Last month, in “Integrating a Single Subwoofer into a Two-Channel System for Beginners,” I wrote about the first steps of that process. This month I go deeper, into various ways of using a high-pass filter with your main speakers, and the advantages of taking more measurements and using equalization (EQ), aka room correction.
First, I’ll talk a bit about two key elements of subwoofer integration: the low-pass filter and the high-pass filter.
A low-pass filter (LPF) lets through all audio signals up to a certain frequency, above which it cuts them off, though not suddenly and all at once -- instead, after the frequencies of the audio signal reach the cutoff frequency, there’s a gradual reduction, or rolloff, of those frequencies. A high-pass filter lets through all frequencies above the cutoff frequency, and instead rolls off frequencies below that cutoff. An LPF is applied to the subwoofer, which reproduces bass frequencies up to the frequency the LPF is set to, after which its output is reduced. A way to set and adjust the LPF is built into most subwoofers.
A high-pass filter (HPF) is applied to the main speakers, which must reproduce the frequencies above the HPF cutoff frequency, their output then being rolled off below that frequency.
Ideally, the LPF and HPF are set to create a seamless blend of the outputs of the speakers and sub.
(If you haven’t read last month’s article, I recommend doing so before reading further here. It explains how to figure out the LPF and HPF frequencies, which will depend on the frequency responses of the speakers and subwoofer, as well as where they’re placed in your room.)
The advantages of using an AVR to integrate a subwoofer into your system
Those looking to buy a complete entry-level stereo system with subwoofer might consider what I’m about to suggest blasphemy, as it runs counter to two-channel orthodoxy: Instead of an audiophile integrated amplifier, use an audio/video receiver (AVR) designed for a surround-sound system. The reason is that even the cheapest AVR has something most integrateds lack: bass management, which lets you choose LPF and HPF cutoff frequencies for, respectively, the sub and main speakers. Would a two-channel integrated, or a pairing of preamp power amp, sound better than an AVR. Probably, all else being equal, and assuming both include bass management. But when integrating a sub, not all things are equal. The audible improvements offered by bass management easily outweigh any improvements a better amp and/or preamp without bass management might provide.
With an AVR, you can run the auto-EQ or auto-calibration software that most of them include to remove much of the guesswork in setup. Then experiment with the frequency at which the main speakers and sub will hand off to each other: the crossover frequency. If you use auto-EQ, once that’s complete, it’s important to identify your speakers as Small in the AVR’s setup menu, regardless of their actual size, and to then verify and adjust the AVR’s subwoofer-volume trim level with an SPL meter, as well as by ear, as described last month. You want the sub to contribute to the overall sound, but not so much that it stands out. In my opinion, even if your main speakers are floorstanders, your sub will likely do a better job of reproducing frequencies below 80Hz, which is why I recommend identifying them in the AVR’s software as Small. If they’re minimonitors, seriously consider crossing them over no lower than 80Hz, and perhaps somewhere between 80 and 120Hz -- this will let the sub provide most of the bass and, because your main speakers are now relieved of most bass duties, increase their midrange clarity.
That said, if you’ve got big, high-quality floorstanders that can reproduce low bass with authority, try running them full-range (i.e., identify them as Large in the AVR’s menu, meaning that its auto-EQ software will apply no HPF), and set your sub’s LPF at or near the frequency at which your speakers naturally rolloff the low end, as described in last month’s article. How well this works will mostly depend on your room’s size and furnishings, the speakers’ positions in it, and the speakers themselves. The idea here is to experiment with the HPF and LPF settings to find what sounds best to you -- and an AVR makes such experiments easy.
I consider myself to be a rather objectively minded audiophile who loves the gear aspect of this hobby as much as the next guy. I’m not suggesting that anyone who feels as I do needs to permanently live with an AVR at the heart of their two-channel system. I am suggesting that if your goal is good sound from a 2.1-channel system at not too high a cost, an AVR is the best way to start.
Using a two-channel preamplifier or integrated amplifier
As you progress in this hobby, you can look for a high-quality, two-channel integrated amplifier, or a pairing of preamplifier and power amp, that offers full bass management akin to that found in an AVR. High-end audio components that satisfy this requirement are few, but things seem to be changing. Two current examples of integrated amplifiers with bass management are Parasound’s Halo P 6 and Anthem’s STR Integrated Amplifier, and they go about it differently. The Halo P 6 manages bass in the analog domain, while the STR Integrated Amplifier does it with digital signal processing (DSP) while also offering room correction. Whether or not analog or digital might be better at this is beyond the scope of this article; all I’ll say here is that, regardless of the domain, having both low- and high-pass filters built into an integrated amp can make accommodating a subwoofer a lot easier.
Anthem STR Integrated Amplifier
But what about the two-channel enthusiast who wants to implement low- and high-pass filters in a system in which he or she has already invested a lot of money, but that has no bass management at all? That was my situation. My system of McIntosh Laboratory amplification comprises separates -- a C47 preamp and MC302 power amp -- with no bass management. Thankfully, as I mentioned last time, almost all subwoofers today come with a built-in LPF, so that was taken care of. But how to set and apply an HPF? For such systems as mine, many options are available as add-ons.
