Lots of Noise From Pre-Amp at DVinfo.net

Brock Burwell · January 27th, 2023, 08:29 AM

I just started a new job and now am shooting with a Canon C70 and R5. One of the microphones I got is the Rode Wireless Go II, but when testing it I'm noticing a lot of noise from the pre-amp in the C70. When I tested it with the R5, I'm not getting the same amount of noise and it sounds good.

I did tons of tests yesterday by turning the pre-amp down in camera and turning it up on the Rode system and then vice versa. I can't seem to figure out what the issue is.

Any suggestions?

Allan Black · January 27th, 2023, 11:50 PM

I haven't got a C70 but looking for something else I came across this. Not sure if it helps, but …

If the C70 fan is on, a sensitive mic will hear it, if the ambience is quiet. The built-in mics certainly will.

Cheers.

Andrew Smith · January 28th, 2023, 04:14 AM

Something that has given me a lot of unwanted noise (that would sound like a very noisy preamp) is accidentally leaving phantom power on when using a mic level input. Drove me batty until I realised what it was.

Might be worth checking for that. As always, let us know how it goes.

Andrew

Paul R Johnson · January 28th, 2023, 10:05 AM

Phantom power, by design cannot add noise, because the voltage is applied identically to the two audio lines - what it can do is 'annoy' a connected device that does not appreciate having 48V applied to it. if switching it on or off with nothing connected raises the noise floor, that's strange.

Andrew Smith · January 28th, 2023, 12:57 PM

Phantom power annoys my RØDELink Filmmaker receiver unit.

Andrew

Paul R Johnson · January 29th, 2023, 04:13 AM

Usually it's an indication the device connected is either not expecting it and the output resistance, as opposed to impedance) is unequal between ground to XLR 2 and 3. In these cases, the unequal resistance doesn't really matter much for the audio, but means that some of the 48V won't be cancelled out, leaving volts on one leg - and that can make things unpredictable. Usually there will be capacitor isolation on output sockets, but some designs didn't consider the possibility of having 48V coming back into the output. Soundcraft - a well respected mixer manufacturer had these things when people connected two mixers together. I discovered it using two, with the first an orchestra submixer. It's stereo out when to channels 1 and 2 of another Soundcraft. These had the phantom power on which went into the mixer outputs. Audio wise - it was perfect, but every LED in the output section - the groups and the stereo out all lit up!

It's possible that the Rode output isn't actually balanced at all - but has one of the XLR pins 2 or 3 tied to pin 1 - this would then work fine with unbalanced camera inputs, and still work with cameras with balanced inputs - but phantom would screw it up.

Allan Black · January 29th, 2023, 03:32 PM

C70 Manual: Options for Fan Mode.

[Automatic]:
The fan runs while the camera is on but not recording, and is automatically turned off while the camera is recording. However, if the internal temperature of the camera is too high the fan will be activated automatically. When the camera's temperature has decreased sufficiently, the fan will be turned off.

Use this setting when you do not want the camera to pick up the fan’s operating sound.

Note: But depending on the ambient temperature and other shooting conditions, the fan may not turn off even if you set its operating mode to [Automatic]

Cheers.

Greg Miller · January 31st, 2023, 01:51 PM

I daresay many source devices, like wireless receivers, have internal power supplies that are positive with respect to ground. If these devices have output coupling capacitors that are electrolytic, the capacitors are installed with their positive terminals connected to the internal electronics, and their negative terminals connected to the device's output connector.

If you now connect that source device's output to the input of a device which has phantom power turned on, those coupling capacitors will now have positive voltage (probably +48v, rarely +24v) applied to the capacitors' negative terminals. This will significantly reverse-bias the capacitors. It can produce unpredictable performance including noisy performance from the capacitors (hopefully not catastrophic failure of the capacitors). So it's quite important that the phantom power is turned off in scenarios like this.

I may be showing my age, but this is a good argument for balanced/floating output coupling transformers, rather than only capacitors. (And a transformer will work equally well when feeding a balanced or unbalanced input.)

Just sayin' ...

Pete Cofrancesco · January 31st, 2023, 02:42 PM

Could be shielding issue on the cable or the receiver. Btw the c70 is xlr input the Rode outputs consumer unbalanced. How are you converting this?

Paul R Johnson · February 1st, 2023, 01:59 AM

Ah, so it may well be squirting 48v up the Rode’s spout! I wonder if the phantom power off mode is still supplying power?

January 27th, 2023, 08:29 AM	#1
Brock Burwell Major Player Join Date: Jan 2014 Location: Charleston, WV Posts: 385	Lots of Noise From Pre-Amp I just started a new job and now am shooting with a Canon C70 and R5. One of the microphones I got is the Rode Wireless Go II, but when testing it I'm noticing a lot of noise from the pre-amp in the C70. When I tested it with the R5, I'm not getting the same amount of noise and it sounds good. I did tons of tests yesterday by turning the pre-amp down in camera and turning it up on the Rode system and then vice versa. I can't seem to figure out what the issue is. Any suggestions?

