hum induced into a symetrical mic cable?

Hi guys!
Just a short question.
I recently wondered about a buzzy noise coming from my G7. It was never there before so I checked the mic for a dirty membrane checked all the connections, nothing.
Then I noticed that the mic cable crosses a switching power supply I use to power my flat screen TFT monitor. I moved it away and the buzz stopped.

I thought it would be close to impossible to induce buzz into a symmetrical connection of pretty low impedance...
Could I messed up some ground paths or does that happen to you , too?


Thank you!
Regards,
Stefan

I think this happens to us all. You could try some Starquad cable if it is a problem for you.

Thanks!
I wasn't aware of the existence of those cables;-)
I searched the net and found out that sommer cable also produces a cable like that at a reasonable price. Maybe, I'll try that one.
http://sommercable.de/2__produkte/2__030_meterware/2__200_0301.html

What mic pre?

Samuel

[quote author="Samuel Groner"]What mic pre?

Samuel[/quote]

Maybe it was a Buzz Audio... :green:

I'm sorry. That was really bad.

First, I used my G9 that by the way sounds terrific.
I thought it could be the pre but my focusrite behaved in the same way, so I sorted that one out.
The cable is of cheap variety so maybe thats the problem.
As I located the problem to be the switching power supply I moved the cable away and, just for curiosity, moved it directly over another (tranny based) supply and I had some low freq hum as well that went away as I moved the cable away from that one, too...

Should I blame the cable?

the problem is the cable. it does not have proper shielding which is causing the issues.

you have to choices move the cable away or get a better quality cable.

The twist in the cable and the quality of this twist is what's mainly responsible for the CMRR before being debalanced at the preamp. Obviously the cores need to be shielded too, but it will not work well if the cores don't have a good twist.

Shielding obviously helps, but the balance between the two cores is essential in order to cancel noise. That's why Starquad (like that Sommer cable you linked) tends to have better noise rejection qualities.

The CMRR of the mic transformer (or the PSRR of the mic) may limit things as well.

Samuel

The CMRR of the mic transformer (or the PSRR of the mic) may limit things as well.

Click to expand...

I never really had prolems of this kind before so I'm not sure it has to do with poor CMRR especially as I had the same behavior and the same level of interference on both preamps...
I'll change that cable and see what it brings.

It's also possible you have a "pin 1 problem". On the preamps which had problems, exactly how is pin 1 on the input jack connected? Ideally it should go to the main ground point, very close by, which is a chassis ground.

Peace,
Paul

It's also possible you have a "pin 1 problem". On the preamps which had problems, exactly how is pin 1 on the input jack connected? Ideally it should go to the main ground point, very close by, which is a chassis ground.

Click to expand...

Interesting.
I know how the G9 ist grounded.
Pin 1 of the input XLR is directly connected to chassis earth...
I don't know how it is done in the focusrite.

A question about XLR cables:
The XLR connector housing itself has a soldering pin, too.
I have seen cables that connect the shield of the cable itself to pin 1 of both connectors and I have seen cables that split the shield and connect both pin 1 and that pin on the XLR housing together.
What is the right way to do it?
Thanks!

The XLR connector housing itself has a soldering pin, too.
I have seen cables that connect the shield of the cable itself to pin 1 of both connectors and I have seen cables that split the shield and connect both pin 1 and that pin on the XLR housing together.

Click to expand...

I do not connect the cable shield to the XLR connector housing. If you do this, you are most likely to have a direct connection between audio shield and preamp chassis (with most preamps).

Yes.

Cheap cables are subject to crosstalk and other problems. Note how when you move the cable the noise goes away. I would suggest getting some better quality cable and making a new one. Quad mic cable IMO sounds better then Mic Cable plus the twisted conductors on the inside off nice shielding. I would say buy mogami but thats more of my opinion.