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
I'm pretty sure I can explain what is happening.

First, the person you spoke with does not know what he is talking about. The amplifier does not see the 4 ohm or whatever impedance of the sub's driver. It sees the input impedance of the sub's amplifier, which is many thousands of ohms (100,000 ohms for the high level input of many of the REL subs), which is therefore a completely negligible load.

The problem is almost certainly that you are connecting the sub's ground to the negative output terminal of the amplifier. The XPA-1 is a fully balanced amplifier, and its negative output terminal therefore drives a signal, rather than being grounded. From the manual for the XPA-1:
DO NOT connect the negative (-) speaker terminal of the XPA-1 to ground, or to the negative speaker terminal of another amplifier. (Do not connect the XPA-1 to any speaker which requires connections between the left and right speakers.) The XPA-1 is a fully differential amplifier and the negative speaker terminal is NOT at ground potential. Connecting the negative speaker terminal of the XPA-1 to ground, or to the negative speaker terminal of another amplifier (including another XPA-1) will cause damage to the XPA-1 or your other equipment.
What you should do is to connect the red and yellow wires of the Neutrik cable to the + output of the amp, and connect the black wire to a circuit ground point on the amp. A way to do that, if you are using the amp's balanced XLR input (and its RCA input is therefore unused), would be to obtain an RCA plug, solder the black wire to the ground sleeve connection of that plug (while leaving the center pin unconnected), and insert that plug into the amp's RCA input connector.

Regards,
-- Al
The problem is HOW you are connecting the subs. And not that they and the speakers are both 4 ohms.
What was explained to you is correct. YOu are conencting 4 ohm subs AND 4 ohm speakers to the same source... Thus making it basically a 2 ohm load. Which not many amps can drive. And I agree it is very unusual for anyone to drive a selfpowered sub from the amplifier outputs (at least for any audiophile.. maybe some kid would do it, thus the persons suprise at it being done at all in a quality system...)
What you must do is find another way to split the signal prior to the amplifier. So you have a signal to the amps and Maggies, And another signal to the subs.(which is NOT from the nmplifier.
If you use a preamp, then a 'y' splitter might work at the preamp output.
Or get a preamp with two outputs..
Elizabeth, you probably posted before seeing my post just above. I always have great respect for your inputs, but in this case you are not correct. REL subs (and many others) often provide high level inputs that are designed to be driven from power amplifier outputs. And that is commonly done in high quality systems. As I indicated above, in those situations the main power amplifier will see the (very high) input impedance of the sub's amplifier, not the low impedance of the sub's driver.

I'm quite certain that the explanation of the problem that I provided is correct.

Best regards,
-- Al
Al, your explanation is great. I did see just what you showed from the Emo manual. I also saw in the REL manual that there is a different way to connect them to amps like these but the connection shown in the manual was for a two channel amp, not monoblocks so I wasn't sure if it would work on them. As soon as I can get the chance I will make the change. Even with my limited knowledge I was pretty sure what the Emo tech was telling me was wrong. Thank you for your help.
Now you know about Emo technical staff so you can rather ask non-Emo tech staff to take care of your equipment :)
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.
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
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.
Thank you Al for mentioning my error. I was not sure about it and was too quick to offer an opinion.
Again,
Thanks for the correction to my statements.
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
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).

Understood, my point was operation with a buffer (Bruce obviously already owned) helped resolve an issue.

Such as In my case, where the JL Audio subs required interconnect cables, unlike the REL which probably works better as designed from speaker terminals.

In my case the subwoofer dragged down the bandwidth of the main system when both sets of interconnect were used.

In cases like this, the buffer saves the day. Literally, only other options are to sell the sub or suffer serious loss of bandwidth.

I don't question the wire suggestion you made for the REL.
Thanks, Albert. Agreed.

To the OP, although I don't particularly recommend it, I'll mention another means of grounding the black wire, that MIGHT work acceptably, depending on the internal grounding configurations of the amp and the sub. That would be to connect the black wire to a chassis screw on the amp. I suppose there would be no harm in trying that, although the result might be a very loud hum. The RCA plug approach I suggested, though, is more technically correct and much less likely to have problems.

