Adding Subwoofer to 2-Channel Stereo – The Definitive Guide


2-channel stereo receiver Denon

No matter your current setup, most of the solutions you’ll find online on adding a subwoofer to a 2-channel stereo system are from home theater forums and are not straightforward.

I’ve read most of Barry Ober‘s work and used his techniques to integrate many of my clients’ subwoofers into a 2-channel system. 

He’s the guy who has more than 30 years of research into the best methodology for the most detailed and complete subwoofer integration and system alignment, including speaker imaging.

I often heard that my bass dynamics, imaging, and clearness are seriously better than any 2-channel system they’ve experienced.

Here’s what you can find in this article:

  • How to Add Subwoofer to 2-Channel Stereo
  • Bass Management & Why High-Level Inputs Can Be a Bad Idea
  • Adding Ported Subwoofer to 2-Channel Stereo
  • Overlapping of Frequency – Crossover Setup
  • Distributed Bass Array – Experiencing the Best Bass
  • Why Don’t Stereo/2-Channel Amps Usually Come with a Sub Output?

If you hope someone can guide you toward the “correct” way to add a sub to your 2-channel system, I’ll do my best.

How to Add Subwoofer to 2-Channel Stereo

There are two ways to hook up a powered subwoofer in a 2-channel setup:

1) Run a line-level signal (RCA’s or XLR) from your preamp or integrated to the sub. 

This can be accomplished by: 

  • using a second pre-out, 
  • splitting an existing pre-out (RCA splitters)
  • using a dedicated “subwoofer out” if your preamp/integrated/receiver has one.

If your amp does have a preamp output, the best way to connect the subwoofer is to separate RCA cables from this output.

That makes it very simple. The sub will receive the same signal going to the main channels.

All that’s left to do is adjust the sub’s crossover frequency. That will filter out frequencies beyond the crossover, so you’re not making bass bloat by reproducing frequencies already covered by your main speakers.

My friend has a subwoofer set up like this, and the crossover is quite close to the lowest possible setting.  

I made a picture for you to see how to do it. If you have the same setup on your stereo, try this.

The method is to run RCA cables from the left and right PRE OUT 1 output of the stereo to the left and right inputs on the subwoofer and then run RCA cables from the sub with outputs to the left and right MAIN IN inputs on the stereo.

With this connection method, the subwoofer will play the bass and dismiss the information under 80 Hz from the signal sent to the mains (enabling them to reproduce the remainder of the frequency range more effortlessly).

sketch of connecting subwoofer to 2-channel stereo

2) Speaker-level connection/high-level inputs

If none of your gear has a sub-output, the option is to purchase a subwoofer with high-level input/outputs. First, you’d run speaker wire from amp to sub. Then speaker wire from the subwoofer to the main left and right speakers.

You’d just Y split the L&R outputs off of the preamp. One set of L&R will go into your power amp, and the other will go into the L&R inputs on the back of the subwoofer. 

The subwoofer will have its own crossover, so you don’t need a dedicated subwoofer pre-out. 

High level input on subwoofers

If your preamp or integrated amp doesn’t have a dedicated sub output, then the standard options are to either:

  • Add a subwoofer to the second set of outputs and utilize the subwoofer dials to alter phase and low-pass filter points.
  • If it has a built-in crossover with analog outputs, run the connection to your subwoofer first to connect your speakers or speaker amp to the subwoofer’s analog outputs. Use the subwoofer dials to adjust for phase and the crossover point.

During their peak, REL, a subwoofer manufacturer, suggested speaker-level connection for two-channel applications vs. using a pre-out.

  • Check the plate amplifier on the subwoofer; it must have high-level or speaker-level input terminals.
  • Connect another set of wires from your stereo’s speaker B to the subwoofer speaker level inputs.
  • Run the speaker wire from your stereo to the subwoofer, then another set of speaker cables to your speakers.

The subwoofer will suck nearly no current from the signal, so your main speakers will still get almost all the juice. 

Don’t use the high-level legacy connection on any subwoofer. This is a configuration for more expensive subwoofers, as they have excellent crossovers you can use.

If you haven’t bought a sub yet, I recently wrote about the four best subwoofers for a 2-channel stereo that I thoroughly tested. I did my best to be as brief as possible.

Bass Management and Why High-Level Inputs Can Be a Bad Idea

Before the beginning of modern AVR, most, if not all, subs were hooked up via full-range pre-outs from the pre-amp. As a result, most home systems had no bass control. 

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The subwoofer crossover controls prevented the overlap, and the phase controls were addressed if the mains were out of phase. Configuration differs from how a modern HT receiver does, but it is very doable. 

High-level inputs can be a bad idea and difficult to integrate as the sub can overlap with the main speakers, sometimes out of phase. Some audiophiles highly discourage this and recommend getting an amp with bass management.

If you add a subwoofer, ensure you can high-pass filter the main speakers and low-pass filter the subwoofer to minimize and control the overlap. 

