Class D Amplifier And Bookshelf Speaker Teardown
Eight parts carry the signal from your source to the air in the room, and one decides most of what you hear: the output filter inductor and capacitor bank on the Class D amp. A weak filter leaks switching noise into the bookshelf crossover, and everything above midrange turns to static and smeared attack.
- The output filter is the gate. A Class D amp chops the signal into rapid pulses. The inductor and capacitor behind the MOSFETs smooth those pulses back into clean audio. Get that pair wrong, and the tweeter picks up harshness the moment the filter starts to ring.
- Bass definition is a pairing problem. The amp’s damping factor controls how firmly it holds the woofer cone. A lightly built amp into a loosely damped cabinet lets the cone keep moving after the note ends, so bass smears and loses its edge.
- Every cost cut leaves a fingerprint. Thin MOSFETs overheat and clip. A small reservoir sags on loud chords. An undersized port makes bass feel thin. Each one is a separate, identifiable sound.
- Moving up one tier fixes one named hole. A bigger filter inductor cleans the treble. A stiffer cone adds midrange body. A longer port extends the bass floor. Each step targets a specific frequency band.
What Is Inside The Box
Reservoir capacitor bank (amp side). The first real part the signal meets is the reservoir. Think of it as a water tank behind a garden hose. When a loud chord hits, the demand for current spikes faster than the wall outlet can respond. The reservoir sits between the mains and the switch stage, soaking up that spike so the voltage behind the MOSFETs stays steady. A well-built example carries a thick bank of capacitors that barely dips on a full orchestra swell.
MOSFET switch array (amp side). Class D works like a bicycle brake. The signal is chopped into very fast on-off pulses, and the MOSFETs are the mechanism that opens and closes thousands of times per second. The pulse width carries the audio information. Two to four MOSFETs handle the job, and they need a heatsink because switching at high speed generates waste heat. The output signal, before filtering, is a jagged, high-frequency wave riding on top of the audio you want.
Output filter inductor and capacitor (amp side). This is the decisive part. After the MOSFETs chop the signal, the output filter smooths that jagged edge back into a clean, slow audio waveform. It works like a mesh sieve between two water tanks: the big, slow ripples pass through; the tiny, fast ripples get blocked. Everything the amp hands to the speaker has to cross this gate. A strong filter blocks switching residue below the 20 kHz upper limit of the audible range. A weak filter lets some of it through.
Crossover network (speaker side). The clean signal from the amp arrives at the speaker and hits a small board of resistors, capacitors, and inductors. This passive network splits the signal by frequency. Low frequencies get routed to the woofer; high frequencies get routed to the tweeter. The crossover is the traffic cop that decides which driver handles which part of the song. In a two-way bookshelf, one crossover point, usually somewhere in the 2 to 3 kHz neighborhood, divides the work.
Tweeter driver (speaker side). A tiny diaphragm, often a metal or treated-fabric dome, sits in front of a small voice coil and a permanent magnet. It handles the upper end of the audible range, above the crossover point. Because the diaphragm is so small, it moves only a fraction of a millimeter, but it does that at very high speed to reproduce cymbal crashes and violin overtones. The voice coil slides in the gap of the magnet, the same basic motor arrangement used in a dynamic microphone, which works in reverse as a generator.
Woofer and mid-bass driver (speaker side). A cone, made of paper, polypropylene, or a mixed polymer, is attached to a voice coil that slides in the gap of a magnet assembly. The suspension, the rubber or foam ring around the cone, controls how far the cone can travel. This driver handles the bass and lower midrange. In a two-way bookshelf, it covers from the top of the port range up to the crossover point. The cone, voice coil, and magnet together form a linear motor that converts electrical energy into mechanical push and pull.
