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Noise when two portable recorders share a power bank.
Old 4th July 2020
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Noise when two portable recorders share a power bank.

Hello! I am new to the forum - I have been lurking for awhile but I have been learning from the forums and I recently purchased some gear that I want to use for classical location recording.

I have a noise issue when I try to use my Tascam DR-40 as a backup recorder for my DR-701D. The DR-701D's 3.5mm line out runs into the DR-40's 1/4" line inputs via a mini TRS to dual TS cable. The two devices are both plugged into a portable power bank.

A digital(?) noise is audible when the devices are both plugged into the battery and I use cheaper Y-cables (Hosa, and other random cables I have tried). When I use a nicer handmade Y-cable (Mogami W2528 + Rean connectors), the sound disappears. Additionally, the sound worsens when I start recording to the SD card of the DR-40. Every time the display updates to change the recording time (every second), there is a noise that is recorded. Using the nicer Y-cable or running the devices on separate power banks eliminates the noise.

I have attached some recordings of the noise. Here are some notes on each recording.
1. Using the good cable results in no noise.
2. When I touch the tip of the 3.5mm end of the good cable to metal on the DR-701D, I can hear the noise.
3. Using cheaper cables results in this noise.
4. Noise occurs only when the DR-40 is plugged into the same power bank as the DR-701D.
5. When the devices use separate power banks, there is no noise.

Would anyone be able to explain why this is happening? I believe it is some sort of grounding issue, but that is the extent of my knowledge on that subject. I would like to be able to use the same battery bank for both recorders, and also not have to worry about what cable I have to use. Why does the nicer cable stop the noise from occurring? Would using USB cables with ferrite cores make a difference?

Thanks for the help, glad to be a part of the forum.
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