Ground Loops and Cameras

[soundmandave]

Hello,

First time poster with a Pro Audio question, but first a little intro to myself.

I have been a daily "lurker" at ASR for over 2 years, but this is my very first post as an official registered member. Came here originally for find a DAC recommendation and eventually bought the Topping D90 after seeing Amir's review and have loved it ever since. I work in the product validation department for one of the top semiconductor companies and have degrees in engineering. Validating our products for release by measuring signals with expensive oscilloscopes and analyzers is our bread and butter. But I am very much into audio as a hobby, both as a listener and creator. Got a separate system for all my home listening on vinyl, digital, SACD, streaming, etc, and separate pro gear for my production work, which I use to create content for work along with non-profit volunteering. I consider myself an amateur/hobbyist when it comes to doing any pro stuff.

Got a question regarding ground loops in preparation for a large live event for which I will be providing the audio visual support. I will be doing live sound with all the sources fed into my mixer, but I will also be making a high-quality video recording of the event for offline playback. I will be using a DSLR camera to shoot the video with a Tascam DR-60DMKII to record the audio. I plan to feed 3 submix outputs from my mixer into the Tascam. The Tascam also has a 1/8" TRS stereo OUT that I will feed to the stereo IN of the DSLR for redundant audio recording and to allow me to sync the multi-track audio with the video in post-prod by using slate tones. Since the camera will be some distance away from my mixer, I will be running the audio over balanced cables. My board's outputs are all balanced, but as the Tascam only has 2 balanced inputs and the other 2 are unbal, I will use a matching transformer on the 3rd input to convert it from bal to unbal right before it enters the Tascam. While both the DSLR and Tascam can run off battery, the program length will drain the battery, so I will power both from AC adapters plugged into the closest outlet I can find near the cameras and it will likely be a different circuit than the one I use for my mixer. I want to make the highest quality recording possible and avoid any unnecessary hums from ground loops.

Question: Since the AC adapters for both the DSLR and Tascam are only 2 prong (Line+Neutral, no PE), do I need to worry about lifting the ground in the balanced cables between the mixer and Tascam? Since 2 of the channels are already balanced, I don't need any transformers, and for the 3rd channel that does require one, a transformer is cheaper than a DI box. I searched around for XLR ground loop lifters, but the price IMHO is overpriced for something that is just "breaking" a wire ($24 USD for this; Hosa adapter is more affordable, but reviews question the build quality and mechanical integrity). Also, the audio I am recording for this event is mostly spoken word, so even if there were a 60 Hz hum somewhere, could I just EQ it out?

Thank YOU, and looking forward to hearing all the responses and ensuing humor!

 

[solderdude]

Question: Since the AC adapters for both the DSLR and Tascam are only 2 prong (Line+Neutral, no PE), do I need to worry about lifting the ground in the balanced cables between the mixer and Tascam? Since 2 of the channels are already balanced, I don't need any transformers, and for the 3rd channel that does require one, a transformer is cheaper than a DI box. I searched around for XLR ground loop lifters, but the price IMHO is overpriced for something that is just "breaking" a wire ($24 USD for this; Hosa adapter is more affordable, but reviews question the build quality and mechanical integrity). Also, the audio I am recording for this event is mostly spoken word, so even if there were a 60 Hz hum somewhere, could I just EQ it out?

When 2 of those 2-prong (class-II) are interconnected there won't be a ground loop. It will also be safe to touch the devices and say the screen of the the audio connectors.
But... each of those mains fed devices have a leakage current. That current will be small (mA at most).
That current flows between mains and the DC output of the power supply. It will differ between power supplies and can be in the audible domain (hum) and HF domain.
Both can become audible (directly or indirectly) and would depend on the used interlinks (RCA, balanced), cable quality (RCA screen resistance) and above all the audio devices internal wire/PCB layout.

The problem with hum is it is almost never 50/60Hz only but also the, sometimes much more audible, harmonics. Hard to filter that out. Prevention is the best way.
In case of leakage issues a DI box (with transformer) could help break that connection.

 

[soundmandave]

@solderdude Thank you. My mixer has 3 prong cord and the Tascam has only 2 (powered by USB charger). If using an balanced cable between the fully balanced inputs/outputs of the mixer/recorder, what would be the risk of hum from any stray currents carried over the shield? Is it worth it to break the connection between pin 1 and shield on one end (presumably the Tascam end)?

 

[solderdude]

I see no bears on this path. Certainly not when using a balanced connection. No need to lift pin1 nor shield.
When you are not plagued with hum everything is fine.

 

[soundmandave]

Thanks @solderdude ! Fully agree there is no problem if there is no hum. Only pause for concern is that I won't be able to setup at this venue until the day of the event, so if I had not bought any lifters and there was an issue, it would be too late to correct. Thankfully, I found this video showing how to make your own lifters at a much cheaper price than buying one, so I think I'll build a few of my own just in case.