Here’s a look at the performance and setup of a pair of REL Acoustics Carbon Special subwoofers, my methods here apply pretty much to any sub from any manufacturer.

If you prefer to watch a YouTube on setup of the REL Carbon Special, click the video below to get started, otherwise scroll down.

Let’s start out with a quick look around the REL Carbon Special.

First a warning….. be prepared for some frustration with this sub as there are some really disappointing elements about its basic design. Be prepared also to ignore some of what’s printed in the owners’ manual as it will inhibit your chances of getting the best out of your subs. Here’s an overview of what we’ll be covering in this article:

1. Design and Aesthetics.
2. Basic Specifications.
3. One Sub Versus Two.
4. Main Speaker Considerations.
5. Setup, And How To Get the Best from Your Subs In A 2 Channel System.

>I won’t be talking about any specific home theatre applications.<

Design and Aesthetics.

The REL Carbon Special is an exquisite piece of engineering and design and is quite visually striking. Clearly, a lot of thought has gone into the aesthetic and how to style the sub such that it isn’t just another plain black box. It looks stunning in the gloss black finish with silver trim, and if you run them without the grills the carbon-fiber drivers are pretty cool looking too.

The silver/chrome trim is really attractive and the pair of sculpted handles on the side of the subs look great but are also functional when it comes to hauling the subs around your listening room, as we’ll be doing later during the setup section.

There is however a niggly flaw in the design of this subwoofer that would prevent me from unequivocally recommending the sub to any interested purchaser. How debilitating the design flaw is will largely depend on how you decide to use the sub. It may seem almost inconsequential in some cases, but believe me, when you come to do a careful setup of a stereo pair of REL Carbon Special subs, it’s going to cause you a lot of frustration. The issue is with the 3 potentiometers on the back of each sub, obviously vital for setup. They do not have a position indicator or scale. And worse, the tactile notches that one is supposed to be able to ‘feel’ as one turns the dial, are non-existent.

The 3 pots (dials, knobs, whatever you care to call them) are for ‘Hi/Low Level’ (volume/gain), ‘.1 LFE Level’, and ‘Crossover’.

Carbon_Special_sub-controls

I’ll describe how the problems manifest in the setup section, but this is a real rooky issue and a major surprise to me given REL’s status and heritage in the sub marketplace.

 

Basic Specifications.

  • The carbon fiber 12-inch driver delivers 50% longer stroke than previous designs.
  • As well as the front-firing 12-inch active driver, there’s a down-firing passive unit also.
  • Low-frequency extension is listed as -6 dB at 19Hz in-room.
  • The Carbon Special is equipped with all of the usual input and output connectors for a sub of this quality, and high-level inputs are via studio-grade Speakon connectors.
  • Power output from the NextGen 5 Class D amp is listed as 800 watts RMS and up to 1,000 watts peak.
  • The Carbon Special has wireless capability via the REL AirShip II which is sold separately.
  • Dimensions are 17″ x 18″ x 21.25″ and the weight is 85 LBS, the sub is available only in Gloss Piano Black..

REL Carbon special connectivity options

So pretty impressive specs from the REL Carbon Special, and they’re surprisingly compact given the driver size, so they can blend nicely into most reasonably sized rooms. And again, they do look quite stunning, particularly the silver handrails on each side and the aluminum feet with the REL logo!

One Sub Versus Two.

This applies to any sub regardless of manufacturer, but generally speaking, TWO subs will always outperform a single unit. In the case of the Carbon Special, it isn’t necessarily about having more output, as a single Carbon Special can provide more than enough low-frequency extension and output in a 2-channel audio system. With two subs, each is working with less effort and you’ll hear a marginal improvement in bass tone and timbre and also more clarity through the upper bass and lower mids. You’ll also hear an increase in soundstage size running two subs versus one, particularly if you can place them outboard of the main stereo pair.

So if you can justify using a pair, …….then do so. If not, you won’t be missing too much and you can always add a second one later when funds permit.

Stacking Subs? – It should be mentioned that REL makes a big thing about the benefits of stacking subs, and the Carbon Special is designed to be stacked but I’ve never used them that way since it doesn’t make much sense with only a pair of subs.

REL Carbon Special sub with proac D40R

Main Speaker Considerations.

I’ve run the REL Carbon Special both with a full-range speaker system that really didn’t need any low bass augmentation, and the subs still managed to improve on the sound. I’ve also run the subs with a smaller floor-stander that rolls off low bass sharply at around 40 Hz, and in that scenario, the Carbon Specials really come into their own.

Obviously, if you’ve a pair of bookshelf speakers then the Carbon Special is going to be a very good choice, but you can get more out of your system with REL subs even if you’re running a true full-range speaker setup.

