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Some time ago, I had the ( stupid ?) idea to use cables which convert asymetrical outputs to symetrical.
This way I attached my old synths to an A16 ultra.
Unfortunately, 50Hz buzz was present more intense than before.
Today I switched back to "mono-cables" and the buzz is gone.

Is there a reason, why I should try to use symetrical inputs for my old synths ?

They are not balanced outputs most likely. There are various ways to break the inverted side of the connection documented online, or you can use a DI box (this can also add some nice color), or you can use this: https://www.musiciansfriend.com/amplifi ... eliminator

no.

whatever way works best is best. it's all about eliminating ground loops.

This is what the Hum Eliminator is doing internally

Here's a good article that covers this in more depth than I can muster right now. I'll attach the relevant image here and reference that it's probably #14 that interests you, but I do suggest reading the article too if you have time.

don't usually need a transformer in the first pic...

anyway, when connecting balanced devices to unbalanced devices, there are often significant level differences as balanced lines are usually +4 and unbalanced lines are usually -10 or -20. there can be impedance mismatches as well. surprisingly, there is no such thing as a standard, although there are conventions...

Agreed on those points as well

I used setup 14, but the cable was 1-conductor shielded. ( so black and green in pic 14 was identical)
When using 15, all is quiet again.

I also tried a DI-Box, but it didn't help. Probably due to my cabling ?

As gary said, I'm happy to find at least one solution, that works.
But being a curious person, I'd can't stop experimenting

here is the "bible" for audio engineers. if you REALLY want to be pedantic, you can buy the latest edition on Amazon or similar...

https://soundsfxedit.files.wordpress.co ... ndbook.pdf

this is all the basic information an audio engineer needs. remember, the guy that runs the train is the engineer. if you own a studio, you are supposed to be one. otherwise, you're just a monkey with his hand on the throttle of the train...