FOH / Mons Mixing / Audio Troubleshooting Decision Guide

Audio Troubleshooting Decision Guide

Reference: SRH-TROUBLESHOOT-01. Good troubleshooting isn't magic — it's logical thinking plus experience, in that order. When there's no signal at all, work the chain from input toward output; when there's an unwanted signal riding on top of a good one (hum, buzz, hiss, static, distortion), you generally work backward from output toward input instead. This page starts with the "dead signal" checklist, then gives you a quick-reference table for the unwanted-signal symptoms below it.

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Key facts: for absence of signal, trace input → output; for hum/buzz/hiss/static/ distortion riding on a good signal, trace output → input instead, unplugging back through the chain until the problem disappears. Change one thing at a time so you know what actually fixed it.

Absence of Signal

Confirm every component in the path is actually powered on and every fuse/breaker is good — an easy thing to overlook, even for experienced techs. Mentally walk the signal path from input to output and check every control and switch along the way: gain/volume levels, correct channel selected and assigned, and nothing muted (or a different channel left soloed). Verify the path is physically complete — check for a missing or unseated connection anywhere in the chain. If nothing above finds it, swap the source itself for one you know is good (a spare mic, a test oscillator, another player) to rule the source in or out. Trace forward from the source: check its output cable, then the next component, then that component's output cable, and so on downstream until the dead link shows itself. Make one change at a time as you test, so you can tell exactly which change restored the signal.

Undesired Signal Quick Reference

For hum, buzz, hiss, static, or distortion sitting on top of an otherwise good signal, the tracing direction flips — start at the power amp end and work back toward the source, unplugging and substituting as you go, until the noise disappears. Use the table below to narrow down what you're likely dealing with before you start pulling cables.

Symptom	Likely Cause(s)	Fix
Hum	Ground loops or faulty ground connections; accidental chassis-to-chassis contact (units stacked, connector shells touching); inductive pickup from a nearby power transformer or a mic line run alongside power cabling.	Review the grounding scheme (aim for one ground path, not two); run a continuity check on suspect shields; don't assume rack-mounting alone grounds a chassis — paint can isolate it; physically separate the offending gear or cables, or cross cables at right angles instead of running them parallel.
Buzz	High-frequency harmonics of the AC line frequency — often a ground loop or ungrounded shield feeding a biamp HF driver (buzzing highs with no matching hum from the lows points here); inductive coupling from SCR light-dimmer circuits into mic lines; electrostatic coupling on a line-level run.	Reposition or reorient the affected mic cabling away from dimmer circuits; add a properly Faraday-shielded signal transformer or change the grounding approach; consider a quad-shielded cable (e.g. Star-Quad style) for better noise rejection.
Hiss	Poor gain staging — noise generated early in the chain gets amplified by everything downstream; less often, oscillation or a fault in a component, a partial cable short, excessive output loading, or (on push-pull outputs into unbalanced inputs) a shorted low-side driver.	Check that every stage is working at a sensible level; the usual culprit is excess power amp gain — back it off so the amp reaches full output around console 0 VU, not before. If the console needs to run near max just to get an audible signal, reduce input attenuation/increase input trim and bring the fader back down. Trace and scope-check any stage suspected of oscillating or faulting.
Static / Crackling	An intermittent signal or shield connection somewhere in the chain; a dirty control; occasionally RF interference.	Signal-trace to isolate and replace the offending cable; exercising a dirty control can mask the problem temporarily, but get it cleaned or replaced properly. For RF-driven static, good grounding practice and, where needed, an isolation transformer usually clears it.
Distortion	Overload of some stage in the signal path — most often a gain-structure problem; also excessive output loading (impedance mismatch or partial short), or a failed input/output transducer.	Check gain structure end to end: back off any stage whose clip light fires well before the system reaches full output, and never feed a −10 dBV input from a +4 dBu output without a pad in between. If a console's input clip lights trip before its output meters approach zero, add input attenuation/reduce trim and raise the output level instead. Check for shorts/mismatch on suspect outputs, and test/substitute to rule out a failed transducer.

Source: Davis, Gary D. & Jones, Ralph. The Sound Reinforcement Handbook, 2nd ed. (Hal Leonard/Yamaha). Section 17.8, pp.343–345. Rewritten in Ryan's own words from the source material.