The speakers used predominantly with the REL Carbon Special in this review were the ProAc D40R, a 3-driver 2-way design with a down-firing port. The ProAcs are very much full-range and are actually quite bass-heavy. I’ll get to a trick that I use for integrating subs into a true full-range setup in a little while.

Setup And How To Get the Best from Two Subs In A 2+2 System

So now we get to the fun part – setting up and tuning the pair of REL subs in a 2-channel stereo rig.

First up is deciding how you’re going to connect the subs in your system, as there are several options to consider with the Carbon Special.

  1. You can connect to the REL sub from the variable pre-output on your preamp, using XLR or single-ended RCA cables.
  2. You can connect to the REL sub using high-level inputs from your power amp(s). This involves using the supplied Neutrik Speakon cables connected to the speaker outputs on the back of your power amp.
  3. You can use the .1/LFE input from an HT Receiver directly into the REL subs (untested).

For the purpose of the review, I used option 2, the high-level inputs wired directly from the power amp, as recommended by REL. According to REL, this has the advantage of ensuring that your sub receives exactly the same signal as the main speakers, which means that the character of the bass from the main system is carried forward into the sub, or as REL likes to call it, the ‘Sub-Bass System’. How important or even noticeable this ‘feature’ is, is a matter of opinion. I’ve run many subs directly from the preamp outputs and I don’t ever recall thinking that there was something different between the character of the low bass and the character of the main speaker system.

Positioning of the subs is the same basic procedure, regardless of which connection option you choose. So, to connect a single sub or pair of subs to your power amp, you’ll need to consult the REL Carbon Special manual as there are some important connectivity options to observe which may also result in some trial and error.

For example, when I ran a Krell KSA 80, which is a single-ended stereo amp, I connected to the speaker binding posts using the recommended method of twisting the red and yellow conductors together and attaching those to the positive binding post on one channel, and connecting the single black conductor to the negative binding post. With that arrangement, the gain was too high, resulting in there being too little adjustment available on the volume pot. Splitting the red and yellow twisted pair and using only the red wire lowered the gain and increased the available range of adjustment from the pot, but not really sufficiently. There really ought to be a gain switch on the sub, to make matching with different power amp designs easier. My Veloce Saetta monoblock amps have a fully balanced differential design, so following the REL manual I made the connection using only the red wire from the three wires in the connecting cables. This resulted in a loud hum from the sub and the main speakers, when I connected the black wire the hum went away and everything worked fine.

Incidentally, I started out with the REL cables that came with the sub but switched them out after a few weeks to a far better quality cable. I went with the Iconoclast/Belden BAV REL Subwoofer cable, which you can order from Bob at Iconoclast Cables here.

iconoclast BAV REL sub-cables

If you get stuck with connectivity issues post a comment below and I’ll try to help.

Positioning Subs In A 2-Channel System.

So with connections from the Iconoclast cables made to the amp, and the Speakon connector plugged into the high-level inputs on the back of each sub, you’re ready to start with positioning.

If you read the REL manual, they’ll suggest placement in the rear corner or corners for most scenarios, where they  say the most linear and efficient low bass can be produced. This is because the sub is able to take advantage of the corner-to-corner axis which is typically the longest axis in a room. Those are mostly REL’s words, not mine.

The manual then goes on to suggest an optional “Expert Set-Up”, which it states is preferable as it brings the subs further out into the room, where they should be placed slightly behind and outboard of the main speakers. Here the subs can be either on the inside of the main speakers, or on the outside, based on available space.

Positioning REL subs for 2 channel stereo

Subs on outsides

 

Positioning REL subs for 2 channel stereo

Subs on insides

 

Let’s talk about these options as it’s important, perhaps the most important aspect of getting the most from your subs.

When configuring your primary pair of speakers, you’re essentially navigating a delicate balance between achieving excellent low-frequency performance and optimizing for other critical factors such as imaging and soundstage. Frequently, there’s a conflict between finding the ideal speaker placement for bass response, versus everything else. Simply put, there’s usually a good position in the room for bass, then a different position in the room for everything else.

To achieve an optimal performance range from around 200Hz and above, it’s advisable to position your main speakers away from the front wall, provide them with adequate space from the sidewalls, set them up with a precise toe-in for good imaging and a well-defined soundstage in terms of width and depth. However, this position rarely coincides with the best spot in the room for reproducing low frequencies effectively. This is where the introduction of subs becomes particularly advantageous. Subs grant you the opportunity to place them in the optimal location within the room for low-frequency reproduction while positioning your main speakers to maximize for everything else.

