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Getting bass right

Paul's Posts — 28 August 2012

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Getting bass right

In yesterday’s post I discussed the fact that there are basically only two ways to connect a modern powered subwoofer: with a high level or a low level input.  The vast majority of subwoofers today no longer offering a high level input because Audiophiles are probably ok adding another cable and with the rise of home theater applications as the primary market for subs, which require a low level input from the surround processor, the fate of the high level input has been sealed.

But what advantages did that high level input offer?  Plenty and, to this day, it is still my favorite for integrating a sub with a system.

Let’s review what the differences between the two inputs are first.  Fact is, both inputs actually go to the same place on a powered sub – they just take somewhat different routes.  The high level input is designed to come from the main power amplifier’s outputs – which is typically 20 to 30 times louder than its inputs.  Subwoofer designers merely take a couple of resistors and reduce this very loud signal down to match the main power amplifier’s input levels.  If done properly the result is that there is no loudness difference between the low levelor the high level input signal that feeds the subwoofer’s internal power amplifier.

There is one big difference, however, and that’s the effect the main loudspeaker’s power amplifier had on the signal we reduced in level. Remembering that all power amplifiers affect the sound of the music that passes through them (phase shift at the lower extremes, tube amps with their output transformers, solid state amps, vs. class D amps etc.), we’d be much better off using the already-amplified output of the power amp to feed our sub because that output will be much closer matched, sonically, to what’s being fed to our main loudspeakers.

If we use, instead, the low level input directly from the preamplifier’s output we have not only lost the advantage of matching the amplifier’s sound but we risk the possible degradation of adding another length of interconnect to the preamp’s output which, in many cases, can do a lot of sonic damage (depending on the interconnect’s length and the preamp’s design).

So with all this in mind here’s my advice.  If you’re forced to connect the subwoofer with its low level inputs, use a Y connector at the end of your interconnect cable feeding the power amplifier if the run is more than 2 meters.

If you’re handy with a few tools and want a better DIY path, just make a 30X resistive divider (use a 30K resistor and a 1K resistor) across the output of your power amplifier and feed the subwoofer’s low level inputs from the junction of the two resistors (the 30K going to the + output of the amp and the 1K going to the – of the amp).

Either way you go, you’ll be delighted you have a sub and connected it right.

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About Author

Paul McGowan

Paul McGowan is the CEO and co-founder of PS Audio Inc. a Boulder Colorado design and manufacturing company of high-end audio products and services. McGowan has been designing and building high-end products for nearly 40 years. Hobbies include skiing, music, hiking, artisan bread baking, kick boxing and cooking. He lives in Boulder Colorado with his wife Terri and his 4 sons.

(7) Readers Comments

  1. paulsquirrel

    August 28, 2012 at 5:55 am

    Hi Paul.

    Until now I could follow all your recommendations but here I see a logical error in your arguments – maybe you are right from the point of practical experience. Ignoring the cable discussion (there are lengthy cables to the main power amps as well) finally the internal subwoofer amp will also add it’s fingerprint (phase shifts etc).
    And if you have active speakers (mains and subs) with digital inputs are your considerations still valid having the additional benefit of active crossover designs in those active speakers?

    And finally: isn’t there still the inherent problem of different “amplification factors” between main- and subwoofer-amps resulting in a loudness mismatch when changing the volume (additionally to the Fletcher-Munson effect – there is a nice idea from TacT with their room correction preamps for compensating the loudness effect))???
    There remains one active-subwoofer-question no dealer could answer to me until now: how can I find an active subwoofer getting maching amps for both main and subwoofer speakers?

    Kind regards

  2. August 28, 2012 at 6:42 am

    You’re correct on the active loudspeakers with digital inputs. In that case you’re rather left out in the cold – so I agree there’s not much you can do.

    The subwoofer’s own amp is always going to be a liability and one you just have to accept (life’s full of these compromises).

    As far as the amplification problem, heck yeah! That’s why subwoofers all have volume controls -= you’ll want to dial in the amount of bass that is perfect – never standing out, blending in perfectly.

  3. alan

    August 28, 2012 at 8:54 am

    Paul,

    First, I want you to know that I enjoy your posts with their resultant discussions. I find it both entertaining and educational.

    Now, if I understand you correctly, you are advocating simply integrating (adding) a sub with full range speakers; I don’t see where the mains are getting any low frequency relief. Other writers have emphasized reducing the requirement for the main speakers to produce a lot of deep bass, citing reductions in distortion plus the ability of the system to produce higher levels overall. But I get the impression from the discussion of your own system that you simply add both mid-bass and sub-bass to your maggies.

    Owners of some of dipoles such as the fabled early Quad electrostatics write that they filtered low bass from the Quads and applied it to their subs with resulting higher dynamic range and less strain on the Quads.

    My own main speakers produce clear, low distortion (apparent) bass down to 20Hz. However, the twin 8″ drivers can produce only so much deep bass before being over-driven. Wanting more, I added twin subs and crossed them over at 40 Hz with the result that a soundtrack from Jurassic Park produces footfalls that are strongly felt long before being heard. Yet I rarely am aware of the subs on ordinary music, except for the occasional recording that seems to expand the size of my acoustic space. Oh yes, those huge pipes on some organ recordings are simply another story altogether.

    • September 4, 2012 at 6:19 am

      Understood and yes, that’s what I am doing and advocating: full range loudspeakers need an extension to their deepest low end. I don’t even roll off the main loudspeakers, instead choosing to buy only speakers that can handle full range audio.

      Certainly your approach can work and has benefits for a speaker that needs a bit of rolling off, but that wouldn’t be my choice.

  4. oliver T. Finch

    August 28, 2012 at 8:58 pm

    Is it possible to post a diagram of the 30K resistive divider and its connections? Regards.

  5. lilleniit

    September 2, 2012 at 10:35 am

    I have a question regarding adding a subwoofer: my subwoofer is fed via the LFE output from my AV receiver where everything has been digitized (allowing EQ among the speakers, including the subwoofer). So when I attach my turntable, am I losing the analogue ‘goodness’ via this A/D conversion and subsequent subwoofer output?

    Lastly, although this is my first post, I want to say I really enjoy reading your posts! They have given me much to think about.

    • September 3, 2012 at 7:18 am

      I don’t know enough about your system to actually give you a good answer but my guess would be yes, you’re probably never listening to pure analog. There are many systems these days that use an A/D converter to digitize the sources because once in the digital domain everything because quite easy to manage and control – not always the best way to go but that depends a lot on the quality of the device and the engineers that designed it.

      Thanks for the kind words.

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