Setup
Subwoofer for a 2-Channel Stereo: When and How to Add One
A subwoofer can turn small bookshelf speakers into a full-range system, or it can add a muddy, boomy layer that ruins the music. The difference is almost entirely in how it is chosen, connected and set up.
This guide covers when a sub actually helps in a two-channel system, how to connect one to amps with or without a sub output, and how to blend it so you hear deeper bass rather than a separate box in the corner.
When a subwoofer helps, and when it does not
Good reasons
Small bookshelf speakers with limited deep bass. Music with significant low bass content: electronic, hip-hop, organ, film scores. A system that doubles for TV and movies.
Weak reasons
Speakers that already reach deep bass in your room. Rooms that already sound boomy. Hoping a sub will fix a muddy midrange; it will not.
One real advantage of a sub is placement flexibility. Bass depends heavily on where the source sits in the room. Your main speakers have to be placed for good imaging, which is not always the best spot for bass. A sub can go wherever the bass sounds most even. Some subs and amps also include room correction, which can help further.
Another advantage, if your amp or a sub's crossover high-passes the main speakers, is that removing the deepest bass from smaller speakers can let them play louder with less strain. Not every setup offers this; check your gear.
How to connect a sub to a stereo amp
| Your amp has | Connect using | Notes |
|---|---|---|
| SUB OUT | A single RCA cable to the sub's LFE or line input | Simplest. Check the amp's manual for whether its sub out is full-range or already filtered, and whether it high-passes the main speakers. |
| PRE OUT (left and right) | Two RCA cables to the sub's L/R line inputs | Common on integrated amps and receivers. Pre-outs are usually full-range, so the sub's own crossover does the filtering. |
| Only speaker terminals | Sub with high-level (speaker-level) inputs, connected in parallel from the amp's speaker terminals | Works with almost any amp, including vintage receivers. The main speakers stay connected as normal. |
| Tape out or record out | RCA to sub line inputs | Usually not affected by the volume control, so the sub level will not follow the main volume. Generally a poor choice. |
Speaker-level inputs explained
Many subwoofers designed for music include high-level inputs. You run a second set of thin wires from your amp's speaker terminals, or from the speaker terminals on the main speakers, to the sub. The sub reads the signal at a high impedance, so it does not significantly load the amp. This also preserves the character of the amp in the bass signal, which some listeners prefer.
Check your amp type. Some amplifiers, including certain class D and bridged designs, have speaker outputs where the negative terminal is not at ground. Connecting a sub's high-level input incorrectly to these can cause problems. Follow both the sub's and the amp's manuals; some subs explicitly state how to handle this.
Setting the crossover
The crossover (low-pass) control on the sub sets the frequency above which the sub fades out. The goal is a seamless handoff from the main speakers.
- Find your speakers' low-frequency limit. Manufacturers often publish a low-frequency extension figure, sometimes with a tolerance such as minus 3 dB or minus 6 dB. Use it as a starting reference, knowing your room will change the real result.
- Set the sub's crossover near that point. If the main speakers roll off around a given frequency, start the crossover around there.
- Listen for a gap or a bump. If bass seems thin around the upper bass, raise the crossover a little. If it sounds thick, boomy or you can hear the sub as a separate source, lower it.
- If the sub's low-pass is being bypassed because the amp's sub out already applies a crossover, set the sub's crossover to its maximum or LFE/bypass setting so you do not filter twice.
If you can localize the sub, meaning you can hear where it is, the crossover is often set too high or the level is too loud.
Phase and level
Phase determines whether the sub's cone moves in step with the main speakers at the crossover region. Subs typically have a 0/180 switch or a variable phase knob. Sit in your listening seat, play a track with steady bass, and have someone flip the switch or turn the knob. Choose the setting that gives the fullest, loudest bass around the crossover point.
Level should be set so the sub blends rather than announces itself. A common mistake is setting the level too high, which sounds impressive briefly and then fatiguing. Start low and bring it up until bass sounds full but the sub does not draw attention to itself. Music with natural acoustic bass, such as double bass or a cello, is a good reference.
Many people find a calibration microphone and free room measurement software useful for this. It shows peaks and dips that are hard to judge by ear, but it is optional.
Finding the best spot: the subwoofer crawl
Bass in a room is uneven, with peaks and nulls that change from spot to spot. The subwoofer crawl is a simple way to find a good position:
- Place the sub temporarily at your listening seat.
- Play music with a steady, repeating bass line.
- Get down on the floor and move around the area where the sub could realistically go, along the front wall and sides.
- Note where the bass sounds most even and well-defined, not just loudest.
- Put the sub in that spot and return to the seat to fine-tune crossover, phase and level.
Corners usually give the most output but often the least even bass. Positions near the front of the room, between or near the main speakers, often blend well. Two subs placed in different locations can smooth room bass further, but one well-placed sub is a fine start.
Choosing a sub for music
- Pick a size appropriate for your room. A modest sealed sub can be ideal in a small or medium room.
- Sealed designs are often favored for tight, controlled bass in music; ported designs can play louder and deeper for the size. Both can work well.
- Look for high-level inputs if your amp lacks a sub output or pre-outs.
- Make sure it has variable crossover, level and a phase control.
- Consider room correction or app control if your room is difficult.
- Check the cabinet finish and size against your space.
Used subwoofers can be good value. Test them by playing bass-heavy music and listening for buzzing, rattles or a cone scraping. Check the amp plate on the back works with every control.
FAQ
Can I add a subwoofer to a stereo amp without a sub output?
Yes. Use a sub with speaker-level (high-level) inputs connected from the amp's speaker terminals, or connect to the amp's pre-outs if it has them.
What crossover frequency should I use for my subwoofer?
Start near the low-frequency limit of your main speakers, then adjust by ear for a seamless blend. There is no single correct number for every system.
Should I use the phase switch on my subwoofer?
Yes. Try both settings or turn the knob while listening from your seat, and choose the one with the fullest bass around the crossover point.
Do I need two subwoofers for stereo music?
No. One well-placed sub works well in most rooms. Two subs can smooth out room bass further but add cost and complexity.
Where should I put my subwoofer?
Use the subwoofer crawl to find a spot with even bass. Positions near the front wall or between the speakers often blend well. Corners give more output but less even bass.
The bottom line
A subwoofer can transform a stereo system, but only if it disappears into the music. Connect it through a sub output, pre-outs or speaker-level inputs, set the crossover near where your speakers roll off, use phase to fill the gap, and keep the level modest. Then spend your effort on placement. A well-blended sub will make your speakers sound bigger, not make you notice the sub.