What Is Line Level Input in Subwoofer

1. I have connected a MartinLogan subwoofer to an amp/receiver in two different ways... line-level (RCA) and speaker level.

   But in each case, it sounds... literally... like the speaker level connection is much stronger and robust than the line level (RCA) connection. The line level is easier to do than the speaker level, but sure doesn't match the response I get from the speaker level connection.

   Can anyone offer an explanation/understanding of this?

   Thanks.

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2. That's probably because your listening to different audio levels. Ideally you want to use a sound pressure meter to set the proper level of your subwoofer, so the two connections would match. Using the line level will take some of the load of the amp.

3. Basically it's just different gain structure through the different inputs.

   FWIW, using the line level inputs vs. speaker inputs will NOT change the load on the amp in any meaningful way. The speaker level inputs of a powered sub are high impedance so the amp doesn't really even know the sub is there.

   For the line level/RCA connections are you using actual pre out or sub out, or are you using a tape out?

   Last edited: Nov 17, 2015

4. I've used both on different amps/receivers. There have been a few receivers that had a dedicated "sub out" RCA, others have had "line out" RCAs that I have used.

   Since the sub I have is a powered sub, it doesn't draw much from the amp/receiver.

5. Yeah, I think it's mainly just a difference in gain. There could be some other factors, considering a frequency limited signal from sub output vs. full range speaker level input, but I think it's primarily just a difference in net gain.

   Does the sub have a level control so you can normalize the sub's volume when you try different connections? That way if you keep the actual volume (not position of the level control) of the sub the same you can evaluate the sound on itself, without the distraction of one way being louder or softer than the other.

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6. When you are using a subwoofer in a home theater application where 5.1 or more channels are decoded, the sub may be better sounding using line level (SUBWOOFER OUT) because it should only get sounds specifically designed to go to a subwoofer.

   If you put the full range signal through your subwoofer (even with a crossover on the sub) you may be asking the subwoofer to play the bass from a mans voice, and it sounds strange, as if darth vader is talking.

7. This sub has a "level" knob as well as the crossover frequency knob, which I have pegged to 150 Hz. I have tried using that level knob when connected via the line level RCA connection, but to get the same output as when using the speaker level connection I have to crank the level knob further to near the max... and the bass performance sounds poor/degraded when I do so. So, I've moved back to the speaker level connection and I like the sound better. It's not as "neat & clean" with the speaker wires as it would be with an RCA cable, but hey... there's no "significant other" around to critique my stringing of cords, cables, and wires all around.

   As for HT use, I don't use one. Been through several HT receivers with "sub outs" but HT receivers and I just can't seem to develop any kind of "relationship."

   "Luke.. come with me and together we can rule the galaxy." MIght be cool to hear that through a sub... who knows?

8. Wait a minute. Your using a line out from a receiver (and I'm assuming it's not a surround receiver, what out jacks did you use from the receiver and what in jacks did you use on the sub-woofer? If you used the record out from your receiver, it does not have any volume control. Surround receivers have a dedicated output that goes thru the volume control.

   If this is the case, you are much better off using the speaker, hi-level inputs for the sub.

   Wayner

9. One possible that has not been said, a line out, or speaker out for that matter, depending on how the preamp is designed or what crossover design is being used in the speakers, can be out of phase, either slightly or completely.

   Lets assume a 12db/oct crossover in a two way speaker. The total phase shift of both speakers is 180 degrees, assuming a total linearity of phase shift at all frequencies, which is not physically possible with most speakers, but this is just for an example.

   This means that the woofer is 90 degrees out of phase with its input at the terminals, and the tweeter is 90 degrees out of phase from the input in the other direction.

   Within the speaker itself, this is not an issue, as one of the drivers will be wired backwards to compensate.

   However, running the speakers with the sub presents a challenge. The sub itself may be out of phase with the speakers, probably more noticeably if the sub is having to do higher bass frequencies. As the mains roll off, and the sub rolls in, the frequencies may not line up resulting in cancellations, and reduced volume.

   That is why better subs have a reverse phase switch, and the best subs may have 90 degree, maybe even 45 degrees as choices too. Yes, the phase issue could be reduced by simply moving the sub around until you get the best response, which many of us have done, but a sub might have the best response in a position where the phase may not. The switch then has the job of correcting for that placement, timing it with your mains.

   This is why instructional manuals for subwoofers say to "select the phase which sounds best," as the manufacturer has no idea where you will be putting it or what speakers you will be pairing it with. There are scientific ways to figure it out, but in this case, listening and adjusting can get you most of the way there.