Given the above, it raises a question about the recommendation made by the manufacturer in their manual to place the subwoofers right next to the main speakers, as it negates one of the main points of using subs in the first place. So my recommendation is to determine the place in your room where the subs perform best for what they do (bass). While I show the subs in some of my photos on the inside and close to the speakers, that’s mostly due to the practicalities of being a reviewer. It’s hard to swap in and out different speakers in my room when the subs are not tucked in close, as shown below.

REL Carbon Special sub with proac D40R

With the ProAc D40R shown above, and using the setup procedure described below, the best position sonically in my room for the pair of subs was something closer to this, but your mileage may vary:

Positioning REL subs for 2 channel stereo

Start the process by getting your main speakers dialed in with the most attention to everything upwards of 200 hertz or so. Then use the subs to smooth out the overall in-room bass response to provide the most seamless and linear transition from low bass through to the crossover or ‘hand-off’ with your main speaker pair. To do this I’ll show you a useful trick that you’ve probably seen before – use it and It’ll help you get things dialed in quickly. Put the sub at the listening seat position, play a bass-heavy test track that you’re familiar with, and then walk around the room until you hear the best bass response, then put your sub in that position. If you’re using the supplied Neutrik cable from REL, it’s pretty long so it shouldn’t be as issue to do this simple procedure. If there isn’t a power outlet near your chair, use an extension cord, it’s fine for this purpose even if you get a little hum from running the sub off a different AC circuit.

If you’re setting up a stereo pair of subs, walk the room and mark two positions as far apart as possible where the bass sounds optimal, and use those two positions for sub placement. For a down-and-dirty setup procedure the above works well.

Once your subs are placed you’ll need to blend them into the main speaker pair by first setting the phase angle and then adjusting the crossover point, followed by the volume.

Setting Phase On Your Sub(s)

To set the phase, there’s a toggle switch marked ‘Phase’ with options for zero and 180. Play your bass test track with one sub connected and your main speakers playing also. In the manual REL tells you to play a bass-heavy recording and set the phase at whichever position results in the loudest bass. In other words, with your main speakers playing, and the sub playing, one of the two toggle positions will increase volume in the HF and the other will decrease the volume, due to cancelation. I disagree with setting the phase for the loudest bass. Set the phase angle at whichever position gives you the most ACCURATE bass. By accurate, I mean taught, tuneful, pure – versus boomy, bloated, etc. You’ll know it when you hear it, it just sounds right one way and not the other. This may or may not be the loudest bass, as preferred by REL, in most of my setups it was not. Why does this clash with the instructions given by REL, surely they must be right? Well, I think in part it’s because REL chooses not to have a potentiometer for phase adjustment which allows for 0-360 degrees of phase adjustment. Think about it. Even if you place the sub directly adjacent to the main speaker pair with the two sets of bass drivers each the same distance from your ears, this does not mean that the subs and the main speakers are exactly in phase with one another. You have to account for time arrival differences due to the effect of the main speaker crossovers and time arrival differences caused by the electronics and circuitry in the subs. Rarely will they be exactly in phase (zero) and rarely will they be exactly 180 degrees out of phase. So REL’s logic is that if you toggle their two position switch and the combined bass output sounds louder, then they must be in phase. If it sounds less loud, according to REL, then they must be out of phase with frequencies  theoretically canceling. None of this stacks up as true. Again, in my experience, when the subs are active and the combined bass output is louder, it doesn’t mean that this is the best position for the phase switch, so try it both ways and listen.

Once you have the phase set properly on the first sub, turn off that sub and move to the next one, then repeat the same procedure to establish the correct phase.

Setting Volume On Your Sub(s)

The next step is to set the volume so that each sub blends nicely with your main speakers. Again, do this one at a time initially, with the main speakers playing and with the other sub turned off.

Start with the volume pot turned way down low then turn it up slowly until you just barely hear the sub. When you hear its effect, back off the pot a fraction. Touching the bass driver will give you some tactile feedback, which helps. In a full-range speaker system, don’t expect the sub’s driver to be moving much.

After you’ve set the level of the second sub in the same way as the first, switch on both subs. With both subs playing, you’ll need to drop the volume slightly on both subs by an equal amount to compensate for the additional output of having two subs running, versus just the one. Be prepared to play around a little with getting the levels just right, and again, touching the main active sub driver will give you some tactile feedback.

With the REL Carbon Special, setting gain and matching them between two subs is made unnecessarily difficult due to the fact that the pots have no visual calibration or markings. You’ll have to eyeball the position of the dial without any calibrated guide. This is really problematic since the range of adjustment with all of the amps I used in my system was very small. Literally turning the pot 10 degrees took the output from very low to very high. Without a calibrated scale it makes adjustment difficult. Also, if like me you occasionally play a concert DVD through your system and like to turn the subs up a little, forget about it. It’s a nightmare to have to reset the volume when you switch back to 2-channel listening. Remote operation should’ve been implemented by REL, particularly on a subwoofer in this price range.