Makers of professional and auto audio gear offer affordable, dedicated, active crossovers with adjustable line-level output to insert between a preamp and amp, or to use with an integrated amp that has preamp outputs and power-amp inputs. A good place to start is Behringer’s Super-X Pro CX2310 V2, an analog crossover rich in features and available from Amazon for about $110. If you’re looking for a more audiophile-approved active, analog, line-level crossover, Bryston offers the 10B-Sub ($3595). I highly recommend Marchand Electronics, which can customize a line-level crossover for you for not much money. After experimenting with the Behringer I bought a Marchand XM446 ($495), which I ordered configured with an HPF at 120Hz with a 24dB/octave slope for both channels, settings I’d arrived at by taking plenty of measurements and doing lots of listening. I measured the XM446 as soon as it was delivered, and found it to be within 1% of those specifications. And because it’s passive, it didn’t audibly contribute to my McIntosh MC302’s extremely low noise floor.
Marchand Electronics XM446
Another way to implement bass management at line level in a system lacking HPF capabilities is miniDSP’s DDRC-24 USB DAC+DSP digital room correction processor ($399), which uses DSP to manipulate the crossover. The DDRC-24 also includes Dirac’s Live room-correction software, which I use, love, and highly recommend. It has one pair of unbalanced analog inputs (RCA), two digital inputs (USB, TosLink optical), and two sets of unbalanced stereo outputs (RCA) configurable for bass management. If you don’t like the idea of the analog-to-digital and digital-to-analog conversions done inside the DDRC-24, miniDSP’s DDRC-22D ($799 with UMIK-1 microphone; I own one of these) is purely digital from input to output, with no conversion to analog -- and thus lacks bass management. (Managing the bass and using Dirac Live with a DDRC-22D would require setting an HPF either by inserting an external component between preamp and amp, or using a preamp or integrated amp with built-in bass management.)
One of the most sophisticated components on the market for making a subwoofer part of a stereo rig is Anthem’s STR Preamplifier ($4000), which offers ways to set up 2.1-channel (one sub) and 2.2-channel (two subs) systems. Not only does it provide full-bandwidth (15Hz-20kHz), user-adjustable Anthem Room Correction (ARC), it also lets the user select independent LPF and HPF cutoffs for two subwoofers -- and for the smoothest transition possible, it can even apply phase-alignment correction to one or both subs and the main speakers. (I recently talked about the STR Preamplifier with its designers, Peter and Mark Schuck; see this space next month.)
Other options are available; these should give you an idea of what’s possible, and where you might start.
Measurements and EQ adjustments
If you’ve chosen a way to set an HPF that doesn’t include room correction, you’ll want to use software such as Room EQ Wizard (REW, discussed last month) to measure your sub’s output from 10 to 200Hz, and look for the smoothest frequency response. (By “smoothest,” I mean the FR with the fewest and/or least-pronounced peaks and nulls.) This will give you the most promising position(s) for your sub. As also mentioned last month, preferential treatment should be given to front soundstage placement if you plan to set the sub’s HPF above 80-100Hz, because of problems caused by a sub placed in a poor location -- e.g., you can hear the subwoofer’s contribution as separate from your main speakers, which isn’t a good thing.
As described last month, for reasons unique to my room, I’ve set the LPF at 130Hz and the HPF at 120Hz, both with 24dB/octave slopes, for my SVS SB-4000 subwoofer and Bowers & Wilkins 705 S2 speakers. The SB-4000 includes built-in parametric EQ (three points), operable with SVS’s free Subwoofer Control app. The SB-4000’s EQ capabilities aren’t full-on room correction, but can be used to smooth out the frequency response. Using the Subwoofer Control app installed on my Samsung S9 smartphone, I smoothed out -- that is, improved -- the system’s bass response to a small degree.
The plot below illustrates the combined in-room frequency response of my Bowers & Wilkins 705 S2 main speakers and SB-4000 subwoofer with no EQ (green) and with EQ (blue).
It should be obvious that the nulls at 63 and 133Hz have been somewhat filled in. The differences may not look big, but in terms of sound they’re significant. Listening with SVS’s Parametric EQ engaged provides bass with better slam and chest pressure.
One last point: For simplicity’s sake, the B&W/SVS graph above plots only two measurements taken at my listening position. Because in-room bass-frequency response can vary significantly even with small changes in mike position, it’s important to take from six to nine measurements, each from a different position within a radius of 1’ to 2’ (high and low mike positions) around the main listening position -- even if that position seats only one person -- and then average them, before deciding which peaks and/or nulls to EQ. Bass isn’t perceived only with the ears -- you feel it throughout your entire body. This is why it’s important not to optimize the bass response based on a single measurement taken at a single position.
As brief as this introduction is, I hope that at least some of you will be able to put it to good use. If you remain unpersuaded to at least try to add, integrate, and optimize a subwoofer into your two-channel system, consider this: I’ve evaluated a variety of high-quality tower loudspeakers in my room, but no matter how good they’ve sounded, or how impressive their bass response, or how high their prices, the bass I hear and feel from them is never as good as the bass I experience when those speakers’ outputs are supplemented by one or two subwoofers in my customized, room-corrected system. That has always produced better sound -- and isn’t that the best reason to give a sub or two a try?
Next time: Peter and Mark Schuck on how the Anthem STR Preamplifier makes integrating a subwoofer into a two-channel system seamless and simple.
. . . Diego Estan