How To Connect A Subwoofer To An Old Amplifier Or Vintage Receiver

Got a vintage amplifier or receiver? When it comes time to add some great low-end bass you might be scratching your head wondering how – and if – you can add a subwoofer. 

The great news is that there are several ways to connect a subwoofer to an old amplifier or vintage receiver.

Even better, it won’t cost a lot, either! Read on and I’ll share with you 3 ways to connect a subwoofer along with clear subwoofer diagrams anyone can understand.

Home stereo subwoofers explained

Passive vs active home subwoofers diagram

Comparing passive (non-powered) vs active (powered) home stereo subwoofers. Powered subwoofers have an internal amplifier and one or more signal input options: speaker inputs, RCA input(s), and in some cases, digital audio inputs.

There are two kinds of home stereo subwoofers: powered (“active”) and non-powered (“passive”).

  • Powered subwoofers use a low-signal signal which is boosted greatly using the built-in speaker amplifier, power supply, and crossover. These types are one of the most common and in many cases use an RCA type input jack to connect to the receiver for sound.
  • Passive (non-powered) subwoofers are simply a subwoofer speaker inside the bass enclosure which is directly wired to the speaker terminals or a passive bass crossover inside. These types are less common.

How does a subwoofer work?

An amplifier boosts the low-level input signal in order to drive the subwoofer’s voice coil with sufficient power and move the speaker cone, producing sound. As the cone moves the air inside of a specially designed enclosure (speaker box) deep bass, contained in the musical input signal, is produced.

They’re designed for only low-end bass and not voice or other musical instrument frequencies.

A powered subwoofer includes an amplifier already inside the subwoofer enclosure. It also has a built-in low-pass crossover to block higher sound frequencies in order to produce clear and great-sounding bass only.

For non-powered subwoofers the problem comes when you connect one to an amplifier or receiver’s outputs without a crossover – it sounds terrible!

Old amplifiers and vintage receivers vs new receivers

Unlike older amplifiers, more modern home stereos and especially home theater receivers have a subwoofer output jack (or pair of jacks) dedicated to this bass signal a subwoofer uses to create sound. This is usually from stereo music signals or the subwoofer (“.1”) channel sound in multi-channel surround sound material such as Dolby Digital or DTS.

For example, you may see terms like “5.1” or “2.1” speaker systems or surround sound audio listed for movies. In this case, the first number represents the number of main speakers. The “.1” is used to represent a sound channel limited to only bass for subwoofer use.

Older amplifiers and receivers don’t provide a subwoofer output so we’ll need to connect a subwoofer using some other ideas.

Powered subwoofer inputs & controls to know

Powered subwoofer example with inputs and controls labeled

Shown is an example of a powered subwoofer’s rear panel with 2 kinds of inputs: speaker inputs and RCA (low level) input jacks. Note that not all subwoofers offer speaker level inputs, meaning if yours doesn’t have them it’s a bit harder to connect the bass signal input.

Powered subwoofers usually have a few different inputs and controls. It always depends on the particular brand and model you buy.

Here’s what you’ll usually find on most:

  • Power input (AC outlet power)
  • On/off switch
  • RCA input jack or a pair of jacks
  • Subwoofer crossover adjustment
  • Subwoofer level adjustment know (the amplifier’s signal boost level)

In most cases, a subwoofer input jack, if you had one on a receiver, is a “mono” (monaural, 2 stereo channels combined into one) signal you connect with a single RCA cable.

Receiver subwoofer output jack example

Shown: What a receiver with a subwoofer output jack looks like as found on many newer receivers. These connect to the RCA input jack on the subwoofer, if present.

Of course, if you’re reading this it’s because you don’t have a receiver with a subwoofer output. In fact, some of the information you’ll find right now on the internet says that you have to buy another subwoofer if you don’t have a receiver with an output jack. That’s simply not true.

Subwoofers with speaker level inputs are great to have for exactly this reason as you can connect them directly to an older amplifier or vintage receiver’s speaker outputs.

Even if you buy (or already own) a subwoofer without speaker level inputs, it’s ok – there’s another way to basically connect it essentially the same way!

Below you’ll find a diagram showing how to connect an old amplifier or receiver without a subwoofer output easily.

DIAGRAM & EXAMPLES: Connecting a subwoofer to an old amplifier or vintage receiver

Diagram showing how to connect a subwoofer to an old amplifier or vintage receiver

1. Connecting an old amplifier or receiver to a subwoofer with RCA input jacks

Example of a powered subwoofer RCA jacks & RCA Y adapter cable

Left: Example of a powered subwoofer with 2, instead of the typical 1, RCA input jacks. Right: An RCA Y adapter that can be used with a line level converter to connect to a subwoofer with a single RCA input jack.

If you’ve got a subwoofer with 1 or 2 RCA input jacks and no speaker level inputs, here’s a simple and high-quality way to connect it: by using a line level converter.

