Forums
https://www.decware.com/cgi-bin/yabb22/YaBB.pl AUDIO FORUMS >> General Discussion and Support >> Subwoofer Hum in Differential Mono https://www.decware.com/cgi-bin/yabb22/YaBB.pl?num=1704216170 Message started by mk60 on 01/02/24 at 17:22:50 |
Title: Subwoofer Hum in Differential Mono Post by mk60 on 01/02/24 at 17:22:50 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!! :) |
Title: Re: Subwoofer Hum in Differential Mono Post by MikeinMontana on 01/02/24 at 18:08:00 We own an older Hsu subwoofer that has speaker high level ( speaker, 5 way posts in and out) as well as low level rca ins and outs.. I wish I could help on your sub..but no regular speaker high level on those. No idea on the newer subs these days...we do use a 3 prong to two prong adapter on the subs plug, making it completely absent of any hum. I don't know if Rel is ok using those though...if Rel ok's this, might be worth a try? Someone will surely get back to you on this... [smiley=icqlite20.png] |
Title: Re: Subwoofer Hum in Differential Mono Post by CAJames on 01/02/24 at 19:34:46 Quote:
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. Quote:
Yeah, good question ![]() Good luck, keep us posted. |
Title: Re: Subwoofer Hum in Differential Mono Post by mk60 on 01/02/24 at 19:46:53 Thanks, CAJames! That's a helpful explanation of the differential operation :) Experimenting a bit with my left amp and sub, touching the black sub wire with my fingers does not change/reduce the subwoofer hum, nor does connecting the black sub wire to ground on my phono preamp. There are definitely plenty of other possible grounding points to try, though. Given your description of differential operation, I'm now curious why the sub isn't connected the same as the speaker: that is, red/yellow to +, and black to -? My guess is it has something to do with how perhaps the sub amp itself is wired, and perhaps this is particular to REL? The instructions to leave the black disconnected when using differential monoblocks is from REL themselves. Maybe I should reach out to them too. |
Title: Re: Subwoofer Hum in Differential Mono Post by CAJames on 01/02/24 at 20:05:11 Quote:
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. |
Title: Re: Subwoofer Hum in Differential Mono Post by mk60 on 01/04/24 at 02:53:56 Hmm I wonder what the right ground is? I tried the grounds on a few different phono preamps and SUTs but no dice. Also, I'm not sure if this is unexpected, but with the the yellow/red connected to + out and black floating, and the subwoofer humming, it seems like the subwoofer isn't really carrying any musical signal at all, just the hum. Even if you turn the sub all the way up, you just get louder hum but no music/bass. Anyway, I've reached out to both REL and Steve. Will wait to try "conventional" wiring before I get confirmation that it won't be harmful, as REL seem to warn against doing this on differential mono amps. |
Title: Re: Subwoofer Hum in Differential Mono Post by mk60 on 01/05/24 at 20:21:38 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 :) |
Title: Re: Subwoofer Hum in Differential Mono Post by briand77 on 01/05/24 at 21:39:49 The best solution for integrating subwoofers is to use an active crossover. Split the signal at 60 or 70 Hz. You no longer have to match the mains roll-off, you will gain some headroom with the amp, and the mains will have cleaner bass with less stress on the woofer. Here is a quality, inexpensive analog crossover: https: //sublimeacoustic.com/products/k231-stereo-3-way-active-crossover (remove space in url, can't post actual links until 12 posts) |
Title: Re: Subwoofer Hum in Differential Mono Post by Geno on 01/06/24 at 00:44:29 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 |
Title: Re: Subwoofer Hum in Differential Mono Post by mk60 on 01/06/24 at 00:55:51 Hi Geno, thanks for the thought. Unfortunately, that trick can only be used when using the amp in stereo mode. When in non-balanced mono, bridging the two negatives compromises the function of the amp (although it does still help with hum, it causes other problems), and while in balanced differential mono the two negative terminals are actually already bridged together :) |
Title: Re: Subwoofer Hum in Differential Mono Post by dank on 01/07/24 at 23:50:12 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 |
Forums » Powered by YaBB 2.2.2! YaBB © 2000-2008. All Rights Reserved. |