How to hook JL F112 sub to both preamp and pre/pro

JL Audio does not support that type of multiple connection, as you know, and I don't think you can get what you want with one sub. I think you probably need to get a sub crossover, Bryston 10b, or 2 way Marchand 44, and run your speakers full out. Set up the Pre/pro with no sub but full range speakers it will send all info including the LFE, through the HT theater bypass to the 2 channel preamp. When you output from your 2 channel preamp to the crossover, it will output all of the lowest frequencies to the sub from both LFE and music to the sub. So if your speakers go down to 40 hrz, you would probably crossover through the electronic crossover at about 45 - 50 hrz with a set slope, testing and listening for the best settings of course, until you get it perfect. All of the LFE effects will come through the sub without damaging the speakers.

texas, are you sure you can't connect both a 2 channel preamp and a HT pre/pro to a fathom sub simultaneously? i was running this setup for a while (on the advice of JL Audio) without any issues. RCA from the HT pre/pro; XLR from the 2 channel preamp. XOver in the HT Processor was set higher than that in the sub. worked great.

Srosenberg, assume your preamp had two sets of outputs for L/R? Also, would assume that one has to be careful not to send signals from both preamp and pre/pro simultaneously, LOL. Seriously though, if that was done in error, could it damage the sub?

I do this right now with an Epik Valor sub.

RCA from the AVR to the LFE input and L/R Line level from my MF A5 Int amp L/R preouts.

For Movies, the A5 is put in the HT Bypass and the preouts become disabled and the AVR drives the sub via the LFE input. Bass Management is via the AVR.

For Music, I turn off the AVR, and use the MF A5 with source components - and the sub is driven from the A5 L/R preouts. "Bass Management" is via the xover cutoff on the back of the sub.

Works just fine.

No, you cannot, you absolutely can get competing signals that can damage the amp/crossover.

I have talked with JL Audio (and Velodyne, and B & W, and Sunfire....) about multiple inputs when I owned an F113. I bought a Rel B1 and sold the JL Audio so I could use one sub for both, with 2 sets of connection. Definitive Technology Subs do this as well.

I really think it's easier, and prob a better set up for a high end system, to buy an external cross over and keep your bad-ass sub like the JL Audio. You are integrating the 2 channel sub anyway, this just take it the next step and takes the lowest frequencies away. If you have speakers that truly go down below 30hrz, you may not want to do that, but then you can just let the full LFE signal go to those speakers anyway without damaging them, and you don't need any connection but the 2 channel, with the processor set to no sub.

Bob, from everything I've read, even the better pre/pros cant compete with a good standalone preamp. Now, to be fair, I havent tested this thesis, but the overwhelming opinion in my research of forums (Agon, AVS, etc) says this is so. Also, adding a good preamp for say 2-3k would be a lot cheaper than paying 8-10k or more for a comparably sounding pre/pro like say a Classe CT or even SSP800, or a Theta Casablanca, or even the soon to be released Bryston SP3 or Cary Cinema 12. Also, the best sound is likely to be from the analog inputs that bypass processing in the these units, and that brings me back to my original problem. I will check out that NHT unit, maybe I can find a used one.

texas, sorry to belabor the point, but i'm very curious on your position where multiple sources to the fathom subs are concerned. if you're still following this thread, would you mind expounding on the circumstances that would result in competing signals? i'm genuinely curious about this, as i ran this way with an F112 without any (perceived) ill-effects for a while after consulting with the tech at JL Audio. am i setting myself up for some issues down the road??

Jeffkad, yes, my preamp (aesthetix calypso) has multiple outputs. i ran the main L/R outs from the HT pre/pro into a bypass input on the preamp, and from there, one set of outs to my main amp, a second set of outs to the fathom sub. additionally, i ran the subwoofer out from the HT pre/pro to the second inputs on the fathom. i made sure the XOver point in the pre/pro was set higher than on the fathom and everything seemed to be fine. Do not get too hung up on finding a preamp with HT Bypass, as any preamp will do, you’ll just have to remember to run the volume up when you select the input from your HT pre/pro.

best to call JL Audio and speak to their tech (Barry) about your specific system. i've also talked with the tech at SVS and they suggested the same setup the JL did, noting no damage would occur.

