Subwoofer hum when BHK 300 switched into standby

I recently installed a pair of REL Carbon Limited subs in my system, replacing a pair of S3/SHO. When I switch each of my BHK 300 amps into standby, a low-level but clearly audible hum is coming from the Carbons. Switch the amps on, it disappears. Switch to standby, and there’s a hum.

This does not seem to me to be transformer hum from the amps, when I switch the subs off, the hum quits and it is most definitely coming from the subs. I’ve reached out to REL, but I thought I’d run it by the PSA pros as well.

Thanks, David

Do a search in these forums using “REL”, “standby” and “hum” as your search terms. IIRC, some work-arounds have been discussed and implemented.

Good luck.

I assume your connecting subs to preamp with RCA? Do you get the hum if the RCA or XLR cable to sub but not preamp when sub is in standby? If so, it sounds like interference. Are the cables to subs really long? Over 8’

If you using the REL connection to amps output, I remember something about not connection the ground wire for some units? Call REL.

I use Iso transformers hi level to XLR, and I get ZERO hum to my JL Audio subs.

@flyingparrotsme-com - Add RCA L/R (if you have stereo) to the BHK300 inputs (assume you are using XLR BHK300s input) going to REL LEH/.1 input and turn the pot CCW. Then turn off the music and allow the REL auto-sense to kick in (30min); then turn the BHK300s to standby. If that is too much to deal with, turn the BHK300s to standby and deal with the hum for 30min (auto-sense on REL turns REL to standby) and then it will go off… there has been tons of posts on this topic… search is invaluable… I assume the red & yel REL wires are going to (+) BHK300 amp and black wire is floating as the REL manual indicates.

This is the approach I have used for my BHK250 and set of BHK300s. There are other approaches with the rel resistive network and PSA network. You may look into that approach. I find this the least drama wrt the network that did not work…

Thank you. My initial search brought up similar, but not exact posts. I suppose my search terms were not specific enough. Thanks again.

@flyingparrotsme-com - all good, we’ll help sort it out. Cause, at the end of the day… it gets dark… :face_with_monocle:


I have the same subs and a pair of BHK 300’s and had the same problem for about a week. I do not have the problem anymore but I honestly am not sure how the problem has gone away. I have the black wire tied to the negative post on the amp and both the yellow and red wires attached to positive and have the subs on all the time. My set up seems to contradict what other folks have said fixes the problem. Not sure if anybody on this post can explain why I am not having this problem. Everything is plugged into a PS 15 Powerplant.

@minnesotafats - there are many ways to configure the REL/BHK. The REL manual indicates float the black wire and hook up the red & yel to the (+). I have tried hooking up the black wire to the REL network with no success. I have tried hooking up the REL black to GND, and or (-) BHK… I guess if it works, go for it… however, I would email/call REL to make sure any set-up is OK just in case of damage, you never know. Also, I would call them to ask why did it suddenly go away… and I assume you are referring to the REL hum when the BHK is placed in standby, and the REL sub is ON. If the REL sub is in standby and the BHK is placed in standby, the REL hum will not happen. At least that is my experience.

If your REL subs are auto-sensing like the S3/SHO for instance, the hum will stop when the auto-sense switches the REL sub from ON (white LED) to standby (amber LED). This may be why you think it suddenly went away. I don’t know, may be it…

When you switch the 300s into standby it disconnects the outputs feeding the REL. You need to connect a separate ground wire (not the the 300’s outputs).

Thanks, Paul, but I thought I did that with the ground wire when I attached it to the small binding post that was included with the BHK 300 amps (the one that gets installed by the balanced input on the amp?) Should that binding post be placed somewhere else on the back of the amp?

Hmmmm. well, that should have worked. It does for us. You can give our folks a shout (or REL) and see what they suggest. I am sitting on a beach right now so hard pressed to do anything other than answer as best I can from memory.

For the love of toast! Don’t answer emails, enjoy the beach!! (But thank you, anyways).

Attach the separate ground wire to a grounding point of another piece of equipment besides the BHK’s. I use the ground lug on a line conditioner. Are the amps on your REL’s Class D or A/B?

The solution to the hum I got with my REL S5/SHO and BHK250 involved a binding post from PSA that replaced a screw on the back of the BHK250 and a resistor network from REL. Hum disappeared.

And that exact solution worked for me too with a REL Carbon Limited and a BHK250.

Yes, I have the small binding posts. I don’t recall ever hearing the same problem when I had my S3’s, but it may not have been as audible as with the Carbons.

With the Carbon’s you have a Class D amp which like the BHK’s does not provide a true ground from it’s negative terminals.

@cardri - Thank you for the tip. I reached out to Rel and they said not to do what I was doing…Yet watch the video on their website and they tell me to do exactly what I did with a BHK 300 monoblock in the video, showing the black wire attached to negative post. and look at this video STEP 3D. CONNECTING A STEREO PAIR OF RELS TO A SINGLE AMP OR MONOBLOCKS USING THE HIGH LEVEL CABLE

The irony is that they sent me an email saying that I could have damaged the BHK’s and the REL. I am nevertheless confused. I followed the video instruction to a tee when looking at the first minute of the video and had no hum problems whatsover. Now I am getting hum whenever the BHK’s are in standby mode. Hoping to get clarificaiton from both PS audio and REl at some point. Will keep you updated.

@minnesotafats - I did what you did day one and it cost me a trip to REL for my right sub. There are many approaches like the network ($150 from ReL)… I have that w/ PSA standoff (free)… if you search you can read my posts with pics for the ADD… I got it working w/ REL and PSA network and stand-off. Paul is right when the BHK goes into standby. My solution is a L/R RCA ( just using the GND) from BHK input (assume you use XLR) to REL LEF/0.1 and CCW pot. I turn my music off, wait for REL to go standby, then BHK Standby and no hum. Or turn BHK to standby and listen to hum for 30 min and then no hum when REL hits standby… or you can go the REL network route… be very careful w/ the black wire… it is the first question they ask when you create permanent hum and have to ship it back… and their service is great and John is just like Paul… a great guy who loves tunes. Unlike Paul, he has a C1 Vette and I own one w/ 1,000 HP, a 61 we had great conversation.

I don’t like the REL network claptrap crap on my amps so I shut off my Roon in my music room and come back out in 30 min and shut off BHK. I also did a test for someone that found out the hum goes away after the REL auto-sense kicks in and RELs subs to to standby. Only S3 and above have auto sense.

So, clap trap works and so does my easy solution on both BHK250 and BHK300s

Just so we are clear S3 is Class D… same as CF sub…