What is a line level converter and how do they work?

Line level converters (also called RCA speaker level adapters) are small electronic devices that connect to speaker outputs from an amp or receiver and scale down the higher-voltage signal to a low level (“line level”). The outputs are RCA jacks which can then be connected to any amplifier or subwoofer with RCA jacks.

 While you almost never see them used for home stereo systems, they’re extremely handy in the car stereo world because they make it possible to connect a stereo without RCA outputs to any amplifier or powered subwoofer.

Likewise, they can be used for home stereo amps and receivers, too!

Image showing examples of line level RCA converters

Shown here are two examples of line level/RCA speaker level converters that work well for home receiver/amp to subwoofer use.

How much do line level converters cost?

Line level converters vary in price a bit depending on the quality and features, selling around $15-$25 or so in many retail stores and online stores.

How to use a line level converter

To convert speaker level outputs from your amplifier or receiver to RCA jack subwoofer outputs, you’ll connect the provided speaker wire connections (marked by colors and striped) just like you would regular speakers. You then connect RCA cables (or a single cable, depending on your particular one) to your powered subwoofer.

The internal electronics not only scale down the speaker output voltage from a receiver but also help prevent noise from the audio path, too. Most do not need a power source and take only a few minutes to hook up.

If your subwoofer has a single RCA subwoofer input jack, you may want to pick up a “Y” RCA adapter to combine both receiver channels on the output side into a single mono RCA plug. Subwoofers with 2 (stereo) RCA jack inputs will need a standard stereo male-male RCA cable.

Will the total Ohms load seen by the receiver be ok with a line level adapter?

Generally speaking, there’s no harm in connecting a speaker level adapter to your speaker output terminals. The receiver’s outputs will still see a safe speaker impedance (Ohms) load.

That’s because these types of adapters often have a very high input impedance (input resistance) and don’t draw any real power from the speaker outputs or pull down the total speaker Ohms load. Their input impedance ranges from values from about 40 Ohms to 80 Ohms and even 1,000 Ohms (1KΩ) although some can be much lower.

With most, the total Ohms load your receiver will have very little change. That’s because a very high impedance connected in parallel with a low one has a total Ohms load that’s close to the speaker Ohms in use.

NOTE: Be aware that a few models have as low as 16 Ohm input resistance, making it a bit low when used alongside some speakers. Whenever possible, try to avoid those or you can add another resistor in series with each input (say 20Ω for example) just to be safe.
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2. Connecting a subwoofer with speaker level inputs

Example of subwoofer speaker level inputs

A subwoofer with speaker-level inputs is especially easy to connect to your older amplifier or receiver! To do so, just connect to the speaker outputs on the source unit using speaker wire and then to the matching inputs on the subwoofer. You can even power speakers from the amp or receiver at the same time.

If your subwoofer has speaker level inputs built-in you’re in great shape! Just connect them directly to your amplifier or receiver’s speaker outputs, either to unused speaker terminals or at the same time (in parallel) with speakers connected to the receiver.

Just like an off-the-shelf line level RCA converter I mentioned earlier, the subwoofer’s internal electronics will scale down the speaker signals to a much lower line level signal the internal amp can use.

You can still expect very nice sound quality as the signal used is just divided down and isn’t changed. Because speaker level inputs have a very high input impedance (total input resistance), in most cases it won’t hurt to connect them to your receiver or amp at the same speaker wire terminals where speakers are already connected.

To do so, you’ll just connect them in parallel: positive speaker inputs to positive speaker outputs and negative speaker inputs to negative speaker outputs.

Additionally, there’s a low-pass crossover built-in as well to produce great-sounding bass and no unpleasant parts of the music – just pure, low-end bass.

Note: Subwoofers with speaker level inputs and outputs provide a way to easily connect both at the same time. The outputs are internally connected to the input connectors, making it easier to add speakers and the subwoofer to a receiver simultaneously.

3. Mini amplifier option for passive (non-powered) subwoofers

Example of a 2.1 stereo mini amplifier with sub output

When wanting to add a single voice coil (SVC) non-powered subwoofer, several issues come up. Hands-down, the simplest solution is using a mini amplifier with a mono low pass subwoofer speaker output.

For this setup, you’ll do the following:

  • Use a line-level RCA converter to get an RCA signal to the amplifier.
  • Connect the passive sub to the subwoofer output.
  • Use the AC-DC power supply to power it.

That’s pretty much it! Having a built-in sub crossover means the sound will be nice, clear bass without vocals much like a self-powered subwoofer provides.

Today’s small amplifiers are typically very compact in size as many use an efficient class D amplifier design. They’re sometimes called a 2.1 amplifier as they usually have both stereo speaker outputs and a dedicated subwoofer channel.

