mysterious mic noises... high-frequency whining [example provided]

Hi!

I have an early 2000's CAD VX2 -- it's a dual-tube, dual-diaphragm LDC.

It has worked fine for years. Today it exhibited a high-frequency whine for a while. After some time, the whine diminished, and then seemed to hold steady at in inaudible level around 12k.

It may have done this intermittently in the past -- it feels vaguely familiar. It started today after I changed the pickup pattern of the mic (but that may have been a coincidence: I don't know if it started immediately thereafter). It had been running for 30-60m or so before this happened, so it was nice and warm (literally -- two tubes, so it makes some heat.)

You can download an audio file of the noise here -- caution, there are some very loud low-frequency components, so watch those speakers!

If you view it in a DAW you can see the large swings of the signal, and if you listen you can here the fainter but obvious high-frequency whine that moves around in pitch. Sounds like something in the circuit oscillating? (It certainly doesn't seem like any kind of EMF?)

In the audio file, the whine starts relatively steady. At 0:04 I start switching the selectable pad of the mic and you can hear the strong effect that has on the whine. At 0:20 I tap on the mic lightly. At 0:23 I switch the polar patterns which has a less dramatic effect than the pad, but still affects it a little. At 0:40 I turn the power supply for the mic off and then on again, and you can hear the dramatic swoop through the frequency range.

After a few minutes, as I said, the whine seemed to get quieter and rise in pitch until it was not discernible, but I could see on a VST scope that there was an inaudible but strong peak at 12k still hanging on.

Any ideas what this might be? Bad component? Old tube?

The schematic for the mic is here; it comes from a technical thread on groupdiy started by mista min, here: http://groupdiy.com/index.php?topic=44060.0

I'd greatly appreciate any input. Thanks!

(Ooops -- this should have been posted in "The Lab", I think, sorry.)

Turns out this wasn't the mic, but apparently had to do with a cable? Somehow replacing a cable between the board and the audio interface made this stop. As described above, the noise was obviously relating to the mic and the mic's power supply in some way, since changing the pad on the mic had dramatic and direct effects on the noise. Perhaps some capacitor somewhere charging/discharging/oscillating in some way, and the cable is transmitting the interference it would normally shield?

And I think the persistent high-frequency peak that remains after the whine goes away is just noise inherent in the prosumer interface I'm using...

Anyway, any ideas are still welcome -- it just happened again on an entirely different channel/mic/cable combination while I was recording drums... annoying, and unpredictable. But this description it's pretty vague at this point, I know; I'm not holding my breath, unless this happens to be an easily-recognizable sound.

Sounds like a SMPS misbehaving or digital breakthrough.

The 60V on the capsule is often generated by a simple 1 BJT SMPS internally but tube designs usually have enough volts to tap for this.

It could also be your sound card.  What are you using?

Check (scope) if HF gets into your 12V or your 200V supply line.

If dried-out PSU decoupling, there's a chance of getting cross-channel interaction enough to start an oscillation.

Strange circuit indeed. I see a high-boost (at tube NFB) - but no low cut.

Adding/subtracting after transformers seem SO unprecise to me - both on high and specially at low frequencies, where lots of parameters like exact value of transformer inductance and feed capacitance influences the signal's time-domain response hugely. It may be a genious design, that I don't understand - but to me it rather looks like doing it differently for difference's sake alone...

And btw.: Using a 12AX7 as condenser microphone front end. Now reeeeeally guys...

Jakob E.

Thanks! - I forgot to mention something important: I was led to the "cable" theory because I unplugged the mic from the board and the sound persisted. So i think it's safe to assume it had nothing to do with the cause?

But I mentioned the mic again in my last post because the mic certainly coupled with the sound in some way, as demonstrated in the example file when the pad is switched, and the pattern is switched, etc. So whatever it is seems to be sensitive to the mic, if not directly caused by it... maybe the noise is just sensitive to whatever signal is being passed and there are loud low-frequency bursts when the pad is switched? Pointing to the interface, perhaps? When it happens again, I'll try to isolate between the board and the interface: I think I did, and that the sound disappears when the connecting cable comes out of the board, so maybe it's the board (Mackie 1202 VLZ Pro)... anyway, i'll confirm next time.

The interface is a Presonus Firestudio Mobile.

After I switched that cable, the noise was gone, and I celebrated, but then it happened with the other channel/cable/mic chain, so I'm not so confident anymore...

@gyraf - thanks; yeah, that other groupdiy thread I linked to has a lot of people scratching their heads over the schematic, too. I wish someone could track down one of the engineers so they could explain themselves.

clepsydrae said:

I was led to the "cable" theory because I unplugged the mic from the board and the sound persisted. So i think it's safe to assume it had nothing to do with the cause?

Click to expand...

Then its NOT the mike that's faulty.  It's your Mackie 1202 VLZ Pro mixer or Presonus Firestudio Mobile.

How old are these items?

Yes.  The mike will interact in some way and should be a clue as to what's wrong.

Medium-aged, I'd say... the board is probably 12 or 14 years old, the interface I'd guess to be 5-ish.

I'll do some more probing next time it strikes. It's interesting how it comes in out of nowhere, hangs around for 2 or 3 minutes, and slowly vanishes.

I looked at the schematic. 

Have you changed the tubes in the microphone?

Can you check the microphone power supply? 

Is it a switcher or linear supply?

Gus said:

I looked at the schematic. 

Have you changed the tubes in the microphone?

Can you check the microphone power supply? 

Is it a switcher or linear supply?

Click to expand...

Thanks a lot -- but I think I ruled out the mic as the cause (see above posts... happened with a different mic as well... I was focusing on the mixer/interface at this point?)

To answer your questions, though: no, I haven't changed the tubes, and I'm afraid i'm ignorant as to the specifics of the power supply.