NAP140 + dual mono REL R-218 subwoofers problem

I encountered an interesting issue when I “upgraded” my CB NAC62/HiCap/NAP140 system’s subwoofers, from a pair of REL T-5is to a pair of older, but beefier REL R-218s.

I had the T-5is high-level inputs connected independently to each loudspeaker, via the speaker terminals, as follows:

  • Connection at the left speaker terminals: Left subwoofer’s speakon cable’s red & yellow to left speaker’s positive, and black to left speaker’s negative.
  • Connection at the right speaker terminals: Right subwoofer’s speakon cable’s red & yellow to right speaker’s positive, and black to right speaker’s negative.

This is the official REL recommended way to connect dual, mono (left and right) subwoofers. I was using this set up for many months, and it worked perfectly.

Now I have swapped in the R-218s, and with both subwoofers connected, I’m hearing much higher frequencies than expected from the subwoofers. In other words, it sounds like the subwoofer’s crossover has been bypassed, which shouldn’t be possible via the REL’s high-level inputs (only via the low-level LFE input). Just to be clear, it’s not a subtle effect, and makes the system completely unlistenable.

If I disconnect the speakon cable from either the left or right subwoofer, the issue goes away, with the remaining connected subwoofer behaving as expected. Otherwise, I’d suspect that one of the R-218s was defective. And in that case, I would expect the problem to persist in just one (defective) subwoofer, whether one or two subwoofers were connected to the system.

Also, if I connect one subwoofer via the speakon’s cable red lead to speaker terminal positive, and yellow to speaker terminal negative (with the black speakon lead floating), things work fine (perhaps the yellow/red lead connected subwoofer needs a higher volume setting, but otherwise seems to behave correctly). However, I don’t consider this a solution, but rather just a data point.

Next, I swapped the T-5i’s back in, just in case I messed something up during the switch-over. But the T-5is worked perfectly like before. No issues at all.

By the way, R-218s have class D amplifiers vs class A-B amplifiers in the T-5is. I do know that the REL speakon cable’s black lead needs to be lifted when connecting to class D amplifiers. But I’ve never read of any special handling that’s required when the REL amplifiers themselves are class D.

So I’m really reaching here, but perhaps my unserviced 140’s negative output has some voltage leakage, which interacts with the dual R-218’s class D amplifiers somehow (if so not sure why only when two are connected)? And on the other hand, perhaps the T-5is are more resilient to voltage leakage?

Anyway, if anyone has any ideas, I’d like to understand the root cause of this behavior, and how I can go about fixing it. I will try REL support next week, but thought I would share here in the meantime.

Please don’t suggest that I switch to using low-level inputs. To each his own, and there’s nothing wrong with that, but I personally prefer REL’s recommended high-level connection method.

Have you tried connecting just black to negative and red to positive, leaving yellow disconnected?

Maybe the new speakers have inverted phase …. Try setting the subs phase to 180 or swapping the pos and neg at speaker ?

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Thank you Chris and Fred for the suggestions.

…black to negative and red to positive, leaving yellow disconnected?

Unfortunately, this makes no difference.

Maybe the new speakers have inverted phase …. Try setting the subs phase to 180 or swapping the pos and neg at speaker ?

Do you mean the phase of my loudspeakers or the subwoofers? In any case, I ran the phase tests from my XLO Test & Burn-In CD, and confirmed that my Audience The Ones are in-phase. Just to clarify - these are the same loudspeakers as I was using previously. I only changed the subwoofers.

I wouldn’t expect the subwoofer phase settings to cause the issue that I’m experiencing. However, these are good suggestions that could provide more data points on possible defects in the R-218s. I tried all combinations of the phase switches. And I also swapped the positive and negative leads at one of the speakers. Unfortunately, none of these changes made any difference. The issue persisted in all cases.

The only work-around I’ve found is to float the black lead of either subwoofer.

So there’s apparently a ground loop issue.

It’s not good to have one subwoofer connected by positive only, so I suppose that I could use the REL recommended connection method for class D amplifiers.

The bizarre thing is that this method is supposed to be necessary when using class D amplifiers, not when the REL amplifiers themselves are class D (as in the case for the R-218s).

I will inquire with REL Support this week.

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I found a work-around that I can live with.

  • Speakon red & yellow leads to each loudspeaker’s positive (alternatively just the red leads).
  • Both speakon black leads to one loudspeaker’s negative.

I’ll still follow up with REL Support this week to see what they say about it.

Turns out that I was experiencing what REL calls “high frequency breakthrough”, not a ground loop (which causes hum).

They let me know that the the grounding design of the R218 is different than with the T5i (resistor capacitor network between signal ground & earth for the R218 vs signal ground bonded directly to earth though a resistor for the T5i).

They couldn’t explain why I was getting high frequency breakthrough with the dual R218s, but said that “if lifting one(or both) of the BLACK wires of the REL high level cable works then that is a valid solution, as long as the REL(s) do not hum. If they do not hum then it just means that that ground connection is getting connected through another path and the BLACK wire is not necessary. That path is likely through the earth connection to the wall AC circuit”.

By the way, they also said that the typical cause of high frequency breakthrough is due to “balanced output amps, because one output terminal is active and moving the ground of the sub around”. However, they said that’s not the case here as I’m not using a balanced output amp.

So the root cause remains a mystery, but I’m going to stick with both high-level cable black wires connected to a single speaker’s negative. I feel more comfortable with that option vs floating black wire(s).

I need to back-pedal…

Don’t connect both REL high-level cable black wires to a single speaker’s negative. After further auditioning, I realized that the sound stage sounded a bit flat and closed in when doing this. More significantly, my NAP140 overheated and did a thermal shutdown. Fortunately, no permanent damage was done.

So the updated connection options for dual REL R-series are:

  1. Float one of the REL high-level cable black leads.


  1. Float both of the REL high-level cable black leads. Make up a Y-cable that ties the REL signal grounds together via the unused LFE low-level input collars (negatives), and also to the grounding screw on the preamp - NAC62 in my case.

I’ve tried both of these options and am getting good results.

I used to connect a sub to my Hicap via a lead I bought on ebay.

Yes, I could certainly go with a low-level connection instead. However, I prefer to connect via a high-level connection when possible, which REL strongly recommends.

Interesting reading. My amp is a balanced bridge design. The manufacturer says in like 5 different places Do not hook up anything except speakers to the ground. Since it’s not a floating ground but an active ground. So I ran left and right Rel leads to positive left and right and ground black to my Shunyata which has a ground post. Dead quiet.

To me it sounds like the 2 subs are out of phase. If all’s ok with only 1 connected, but the sound “thins” when the 2nd is activated, then they cancel each other.
I had a rather similar, but very strange experience with Rel phasing myself. When I added the 555DR instead of the XPS DR to a 272, in the end I also found out that I needed to switch the phase on the Rel.

Yes, for balanced amps, REL advises to float the high-level black lead(s), or ground them to your amplifier chassis.

It’s true that my description sounds like an out-of-phase issue, but to clarify, it sounded more akin to having a Naim (Black series) signal ground switch (float vs chassis) in the wrong position. Of course I don’t have such a switch on my CB gear. But I’m just trying to illustrate the effect that I heard when I connected both REL high-level black leads to a single speaker’s negative post.

Btw, I checked for incorrect phase issues earlier. See my 2nd post in this thread for details if interested.

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