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Why are they doing this?

Kinda vague, huh? I'll elaborate. I have Emotiva XPA-1 monoblocks driving Magnapan 20.7's. Before that they were powering Mag 3.7's. They are 1000 watts into 4ohms. They never clipped. I did play them loud but not ridiculously loud. These amps have blue led meter lights that move in sync with the amp output and they would peak a little passed halfway. Occasionally a bit higher. Recently I added a pair of REL G2 subwoofers. They are connected one left and one right as per the manual. That would be to the speaker posts on the amps with the supplied neutrik cable. Upon connecting the subs, the blue led's no longer move. (Yes, I've checked the switch that turns these on and off) Also, three times now, the amps have clipped, once left channel and twice right channel. It was at a pretty good volume but not where it was outrageous. I talked to a tech at Emotiva and first off, he had no knowledge of REL subs and how they are connected. After I explained it he was kinda at a loss for words having obviously never heard of connecting subs to the speaker posts on the amps. His explanation for what was happening was that the subs are 4ohm and the 20.7's are 4ohm so that's driving the load down to 2ohms and that's causing the clipping and the amps cannot detect 2ohms so that's why the led's are not functioning. If this is true then Rel subs cannot be used with any 4ohm speaker or the same problem will occur. I'll be honest, when you start talking ohms splitting and other electronic stuff I tend to get lost. Anybody care to take a stab at this?

mrschret
bifwynne
2,206 posts
 
Thanks Al. I suppose my first post had more to do with my surprise that the OP's sub could be hooked up to the power amp outputs. Admittedly I am not familiar with his sub. I thought most subs are self powered like mine. But even if not, if this were my rig, as a threshold matter, given the complexity of optimizing amp and speaker compatibility, I would have been somewhat circumspect about throwing another load into the mix - namely a sub.

You raise a couple of interesting points about my current interconnect hookup. In fact, the Ref 5's Main Outputs are not buffered. That's why using a "Y" connector to sum the L and R channels was not an option, ergo one of the little problems that Tom's buffer addressed.

The other point deals with combined output capacitance. Certainly, the combined resistive impedance side is covered. Just checked the ARC web site about the capacitance side of the house. ARC advises that maximum capacitance should not exceed 2000 pF.

In my case, the Ref 5 Main One I/C run to the amp is about 10 feet. The other I/C run to the buffer is about 5 feet. Not sure how combined capacitance is calc'd in this context, but even if its summed, to exceed 2000 pF would mean that capacitance per foot would be roughly 135 to 140 pF per foot. That impresses me as quite a bit. I'll double check the specs on my Kimber Heros and come back if an issue. But I suspect not.

Thanks Al.

Cheers,
almarg's avatar
almarg
9,670 posts
 
Hi Bruce,

As you've probably already found, it looks like the capacitance of 1 meter of Kimber Hero balanced is a VERY low 33.1 pf, including connectors (and 78.8 pf for 1 meter of the unbalanced version). So as you suspected there is no problem re the 15 foot combined length.
I thought most subs are self powered like mine. But even if not, if this were my rig, as a threshold matter, given the complexity of optimizing amp and speaker compatibility, I would have been somewhat circumspect about throwing another load into the mix - namely a sub.
Like many subs having speaker-level inputs, the OP's REL sub IS self-powered. See my first post in this thread. The load that is imposed by the sub on the main power amp corresponds to the input impedance of the sub's amplifier, at its speaker level inputs (which is usually much higher than even the input impedance of the line level inputs, for subs that provide both kinds of inputs). As I indicated, in the case of many of the REL subs that speaker-level input impedance is 100K, and hence represents a completely negligible load.

Best,
-- Al
oddiofyl's avatar
oddiofyl
4,084 posts
 
The connection of the REL Neutrik connector should not present a difficult load to the amp. It is a 150k input, it should not cause the amp to clip. The emo guy does not know what the hell he is talking about.

If it is a fully balanced differential amp you must connect the Neutrik with a separate ground to the chassis, you can not attach the (-) signal wire to ground.

I would connect just the subs while you are troubleshooting
almarg's avatar
almarg
9,670 posts
 
If it is a fully balanced differential amp you must connect the Neutrik with a separate ground to the chassis, you can not attach the (-) signal wire to ground.
With due respect, although this statement is a little ambiguous, if I am interpreting it as it was intended it is not correct. What should not be done is to connect the (-) signal wire from the sub (the black wire) to the (-) output terminal of a fully balanced amp, because the (-) output terminal of a fully balanced amp is NOT ground.

As I indicated earlier, ideally the (-) signal wire from the sub (the black wire) should be connected to a circuit ground point (also known as a signal ground point) on the amp. Some Pass Labs balanced amps, for example, provide a circuit ground binding post specifically for this purpose.

Connecting to chassis instead of circuit ground will work in many cases, however, as it did for the OP.

Regards,
-- Al
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