High level vs. low level subwoofer inputs

November 11, 2022
 by Paul McGowan

18 comments on “High level vs. low level subwoofer inputs”

  1. Paul, permit me to disagree somewhat. I don't think amplifiers convey a "sound" to subs via the high level connection. Maybe there is an initial variation depending on speaker wires vs interconnects being used. However, considering that the signal must pass from the high level input directly through the low level filters and crossover on its way to the sub amplifier and woofer, whatever the "sound" characteristic of the main amplifier, it will have been obliterated by the subsequent conversion and filtration. I believe the high level connection is an artifact from the days when tube based preamplifiers and receivers could not effectively pass a low level signal to subs via long cable runs. Today, that high level connection is more of a marketing differentiator than a real life audio improvement, unless we're talking about the set up Vandersteen uses, which is very different from Rel. But, to each his own.

    1. Good afternoon jimkannry!
      It all depends on your equipment.
      It's not just cables, but it is a lot of things going on there.
      You just have to be able to listen for it.
      And also, being skillful enough to know what to listen for.
      Because if your equipment is setup in such of a way, you'll be able to hear the characteristics of your amplifier threw your subwoofers with the high level inputs in use.
      Some lower priced subwoofers yay thoe they may have high level inputs on them, won't pass the sound of your amp threw them.

  2. I wonder whether the real advantage of high level inputs from the power amp, or in my case from the speaker terminals, is that there is a better chance that the subwoofer and the woofer drivers of the main speaker are all in phase. I wonder if Paul could comment?

    1. By in phase are you referring to the idea that perhaps the slight amount of delay from the signal passing through the power amp might make things more in synch? If so, I rather doubt it. The latency of a power amplifier is negligible.

      No, I think what I find is the sound quality changes because our amps all have a "sound" to them that is then passed on to the subwoofer.

  3. Well, although I don't use subwoofers there are some points to think on.
    Depending on the input characteristics of the subwoofer amplifier input there ist a chance of a ground loop when the subwoofer amp is safety grounded. Which means that hum can be injected. Best would be a balanced input with good CMRR which can be used non-symmetrical without a ground loop. There I guess is no major difference between low and hi input. Phasing is anyway an extra subject depending on the main loudspeaker crossover characteristic and the lowpass filter of the subwoofer amp as well the spatial placement of the subwoofer.

  4. What's the best way to hook up two subwoofers? Also dealing with two subwoofer amplifiers should each one be plugged into wall outlets on separate circuits so not to overload the AC outlet?

    1. Good afternoon Joe!
      I can get you started in the right direction with that.
      But first, I need a little information about your powered subs.
      Do they have class D amps built in to them?
      If so, then plugging them in to the same outlet won't overload it.
      But if the amps are either class A or A-B, then you'll want to plug them in to separate cerkets.
      But make sure you have 15 to 20 amp brakers on each one of them.
      And this is baste on the power output of the amps that are built in to your powered subs.

      1. Class D might maximize the efficiency of the power it draws from the wall and give you a lot of watts to your speakers but I'm not sure how that effects what it draws from the wall if the resulting wattage to the speakers is very high. I have 5 Sunfire Carver signature subwoofers that each have a high efficiency amplifier rated at 2700 watts. Im only using 2 in my set up to achieve stereo sub operation. At its maximum rated output it uses 800 W average, 16 Hz to 100 Hz, at absolute maximum continuous output. 8 W at idle. 2700 W peak, time limited basis. Heres the specs....

        Specifications
        Amplifier Output
        2,700 W rms (3.3 Ω impedance)

        High Cut Filter
        30 Hz - 100 Hz adjustable.

        Frequency Response
        16 Hz - 100 Hz.
        Low pass is -6 dB @ 80 Hz

        Power Line Voltage
        AC 120 V, 50/60 Hz

        Dimensions
        13" x 13" x 13"

        Weight
        53 lbs

        Finish
        Black anodize and black lacquer.

        Line power consumption:
        800 W average, 16 Hz to 100 Hz, at absolute maximum continuous output.
        8 W at idle.
        2700 W peak, time limited basis

        Output levels:
        Greater than 116 dB peak SPL (includes room gain) from 16 Hz to 100 Hz.
        Measurement method is one meter, anechoic, including room gain.

        A typical Sunfire subwoofer can be expected to ‘best’ its specified minimum peak SPL by several decibels.

