Thanks to Stfoth for providing the data on the SoundPath RCAs. Their price is indicated at the SVS website as $89.98 for a stereo pair of 8 meter cables, and those are the only interconnects listed, so I assume those are what RV was provided with. RV, given the lengths that are involved (even if you were to change to 16 feet), and given the somewhat highish output impedance of your preamp, and the unknown but very possibly highish output impedance of the sub’s high passed line-level outputs, it is particularly important for the cable to have low capacitance per unit length. The SoundPaths provide that, at 39 pf/meter. But of course a shorter length, such as 16 feet, would reduce the total capacitance correspondingly, and very possibly be beneficial in other ways. I could probably get away with 15’ cables. I understand from SVS that’s the next size down, but I’d feel comfortable with a foot or more longer than 15. The SVS website lists the SoundPath offerings only in metric lengths, including 5 meters, which is 16.4 feet. Another possibility for a modestly price cable having very low capacitance (12.2 pf/foot), which can be ordered in any desired length, would be Blue Jeans LC-1. Cable differences are generally system dependent and hard to predict, of course, but I wouldn’t be surprised if changing to a 5 meter length of the SoundPath cable, or a 16 foot length of the Blue Jeans cable, would provide a worthwhile benefit. Best regards, -- Al |
RV, are the 25 foot cables connecting the preamp to the sub AND the sub to the power amps, or are they just being used for one of those two interconnections? And if the latter, which one? And how short would it be feasible for the cables to be for each of those two connections?
Best regards,
-- Al
|
RV, that sounds like either you spoke with a very confused rep, or there was a miscommunication. The high passed line-level outputs of the sub would have no way of knowing whether they are connected to a speaker having a built-in amplifier, or to an amplifier that in turn is connected to a passive speaker.
You would simply connect the preamp outputs to the sub’s line-level inputs, and connect the sub’s high passed line-level outputs (left and right) to the inputs of your left and right monoblock amplifiers, respectively. You would probably then want to set the sub’s internal low pass filter to the vicinity of 80 Hz (and then fine tune by ear), since the high pass function has a fixed setting of 80 Hz.
An uncertainty pertaining to how good the resulting sonics would be, though, is introduced by the fact that the output impedance of the sub’s line-level outputs does not appear to be specified. Although I wouldn’t be surprised if that proved not to be an issue with the 22K input impedance of your amps.
Best regards,
-- Al
|
Rvpiano 6-9-2017
The SVS guy told me to set the phase to zero. How do I determine phasing
differences? What do I listen for? I really couldn't hear differences
when I first set up the sub. From the manual for the sub:
Phase Control:
This control delays the signal being processed through the amplifier. For connection to an A/V receiver, it should be set to 0 degrees. For 2-channel applications, it should be adjusted to obtain the most coherent and stable soundstage and transition between the loudspeakers and the subwoofer.
Although it isn't stated clearly, I assume "the amplifier" in that statement refers to the amplifier in the sub, not to the external amplifier which drives the main speakers. Keep in mind, also, that the optimal phase control setting is likely to vary somewhat depending on where the sub is located. Also, I'm not sure how the SVS person could make a blanket recommendation of zero degrees without knowing the phase shifts (or possibly even 180 degree polarity inversions) that might be introduced by your amplifier and/or your main speakers. Regarding the high pass filter in the sub, note that it has a fixed corner frequency of 80 Hz, and rolls off at 12 db/octave below that frequency. The sub's low pass filter also rolls off at 12 db/octave, above whatever corner frequency its control is set to (between 50 and 160 Hz). Therefore if the high pass filter function is used (and it is certainly worth trying, although depending on many variables its use may or may not prove to be optimal), the low pass corner frequency should probably be set in the vicinity of 80 Hz, with fine tuning by ear. Regards, -- Al |
Rhljazz, thank you. I believe the phenomenon referred to in your first paragraph applies to the situation where a y-connector is used to sum left and right line-level outputs to a mono signal, with the resulting mono signal provided to a sub having only a single line-level input. If the outputs which are summed are common with the main signal path, or are just isolated from it via relatively low value resistors, the channel separation of the main signal path can indeed be compromised. Although some people apparently get away with doing that, depending on the specific design. But it is poor practice at best, and conceivably could even degrade long-term reliability. But none of that is applicable here, as the SB-2000 provides separate left and right inputs. RV’s y-adapters, given that two are being used (as Yogiboy points out should be used), are for the purpose of splitting left and right such that each is routed to the corresponding inputs of the monoblock power amps and the sub. RV, earlier in the thread I had recommended the Audioquest "hard y adapters." I feel it would be good practice to use those, or something equivalent, rather than y-cables. Although I suspect the use of the cheap Radio Shack y-cables is NOT a major contributor to the issue you described. You may have provided some or all of this information earlier, but could you recapitulate a description of the room dimensions, the distances of the