Facmogu ST-838 2.1 Mini Amplifier With Subwoofer Output
20Wx2 stereo + 40Wx1 subwoofer output. RCA and 3.5mm input jacks. Easy to use speaker terminals. Bass, treble, and volume controls. Includes power supply.

Cost and other notes

They cost around $35 to $50 or so depending on the model. The downside is that if you want a ton of power, they’re not the first choice as many offer about 34W to 68W sub power. If that’s not enough you’re better off getting a more expensive amplifier.

Note: Mini amplifiers may or may not include the power supply so be sure to check when shopping.

4. Connecting speaker outputs to a passive sub

Example of a passive subwoofer low pass crossover

A passive subwoofer low-pass crossover, unlike an electronic crossover, works using capacitors and inductor coils instead of electronic components to filter out the unwanted higher-frequency sound that would otherwise go to the subwoofer. This lets you power the subwoofer with only a lower bass sound similar to how a powered subwoofer works.

Using a passive (non-powered) subwoofer is definitely not as easy as a powered one. The good news is that it can be done, and relatively easily, too. In fact, it you don’t have to worry about going broke, either, although you will need to do a bit of shopping.

To connect your amplifier or receiver to a non-powered subwoofer as is shown in the diagram above, you’ll need to pick up a low pass crossover that you’ll connect between the amp or receiver and the subwoofer.

These will filter out sounds above the crossover frequency and provide only a nice bass sound to it.

How to choose a subwoofer crossover

Speaker crossovers like the one shown are sold both in a single (one speaker) or dual (2-speaker) models depending on the brand & supplier. They also have to be matched correctly to the impedance (Ohms rating) of the sub.

For example, for an 8 ohm subwoofer, you’ll need to use a crossover designed for 8 ohm speakers. Otherwise, the sound filtering is radically different and won’t sound right since the speaker load will change how the crossover filters the sound quite a lot.

Normally you’d choose one with a low-pass frequency of close to 100Hz or close to that. You may need to shop around to do so.

Where to shop for passive subwoofer crossovers

Speaker crossovers are sold where speaker parts & related components are sold as well as marketplaces like Amazon or Parts Express. Other speaker specialty stores where replacement speaker parts are sold may have them, too.

Stereo vs surround sound receiver subwoofer output comparison

Stereo vs surround sound receiver differences diagram

Unlike older or standard stereo receivers, surround sound receivers have a unique output that comes from the surround sound movie or music source. However, in regular stereo listening mode, they act the same as regular receivers.

Just as a side note, one thing to be aware of is that when you connect a subwoofer to an old amplifier or receiver you won’t be able to get the separate dedicated bass sound channel (.1 channel) like you can with a surround sound receiver. 

Those are able to extract the dedicated bass sound from a DVD or other media and route it to the subwoofer output jack. On the other hand, it might not even be a problem.

Did you know? The surround sound receiver “LFE” (low-frequency) output is considered optional – hence the “.1” name. 

Surround sound receivers and amplifiers are designed so that you can play nearly all the sound through the main speakers if needed.

In some cases, for example, some movies and music use the bass channel to really draw you into the experience. Using a receiver without that output means you can’t get the same effect, but that’s only for surround sound mode.

The good news is that in stereo mode, both new and old receivers & amps have very similar subwoofer behavior when connected as you’ve seen here. In other words, you probably won’t really miss it if you’re using an older receiver or amp.

That’s just something to be aware of in case you’ve considered upgrading at some point.

Marty

About the author

Marty is an experienced electrical, electronics, and embedded firmware design engineer passionate about audio and DIY. He worked professionally as an MECP-certified mobile installer for years before moving into the engineering field. Read more »

Your comments are welcome.
  1. So, Marty…I have a peculiar situation her that I’m not confident is addressed in your subwoofer connection tutorials.

    I have an old Klipsch KSW 10 active sub with a fried amplifier. Rather than replace the plate amp I want to use an old receiver I have sitting around (Denon AVR-2106) as an external amp but I don’t want to blow the receiver’s amp section doing it.

    The way it’s now set up is that the Klipsch sub is just a driver wired directly to the High Level “Speaker In” connectors on the back of the cabinet. There are two Speaker In connectors (+/-) for the left and two for the right. Those four wires are now spiced onto the two wires going to the driver — the two black (-) wires from the left and right Speaker In terminals into the single black wire coming off the driver and the left/right red Speaker In wires into the single red driver wire.

    I intend to hook up the Denon receiver powering the sub to an Onkyo receiver providing the sub Output signal with a RCA cable from the Onkyo’s two sub Pre-Out connectors to an input source on the Denon which will then output that signal to the sub over speaker wires.

    From what I’ve read I *think* that I can only connect one channel (either left or right Main) of the Denon to the sub. Otherwise I would likely blow the Denon’s amp section either from wild impedance conditions or shorting the left/right channels on the Denon at the sub’s Speaker In plate since both sides are now connected directly to the corresponding driver wire (without the protection provided by the internal amp).