Regards,
-- Al
Very much appreciate the feedback here. To bring things up to date, I contacted REL and their answer was exactly what Al says here in the last post. Take the red and yellow wires, twist them together and attach to the positive post on each amp. Take the black wire and attach to a chassis screw for grounding. Ok, so that I don't seem like a complete idiot, the REL manual does show this connection for fully balanced amps and I DID look at it. My problem was that it shows only for a single sub and not monoblock amps. I didn't want to take a chance and connect them this way unless I was completely sure it was safe. So, that's the way their connected now and everything works fine. The led's are even metering again.
As for the suggestion to connect the RCA plug, maybe that would be a good way but I really don't think the folks at REL intended for anyone that bought their subs to have to go through those extra steps. I did learn soething here though and I thank you for that.

Just to let you know about REL subs as I have a pair of Stentors III connected to monoblocks. I use the red and yellow together for each sub on the + terminal of both right and left monoblocks so each Stentor is connected to one monoblock each. I do not connect the black wires. You only need to connect the black to ground if you have a hum.
01-31-14: Coxhaus
... You only need to connect the black to ground if you have a hum.
While with some amplifiers and subs not connecting the black wire will work, in general I would strongly recommend against it. As I'm sure everyone here realizes, for current to flow a complete circuit is required, in this case from the amplifier to the sub and back to the amplifier. Without the black wire being connected, instead of being through that wire the return path would be from the sub's circuit ground via some design-dependent impedance to its AC safety ground, then through the AC power wiring to the amplifier's AC safety ground, then through some design-dependent impedance to its circuit ground. In addition to creating a susceptibility to pickup of low level hum and noise, I would expect that to result in at least a few cases (depending on the internal grounding configuration of the sub and the amplifier) in hum that is loud enough to be destructive.

And particularly so in the situation where there is initially a low level hum, which the user then sees fit to try to eliminate by putting a 3-prong to 2-prong cheater plug on the power cord of one of the components. Thereby leaving the grounds of the sub and the amp "floating" relative to one another, with AC "leakage" paths (e.g. in power transformers) being the only means of signal return. I would not want to be anywhere in the vicinity in that situation.

Regards,
-- Al
Al, you responded to Albert's point about connecting a self powered sub to the linestage/pre as follows:

"[C]onnecting 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 ...."

Of course I get your points, which are fair. Just a few clarifications please. First, in my case, I suspect the sonic downsides may be acceptable because my linestage has two (2) XLR Main Outputs -- one for the power amp and the other for whatever turns you on. In my case, it's the self-powered sub. So the linestage's output signal did not have to be split. Instead, as mentioned below, it needed to be summed.

Second, while there may be some qualitative sonic costs to the hook-up just described, I suspect that it's minor because my sub is configured to blend into the fronts at about 40 Hz and below, which is the roll off point of the fronts. So .... the main signal from the linestage is direct, i.e., no artifacts, to the power amp and then of course to the speakers.

Third and last, I had no choice. My sub is NOT designed to accept an input signal from the power amp. Hence, a hook-up of some sort had to be made from the linestage/pre.

As you may recall from my posts about this issue some years ago, my impedance buffer had to solve three (3) problems at one time: (a) asymetrical loading of the linestage (i.e., XLR -- linestage to amp; RCA/SE linestage to sub); (b) summing the L and R channels without shorting the linestage's Main Outputs because I am running a single sub; and (c) increase the impedance presented to the linestage or else chance overloading the linestage's outputs, thereby causing possible sonic degradation.

Ergo, a custom impedance buffer designed and made by Tom Tutay (located in Ft. Walton Beach, FL). Considering Tom's fee, which was extraordinarily reasonable, he solved a lot of problems for a paltry sum indeed.

Cheers,
Hi Bruce,

Yes, of course I well recall the solution you arrived at, revolving around Tom Tutay's buffer. And it is certainly an excellent solution in your case.

What precipitated my comments that you've quoted is that the wording of your initial post above, the perhaps also Albert's initial post following my response to it, might have been construed by some to be implying that Mrschret should abandon the speaker-level approach he was trying to implement, and go for a line-level approach instead. You perhaps meant no such implication, and Albert subsequently made clear that he didn't, but your and his initial wording, it seemed to me, could easily have been interpreted that way.

Given particularly that your sub is pretty much just filling in the bottom octave, and assuming that your interconnect cables at the preamp outputs are not especially long and do not have particularly high capacitance per unit length, I see nothing less than ideal about your setup. (The reason I mention the last point is that if the two XLR outputs are both used, but are not individually buffered within the preamp, the interaction of preamp output impedance with the TOTAL of the capacitances of the cables attached to BOTH sets of outputs may affect the high frequency components of the signals sent from one of those output pairs to the main power amp).

Best,
-- Al
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,
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
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
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