speakers to the closest walls, the distance between the speakers, the physical location of the sub, and the location of the listening position within the room. BTW, I have about 40 or 50 Telarc classical LPs, mostly from the 1980s, which I’ve listened to on several different floor standing speakers that reach down to the vicinity of 24 to 28 Hz, and I am very familiar with their sonics. Although I don’t use a sub, it may be relevant that in my room (see my system description) when deep bass weakness occurred in the past on Telarc bass drum beats and other low frequency notes it was caused primarily by rear wall reflections, even though the central third of the wall behind the listening position is open to another room. Such reflections will result in a suckout at a range of frequencies centered at a frequency (in Hz) of about 281.5 divided by the distance in feet between the listening position and that wall. (281.5 feet per second is about 1/4 of the speed of sound in dry air at room temperature, which would cause a rear wall reflection to arrive at the listening position out of phase with the original arrival at the listening position, for frequencies in the vicinity of 281.5/that distance in feet). I was able to improve that issue significantly in my system when I upgraded my preamp to a DEQX HDP-5, which provides myriad signal processing and room correction functions but costs a pretty penny. Finally, of course, in addition to placement considerations your sub provides a number of adjustable parameters, which I assume you’re in the process of fine tuning. And I would expect that if those aren’t optimized, especially the phase control, the acoustic output of the sub may even be partially cancelling the bass outputs from the main speakers somewhat. Good luck. Regards, -- Al |
@steakster
That sounds MUCH more promising. I would expect a reasonably well designed transformer coupled output stage to not have much variation in output impedance over the frequency range. Consequently I would expect that even if the output impedance is 630 ohms there would be no problem even if the overall load impedance is much less than 12K, and certainly less than the parallel combination of 12K and 50K which is 9.7K. (The 100K balanced input impedance of the power amp probably represents a load of 50K on each of the signals in the balanced signal pair).
And capacitance won’t be an issue either, given the short length of the cable to the power amp, and its exceptionally low capacitance per foot.
The one thing I’m left wondering, though, is if separate transformers are used for the RCA and XLR outputs, or if the circuit is somehow configured such that the same transformer is used for both. Or even if separate transformers are used, if there might be interactions between the two outputs as a result of effects that occur on the primary (input) side of the transformers. For example, effects that might result in a slight impedance imbalance between the balanced pair of signals going to the main power amp. An experiment that might be worth doing would be to turn the level controls on the subs all the way down, and seeing if there is any difference in the sound coming from the main speakers when the cables to the sub are disconnected from the preamp vs. when they are connected. (Of course, have everything turned off when changing connections).
Best regards,
-- Al
|
@Steakster, I don't think it's possible to answer your question without a lot of additional
information, including the things you mentioned that cannot be determined, and
also:
-- The input impedance of your power amp.
-- The length of the cables that connect the preamp and power amp.
-- Whether the outputs of the tube preamp are capacitively coupled or
transformer coupled. (Or if it is one of
the very few that are direct coupled, e.g., those made by Atma-Sphere).
Generally speaking, though, I would consider using a tube preamp to drive both
a power amp and a pair of subs, at least one of which has an input impedance as
low as 12K, to be cause for concern. Especially if the tube preamp has
capacitively coupled outputs (as most tube preamps do), since the resulting
output impedance most likely rises considerably in the bottom octave or two
from what it is at higher frequencies. (The impedance of a capacitor
increases as frequency decreases).
Something you might consider, though, would be trying to
find some sort of buffer stage, that would provide a high input impedance and a
low output impedance, and inserting it in the path between the preamp and the
subs. Or, if the main power amp also has
a low input impedance, perhaps inserting it in the path between the preamp and
both destinations. Something like the Burson
Audio buffer stage that was marketed some years ago, but is no longer
made.
Best regards,
-- Al
|
Shadorne, thanks for the excellent comments. Regarding cable capacitance I’ll mention that I had raised that issue earlier in the thread, and it turns out that the OP’s existing interconnects are only 2 feet long. Given the preamp’s nominal output impedance of 500 ohms, and assuming the length of the cables to the sub is not unusually long, cable capacitance (which of course is proportional to length) is therefore unlikely to be an issue in this case.
I’ll mention also, as I’m sure you realize but others may not, that the capacitance of the cables connecting the preamp to the sub can affect what is heard from the main speakers just as much as the capacitance of the cables connecting the preamp to the main power amp. The reason being that the capacitance of the cables to both destinations will interact with preamp output impedance and thereby affect the signals that are present at the preamp’s output terminals, that in turn are sent to both destinations. The resulting effects, if any, being most likely to occur in the treble region.