Another option you might consider are Rythmik subs. they are FANTASTIC and their DI amp option (soon to be replaced by the PEQ3) is specifically designed to work with both a 2-channel preamp and a HT pre/pro.

to your comment on high end pre/pros, my aesthetix calypso was night and day better than my theta casablanca. to my surprise, using my Wyred4Sound DAC2 as a preamp was also better than the casablanca and very close to the aesthetix.

Srosenberg, thanks much for the input. I actually did have an email exchange with Barry at JL and he confirmed what you said, its perfectly fine to run both inputs, LFE/RCA out from pre/pro and balanced/XLR out from preamp.
Not surprised your calypso was better, as I have heard great things about that unit, but VERY surprised about the W4S being so good. What gear are you running? Wondering if there is a certain threshold of gear quality above which such distinctions become more pronounced. I'm running B&W N803 as mains (HTM1 ctr and DS7 surrounds), with Sunfire TGA5200 amp ( a very good and underappreciated amp) and a Sunfire TGP5 pre/pro fed by both a modded Denon 3930ci and a Sonos-BelCanto Dac2 combo. Although not heavy high end, would assume its good enough to hear a difference?

The below is from the JL Audio F112/3 manual. As you can see, they specify NOT connecting 2 sources to one sub, as the tech at JL Audio told me previously. I don't know who Barry is, but I would check for clarification. I don't think you can do this with a JL Audio, I think it could fry the crossover and amp, unless they have changed the electronics significantly. Look again at the last sentence of the setup from the manual below.

SYSTEM CONNECTION DIAGRAM 1:
One Fathom to
Home Theater Receiver or
Home Theater Preamp/Processor
Most home theater receivers and
preamp/processors provide a single
(mono) subwoofer output. When
connecting a mono subwoofer output
to your Fathom, you will only use the
Fathom’s “Left or Mono” input.
Two connection types are available
for connecting the Fathom to your home
theater system: balanced (XLR or 1/4-
inch TRS connector) and unbalanced
(RCA-type connector). Balanced
connections provide superior noise
rejection and ensure proper grounding
between components. If your receiver or
processor has balanced outputs, we highly
recommend that you use them.
In the connection diagram at left ,
balanced connections are shown as solid
lines, unbalanced connections are shown
dotted. You will only use one of these
input connection methods (not both).

texas,
that's really odd that the manual is SO explicit on this point, while their technician is clearly telling end users something different. point taken, thanks for your comments - perhaps the OP should bring this up with JL.

Jeff,
my rig isn't the final word in high end, but i've spent the last several years going through several different amps, preamps, sources, DACs, speakers, etc to get to a point now where i'm comfortable evaluating gear IN MY ROOM to MY PREDILECTIONS. those are important to note, as we all listen for different things, and my gear might sound dramatically different in another room.

i'm currently running Wilson speakers and VAC monoblock amps. the aforementioned DAC2 serves as DAC and preamp and is surprisingly capable in that role. that piece was really something of a surprise, as i never expected it to better my Weiss Minerva, and certainly never thought I'd be running it as a preamp.

Mac:

Barry Ober is a technical consultant for JL. Check out his own personal website at soundoctor.com ( a wealth of audio info and white papers). This guy has an uber-impressive resume, he's an audio engineer and has built recording studios. A very knowledgeable dude.

I think he's credible, to say the least.

