AKG C451 EB High pitch whine?

[mkiijam]

I have this mic that is making this intermittent 2k whining noise. I have seen it (on a spectrum analyzer) twice act up, but mostly not... any thoughts (any mic) would help as this is very strange.

 

[cyrano]

My C451s also make noise, but only if the phantom power provides not enough juice, like on USB powered interfaces. Sometimes it happens only on startup and goes away after a minute or so. I suppose the caps have become leaky. It's not a 2K whine, though. It changes from motor boating (on a Sound Devices USB interface) to a kind of gurgling (on a Focusrite Solo). No problem with an RME or on a DDA mixer. I've seen this with other old mics too, but not as consistent as the C451s.

Have you tested your mic with another preamp?

 

[mkiijam]

Have you tested your mic with another preamp?

Yes, that's what kinda through me at first. I heard the noise and changed in over to another pre and the issue went away. I think I will play with lowering the 48v and seeing what happens. Thank you so much!

 

[soapfoot]

this era AKG microphone can be very sensitive to the quality of phantom power (and the precise voltage)

 

[babisbabis]

I would check the phantom power circuit section (power supply, capacitors etc) of the pre with which the mic has these issues. Also measure the actual voltage of the phantom power, although the mic accepts 9 to 52 V (I read somewhere that it was first designed for 12V).
I hope you'll solve it.

 

[mkiijam]

Any body have a quick trick to make an adjustable 48v mod? This seems like a silly question. I think I have to build or mod a pre and then make a LM317HVT or something that is adjustable

 

[mkiijam]

Okay, I've set up a POT voltage divider and am able to lower the 48v into the mic. I would like to go very low, but am concerned about damaging the power supply. Would there be a way to protect the PSU, but lower the 48 all the way to 0V? Here is a picture of my current setup / circuit.

 

[Khron]

You're better off with a pot controlling the voltage setting on an LM317.

 

[mkiijam]

You're better off with a pot controlling the voltage setting on an LM317.

Yes, I thought about that. I was going to do a LM317HVT, but they are out of stock with my favorite parts suppliers, so I moved on to this way. I've brought it down to about 30v and haven't got the "whine", but would like to go lower... below 12v I think.

 

[dmp]

So these mics are kind of interesting. They take the phantom in and drop it to 10 VDC. You can see the schematic here and there is another schematic attached with component values. Voltage over 10V is dissipated through the transistors to the right of U35 (the output transformer) and stabilized by a couple electrolytics. This transistor pair might be replaced with a zener in some models. Then, there is an oscillator at the far right to create 30vAC with the U26C transformer. This is rectified with the diode and smoothed with the 4n7 cap to provide 38vDC for the capsule. This is probably where the whining noise you are hearing is coming from - leak of the oscillator voltage. R10 (150k) and C10 (470p) sets the frequency of the oscillator, 2257 Hz = 1/(2piRC).
I suspect some parts in your mic are out of spec, particularly around the 38VDC created to bias the capsule.

The first thing to do is check/replace the electrolytics in the mic, assuming it is an older one. C9 (10uF) & C13 (3.3uF) stabilize the positive voltage rail.

 

[mkiijam]

Yes, interesting. VERY interesting that you math out 2khz, for that is the tone that I heard (saw) on my spectrum analyzer. It is no longer making the tone. And, that is why I wanted to see if I could reproduce it again before shot gunning parts.

 

[mkiijam]

Well, I ran it all the way down to 1.2Vdc and haven't had the issue come back. I'm hesitate to replace parts without getting to act up again.

 

[mkiijam]

So, we have done more tests and the issue seems to follow the length of cable we use and perhaps how many connections it goes through. The longer the cable to more the noise is there. The issue shows up very strong with about 50' of cable, and drops to nothing at 3'. The noise also seems to be more like 5kHz, but may have been different the day I did the 1st test.

Something with microphones and long cable runs?

 

[Gus]

Emitter followers can turn into oscillators when loaded with more capacitance. I bring this up because you wrote about the cable length
Does it oscillate more when the highpass filters are off?

You can sometimes tame the oscillation in a follower with a series resistor to the base or a resistor in series with the output cap.

I have not worked on a 451 what type cap is C7? is it the original transistor?

Search for things like "emitter followers oscillations" it is something to know about when using followers.

Are there more reports of of 451s having this issue?

 

[Big Valley Audio]

So, we have done more tests and the issue seems to follow the length of cable we use and perhaps how many connections it goes through. The longer the cable to more the noise is there. The issue shows up very strong with about 50' of cable, and drops to nothing at 3'. The noise also seems to be more like 5kHz, but may have been different the day I did the 1st test.

Something with microphones and long cable runs?

Here’s a couple of quick things to try to rule out RF interference ( please ignore if you’ve already done this)
1) while listening to the mic turn the mic around the room and see if the noise gets louder quieter
2) when you hear the noise, try wrapping your hand around the female XLR and see if that helps.

If it is RF then try connecting pin 1 of the female XLR to pin 0 of the female XLR ( the pin that shorts to the metal case of the XLR connector.

I know this is pretty basic so I’m not trying to offend anyone here.
RF (specifically 5G at 600MHz ) is causing incredible amounts of interference here in Vancouver so it’s the first thing I try when dealing with condenser mic noise.

 

[mkiijam]

It's not RF. We did those tests. We have two of them and the other seems much better, but actually has noise at like 10khz!