Bob, in answer to your questions I would put it that induced noise can typically be viewed as emanating from a high source impedance. Therefore the lower the load impedance the more the noise voltage appearing across that load will be attenuated, due to the voltage divider effect that will occur between the load impedance and that source impedance. And since the receiving device presumably responds to the voltage appearing across its input impedance, that voltage (and not current) is what matters.
Best regards,
-- Al
|
Jafant, see the first of my two posts dated yesterday (6-4-2017). Most or all REL subs are not suitable for this particular application. Also, the OP has already made his decision and his purchase.
Regards,
-- Al
|
Hi JohnnyR,
As I mentioned earlier in the thread the line-level input impedance of the SVS SB-2000 is 47K, which is much higher than the line-level input impedance of many and probably most other powered subs. In combination with the 22K input impedance of the OP's amps that will result in a 15K load being presented to the preamp, significantly higher than the 10K minimum that is recommended by Conrad Johnson for the particular preamp.
Regards,
-- Al
|
Wolf_Garcia 6-4-2017
So, be like me, do what I do, and everything will be fine. Hi Wolfie, Your advice is good, as usual, for many circumstances. But I suspect that understandably you haven’t read through a lot of this lengthy thread. If you had done so you would have seen that the outputs of the OP’s amplifiers cannot be used to drive a powered sub, as their + and - output terminals are offset from ground by 24 volts. Also they are monoblocks, which presents additional complications. And while most or all REL subs have 100K input impedances on their speaker-level inputs, as you mentioned, most or all of them have 10K input impedances on their line-level inputs. Which is too low for his preamp to drive with good results, in combination with the 22K input impedance of his amps. So while you and a number of others in the thread have suggested REL subs, they would not be a good choice in this particular situation. Tim (Mr_m), you’re welcome! Best regards, -- Al |
PPS: Rvpiano, ignore my last post. In addition to the fact that you’ve already ordered another sub, I’m uncertain as to whether the DC blocking devices I referred to would be suitable for use when monoblock amplifiers are used in conjunction with just one sub, given that the sub probably connects its negative input terminals for the two channels directly together.
Regards,
-- Al
|
Rvpiano, one further thought. I recall reading several years ago that at some time in the past DC blocking devices were marketed specifically for the purpose of connecting powered subs to amps utilizing the ICEPower-based class D modules that had the substantial DC offsets. Although I suspect such devices are no longer available new, it may be possible to find some used ones, which would presumably make it possible to use the 2Wq you already have.
Edit: This was written before seeing your post just above.
Regards,
-- Al
|
Rvpiano, the SB-2000 is described as having the same 47K line-level input impedance as the SB-1000, so using the PV11 to drive both the SB-2000 and your amps should be fine.
Tim (Mr_m), first I'm pretty certain that the DC offset issue which Rvpiano experienced using his amps with the 2Wq doesn't apply in your situation. I believe that issue applies mainly (or perhaps entirely) to certain older designs utilizing ICEPower modules, while the Pharaoh is Hypex-based. Also, I don't see anything in the manual for the Pharaoh cautioning against connecting a sub or any other active device to its outputs.
As far as the issue you described is concerned, I'm understanding that the symptom just occurs on the output of the sub, and not on the outputs of the main speakers. And I assume it occurs on musical passages for which imaging is centered between the speakers when the balance control is set normally. If so it would certainly seem to point to an imbalance between channels in the input circuit of the sub, which would be surprising. And if so it may be worth contacting RV (the sub designer, not the OP :-)) and seeing if he can shed any light on it. Beyond that I have no particular thoughts about the issue.
Regards,
-- Al
|
RV, I have no first-hand knowledge of the sonics of SVS subs (or subs from any of the other manufacturers that have been mentioned, as I don't use a sub in my system). But FWIW the comments I can recall seeing here and elsewhere about the SVS subs have been very favorable.
Regards,
-- Al
|
OK. The manual for the PV11 indicates a nominal output impedance of "less than 500 ohms" (presumably at mid-range frequencies), and recommends a minimum load impedance of 10K ohms. Given that output impedance and the short length of the cables to the amps the added capacitance of line-level interconnects that would connect the preamp to a sub won’t be an issue, for any reasonable length. But I would look for a sub providing a line-level input impedance of at least 20K, as the parallel combination of 20K and the 22K input impedance of your amps results in an overall load impedance that would be seen by the preamp of: (22 x 20)/(22 + 20) = 10.5K. Unfortunately the specs I can recall for many of the REL models indicate line-level input impedances of 10K, which with the 22K input impedance of the amps would result in a combined load impedance of only 6.9K, well below CJ’s 10K recommended minimum for the PV11. The SVS SB-1000 that was suggested has a line-level input impedance of 47K, which would be great. Some of the relatively expensive JL Audio models have unbalanced line-level input impedances of 50K, although if I recall correctly the input impedance of some of their other models may be too low to be acceptable. You may have to contact some of the other manufacturers that have been suggested to determine the input impedance, if it isn't indicated at their website or in the manuals that are provided online. Good luck. Regards, -- Al |
Rvpiano 6-3-2017
Does anyone know of any subs that are compatible with my type of amps.
