Making subwoofer components is one of our big additions to Audio Club for Spring 2026. In order to make a viable subwoofer to use with the desktop speakers, we need two important items: (1) the subwoofer itself, consisting of a large driver and its enclosure and (2) a bigger amplifier. (The GTDT and 3122 Class-D project are adequate for driving the desktop speakers, but are not sufficient for a subwoofer.)

The speaker portion of the subwoofer sub-system is fairly easy and not too expensive — a medium-sized sub with enclosure and connectors can be built for around $100. There are two options, a sealed sub or a vented one. (I wanted to hear and measure for myself the differences between sealed and vented subs, so I decided to build examples of both and offer both as possible club projects.)

The amplifier must have much higher power and so will be more much more expensive than the GTDT or the 3122 Class-D. Most of the added expense is in the power supply, and most of the power supply expense is in the transformer. (Currently, the preferred transformer carries a huge tariff, making it even more expensive.) There are alternatives to lower the cost, as described below.

Conceivably, we could mount the amp into the subwoofer enclosure to make an active subwoofer, but the total cost would be well over $200. At that level, I suspect that many club members would be priced out.

Instead, we will take a more flexible approach and build a passive sub with a separate amplifier. And we will go a step further and separate the basic amplifier circuitry (not expensive) from the power supply (quite expensive). So to add a subwoofer to your small stereo will require three pieces: (1) the subwoofer speaker itself, (2) the amplifier circuit, and (3) the power supply. There are project options and alternatives for all three pieces. This modular approach provides flexibility so that you can choose the level of effort and cost that you want to commit in order to make a subwoofer.

Below is a list of the sub-projects and a brief description of each, along with some alternative approaches. More detailed descriptions, costs, and build instructions are provided on separate project pages.

• SUB-1S. A sealed passive subwoofer. It is very simple: a GRS 8SW-4HE 8-inch subwoofer, MDF panels for an enclosure, a bunch of screws, a pair of connectors, and some wire.
• SUB-1V. A vented passive subwoofer. Also very simple, it uses a Dayton DCS205-4 8-inch subwoofer, a 2.5-inch diameter adjustable port, MDF panels for an enclosure, a bunch of screws, a pair of connectors, and some wire.
• SUBAMP-1. A simple monaural subwoofer amplifier using the venerable LM3886 audio amp. It is a pure analog amp with Class-AB output, producing a maximum of 68 W. That should be enough to make the either of the above subwoofers work reasonable well. It is fairly inexpensive and requires only a few extra components. But it does need a bipolar power supply, with ±25 V needed to get to the full 68 W output. This is intended to work together with the GTDT or 3122 Class-D amps. The stereo signal would go into the sub amp and be filtered to split off the low sub frequencies and combine the two channels to create a low-frequency monaural signal. The "sub" signal is amplified by the LM3886 and sent to the subwoofer. The two stereo signals containing the higher frequencies are passed on the smaller stereo amp, which would amplify those frequencies for the stereo desktop speakers.
• SUBAMP-2.1. In audio-system jargon, a 2.1 amp has a section with a small- to medium-power stereo amplifier for higher frequencies going for the stereo speakers and large monaural amp for the low frequencies going to the subwoofer. In essence, it is a combination of a GTDT amp and a SUBAMP-1 on a single PCB. It will require a ±25-V supply — most likely a separate unit (see below), although it would be possible to find a single box big enough to hold both 2.1 and power supply.
• PM25V. A classic linear power supply, producing positive and negative 25 V. It uses a dual transformer (either 18 or 24 V_RMS), bridge rectifiers and big capacitors to produce the unregulated voltages, and two LM338 voltage regulators to provide the clean regulated output voltages. This is the most expensive item of the sub-system.
• Power supply alternative. Instead of building a linear power supply with its big transformer, you could just by two 24-V DC switched-mode power supplies and connect them to make a ±24-V supply for the sub amp. Buying these would be less satisfying that building your own, but two of them would be less than half the cost of the linear supply.
• Just buy an amplifier. You don't have to build everything. You could just buy an amplifier — there are many available at Crutchfield, Parts Express, B&H, Best Buy, Amazon, and other places. Some of the prices may make your eyes water. Note that many dedicated subwoofer amps are intended for use in cars, so be sure to check that any amp you might choose will work off the AC power in your house.
• Design your own passive sub. There is nothing particularly unique about the two sub designs listed above. The drivers are good quality, but not too expensive. The enclosure designs are straight-forward — the sealed design is a simple cube and vented design is retangular box with an extra hole. If you wanted to choose your own driver and design your own enclosure, I would be happy to work with you to cut the MDF panels according to your design. Come talk to me if you are interested in this approach.

Links:

• 24-V DC power supply from Amazon.
• Examples of subwoofer amplifiers at Crutchfield, Parts-Express, Best Buy, and Amazon. These are just some random examples — there are many more options available.