THE 12 VOLT ENCYCLOPEDIA · VOLUME XIII
Turn-On Thumps, Silent Moving Subwoofers and Missing Full-Range Factory Signals
Three failures often look unrelated but share one lesson: movement and voltage do not prove correct timing. A remote wire can switch equipment in the wrong order, subwoofers can move against each other, and a factory wire can carry clean audio that lacks most of the audible spectrum.
What creates a turn-on thump
A thump occurs when a transient or DC offset reaches the amplifier output during startup or shutdown. Sources, processors and amplifiers stabilize at different times. A component without adequate muting can pass the transition. Determine whether the thump occurs at turn-on, turn-off, source change or remote-wire chatter.
What a standard relay can and cannot do
A standard relay lets a small radio remote output switch a stronger fused 12-volt feed for multiple devices. Terminals 85 and 86 energize the coil, 30 is common power, 87 is normally open output and 87a is normally closed and unused in this application. A basic relay adds no controlled delay and can click or bounce.
Actual sequencing solution
If the source produces a transient before becoming stable, a time-delay module or sequencer should turn processors and amplifiers on in the correct order and reverse that order at shutdown. A relay may be part of that circuit. If DC exists on the RCA line, repair or isolate the faulty source rather than hiding it with timing alone.
Subwoofers moving but cancelling
Two drivers wired in opposite acoustic polarity create pressure in opposite directions. At equal level and location, their outputs cancel strongly even though both cones move. Confirm coil wiring, amplifier channels and enclosure orientation. An inverted driver may require electrical polarity reversal depending on the desired acoustic relationship.
Polarity versus phase
Polarity is a fixed electrical inversion. Phase is a frequency-dependent timing relationship created by distance, filters and enclosures. Flipping wires changes polarity by 180 degrees, but it does not correct every phase mismatch. Measure combined response around the crossover and across the passband.
The wrong factory tap
A factory tweeter wire may contain only high frequencies; a woofer wire may contain bass and lower midrange; a center channel may carry processed mono information. A line-output converter can reproduce only what it receives. An amplifier makes the selected signal larger—it cannot invent missing frequencies.
Reconstructing full range
Measure every candidate factory channel with sweeps at multiple volume levels. Select complementary bands and feed them into a DSP or integration device with summing capability. Correct input levels, polarity, delay and all-pass behavior before combining them. Automated summing is helpful, not magical.
One diagnostic mindset
For a thump, map time. For cancellation, map polarity and arrival. For missing bandwidth, map frequency. Isolate one dimension at a time and compare before/after measurements. Replacing the amplifier will not repair incorrect timing, opposing waves or absent input content.
Diagnostic Reference
| Symptom | Dimension to test | Correct tool |
|---|---|---|
| Turn-on/off thump | Time/DC transient | Scope + sequencer test |
| Remote output weak | Switching current | Loaded voltage + relay |
| Two subs weak together | Polarity/phase | Battery polarity + acoustic measurement |
| New speakers lack bass | Input bandwidth | RTA/sweep each OEM channel |
| Summed signal has holes | Input phase/delay | DSP input analysis |
Frequently Asked Questions
Will a normal relay always stop turn-on thump?
No. It strengthens switching but does not automatically delay or mute a transient.
Which relay terminal is unused?
87a is normally closed and commonly unused here; verify the molded diagram.
Why do two moving subs make almost no bass?
They may be producing opposite pressure and cancelling.
Can polarity be fixed with amplifier phase control?
Sometimes at selected frequencies, but polarity, delay and filters must be understood.
Can auto-summing restore every OEM system?
Not when channels contain incompatible delay or all-pass processing without correction.
Final Word
Successful troubleshooting replaces guesswork with evidence. Power, signal, load, timing, temperature and mechanical integrity can all create similar symptoms; a disciplined test sequence separates them without damaging good equipment.