        THD:
        Less than 1/10 of the fundamental between 18 and 80 Hz

        Input sensitivity for full output:
        250 mVrms from left input with volume control at 0dB, 60 mVrms with volume control fully clockwise*
        0.75 Vrms from right input with volume control at 0dB, 200 mVrms with volume control fully clockwise*

        Input impedance:
        15 KΩ for Line-Level inputs
        15 KΩ for Hi-Level inputs

        Driver:
        Extra large magnet and long throw mechanical design yield very high back-emf. The result is extraordinarily high operating efficiency – that is, more acoustic output for each watt of input.

        Internal system gain:
        54 dB from left input jack to speaker with the Bass Level control at 0 dB, 68 dB with the control fully clockwise*
        44 dB from right input jack to speaker with the Bass Level control at 0 dB,
        46 dB with the control fully clockwise*

        (* The Sunfire Subwoofer utilizes differential gain on the left and right inputs to retain both L+R and L-R information.)

        1. The subs I have are the 12" woofers with a 12" passive radiators so very efficient, I don't think I will need to use the amplifiers full power output capabilities to drive it to its maximum rated 116 db output thats conservatively rated and can reach close to 120 db.

          Wondering if using two of these subs should I need to have them on seperate AC lines and if using two subs if the high level inputs are still the best way to go. I assume it is.

          I have 5 of these subs so using two on each side stacked would give me unbelievable low bass output. I might have to clamp them together to keep the upper sub from vibrating off the lower sub...lol.

        2. Good evening Joe!
          I know Sunfire subs very well.
          And yes, they are in deed very powerful.
          But they can also be energy hogs too.
          You said that they both draw 800 watts from the power outlets they're plugged in to.
          800 watts=15amps.
          Yes you will want to put those on separate cerkets.
          Putting them both on the same cerket will pop the braker.
          The amps in those subs, are class AB.
          As far as I'm aware of, Bob Carver doesn't like class D amplifiers.
          He said that, he couldn't get rid of the high frenclicy noise that comes from the switching power supply.

          1. 15 amps might be a tad higher than 800 watts average and it could draw more power in some cases so unless I have more devices on that circuit it should be OK. Might even be OK with 2 subs on the circuit if Im not pushing the amplifiers to their maximum.

            Carver probably didn't like D on a full range main amplifier but in the case of cramming a 2700 watt amplifier into a small box that has two 12" woofers he may not of had a choice. These are Sunfire amplifiers so they could have had a say on what went into these and also had no choice than to put a D amplifier in there. Theres no extensive heat sinks so I doubt they are AB amplifiers. And they call them high efficiency amplifiers without specifying the class.

            Thanks for your input John.

            1. Good morning Joe!
              Just one quick question.
              How long ago did you pick up those Sunfire powered subs?
              The reason I'm asking is because, I was very curious about what's going on inside of one of those Sunfire powered subs.
              And so, I opened one of them up and took a pepe inside of it.
              I found a very sizable power transformer in there that was cranking out 90 volts.
              Hints the 2700watt rating.
              You may not see the heat sink from the outside, but it does have one.
              It has 16 transistors on it.
              It's push pull class AB.
              But this was about 25 years ago that I opened up a Sunfire sub.
              Bob is the CEO of Sunfire.
              But he also has a parenting company that he put his last name on.
              But if you ask me, both Sunfire and Carver are one of the same.

  5. After removing the preamp from my system (used the line level RCA LP Sub output for the past 12 years), went with the main amp's speaker outputs to include the Sub in the new configuration (sub has high level inputs). As Paul advocates, found my stand monitors playing full range Integrates Beautifully and Seamlessly with the sub (w/sub amp internal LP crossover set @60hz)! AMAZING tonality/timbre blend and total system balance with soundstage/imaging integration that says, "LIVE, I Am THERE"!

  6. Using the speaker level inputs of your subwoofer can damage your amplifier if your amplifier is not common ground. Is there anyway to tell if your amplifier is common ground besides contacting the manufacturer? I'm surprised that Paul didn't mention this when recommending using the speaker level inputs versus the RCA inputs.

    1. Good morning Joe!
      Yes there is.
      Disconnect the negative wire of one of the speakers that you have hooked up to your receiver and or power amp, and touch it to the frame of the amp's chassis.
      If that speaker makes music, then your receiver and or amp is commonly grounded.
      But if the speaker makes a popping sound, then that's how you know it's not commonly grounded.
      Hope this helps.

    2. It is highly unlikely that any modern amplifier would be damaged by grounding one of its outputs. That simply triggers the protection circuit.

      If you have a very old amplifier that you worry might be damaged then John's method will work and be worth the effort.

  7. A safer way (for the amplifier) to do this is with the amplifier powered off set your multimeter to resistance connect one lead of the meter to the right channel negative binding post and the other lead to the left negative binding post it you get zero on your meter you have a common ground amplifier.

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