The easiest way to do this is to get a stereo with bass management. Not all stereos with a sub-out have bass management, but almost all AVRs do.

Running the mains and the subwoofer from the same binding posts

The main problem is you have no choice regarding at which frequency to make the transition between the sub and your mains. 

There will usually be some range of frequencies that your sub and main speakers can both deliver. For instance, your main speakers might be capable of producing bass down to 80 Hz, and maybe the sub can produce bass up to 140 Hz.

You don’t want them both playing the bass from 80-140 Hz as then that frequency will be magnified, and it will sound boomy and muddy. 

So ideally, you’d choose some frequency like 105 Hz and send everything below that to the sub and everything beyond it to the main speakers. Actually, it’s more of an incremental transition, but 105 Hz is the point where it’s fifty/fifty.

If your stereo supported a subwoofer directly, it would have controls for this and would guarantee that sounds above 105 Hz don’t get sent to the subwoofer, and that sounds below 105 Hz don’t get sent to the mains.

Many subwoofers include a frequency cutoff knob on them, but it only involves the sub and not the main speakers. So instead, you’ll need to figure out the frequency below which your speakers inherently stop producing bass and then modify the subwoofer to match. 

So if your speakers produce 80 Hz fine but drop off rapidly below that, you’d set the subwoofer knob at 80 Hz. This will take some manual tweaking to try to get it right.

There are two main reasons this matters:

1) One use of a subwoofer is to take a little of the hard work away from the main speakers (and your stereo). 

Often, speakers will sound more pleasing if they aren’t going wild trying to produce deep bass. So we’re not really losing quality here; it’s more than we’re unable to unlock a boost in quality.

2) We might not be able to blend subs as smoothly. 

Ideally, this transition is controlled so that as the frequency drops, if the main speaker’s volume drops by 10%, the sub’s volume has a corresponding gain of 10%, and so on. 

This is straightforward to do if you’re utilizing a circuit built to do that. But if you’re relying on what your speaker physically happens to do, it may not drop off as neatly as frequency declines, so there may be no way to correspond entirely with the subwoofer.

However, neither of these issues is actually a big problem at all. You may have other problems (like room acoustics or subwoofer placement) that make a more significant difference.

Adding Ported Subwoofer to 2-Channel Stereo – Why You Need a Crossover 

A ported subwoofer is an affordable way to get free bass from an enclosure and/or driver that’s too small. 

It’s a holdover from the 1940s when you had to do everything feasible to improve the useable output over the preferred range of low frequencies because of driver inefficiencies.

When the subwoofer cabinet manufacturer defines the frequency response (i.e., 36 Hz – 21kHz +/- 3dB), this is determined by the whole structure of the port, the air in the cabinet, and the driver. 

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At some low frequencies, the port air is precisely out of phase with the driver air pressure, and since they cancel, there is NO output from the cabinet into the room. 

Therefore with a ported cabinet, the whole sloppy concept is this juggling game between the drivers’ response under air pressure, the passive crossover inside the box, the port size, and placement.

Connecting a subwoofer to existing main speakers (or amp) terminals is the most inferior possible way. 

Everything scientific and acoustic concerning this method is wrong, from the additive delay problems to the back EMF of the main speakers influencing the low-frequency signal. 

However, plenty of people don’t comprehend correctly integrated bass, and they will be reasonably satisfied simply sticking another box onto their system without reference to timing, phase, and frequency problems. They’ll think it sounds “ok” or “good,” and it doesn’t matter to those people.

Some audiophiles don’t want to introduce another active “something” in their beloved signal path, not recognizing that adding the crossover is the lesser of two evils.

This is why adding a crossover is an excellent idea:

1) Since you are now NOT putting in 20 Hz – 80 Hz into the main speakers, you’re not using up the available low-frequency cone movement with bass, so the LF cone in your main speakers can play its higher frequencies (up to its crossover point) much more purely. 

You get an apparent 6dB or more dynamic range. So you can play your system LOUDER with less compression distortion in the low-frequency driver.

2) Since you’re not putting bass into that same driver, you’re not Doppler modulating everything between 80 and 600 or whatever the following crossover point is. So that means cleaner mids by far.

3) You’re not sucking current from your main power amp at LF, so there’s more current reserved to play those high frequencies louder.

4) Since the cones aren’t moving as far at the low frequencies, the driver itself is not generating as much back EMF. Thus the damping factor and all of its issues are significantly negated. And you don’t need to run silver-plated cold water pipes to your main speakers as speaker wires because the speakers draw less current.

5) Frequencies below 80 do not cause transient intermodulation distortion with the higher frequencies (and vice versa) in your power amp.

If you have a 2-channel-only system and do not correctly use a crossover, you are wasting your time and will be frustrated.

If your speakers are ported, you MUST close (seal) the ports. 

Towels will do for a test, but you might consider purchasing a 

3″, 4″, or 5″ thick slab of “foam” at a notions/sewing store. 

You’re trying not to have multiple sources of different phase relationships (the main driver, the port air, and the subwoofer driver) at or near the crossover frequency.

