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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
 
Al and Mrschret ... I also use a self-powered subwoofer which is connected to my linestage via a 330K ohm impedance buffer. As just stated, the sub is connected to my linestage ... not the amp.

Why can't Mreschret connect her REL sub to his/her pre/linestage?? Why mess with the amps.
almarg's avatar
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almarg
9,670 posts
 
Bruce, as you realize connecting via line-level can often be a good approach. REL, though, usually recommends connecting at speaker-level if possible, their theory being that it is preferable for the sonic effects of the power amp to be reflected in the signals received by both the sub and the main speaker. While that theory is debatable, and it can certainly be expected that there will be some situations in which that approach would not prove to be optimal subjectively, I don't think it can be said to be an unreasonable recommendation.

Also, as you realize, impedance issues have to be considered. I couldn't readily find input impedance specs for the G2 sub, but I recall that the line-level input impedance of some of their subs is only 10K (unbalanced). The input impedance of the XPA-1 amp is spec'd at 23.5K unbalanced and 33K balanced. I don't know if the connections in this case would be balanced or unbalanced, but 10K in parallel with 23.5K is only 7K. Providing good performance, and especially flat frequency response at deep bass frequencies, when driving a 7K load would certainly be a problem for some preamps, especially tube preamps that utilize a coupling capacitor at their output.

Best regards,
-- Al
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albertporter
4,949 posts
 
Almarg

Also, as you realize, impedance issues have to be considered. I couldn't readily find input impedance specs for the G2 sub, but I recall that the line-level input impedance of some of their subs is only 10K (unbalanced). The input impedance of the XPA-1 amp is spec'd at 23.5K unbalanced and 33K balanced. I don't know if the connections in this case would be balanced or unbalanced, but 10K in parallel with 23.5K is only 7K. Providing good performance, and especially flat frequency response at deep bass frequencies, when driving a 7K load would certainly be a problem for some preamps, especially tube preamps that utilize a coupling capacitor at their output.

Al, that should be resolved by what Bifwynne stated:
Bifwynne
Al and Mrschret ... I also use a self-powered subwoofer which is connected to my linestage via a 330K ohm impedance buffer.

I had the same issue with my JL Audio subs and used a 480K JFET unity gain buffer designed as true balanced with option to operate RCA.

Even with tube preamp the 480K load on the preamp helps out and should allow 25.5K to operate fine. In my case it was a slam dunk since my tube amps are 150K.

150K plus 480K made the tube preamp very happy.
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almarg's avatar
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almarg
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Thanks, Elizabeth. As I indicated I always have great respect for your knowledgeable inputs. I've learned from them over the years as well.

Albert, thanks also. I realize that. In this case, however, the tradeoff appears to be between (a)connecting at speaker level, which would require soldering a wire onto an RCA plug, for each channel, and (b)connecting at line-level, which would require purchasing additional interconnect cables, possibly purchasing splitters and introducing them into the signal path, and possibly spending several hundred dollars or more on a buffer, while losing the possible sonic benefit of providing the same signal to both the subs and the main speakers, and introducing the possible sonic effects of the splitters or buffer, if needed, and of the additional interconnect cables (which for several reasons I would expect to be more significant than the effects of the Neutrik cable).

The answer to Bruce's question "why mess with the amps" seems clear in this case. Why mess with the alternative?

Best regards,
-- Al
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