Srosenberg, thats a mighty nice system, and certainly more high end than mine by a longshot. As I stated earlier, I wonder if I'll be able to hear as much of a significant difference with preamp changes as you would in your system. Guess the only way to find out is try it! The W4S Dac seems like a real giant killer based on your experience, so I may give that a shot at some point. By the way, Classe just came out with the CP-800 a killer preamp/dac/async USB all-in-one unit that also has sub-out. $6k pricetag, but thats pretty reasonable for everything it does (as long as it sounds good of course)

Bob, OK, so you are firmly in the camp of processed sound, if done right, sounding better. I'm ok with that (I think Kal at Stereophile agrees with you too). Of course I cant agree or disagree as I've not compared it much, but assume you have and to your ears processed is better? Clearly there are two distinct camps on this, and as always, only your own ears will tell you what you like. Some people firmly believe that room correction is the only way to hear music properly. However, I think the argument on pre/pro vs preamp assumes a certain level (pun?) playing field and, given such, a preamp provides a better, cleaner, clearer picture than does an average pre/pro. Again, assumption on my part from reading forums. No doubt that, in a poor room, processing/correction can probably make a bigger difference and provide better SQ than a good preamp.

A pre/pro is a swiss army knife that is great for doing a bunch of stuff at the sacrifice of doing just a couple really well.

I'd prefer a high end preamp/amp and take the extra time to do my own sub integration.

[From the description, it seems that Jeff's original idea of a line level switch box would be a simple solution. Decware's box comes to mind]

No, actually, according to Barry, just connect the two units directly to the sub: pre/pro LFE out via RCA to sub, and preamp L/R out via XLR to sub. Just dont send signals out from both units simultaneously!

Although I am of course not familiar with the internal design of the JL sub, I absolutely can't envision a way in which driving both inputs at once could cause damage to it, assuming that both inputs are not simultaneously overdriven with absurdly high volume signals. My guess is that the reason the manual refers to not using both inputs simultaneously relates to sonics.

First, with both inputs simultaneously connected to different components there conceivably could be an increased chance of ground loop issues arising.

Second, my guess would be that the unbalanced signal input is simply connected directly to one polarity of the balanced input. If so, that would mean that whichever component is providing the input signal that is being used would have to drive a load impedance approximately equal to the output impedance of the component that is connected to the other input. That will be a much lower impedance than line level output stages are normally called upon to drive, and, depending on the design of the particular components, compromised sonics might result. And I suppose that a poorly designed output stage in one of those components might even suffer degraded long-term reliability or damage as a result. Finally, it would mean that the balanced input is no longer truly balanced, because the two signal polarities would be loaded in a very unequal manner.

As far as line level switchboxes are concerned, the DB Systems DBP-2JAU/5 from DB Systems may be of interest. I've had occasion to use one in the past. It is nicely made and does what it is supposed to. It costs $110 for the version with gold-plated jacks. My one concern about that approach would, again, be the possibility of ground loop issues arising.

Regards,
-- Al

A follow-up to my previous post: If you have a multimeter you can easily determine whether or not the unbalanced signal input is connected directly to a balanced signal input.

Disconnect the sub from everything (including power), and for each channel check for continuity between the center pin of the rca input jack and pin 2 of the xlr input connector. If there is no continuity, check for continuity between the center pin of the rca input jack and pin 3 of the xlr input connector.

Regards,
-- Al

It will send signals at the same time, and I am 100% certain the manual is correct, and the tech, don't know the name, at JL Audio told me explictly that you CANNOT send both signals, you have to only have one going at a time, unlike Rel's or Def Technology.

Hi Mac,

Yes, of course you don't want to input two ACTIVE signals at once, given that there is no provision for selecting between them in the sub. If you did, you would hear some highly distorted combination of the two signals.

My comments were addressing the questions of whether DAMAGE would occur in that situation (I don't think it would), and whether or not sonic quality would be compromised in the situation where both inputs are connected but only one input has an active signal on it (I think that sonic quality MIGHT be compromised in that situation, depending on the factors I described).

Best regards,
-- Al