I doubt that a sub can be found that would be suitable for connection to the outputs of your amplifiers. But a considerable majority of subs provide line-level inputs, in addition to or instead of speaker-level inputs. That would include most and possibly all of those that have been suggested by the others, aside from the 2wq. So the approach I would suggest is to choose a sub which provides line-level inputs, and connect it to the outputs of your preamp. If the preamp just provides a single pair of output jacks you could connect it to both the sub and the NuForce amps via a pair of Audioquest "hard y-adapters," or equivalent. In doing so, however, from a sonic standpoint it is important to be sure that the preamp has a low enough output impedance to be able to drive the combined input impedances of the amp and the sub with good results, and also to be able to drive the capacitances of both sets of interconnects. In most cases that same caveat would apply even if the preamp provides two sets of output jacks, and a y-adapter is not necessary, since in most such cases the two sets of output jacks are wired directly together inside the rear panel, and driven by the same output stage. The line-level input impedances of many subs are relatively low, often between 5K and 20K, which can make that issue a particular concern. And the output impedance of some preamps, especially tube-based preamps, often rises to much higher values at deep bass frequencies than the specified nominal impedance, which is usually based on a mid-range frequency such as 1 kHz. That would also add to the concern. Again, though, these concerns just relate to sonics, not to any possibility of damage. If you'll let us know the specific model of your CJ preamp, and whether the modifications that were made to it included an upgrade of its output coupling capacitors (especially if it is a tube-based model), and the lengths of the cables that would connect it to the power amps and to the sub, I can comment further on this. Regards, -- Al |
Rvpiano, glad I was helpful. Unfortunately I didn't read the thread in time to recommend against the purchase in the first place, given your mention of the NuForce amps.
4hannons and Bob (Gdnrbob), yes I had recommended that cable, while also pointing out that given the very high input impedance of the 2wq (described as being more than 100,000 ohms) it is overkill in terms of gauge. Given the very high input impedance the thinness of the gauge that could be used would only be limited by the physical fragility of the wire and its connections. "Speaker wire" in the traditional sense of a heavy gauge is not necessary from an electrical standpoint.
Regards,
-- Al
|
Rvpiano 6-1-2017
After much difficulty I finally got to set up my used 2Wq sub only to find that the left channel doesn’t work. I followed the directions in the manual exactly in setting up the wx-2 variable crossover, matching it to my amp. I checked all the connections several times for tightness and correctness.
I even reversed the connectors to to the amp and the left channel still was out!
You indicated earlier that you are using modified NuForce amplifiers. Like some other class D amps some or many of the NuForce models have their + and - output terminals offset from ground by 24 volts, or some other comparably high voltage. Depending on the internal grounding configuration and other characteristics of the design of the particular sub, connecting a powered sub to that kind of amplifier output can cause either improper operation, or no signal to be heard, or damage to the amp or the sub. For example, from this manual for one of the NuForce amps: WARNING: Do not connect NuForce speaker-level outputs to the line-level inputs of active devices such as active subwoofers or semi-active speakers with powered woofers. If you use semi-active speakers where the woofer is powered, or a subwoofer that accepts speaker-level signal from the NuForce, or sums the left and right input signals from the speaker outputs of two NuForce mono amplifiers, please consult NuForce or your dealer before proceeding....
Floating Speaker Output – The speaker output terminals are floating, with a 24VDC offset (DC across the speaker terminals is still 0V). Please consult NuForce Support (support@nuforce.com) if your setup requires any of the following configurations:
1. Multiple amplifiers to drive a single speaker driver (Never do this!).
2. Subwoofer that accepts and sums left and right input signals from two NuForce amplifiers’ speaker terminals.
3. Grounding other audio signal grounds to any of the speaker terminals on the NuForce amplifier.
4.Using two amplifiers to drive a bi-amp, series-crossover speaker (uncommon setup). The fact that you "even reversed the connectors to the amp and the left channel still was out" would seem to confirm that the problem is related to the amp/sub/speaker interface, not to the crossover. But the fact that "both channels work fine without the wx-2 and sub attached" presumably means that the amp has not been damaged, at least not yet. So depending on your specific NuForce model it might not be possible to use it with any powered sub that only provides speaker-level inputs. Good luck. Regards, -- Al |