Enclosure and bass reflex port (speaker side). The drivers mount on a rigid box, often made of wood or medium-density fiberboard. The box baffles the cone so the rear sound does not cancel the front sound. A tube, or port, carved into one wall lets the air inside the box work with the cone at low frequencies. This is the bass reflex system, the hole you see below the lowest woofer on most bookshelf speakers. The port size, length, and the box volume together set where the bass response starts to roll off.
Internal damping material (speaker side). Fibrous pads, often fiberglass or melamine foam, line the inside walls of the enclosure. They absorb standing waves that would otherwise bounce between the box walls and color the midrange. The amount and placement of this material is a tuning choice: too much kills the bass extension, too little leaves a boxy resonance you can hear as a one-note hum in the lower mids.
What Each Part Does
| Part | Its single job | Well vs. Poorly |
|---|---|---|
| Reservoir capacitor bank | Supply steady voltage during sudden current demand | Bass holds on loud passages vs. bass sags and thins out the moment a chord hits |
| MOSFET switch array | Chop the audio signal into fast on-off pulses | Midrange stays clean and even vs. upper mids turn to a harsh, buzzy edge on sustained notes |
| Output filter inductor and capacitor | Smooth the pulse train back into a clean audio waveform | Treble stays smooth and transients snap vs. treble gets a layer of static and attacks smear into a mushy wash |
| Crossover network | Split the signal so each driver only receives its assigned band | Vocals sit in a tight, focused spot vs. vocals smear and lose their forward position |
| Tweeter driver | Convert electrical signal into fast air movement in the high band | Cymbals and overtones stay detailed and separated vs. highs turn to a single, fatiguing wash |
| Woofer and mid-bass driver | Convert electrical signal into slow, large air movement in the low band | Bass notes have a clear start and stop vs. bass drones and muddies the low mids |
| Enclosure and port | Shape the bass response by letting internal air reinforce the cone | Bass extends evenly to the port tuning frequency vs. bass is thin above the port and boomy below it |
| Internal damping | Absorb standing waves inside the box | Midrange stays neutral and airy vs. midrange gets a boxy, one-note coloration |
The Corners That Get Cut
Small reservoir capacitor (amp side). The physical cut is using fewer or smaller capacitors in the power supply. The reservoir acts like a water tank: the smaller the tank, the faster the water level drops when you open the tap wide. The audible symptom shows up on a loud, low bass passage. The voltage sags, the MOSFETs cannot switch at full speed, and the bass loses its footing. It does not sound quieter; it sounds thinner and less controlled, like a bicycle brake that grabs and then slips.
Thin MOSFETs with a small heatsink (amp side). To save board space and component cost, a budget Class D design uses smaller MOSFETs and a heatsink that barely covers the chip. The physical result is that the MOSFETs run hot under sustained load. Before 1934, when Harold Black introduced negative feedback to reduce the typical 5 percent distortion of early tube amps, designers had no easy way to force the output to follow the input. Today, feedback still helps, but a thermally stressed MOSFET cannot respond fast enough. The audible symptom is that on a long, loud passage, the amp compresses. Cymbal crashes lose their leading edge, vocal peaks flatten out, and the music sounds like it is being played through a thin rubber membrane.
Thin, stiff cone material (speaker woofer side). A cheap woofer uses a thin, flat cone that bends rather than moves as a single unit. The physical cause is that the cone material cannot hold one uniform shape as it travels back and far. The audible symptom is that bass notes sound smeared and uneven. A single bass drum hit rings with a slight, unnatural timbre because different parts of the cone flex out of sync. The low mids turn to a muddy, undifferentiated blob, and the midrange loses its clarity.
Undersized or short bass reflex port (speaker enclosure side). To save cabinet depth, a budget bookshelf shortens the port tube or narrows its diameter. The physical effect is that the port cannot move enough air at the tuning frequency, so the bass reflex reinforcement runs out early. The audible symptom is that the bass feels thin above the port’s actual cutoff and becomes a single, boomy bump below it. There is no smooth, extended low end. Instead, you get a one-note thump where a full range of bass notes should be.
