Random hiss that comes and goes with mic/cable position.

[k brown]

Has anyone else had this problem with their DIY mics?

No hum or buzz, but high frequency hiss that comes and goes depending on mic and/or it's cable's position in the room. As I wave the mic(s) around in random circles, the hiss appears, then disappears. If I hold the mic in a fixed position, but move the cable around - same thing. I've never experienced this before, and I've been building and modding mics for over 30 years.

Is this some sort of recent cell phone interference phenomenon? Do I have some sort of transmitter in my area? Wireless modem? Apple Time Machine?

The mics in question are recent builds, and haven't yet had an opportunity to test them away from home - but has anyone else had this kind of noise problem?

 

[TwentyTrees]

Could movement of the mic or cable be putting stress on a marginal joint in the connector assembly to the mic, or the cable itself perhaps?

 

[k brown]

Not a dodgy connection - the hiss smoothly fades in and out; doesn't instantly cut in and out like a loose connection.

 

[TwentyTrees]

Intriguing! Is it always in the same place within the room?

 

[k brown]

Doesn't seem like it, which makes it even more mysterious.

 

[luckythedog]

Rf pickup? Inaudible oscillation at rf or ultrasonic from mic/cable preamp changing circuit bias somewhere in the chain? Do you have a scope or maybe record at high bandwidth/sample rate and look at spectrum?

 

[Ike Zimbel]

This is probably RF noise from either a new cellular installation near you, or some of your own WiFi equipment (router, phone, WiFi extender etc.). I have done some troubleshooting of these types of issues and I highly recommend getting at least one mic cable with the Neutrik EMC connector on the female end. Plug that cable into your mic and see if the problems are still present.

 

[k brown]

My suspicion, as well - thanks for the suggestion.

If that turns out to be the issue, then the task becomes why most of my mics aren't picking this up, and what's different about the ones that are?

 

[dmp]

Have you checked the grounding of the mic? Is the mic body connected to chassis ground? Is 0v connected to chassis ground in the mic?

 

[k brown]

Would that not also produce hum and/or buzz? - all I get is pure white noise (added to the audio).

Apparently it varies with time of day, as well; this morning I'm getting next to nothing, with the same mics last night it was very bad.

Could a Ham radio in the neighborhood do such a thing? I would think that would produce a very different kind of 'modulated' noise.

 

[Ike Zimbel]

Making sure the the Pin-1 connection to the body of the mic is good is very important, in my experience, and as mentioned above.

 

[k brown]

Again, that could produce pure, smooth white noise, rather than hum/buzz?

 

[dmp]

The job of shielding with proper grounding is to prevent the mic picking up anything, hum, buzz, high freq noise. By your description it seems the mic is picking up the noise since it depends on the orientation.

 

[k brown]

I suppose in this new RF-saturated world we live in, too-open grille mesh is also a bad idea, even if there are no hum problems - HF noise can still make it's way in.

I wonder if current users of the original AKG C12 are having this problem, with it's very open single-layer mesh?

 

[MisterCMRR]

This is more than likely, as Ike and others have pointed out, RF pickup ... and it's likely being demodulated in the microphone itself. A busy digitally modulated RF carrier will sound like white noise when AM demodulated. Because today's digital communication channels operate in the GHz frequency range, wire lengths connecting pin 1 to the housing and electronics "ground" in the mic circuitry is critically important! Remember that, at these frequencies, 1 or 2 cm of wire has high impedance and becomes a good receiving antenna! My advice is to use the shortest possible wiring to connect XLR pin 1 to the mic enclosure/circuitry and install a ferrite bead or two on the tightly-twisted pair of wires from your circuitry to pins 2 and 3. It's also good practice to put a 47 pF to 100 pF capacitor, with extremely short leads, across the output of the mic electronics as close as possible to the semiconductor(s) that drive it.

 

[dmp]

You can test if the noise is originating in the capsule by replacing it temporarily with a small value capacitor. I doubt that's it. More likely a problem in the ground scheme

 

[k brown]

Interesting that you say to put the shunt caps as close to the semis as possible - I usually see them mounted right on the XLR pins. I always thought that was to short the RF to gnd as soon as possible.

 

[gswan]

Maybe your DIY mic is oscillating at VHF. I've heard this in old DIY mixer channel strips which started oscillating at 20MHz+. Not audible, but it generated a lot of hiss. Get the scope on the mic when it's hissing and wind it out to see.

 

[k brown]

So, the hiss is like a sub-harmonic of the ultrasonic oscillation?

 

[CHICAGOAUDIO]

This is probably RF noise from either a new cellular installation near you, or some of your own WiFi equipment (router, phone, WiFi extender etc.). I have done some troubleshooting of these types of issues and I highly recommend getting at least one mic cable with the Neutrik EMC connector on the female end. Plug that cable into your mic and see if the problems are still present.

What Ike says. RF that’s getting into the mic, the preamp, or both.

Without an RF analyzer, impossible to say what the source is.

Reusan shielded cable with good quality connectors is your friend…

Make sure you don’t have any pin 1 problems to start with.