Setting the crossover points.

You’ll do this one sub at a time, so leave one sub switched off while you’re working on the other sub and vice versa.

Once again, setting the crossover point on a Carbon Special is a bit of a pain due to the poor design of the crossover potentiometer – there’s no calibrated scale. Supposedly the potentiometer dial is ‘notched’, so you can feel the increments as you turn the pot. I can’t feel any notches when I’m turning the pots on either of my subs and since there’s no scale or pointer on the pot adjustment, it’s impossible to know accurately where the crossover point is set.

It’s frustrating, and you’d expect better from a company like REL.

With the full-range ProAc speakers delivering output to around 30hz, I set the crossover point quite low. It’s impossible to say exactly where it’s set due to the potentiometer design, but from listening and playing back bass test tones it appears to be set at around 40-45 hz.

With main speakers that offer less low-frequency extension, you’ll want to set the crossover higher, though I always lean toward the lowest possible setting. I can’t think of any reason why both crossovers shouldn’t be set at the same values, so once you’ve completed the setup on the first sub, just mimic the pot position on the second sub – yes, you’ll just have to eyeball it the best you can since there’s no marking on the pots.

Using a good CD with bass test tones is essential for setting up the crossover points properly, and there are a number of good CDs available, including several published by Stereophile (magazine) and Chesky Records. I believe there are also test tones on Paul McGowan’s ‘Audiophile Guide’ CD/SACD, and I have a copy of that somewhere around the place that I haven’t gotten around to using yet. Having a good SPL meter is also useful, and for the purposes of what we’re doing here, you can download a free SPL app for your smartphone. You’ll use the test CD to play tones at different frequencies starting low, at around 20hz, and going up past the crossover point as high as around 150hz. As you play each tone, measure and make a note of the SPL measured by the app or meter and you shouldn’t see any large holes or peaks in the frequency response. Pay careful attention to the measurements on either side of the crossover point. If there’s a gap or dip in LF output around the XO point, then set the crossover point higher to fill the gap. If there’s a peak around the XO point then turn the XO down a little a re-measure. Don’t expect a perfectly linear response in terms of measured SPL through the frequency range as you’ll never achieve anything close to that in most rooms without some form of EQ.

At this point, you’ll have the subs roughed in and ready to play some real music. When you start to flick through some tunes on your playlist, avoid the temptation to jump up and make adjustments whenever you hear a track where the bass doesn’t sound quite right. A lot of recordings just have poorly mixed bass and if you try to adjust your system to this it will never sound right.

If you’ve used a good test CD and followed the procedure above there shouldn’t be any obvious ‘holes’ in the frequency range. If you do hear what sounds like a segment of missing notes in low-frequency playback it could be one of several things.

  1. Just a poorly recorded or mixed track. Don’t react to music that you’re largely unfamiliar with!
  2. The crossover point is set too low. Simply nudge up the crossover points on both subs until the missing frequency range becomes audible and at the same levels.
  3. Cancelation due to room nodes. Hopefully, this won’t happen as it’s the most difficult thing to fix.

If you do have problems you may need to investigate some additional room treatments, and/or DSP/EQ, or start over by moving one or both subs.

You can spend an hour or two playing with the gain settings until it sounds just right. Remember, the idea is that the subs blend seamlessly with your main speakers. Your ears/brain can’t effectively localize low frequency sounds, so if you can hear the subs as the source of sound then likely the XO is set too high and/or the gain.

Also, you shouldn’t be hearing the volume increase or decrease as notes drop down into the lower octaves, if you can, it’s because you have the volume set too high or too low.

rel-carbon-special-review

When properly set up, subwoofers should add the lowest octaves tunefully and accurately, they should augment the main speakers and sound as though everything is cut from the same cloth. In a perfect setup, you won’t know that there are subwoofers in the system, other than from the obvious rumblings and hammering sounds coming from the adjacent neighbor’s wall.

A well-integrated pair of high-quality subs like the REL Carbon Specials provides much more than just low bass. You should hear improvements in the midrange too with more solidity and presence on instruments and vocals. The soundstage should extend in all directions and the music should take on much more of a live feel.

While I’ve used subs from companies like Aerial Acoustics, HSU, and Martin Logan, the pair of Carbon Specials have outperformed any other subs I’ve used in my various systems, even in spite of the annoying design flaws around the potentiometers. At around $9,000 for the pair, they’re quite expensive, but in the context of a high-quality audio system, they may be worth the investment. I have a hard time recommending them unequivocally due to the unnecessarily difficult setup process born from poorly thought-out pots/dials.

If the above hasn’t resolved your specific subwoofer problems, drop a note in the comments below and I’ll try to point you in the right direction.

CAH