Helicopter Noise from mics
 

Several times I have experienced "helicopter noise" coming from my mics while setting up. Sort of a pumping in-and-out white noise with a frequency approximately 4Hz. In all cases it has gone away after I fiddled with connections, gain, and flipping the 48V, limiter, and Low pass filter switches, but I haven't been able to narrow down exactly what I did that made it go away. I have experienced this with both my ME66 and cs-3e mics, while feeding the Mixpre.

I have noticed that when it's an issue, it happens only when the gain is turned up past a certain amount. This makes me suspect a voltage supply instability in the mixer. The fact that I have only experienced it while setting up suggests that perhaps it is compunded by cold batteries producing lower voltage, or possibly condensation on the mic?

Re: Helicopter Noise from mics
 

The Rode NTG 2 did pick up micro vibrations sounding like a helicopter when the internal 48V from a AA battery was used. It simply seems like one AA cannot power the mic sufficiently. When switching on the phantom power inside the camera or a mixer, the sound goes away.

Re: Helicopter Noise from mics
 

I would try it without a mic connected to the mixer. Of course, it is never legitimate to test a mic preamp without some sort of termination. A dead short across XLR pins 2-3 at minimum, or more properly, a resistor around 150 to 300 ohms across XLR pins 2-3 makes a proper terminating plug that should be in any good audio kit of bits and bobs.

Some mixer designs have internal feedback issues when operated at high gain. As Mr. Krol suggested for the microphone, I would also test the preamp/mixer with an external power supply and known good fresh batteries. The power supply/battery is an important part of the circuit to stabilize the preamp power.

Re: Helicopter Noise from mics
 

Doesn't have your mics listed but something to be aware of maybe?

Oscillation with Transformer-Balanced Inputs|Sound Notes|Sound Devices, LLC

Re: Helicopter Noise from mics
 

The Rode NTG-2 is designed to work equally well powered by a charged battery or 48volts.

If you use 48volts most of the time don't leave the battery in it and check it has a suitable charge before installing it.

If it's your NTG-2 you're talking about, make sure a battery hasn't leaked and corroded the mics battery terminals ..
then try it with a new *name* alkaline battery, not a cheapie from the East.

Or if it's registered for its 10yr warranty with Rode, email support in your area to get it repaired.

Cheers.

Re: Helicopter Noise from mics
 

This is sometimes also known as "motor boating" as it can sound like an outboard engine.

It can be caused on some mic's if you have the battery in place and also try to connect 48 volt phantom power from an external mixer at the same time.

It can also be caused if the phantom power is too low or too high on some mic's but can also happen if the earth +ve and -ve balanced connections are intermittent and the power is not being fed to the mic correctly.

One other possible other problem may be if several mic's are being used on a battery mixer and it doesn't have enough power to feed all of them together.

I must admit though that I haven't heard this sound for a few years as most modern mic's and mixers tend to be more tollerant than they used to be but the most common reason is that battery and phantom, is being applied to a mic such as the NTG2 at the same time.

Re: Helicopter Noise from mics
 

Thanks. That SD Sound Note looks like what I'm experiencing:

Oscillation with Transformer-Balanced Inputs|Sound Notes|Sound Devices, LLC

I think next time this happens, I will try the experiment of plugging the Mixpre into an AC adapter. That should show me whether this is indeed the issue I'm seeing.

Perhaps it only happened while setting up because I had the gain too high before I trimmed it.

Re: Helicopter Noise from mics
 

That's very interesting Tom and I recall hearing this a lot more in the old days (25 years ago) when transformer balanced inputs were more prevalent.

Re: Helicopter Noise from mics
 

Hello Tom,

It has been noticed that some very current hungry mics will exceed the current capacity of the battery causing motorboating. This may not happen until or unless the headphone amp on the device is turned up A LOT. That extra drain on the power supply puts the power supply into oscillation.

Regards,

Ty Ford

Re: Helicopter Noise from mics
 

I had a chance to sit down and experiment with this helicopter noise. I found that I can get it to occur reliably by plugging in either an ME66 or cs-3e shotgun into my SD MixPre, and turning up the gain dial all the way (max). When I turn the headphone dial up to approx halfway (give or take a notch or two depending on mic position, low cut, and limiter switches, and power source), the motorboating starts occuring. It sounds like the background noise cuts out periodically; first at low frequency then at faster frequency as I turn the headphone level further up.

Strangely, it occurs more and at lower levels when I plug in with the SD AC power adapter, versus running off of two used AA batteries.

So I think it's not something to worry too much about; I never turn the mic gain that high in practice so this is more of a theoretical than actual problem, and if it becomes a problem I know that the emergency solution is just to turn down the headphone gain.

Re: Helicopter Noise from mics
 

Tom,

I think that may well have been a problem with older MixPres.

Here you go: Oscillation with Transformer-Balanced Inputs|Sound Notes|Sound Devices, LLC

I have a Sound Devices 442 and have never had the problem. I run it off a very good external battery.

Regards,

Ty Ford

Re: Helicopter Noise from mics
 

Are you sure it isn't coming from cell phones?

John

Re: Helicopter Noise from mics
 

Or maybe solar flares?