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Wierd issue with a DIY cable for a REL G2 sub

I built a cable for my uncle for his REL G2. He wanted a cable shorter than the 35' one that came with the REL. It is causing a thermal issue with the McCormack amp that is being used for the high level source. He says it plays fine for about 5 minutes, then thermals and shuts off. The OE REL cable does not have the problem (obviously). Here is the breakdown of the cable:

Neutrik NLT4FX speakon connector (I used this so I could have a solder connection instead of compression, also the aluminum casing is just nicer)

15' Belden 5102UP 14ga speaker cable (using 3 of the 4 conductors)

14ga spade terminals

Connections at the speakon end are soldered with WBT silver bearing solder and connected to the same lugs in the speakon as the original cable.

At the spade end, the connections are crimped, then soldered with WBT silver solder as well.

All 3 wires for the cable show continuity to the appropriate point on the speakon, and show no continuity to each other.

All 3 connections read .3 ohms of impedance between the spade and the speakon. As well as .3 ohms when testing between the input and output of the speakon.

I'm stumped. The OE cable didn't look like it had any kind of special circuit attached with it so I assumed it is just copper wire.

At this point I am thinking voodoo devil magic.

Anyone have any thoughts or input?
diamondjoequimby
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almarg
9,670 posts
 
Is the McCormack amp one of those having balanced outputs, such as the DNA-500? If so, is the negative input to the sub (the black wire on the original cable) being connected to a negative output terminal of the amp? If so, that is probably the cause of the problem.

On a balanced amp the negative output terminal has a full amplitude signal on it, just as the positive terminal does, and that signal is in effect being connected to the amp's ground through a very low resistance when it is connected to the sub's ground via its negative input terminal. Probably the higher resistance of the original cable (it being much longer and of much narrower gauge) was high enough that the amp could deal with it.

If all of that seems applicable, try connecting the negative wire of the sub cable to a chassis screw on the amp. That is likely to fix the problem, if my diagnosis is applicable. Or alternatively, as an experiment, temporarily defeat the AC safety ground connection of the sub's power plug with a cheater plug (a 3-prong to 2-prong adapter), since if my diagnosis is applicable the AC safety ground wiring would be part of the path through which the amp's negative output terminal is being shorted to the amp's internal circuit ground (which is what causes the problem).

Regards,
-- Al
diamondjoequimby OP
9 posts
 
Thanks. That makes sense. I'll double check with him and see which amp he has.
mitch2
3,969 posts
 
Ok, so I found this;
REL manages the lower frequencies differently than other manufacturers—and does so with excellent result. While the company offers line-level RCA inputs, these should be used only as a last resort. The supplied Neutrik speakON connector utilizes a high-level connection that goes directly to your power amplifier’s speaker outputs. The sub’s high impedance does not affect loading of the main speakers, thus allowing the character of your amplifier’s sound to carry forward into the subwoofer.
My first thought was to recommend trying the line level outputs from the preamp but it appears that using the high level speaker cables to connect the sub to the speaker cable output terminals of the McCormack amp is the preferred approach, since that is designed to result in superior blending with the sound of the amplifier that is being used to power the upper frequencies.

Hopefully, this is a grounding issue, as Al suspects, and one of his recommendations will solve the problem.

I do have a couple of additional thoughts. First, I noticed you used only three of the four wires in the Belden speaker cable, which would result in an unbalanced configuration with one polarity having a larger gauge wire than the other (whichever side has two wires vs. one). Since most speaker cables use the same gauge wires for the positive and negative poles, you could try connecting the cable in a star-quad configuration where the two wires opposite each other are connected together to form each polarity. Based on the pictures I found, it appears you would connect the black/white wires together and red/green wires together.

Also, you might check the polarity of your wire connections in both channels to make sure they are consistent.
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almarg
9,670 posts
 
Tim (Mitch2), thanks for the additional thoughts. Be aware, though, that the three wires which connect to the speaker-level inputs of a REL sub are not used as you appear to be envisioning. One wire carries the + signal of one channel; another wire carries the + signal of the other channel; and the third wire provides a return (-) path for both of the channels. (When monoblock amps are being used in conjunction with a single sub, REL recommends connecting that single return wire to a ground point on the preamp, rather than on one of the amps).

Also, as with most powered subs having speaker level inputs the impedance of those inputs is extremely high, 100K for many of the REL models, so essentially negligible amounts of current are being conducted. As you'll realize, a single 14 gauge conductor is overkill by a wide margin. And of course effects that the cable might have on mid and high frequencies won't matter.

Best regards,
-- Al
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