connecting a REL subwoofer: why high level, and when not to
Most subwoofers take a lead from the amplifier's sub output. REL ask you to wire theirs to the speaker terminals instead. Here is why, and when not to.
You look behind your amplifier, find a socket marked SUB OUT, and reasonably assume that is where the subwoofer goes. For most active subwoofers it is. REL are the exception, and if you follow the obvious route with one of theirs you will be ignoring the advice of the people who built it.
Where a REL has a high level input, REL normally want it connected to your amplifier's speaker terminals, the same posts your main speakers are wired to, even when a subwoofer output is sitting there unused. Low level is their fallback rather than their preference, and the dedicated LFE input is for something else again.
why REL ask for the speaker terminals
An ordinary subwoofer connection takes a line-level signal from a preamplifier or a sub output. REL's high level connection samples the signal at the speaker terminals instead, after the amplifier has done its work.
Their reasoning is integration. The subwoofer hears the same left and right signal your speakers hear, carrying the tonal character and timing of everything upstream of it, the amplifier's own signature included. You then use the REL's crossover and level controls to bring in only the deepest bass underneath that, so the subwoofer extends the speakers rather than announcing itself.
The important thing to understand before you start: the REL does not replace your speaker connection. Your left and right speakers stay wired to the amplifier exactly as they are. The REL's cable shares those same terminals, taking a very high impedance signal from them, which is why it can sit alongside your speaker cable without stealing anything from it.
what the cable actually does
The supplied REL lead has three conductors at the amplifier end and a locking Speakon plug at the subwoofer end. In REL's standard arrangement for a single subwoofer, red goes to the amplifier's right positive terminal, yellow to the left positive, and black to a ground point, normally a ground lug or a chassis screw. The Speakon plug pushes into the REL's high level input and twists to lock.
Switch the amplifier and the subwoofer off before you make or change any of it, and check that no stray strand of wire can touch a neighbouring terminal.
That colour scheme is REL's standard arrangement. It is not a universal instruction, and the next section is the one to read before you wire the black conductor anywhere.
the amplifier decides the wiring
Class D, balanced, differential, bridged and monoblock amplifiers can all need a different grounding method, and getting this wrong is the one part of the job with real consequences.
On those designs the negative speaker terminal may not be signal ground at all. Connecting the REL's black wire there without verified instructions can produce hum, trip the amplifier's protection, or damage something. REL's own advice is to identify the amplifier's output design first, then follow the scheme REL or the amplifier's manufacturer publishes for it.
Manufacturer guidance wins over the general rule every time. Primare, for instance, publish different REL connection methods across their range, and tell owners of several models to use stereo low level with a single subwoofer rather than high level at all. If you cannot confirm the right scheme for your amplifier, use low level, or ask the manufacturer, REL, or a dealer before connecting anything.
sharing a binding post without loosening anything
Five-way binding posts will usually take two different kinds of termination at once, and the rule is to protect whichever connection is holding your main speaker cable.
If your speakers are on banana plugs, leave those bananas where they are and clamp the REL's bare conductors under the binding post collars. Bend each stripped end into a small hook, keep the exposed copper to a minimum, and tighten the collar firmly.
If your speaker cables are on well-clamped spades, leave the spades undisturbed and fit banana plugs to the REL's conductors instead, so they use the sockets in the ends of the posts.
What you should not do is force two badly secured conductors under the same collar. That is not a tidiness point. A loose strand bridging two terminals can short the amplifier's outputs.
when low level is the right answer
High level is REL's preference, not a requirement, and several ordinary systems cannot use it.
If there are no accessible speaker terminals, the question does not arise: active speakers, a television, a computer, or a preamplifier feeding a power amplifier you would rather not touch all point to low level. Take the amplifier's variable preamplifier or subwoofer output into the REL input its manual specifies.
REL also ask for low level when one subwoofer is working with a pair of monoblock power amplifiers. A single high level cable would span two separate chassis and can create a ground loop between them. Their preferred high level answer for monoblocks is two subwoofers, one per channel, each wired to its own amplifier.
And some RELs simply do not have a high level input, and the models built specifically for home cinema are the ones to check. Do not assume the socket is there because the badge says REL. Look at the back panel, or the manual for your exact model.
One thing not to do: the LFE input exists for a genuine surround effects channel. Do not use it for ordinary stereo bass just because the plug fits.
films as well as music
A surround soundtrack carries a dedicated low frequency effects channel, the .1 in 5.1. That belongs on the REL's LFE input, fed from your AV receiver or processor's subwoofer output with an ordinary RCA subwoofer cable.
On RELs that have both, high level and LFE can be connected at the same time. High level handles the front left and right channels for music, LFE carries the effects channel for films, and REL describe these as separate circuits. If you want your front speakers' bass to reach the subwoofer through the high level connection in that arrangement, the receiver needs those speakers set to Large or Full Range.
What must never be connected together is ordinary high level and low level. REL are explicit that those two share internal circuitry, and the combined gain can overtax it.
getting it to disappear
Wiring it correctly is only the first half. Start with the gain and crossover low, then bring them up gradually until bass lines gain weight and scale without pulling your attention to the corner of the room.
The honest test is to switch the subwoofer off after a few familiar tracks. A well-integrated REL rarely sounds loud while it is playing. What you notice is its absence, the system suddenly sounding smaller and thinner than it did. If you can point at the subwoofer, or every kick drum has grown, take the level or the crossover back down and listen again.
before you start
Confirm your exact model has a high level input rather than assuming it. Work out whether the system is stereo, home cinema, or both. Identify the amplifier as conventional, Class D, balanced, bridged or monoblock, and check REL's instructions and the amplifier's manual for anything model-specific before you wire the black conductor. Prefer high level for a compatible stereo system even when SUB OUT is sitting there. Keep the main speaker cables connected as they are, use complementary terminations so nothing works loose, and switch everything off before you begin.
REL's approach looks strange until you see the point of it: for music, let the subwoofer hear the same signal as the speakers, wherever the equipment safely allows it.