Rack unit too hot, too quiet, or noisy in your FX loop? It's almost always a loudness mismatch (-10 dBV vs +4 dBu). Pick your two below.
⚡ What's wrong with my loop?
Pick your amp's loop and your rack/pedal gear — I'll tell you the problem and the fix. Not sure which you have? Check the gear list further down.
The two standards
-10 dBV
316 mV
Consumer / instrument level. Most guitar amp FX loops, pedals, and home gear. Also called "instrument level" or "line level consumer." Unbalanced (TS) connectors.
+4 dBu
1.23 V
Pro / studio level. Rack units (compressors, EQs, multi-FX), studio interfaces, and PA mixers. ~11.8 dB louder than -10dBV. Usually balanced (TRS / XLR) connectors.
The gap between them is ~11.8 dB. Plug a +4dBu rack unit into a -10dBV FX loop and you're already clipping before you touch the send level. Plug the same unit in reverse and everything's noisy and thin.
Level converter
Enter a level in either standard to see the equivalent.
⟷→
dBV = 20 × log₁₀(V). dBu = 20 × log₁₀(V / 0.7746). The 0.7746V reference for dBu comes from a 1mW signal into 600Ω.
Common gear reference
-10 dBV loops (most guitar amps): Fender, Marshall (most models), Vox, Orange, Peavey, Boss GT series, most stompbox-style multi-FX. +4 dBu loops: Mesa Boogie (many models), Kemper, Fractal AX8/FM9/Axe-Fx, Line 6 Helix, ENGL Invader. Switchable: Mesa Boogie Mark V, Bogner, some newer Mesa models. Check the manual — it's usually a rear-panel switch or dip switch. Typical rack gear (+4 dBu): dbx compressors, TC Electronic rack FX, Eventide H-series, Lexicon reverbs, Roland SDE series, most studio-grade EQs.