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 |
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 |
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 |
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 |
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 |
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 |
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 |
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 |
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 |