How The Whole Thing Sounds
Bass. The bass you hear is a conversation between three parts. The reservoir sets the floor: if it is undersized, the whole low end loses pressure on loud passages, and the bass sounds like a weak hand on a door handle. The MOSFET switch stage and the output filter together determine how cleanly the amp hands off the low-frequency current to the woofer. A sloppy filter lets switching residue into the bass band, and the woofer cone gets a jittery, undulating push instead of a clean, firm one. Finally, the enclosure port and the damping material shape where the bass extends and how it decays. A well-damped, properly ported box gives bass that starts and stops on the beat. A loosely damped box with a short port gives bass that rumbles on past the next note.
Vocals. The vocal range, from a few hundred hertz to a few kilohertz, lands on the upper part of the woofer and the lower part of the tweeter, right where the crossover is doing its job. If the crossover is a low-order, cheap network, the hand-off zone is wide, and the vocal sits in a smeared, unfocused spot in the soundstage. The damping material matters here too: boxy resonances in that range color the voice with a one-note hum. When the filter on the amp side is clean, the crossover gets a smooth signal to split, and the vocal comes forward with a sense of physical presence. When the filter is weak, the crossover receives a noisy signal, and the vocal gets a thin, reedy edge.
High frequencies. The tweeter is the most sensitive part of the chain to amp-side errors. The output filter inductor and capacitor are the last gate before the signal reaches the tweeter. A weak filter leaks high-frequency switching residue, and the tweeter, which is already working at the limit of its small diaphragm, translates that residue into a harsh, static-like layer over every cymbal and overtone. The tweeter magnet and voice coil quality also matter: a small magnet gives the diaphragm less force, so high notes lose detail and separation. In a well-built pairing, the highs are open and airy, with individual instruments you can point to. In a weak pairing, the highs blur into a single, fatiguing wash.
Build Quality Tells
Finish and panel gaps. Hold the enclosure under a lamp. A well-built bookshelf has tight, even gaps between the front baffle and the side panels. The internal damping material is cut to fit the walls without leaving visible strips at the seams. A sloppy build shows uneven gaps, exposed glue lines, and damping pads that curl at the edges. The crossover board, visible through the rear, is soldered with short, neat joints rather than long, dangling wires.
Weight of the power section. Pick up the Class D amp. The reservoir capacitor bank and the heatsink are the heaviest parts. A heavier unit in the same form factor usually means a larger capacitor bank and a bigger heatsink, which in turn means the MOSFETs have room to run cool under load. A light, plasticky amp chassis is a rough proxy for a small reservoir and a minimal heatsink.
Connector feel. Push the speaker cable into the binding post. A well-made post gives a firm, even squeeze with no wobble. The metal shell of the post is thick and cold to the touch. A cheap post feels loose, the threading strips quickly, and the contact plate is thin stamped metal that flexes when you turn it. On the crossover side, a good terminal board uses large, solid brass columns. A cheap one uses small, hollow wire posts that the speaker cable barely grips.
Behavior when driven hard. Turn the volume up to a level you would use for a full album and listen for the last thirty seconds of a loud track. A well-built pairing keeps its tonal balance: the bass does not suddenly get boomy, the mids do not thin out, and the treble does not spike. A poorly built pairing starts to change character under load. The bass gets muddy, the midrange gets recessed, and the treble gets a harsh, sibilant edge that was not there at lower volume. Thermal compression from the amp and port tuning collapse from the speaker both show up as a sudden, uniform darkening of the whole sound.
What Each Step Buys You
Entry tier. At the entry level, the Class D amp uses a small reservoir, thin MOSFETs, and a compact output filter. The bookshelf has a thin cone woofer, a small tweeter, a short port, and minimal damping. What you get in sound: the bass is present but thin, the midrange is flat and slightly recessed, and the treble carries a faint static edge on complex passages. The music is listenable but lacks depth and separation. The crossover hand-off is wide, and the vocal sits back in the stage.