  • Invert the polarity of the MAIN speaker the subwoofer is CLOSEST TO. Detach all the other speakers in the room. Put your head midway between the subwoofer and the speaker it is closest to. 
  • Play the 80 Hz tone. Modify the phase control, level control, and polarity switch settings until you hear a different NULL. (it might fully fade) 
  • There should be some setting of the two controls on the subwoofer, which will provide a rather sharp null – this is an essential setting, and you might find it very acute. 
  • Now place the wiring back the proper way to that one speaker. 
  • Reconnect the other speaker, and you should be done.

Overlapping of Frequency – Crossover Setup

If your subwoofer crossover at the lowest goes to a higher number of hertz than speakers, you have an overlapping of frequencies and need help with the crossover setup.

Even with very low volume, this will feel like a bloat in the bass.

If your amp and preamp have no subwoofer management and the subwoofer does not have eq, you have an even bigger issue.

I’d recommend getting an external DSP like miniDSP (Amazon link), even if you think an extra cable will indirectly affect your system’s performance.

The quality “gain” from implementing a miniDSP EQ’ed subwoofer would probably be huge. 

Nine times out of 10, a sub demands EQ because the bass waves are 20-70ft long, which usually generates an uneven response when sealed in a room smaller than an Airbus factory.

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High passing your mains will decrease distortion significantly. No more cones flapping around at frequencies it can’t adequately reproduce, eating up Xmax.

The latest model miniDSP 2×4 HD kit has solid processing power and can operate at 24/96. In addition, you can correctly time align the subwoofer with your mains for better subwoofer integration.

And this option also opens up the proper room correction EQ with REW or Dirac if you get the UMIK-1 on Amazon.

Distributed Bass Array – Experiencing the Best Bass

The best bass I’ve experienced with a 2-channel system is using something dubbed a Distributed Bass Array. 

This is the use of (at least) four subwoofers placed asymmetrically in the room to break up standing waves. 

Standing waves are expected in regularly dimensioned rooms. This is because the bass frequencies (at 80 Hz, the bass waveform is roughly 14 feet) can bounce off of walls and come back at the listening chair with an abundance of energy to cancel out the incoming bass signal at specific frequencies, depending on the placement of the listening position and the room size.

By placing the subs asymmetrically, you get lots of much smaller peaks and valleys evenly distributed about the room. 

Since the bass waveform is so long below 80 Hz, unless you have an enormous room, the bass is entirely reverberant within two iterations of 80 Hz. For this reason, 80Hz and under can generally be driven with a mono signal.

The trick is to make sure the subwoofers don’t draw attention to themselves – so they should be crossed over enough below 80Hz that the main speakers persuade you that the bass sound is arriving from in front of you – by the harmonics of the bass instruments.

Why Don’t Stereo/2-Channel Amps Usually Come with a Sub Output?

It’s not a new trend; stereo receivers have always been like that before there were subs, which these days are more frequent for AV surround sound receivers, not so much stereo receivers. 

Still, a subwoofer can easily be connected to a stereo receiver that doesn’t have a sub-output.

To answer the question, you’re looking at different product segments. The subwoofer out will be part of the preamp (the input selection part), not the amp.

Stereo receivers will usually have a display, amp, preamp (possible sub out), tuner, an EQ, room correction, or other built-in features, like an all-in-one system. This is generally but not always the budget feature.

Stereo amplifiers are normally part of a component system, with an independent preamp (that may or may not have a subwoofer output). You would add a separate tuner or DAC, as needed. This is frequently but not always the higher-end stuff.

There are also integrated amplifiers, one unit with an amp/preamp, occasionally a DAC, and occasionally with a subwoofer output. Still, without the other perks you’d find in a receiver (no tuner and typically no display). This is commonly the middle of the road for price/quality (but not always).

Some integrated amps will have RCA pre-out connections that we can use to bypass the pre-amp or add a subwoofer. 

For example, in the case of my Denon PMA-600NE (Amazon link), there’s one set of pre-outs that I can use to connect a sub via the Left or Right channel into my subwoofer’s LFE input.

Most subwoofers have a low pass filter on the inputs. Therefore, we can take the line-outs of most stereo/integrated amps and connect them to the inputs on the sub. 

Things that are just straight power amps generally don’t have any type of output. We could take the “line out” or “tape out” and connect them to the inputs of, for example, this sub from SVS (Amazon link) by just setting up the low pass on the subwoofer and be good to go. 

A proper subwoofer output sends only the low frequencies no matter the source, usually by selecting “small” front speakers in the receiver setup.

Many subwoofers have low-impedance inputs (aka speaker cable inputs, not line-level inputs). In this example, you can see a pair of speaker inputs and outputs. This means you can send the output from your posted receivers to the subwoofer and then to your speakers.

Tray Fiddy

Tray has come to terms with the fact he will probably never be a famous DJ.... but that hasn't stopped him from mixing and researching audio equipment. Tray has over 12 years of experience DJing at home and events.

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