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EQUIPMENT FORUMS >> SE84UFO25 >> Negative to Negative for hum reduction in mono
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Message started by mk60 on 06/12/23 at 22:53:32

Title: Negative to Negative for hum reduction in mono
Post by mk60 on 06/12/23 at 22:53:32

Hi all, I have a pair of UFO25s each bridged into mono (left ch + bridged to right ch -, speaker - to left -, speaker + to right +)

I have hum from both amps, and had read that one possible solution for this is to connect the negative terminals together (left ch - to right ch -).

Since mono isn't the typical use case, I just wanted to double check that connecting the negative terminals is safe when the amp is bridged into mono?

For clarity, this would mean that each amp has the following connections between terminals:

left ch + to right ch -
left ch - to right ch -
left ch - to speaker -
right ch + to speaker +

It does seem to help with (though doesn't completely eliminate) the hum. But again just want to make sure that this connection is safe in my application.

Thanks all!

Title: Re: Negative to Negative for hum reduction in mono
Post by CAJames on 06/13/23 at 00:15:37

Is it safe? I don't see why it wouldn't be, but that is JMO/FWIW and all that. If you want to know for sure I would email Steve. That would probably be a good idea in any case.

Does it make sense? That I am less sure about. It may be the reason the hum is reduced is because you are no longer connecting the power tubes in series and are reducing the power, along with the hum. I would be more interested in finding the source of the hum itself. Do you get it if you use either amp in stereo? Do you get it with no source connected to the amps? What is your power situation? It could be you have a ground loop, or DC offset on your AC. Steve has helped a lot of people track down these issues as well, so all the more reason to contact him.

FWIW I had a pair of UFOs configured as balanced monos and they were fantastic. So good I got back on the waiting list for a pair of UFO25s and my order just reached the first page. So I'm very excited about joining the "mono 25th" club.

Title: Re: Negative to Negative for hum reduction in mono
Post by mk60 on 12/14/23 at 21:17:56

After talking to Steve, it turns out that the answer is no, one should not do this when running the amps in mono!