January 27th, 2023, 11:50 PM	#2
Allan Black Inner Circle Join Date: Sep 2006 Location: Sydney. Posts: 2,931	Re: Lots of Noise From Pre-Amp I haven't got a C70 but looking for something else I came across this. Not sure if it helps, but … If the C70 fan is on, a sensitive mic will hear it, if the ambience is quiet. The built-in mics certainly will. Cheers. __________________ Drink more tap water. On admission at Sydney hospitals more than 5% of day patients are de-hydrated.

January 28th, 2023, 04:14 AM	#3
Andrew Smith Equal Opportunity Offender Join Date: Feb 2009 Location: Brisbane, Australia Posts: 3,066	Re: Lots of Noise From Pre-Amp Something that has given me a lot of unwanted noise (that would sound like a very noisy preamp) is accidentally leaving phantom power on when using a mic level input. Drove me batty until I realised what it was. Might be worth checking for that. As always, let us know how it goes. Andrew

January 28th, 2023, 10:05 AM	#4
Paul R Johnson Inner Circle Join Date: Nov 2005 Location: Lowestoft - UK Posts: 4,045	Re: Lots of Noise From Pre-Amp Phantom power, by design cannot add noise, because the voltage is applied identically to the two audio lines - what it can do is 'annoy' a connected device that does not appreciate having 48V applied to it. if switching it on or off with nothing connected raises the noise floor, that's strange.

January 28th, 2023, 12:57 PM	#5
Andrew Smith Equal Opportunity Offender Join Date: Feb 2009 Location: Brisbane, Australia Posts: 3,066	Re: Lots of Noise From Pre-Amp Phantom power annoys my RØDELink Filmmaker receiver unit. Andrew

January 29th, 2023, 04:13 AM	#6
Paul R Johnson Inner Circle Join Date: Nov 2005 Location: Lowestoft - UK Posts: 4,045	Re: Lots of Noise From Pre-Amp Usually it's an indication the device connected is either not expecting it and the output resistance, as opposed to impedance) is unequal between ground to XLR 2 and 3. In these cases, the unequal resistance doesn't really matter much for the audio, but means that some of the 48V won't be cancelled out, leaving volts on one leg - and that can make things unpredictable. Usually there will be capacitor isolation on output sockets, but some designs didn't consider the possibility of having 48V coming back into the output. Soundcraft - a well respected mixer manufacturer had these things when people connected two mixers together. I discovered it using two, with the first an orchestra submixer. It's stereo out when to channels 1 and 2 of another Soundcraft. These had the phantom power on which went into the mixer outputs. Audio wise - it was perfect, but every LED in the output section - the groups and the stereo out all lit up! It's possible that the Rode output isn't actually balanced at all - but has one of the XLR pins 2 or 3 tied to pin 1 - this would then work fine with unbalanced camera inputs, and still work with cameras with balanced inputs - but phantom would screw it up.

January 29th, 2023, 03:32 PM	#7
Allan Black Inner Circle Join Date: Sep 2006 Location: Sydney. Posts: 2,931	Re: Lots of Noise From Pre-Amp C70 Manual: Options for Fan Mode. [Automatic]: The fan runs while the camera is on but not recording, and is automatically turned off while the camera is recording. However, if the internal temperature of the camera is too high the fan will be activated automatically. When the camera's temperature has decreased sufficiently, the fan will be turned off. Use this setting when you do not want the camera to pick up the fan’s operating sound. Note: But depending on the ambient temperature and other shooting conditions, the fan may not turn off even if you set its operating mode to [Automatic] Cheers. __________________ Drink more tap water. On admission at Sydney hospitals more than 5% of day patients are de-hydrated.

January 31st, 2023, 01:51 PM	#8
Greg Miller Trustee Join Date: Jan 2011 Location: Pennsylvania Posts: 1,792	Re: Lots of Noise From Pre-Amp I daresay many source devices, like wireless receivers, have internal power supplies that are positive with respect to ground. If these devices have output coupling capacitors that are electrolytic, the capacitors are installed with their positive terminals connected to the internal electronics, and their negative terminals connected to the device's output connector. If you now connect that source device's output to the input of a device which has phantom power turned on, those coupling capacitors will now have positive voltage (probably +48v, rarely +24v) applied to the capacitors' negative terminals. This will significantly reverse-bias the capacitors. It can produce unpredictable performance including noisy performance from the capacitors (hopefully not catastrophic failure of the capacitors). So it's quite important that the phantom power is turned off in scenarios like this. I may be showing my age, but this is a good argument for balanced/floating output coupling transformers, rather than only capacitors. (And a transformer will work equally well when feeding a balanced or unbalanced input.) Just sayin' ...

January 31st, 2023, 02:42 PM	#9
Pete Cofrancesco Inner Circle Join Date: May 2004 Location: USA Posts: 3,005	Re: Lots of Noise From Pre-Amp Could be shielding issue on the cable or the receiver. Btw the c70 is xlr input the Rode outputs consumer unbalanced. How are you converting this?

February 1st, 2023, 01:59 AM	#10
Paul R Johnson Inner Circle Join Date: Nov 2005 Location: Lowestoft - UK Posts: 4,045	Re: Lots of Noise From Pre-Amp Ah, so it may well be squirting 48v up the Rode’s spout! I wonder if the phantom power off mode is still supplying power?