EVen the guy who was helping me didn't see it and he said that all voltages seemed ok, but I can't fully verify that! Anyway, mic built and working, had it in the studio last night and very happy with it, even the engineer seemed pretty impressed!
Now, with all my spare parts and new-found understanding, onto mic build number 2 so that I have a pair!!

One thing that some of you may be able to advise me on however, as the mic was built for a college project, I'd love to be able to carry out a frequency response test. A college lecturer had made arrangements with Tannoy to be able to use their anechoic chamber but he recently e-mailed me saying that it might not be possible as their chamber is obviously set up to check freq. response of loudspeakers etc. and he would have to look into finding out what different equipment I would need to test the mic there. I've left it with him as I have a huge write-up to do, but can anyone advise on what different equipment might be needed? I would assume that it would be fairly similar to that needed to test a loudspeaker - a measuring mic and some spectrum analysis software. Any advice?