Mid tier. One step up, the amp gets a larger reservoir, a proper heatsink, and a bigger inductor in the output filter. The bookshelf moves to a stiffer cone, a larger tweeter magnet, a longer port, and real damping material in the enclosure. What this buys in sound: the bass gains a firm, controlled bottom. The treble static layer disappears, and cymbals ring with a natural decay. The vocal gets a defined, forward position in the stage. You are not hearing better sound in a generic sense; you are hearing that one specific frequency band, the one that was weakest, has been filled in. The port extends the bass floor, and the damping tightens the midrange.
High tier. At the top, the amp uses a large, multi-capacitor reservoir, oversized MOSFETs with generous heatsinks, and a high-quality output filter with a low-resistance inductor. The bookshelf uses a composite cone, a large-diameter tweeter, a long, wide port, and precisely placed damping. What this buys: the bass extends lower with even output across the whole low band. The midrange gains a sense of air and detail between the notes. The treble becomes separately resolvable, so you can hear the reflection of a hall behind a solo cymbal hit. Each step up did not make everything better at once; it fixed the one band that was the limiting factor. The dynamic speaker, first built in 1925, has not changed its basic motor-and-cone principle. What has changed is how carefully each part in the chain is built to do its one job without interfering with the next.
Questions About The Build
Why Does A Weak Output Filter Make The Treble Harsh Rather Than The Bass?
The output filter sits in front of the entire speaker. But the tweeter is the smallest, most sensitive driver in the chain. It has a tiny diaphragm that already works at the edge of its physical range. Any switching residue that leaks past the filter hits the tweeter first and hardest, because the tweeter has no physical damping to soak it up. The woofer, with its larger mass and stiffer suspension, mechanically filters out most of that residue before it reaches the cone.
How Do I Tell If The Bookshelf’s Port Is Undersized Without Measuring It?
Play a track with sustained low bass, like a cello or a synth pad, and listen to the last two seconds after the note stops. A properly sized port lets the air inside the box move freely, so the bass decays smoothly. An undersized port chokes that airflow, and you will hear a short, hollow bump where a smooth, full decay should be. The note sounds like it is being cut off by a hand over a glass. The Thiele and Small parameters for the driver tell the designer what port volume is needed, but you do not need those numbers to hear the result.
What Does Damping Factor Actually Do To The Woofer, In Plain Terms?
Think of a bicycle brake. The amp is the brake, and the woofer cone is the wheel. A high damping factor means the brake grabs firmly: the wheel stops the instant you squeeze. A low damping factor means the brake is soft: the wheel keeps rolling a little after you let go. In audio terms, a high-damping amp tells the cone to stop the moment the signal stops. A low-damping amp lets the cone wobble on, and that wobble is the smear you hear in the bass, especially on fast, repeated notes.
Why Does The Crossover Matter More When The Amp Is Weak?
The crossover is a passive, fixed network. It cannot compensate for a noisy signal coming from the amp. If the output filter leaks switching residue, that residue rides into the crossover and gets split across both the woofer and the tweeter. The tweeter, being the more sensitive driver, makes the residue audible as harshness. A clean amp gives the crossover a clean signal to work with, and the crossover’s only job is to split frequencies, not to clean up noise that the amp left behind.
Can I Improve The Pairing By Upgrading Just One Side?
Yes, and the improvement will show up in a specific band. Upgrading only the amp’s output filter cleans the treble and tightens the transients, but the bass is still limited by the bookshelf’s port and cone. Upgrading only the bookshelf’s woofer and port extends the bass, but the treble still carries the amp’s filter residue. The most noticeable single upgrade depends on which band is currently the weakest link in your specific pairing. You do not need to replace both halves at once to hear a real, named change.
(Written for and first published on hifireport.com. Republishing requires permission.)