2 Channel Subwoofer connection

Regarding three-wire vs. four-wire connection of a sub to the outputs of an amplifier:

In most stereo amplifiers (as opposed to monoblocks) the negative output terminals of the two channels are connected directly to the amp's circuit ground, and are therefore connected directly to each other within the amp. Exceptions to that are mainly amps whose outputs are balanced or bridged, since in those cases the negative output terminals have full-amplitude signals on them, as opposed to both terminals being connected to the amp's circuit ground.  Certain older class D designs that have substantial DC offsets on both their + and - output terminals are also exceptions.

When the three-wire cable that is used to connect the high level input of a REL sub is connected to the output of most stereo amps, one wire goes to the positive output terminal of one amp channel, another wire goes to the positive output terminal of the other amp channel, and the remaining wire (the ground wire, which is colored black) goes to the negative output terminal of either channel. The reason that works well is, as I said above, that with the exceptions I noted the two negative output terminals of the amp are connected directly together within the amp.

So if the high-level inputs of a single sub are connected to the outputs of most stereo amps a four-wire connection amounts to having a three-wire connection plus an additional connection to the amp's circuit ground in parallel with the ground wire of a three-wire connection. The load presented to the amp by the sub is the same in either case, and will be negligible assuming the sub is active (i.e., that it has a built-in amplifier).

Special considerations come into play if the outputs of the amp are balanced or bridged, or if monoblocks are being used, or in the case of certain older class D designs as I mentioned above.  In those cases, depending on the designs of the amp(s) and the sub, and on whether one sub or two is being used, specific connection approaches are necessary to avoid the possibilities of hum problems or even amplifier damage.  These issues are discussed in the manuals for most REL subs, and need to be considered regardless of whether the connection is via four wires or three.

Regards,
-- Al
 

Subs that use 4 (2 neg and 2 pos) wires from the amp to receive the signal is an old outdated way of running a sub, how can you say the active sub dosent see the 2 additional wires connected to it..it does,

The subwoofer plate amp sees a signal from the speaker terminals of the main amp, but the subwoofer driver load is seen only by the plate amp itself, not the main system amp.  If I understand the OP's situation, he can connect any active sub with high level (speaker level) inputs to the speaker terminals of his Jolida 202 and the Jolida will still recognize ONLY the load from his B&W 705s.

If I understand the OP's situation, he can connect any active sub with high level (speaker level) inputs to the speaker terminals of his Jolida 202 and the Jolida will still recognize ONLY the load from his B&W 705s.

I agree.

The one qualification I would add to that is the slight possibility that electrical noise picked up by or introduced into the sub cable could conceivably have audible effects on the signals provided to the main speakers as a result of entering the feedback loop of the integrated amp. (The 202 is described as using "less than 9 db" of negative feedback). That kind of effect, by the way, might be one of the reasons people report finding sonic differences among different sub cables.

To the extent that might be a possibility, four wires could very possibly be better than three, depending on the specific cable designs.

Regards,
-- Al

Thanks to you, Al, and hifman5. I knew a  lot of these things both of you mentioned, but just couldn't put it into words;)

+1 almarg!  Very easy to follow explanation.  I appreciate the clarity.