    Do I have that right? That I cannot safely connect both the left and right channels of the Denon to the Left and Right Speaker In terminals on the sub?

    I apologize for making this such a long ramble.

    FYI, The Denon is supposed to be able to handle 4 ohm loads from the Main terminals and the amplifier specs for the Klipsch sub have it into 4 ohms, so I’m just assuming that the driver is a nominal 4 ohms. That also seems to be fairly representative of many smaller sub drivers.

    My thanks for any opinions you care to offer!

    Grant

    Reply
    • Hello there, Grant.

      > Do I have that right? That I cannot safely connect both the left and right channels of the Denon to the Left and Right Speaker In terminals on the sub?

      Yes, that’s correct, you can only use one speaker output channel to power the subwoofer as it’s not a dual voice coil (DVC) subwoofer. There’s an issue with left/right stereo sound when doing so normally, but if you’re going to use a “LFE” (subwoofer) RCA output from the other receiver to go to the Denon, it should be mono and avoid those issues.

      Originally in the plate amplifier the speaker level inputs go to input circuitry and are safely combined to a mono signal which is then amplified to power the sub. Sounds like in this case you’ll essentially acheive the same thing using one speaker output channel from the Denon and a subwoofer output signal from the Onkyo. Best regards!

      Reply
  2. Hi, Marty, when I press both Speakers A and Speakers B, my speakers would still play, so I’m assuming I can connect another set of speakers and play all 4. That being said instead of using the same L & R speaker terminals as diagramed in Example 1 & 2, can I just connect the subwoofer to speakers B, leaving the regular pair of speakers to Speakers A? Thank you.

    Reply
  3. As a Senior (75y) that’s been fiddling around with stereo systems (Loved my old Maranz), since late teens I found your article very clear, concise and helpful…

    Reply
  4. Hi Marty,
    I followed your rather excellent instructions and purchased a Scosche LOC80 and connected up my powered sub. There is no sound coming from it however – is there something obvious that I’m doing wrong?

    Reply
    • Hi there Gary. How did you hook it up, and have you verified you’ve got sound on those speaker output channels from your receiver?

      You can verify that by connecting the line output converter in parallel with a speaker/pair of speakers and see what you get. You can also troubleshoot, just in case, the powered sub by connecting a different line level audio source to it in order to verify it’s working right as well. This could be a smartphone headphone jack, MP3 player, etc.

      You may need to try it one channel at a time as well, to eliminate something going on with having both channels connected to the LOC at the same time. Ordinarily this should be pretty simple, though, so I would suspect something isn’t set up as expected, etc.

      Best regards!

      Reply
      • Hey,
        Thanks so much for your reply.
        Yeah, everything checks out. I have original speakers coming out of the amp and it all works great. Just connecting the sub does nothing. I even tried another sub and nothing either. I also messed around by leaving L connected- then R outputs. Subs definitely work – just not through the Scosche. I followed example 1 above (sub is powered).
        It’s ok if you can’t think of what it might be – you’ve been more than helpful to date and I don’t want to use up your time.
        Thanks again

        Reply
        • Hi Gary & sorry to hear you’re still having a problem with it. This is an usual problem going on; I certainly don’t mind trying to help.

          I would try turning the adjustable outputs on the Scocshe unit just in case and see if you get anything. There’s always the possiblity that the unit is defective, unfortunately.

          It’s a bit hard for me to troubleshoot it from here, but I would verify 100% everything is wired correctly and only connect one input channel first. *Then* the 2nd channel if that works.

          Reply
          • Hey, thanks so much.
            Yeah, I tried turning the outputs too…

            So, I have the Scosche connected to left and right speaker outputs on the back of the amp… then the two RCAs going to. 3.5mm jack (as it’s a line-in input on the back of the subwoofer).

            I wonder is it that the Scosche can’t process an output to a 3.5mm jack?

          • Hello Gary. If it’s simply a standard 3.5mm line level input jack it shouldn’t matter. I would try connecting *only* one speaker output channel to begin with and see what you get.

            What brand & model powered subwoofer is it, and what type of receiver do you have?

    • Hi Gary this is not a traditional powered subwoofer but rather one of the “3 in 1” sets with some parts integrated. They don’t always work with a receiver etc.

      I assume you have the volume control that came with it plugged in? If so I think it may work with the power switch on and using the green input jack to connect to the LOC. Without the volume control connected I suspect it won’t work, as this is often the case for other brands as well.

      You may also want to see if it begins working with the small stereo speakers included with the subwoofer plugged in as well. Best regards.

      Reply
  5. I just want to thank you this helped me out tremendously!! After researching for days and days I finally found ALL of my answers!! Thank you

    Reply
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