   In your situation, though, I think that your sub out jack simply has lower gain that what is offered by the speaker terminals.

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10. 

    GuyK Addicted Member

    If you are using a line out from something like a tape monitor circuit, it will have no attenuation from the volume control, and is going to be a fixed voltage. A speaker level connection will have attenuation (or not, depending where the volume knob is), and could very easily have a higher level signal.

11. If the sub woofer has its own amplifier (a "line level" input shows that it does), it does NOT put an extra "load" the main amplifier at all!

    What happens is that the main amp. still "sees" your speakers as if the sub was not connected, OR gets gradually "disconnected" from your main speakers at a low frequency by way of a crossover capacitor being added to the main speaker path.

    With a large capacitor being added to your main speaker line, the lower you go in frequency, the less of a load your main speakers have on the main amp.

    This way, the sub woofer "picks up" the range your main speakers are "dropping off" via the subs speaker path crossover (if the sub woofer has NO crossover at the speaker terminals in\out, your main speakers just continue to play as they always do with the sub "brings up" the low bass, augmenting what your main speakers are doing).

    It is a different story for a "passive" sub (NO electronics, NO line level input, NO AC cord connection), these units still drop off your output from the main speakers as you go lower in frequency, BUT through a large COIL, send your main amps output to the large sub woofers voice coil, and at the frequency that is in the sub woofers range, your main amp now just sees the larger speaker as your main speakers are dropping out.

    This is like adding another larger woofer to a 2 or 3 way existing speaker system, you are adding more crossover parts to enable the "new" woofer to work with the existing system, even thought the "new" (sub) woofer) is in its own cabinet.

    With a "powered" sub woofer, the load on your amp. remains the same at lower frequencies (or may be lightened if the sub has a crossover).

    With a "passive" sub woofer, your amp. continues to be "loaded" even as you "go lower" and the sub takes over (as its crossover "rolls off" your main speakers while now putting power into the subs speaker.

    If the (passive) sub has the same impedance as your main speakers, there should be no "overload" as the main amps "crossover" from your main woofers to the sub woofers.

    On a powered sub, your main amp sees nothing "new" with your main speakers (a sub with NO crossover) OR sees less of a load as the frequency gets lower ( and drops your main speaker output ) as the sub "takes over".

    None of the above applies (as far as amplifier load) if you are using the RCA line level inputs (passive subs need not apply here!)

    Mark T.

12. You are correct!! hours after i posted that reply i realized i was thinking of a passive sub. Excellent description on how subs work.

13. When using a receiver/integrated w/ A-B speaker outs, I like putting the sub on B out.

    Basically for the convenience of being able to remotely mute it by shutting off the B out.

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14. I have been wondering about this very thing. I have a power amp in route and it has connections for two pairs of speakers, A & B. I had thought, since I use the "speaker level" sub connection, of connecting my powered sub to the B speaker terminals.

    Thanks for sharing this.

15. It works great for home theater and music systems. Sometimes you don't want a sub on at all when you are just listening to music, However, be mindful when you are switching into home theater mode and that you are not sending high-pass only signal to the speakers. If that is so you would also be doing that to the subwoofer when connected to speaker outs whereas the subwoofer line out will not,.

16. I don't have a home theater receiver. I have a tube preamp going into a power amp.

    It might be interesting to hear what the Infinity RS 5bs will sound like alone without the sub. The Infinitys have 10 inch woofers and my MartinLogan sub is 8 inches.

17. you probably just like the sound of the built in crossover of the sub. when using high level inputs you use the subs internal crossover. using a sub out on an amp or receiver, you're using the built-in active filter of the amp/receiver that bypasses the sub internal filter.

    there is probably way different slops between two. if the internal sub has a smaller slop like butterworth/linkwitz 12/18db filter compared to higher slop of active filter in amp/receiver which might be 24/db slop, that alone will make huge difference. the lower slop will make seem like woofer is stronger.

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18. That causes too many problems, IMO. Invariably someone tries that but their receiver or whatever put A+B in series then it doesn't work at all due to the high impedance the sub.

    It's better just to keep it simple. Connect the sub to the same terminals as the main speakers. If you don't want the sub on then turn it off or down. Realize this is my opinion, but in the last few weeks there have been a number of problems in exactly that regard, that wouldn't have been a problem if they weren't trying to outhink the system, so to speak.

19. How does the sub's high input impedance cause too many problems exactly?

20. How much sound comes out of an 8 ohm speaker in series with 10,000-20,000 ohms?

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What Is Line Level Input in Subwoofer

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