Hi all, sent an e-mail to Steve about this, but thought I'd also ask the community to see if anyone else knows the answer

I just reconfigured my two SE84UFO25 monoblocks to run in "differential balanced mode" using custom XLR->RCA adapters. My speakers sound really great!

There is a problem though: my subwoofers do not. I have two REL 212/SX subs that I connect via high level connection. When I was running the amps in "non-differential non-balanced" mono, the connection was simply red+yellow wire to the +'ve speaker out and black wire to the -'ve speaker out, on each amp.

Looking at the REL 212/SX manual, it says that when running differential mono blocks, it should be red+yellow to +'ve speaker out and to leave the black wire totally disconnected and floating.

When I do this, I get pretty bad 60 Hz hum from both subwoofers, and I don't really think they're usable like this.

My question is: has anyone else encountered this specific issue when running their SE84UFOs in differential mono, and is there a safe solution for where to connect the subwoofer's black wire to decrease/eliminate the hum? I don't really know how differential balanced amps work and am afraid to experiment and start connecting the black wire to things when there's a chance I could damage my amps.

Thanks for any advice, and Happy New Year!!

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I can't help you with your actual question, but maybe I can explain how the differential balanced amps work and that will point you in the right direction.

I think of "regular" stereo as a two channel system, where you have left and right channels (the positive phase) that are both relative to ground. Balanced is a 4 channel system where the L and R channels are relative to a mirror image of themselves, the negative phase. So, traditionally a balanced signal is carried on a 3 pin XLR cable, where pin 1 is ground pin 2 is the positive phase and pin 3 is the negative phase. You XLR to RCA cables break out the + and - phase of each channel and then you use the L and R channel of your "stereo" amp for the + and - phase of the balanced signal. So when you connect the L and R positive speaker outputs to your speakers you are differencing the + and - phases (rather than just the + phase relative to ground as in "regular" stereo) and since 1 - (-1) = 2 you get twice the output voltage, in addition to all the common mode noise rejection.

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Yeah, good question . Assuming you're sure you're getting the correct (i.e. positive) phase from both your amps, my guess is the black wire needs to be grounded, but to what? Your source or maybe negative (unused) speaker outputs on the amps would be the logical choices IMO, but that is a (somewhat) educated guess. I think as long as you are connecting black to ground you are safe from doing any actual harm esp. if you just briefly touch the wire to something to see if the hum gets better. You could also try connecting the blacks together but not to anything else.

Good luck, keep us posted.

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Maybe because that is too much voltage for sub? Although you could try it....

To me, it seems like you want to want to wire it the same as "conventional" stereo, with red/yellow to + and black to ground. But the magic of the UFO amps is that the output transformers are not grounded, i.e. they float, so there isn't just one ground, and that is why you are getting 60 cycle hum with black floating. So, again JMO, but you need to find the right ground for the black wire.

Got REL on the phone yesterday and they were totally baffled. They said that either connecting the black wire to a phono ground, or connecting the sub's LFE in to an unused RCA somewhere else on my system should solve the problem by providing the appropriate ground to the subwoofer. Neither worked.

I have devised one possible way around this -- split both of the balanced outputs of my ZTPRE with XLR Y-splitter. From each splitter, send one output to an amp and the other to my ZBIT which can then run RCA to subwoofer. It'll work, but I would really prefer to get the high level connections working, as they tend to sound much better.

Fingers crossed that Steve will have a fix when he gets to my e-mail

MK,

I had this issue running a single SE84 with a subwoofer run high level.

Steve had me run a single jumper wire between the two negative speaker terminals. This eliminated the hum.

Not exactly sure how you would replicate that with two amps, but something similar should get you back on track.

Good luck!

Geno

MK

I think I might know what's going on.  I'm assuming you have two SE84's, each with 2 dual banana output jacks.  You are currently using the two red banana's on each amp (L & R) to drive your 2 speakers (L & R), leaving all 4 black banana's not connected to anything...right?

I suspect you might need to connect all 4 black banana's together and then tie that to the two grounds on the 2 channels of your sub...it might not be enough to do each channel separately as you may end up with the grounds at different levels which could be the source of your hum.


Dan