How to check a used mixer for scratchy pots and noisy preamps

You meet the seller at a cafe. They pull a compact mixer from a soft bag. It looks clean, but looks can lie. You want to know if the pots scratch and if the mic preamps hiss before you hand over cash.

• Bring good cables and headphones

• Bring a small mic (or use your phone as a test source)

• Bring power adapter for the unit if it needs external power

• Inspect the jacks and knobs for wobble or grime

• Ask to plug in a simple source and a mic

• Test every channel, one at a time

• Listen at low and high gain settings

Quick visual and feel checks

Look for shoved-in knobs or paint worn on faders. Loose knobs that tilt are a warning. Check the jacks for bent pins or heavy corrosion. Roll each pot and fader slowly. If a pot feels gritty or jumps, it likely needs cleaning or replacement.

Take the case off only if the seller is okay. A quick look inside can show crusted dust, leaking capacitors, or chewed wires. Those are signs of long neglect. Close the case and move on if the seller won’t allow it.

How to test preamp noise — a simple audio test

Plug a mic into a channel and set levels low. Use headphones so you hear details. Raise the preamp gain slowly while watching meters. If you hear a steady hiss that grows with gain, that is preamp noise. Some hiss is normal on budget mixers; loud hiss at medium gain is a problem.

Now switch the mic to another channel and repeat. If only one channel is noisy, the preamp on that channel may be bad. If all channels are noisy, the noise could be the power supply or a bad design.

Micro-moment: You press your ear to the headphone cup, raise the gain, and hear a high, steady hiss. The seller shrugs. You ask to try another mic and the hiss stays. You slow your raise of the gain and note how quickly the noise appears.

Finding scratchy pots and dirty contacts

Turn every knob and slide every fader while music plays through the channel. Scratchy pots make crackles when you move them. Try moving them slowly and fast. Some pots only crack when moved quickly.

Use the mute and solo buttons. A dirty contact can cut sound for a second when you press mute, or make a channel pop when you hit solo. Tap the buttons and listen for intermittent cuts or pops.

Tests to separate preamp noise from wiring and mics

Swap cords and mic to rule out bad cable or mic. A bad cable can make crackles that look like pots or bad jacks. Try the same mic and cable on another known-good mixer or a simple interface, if the seller allows.

If the noise changes when you touch the case or the grounding, it could be a ground issue. Try moving the mixer away from phones, power bricks, and strong Wi‑Fi devices. Some noise shows only with certain power adapters. If possible, try the mixer on battery (if it supports it) or a different power supply.

Simple repair and negotiation levers

Many scratchy pots respond to contact cleaner. If you can test with cleaner applied (seller willing), the noise may drop. That fixes many dirty pots but not worn carbon tracks.

If a single channel preamp is noisy, that channel might need a new op amp or cleaning. That repair has a cost. Use that cost to negotiate. Ask the seller for a price cut or for a partial refund to cover a shop repair.

Red flags and when to walk away

If multiple channels hiss loudly at low gain, the power section or design might be to blame. If pots crack even when the unit is warm, internal wear may be advanced. Heavy corrosion inside or bulging capacitors are a hard no for most buyers.

If the seller refuses a short on-site test, refuses to swap cables, or blocks you from listening with good headphones, treat it as a red flag. A seller confident in gear will let you try basic checks.

Final checklist before you pay

Make one final run: all channels, all pots, mute/solo, phones, main outs. Note any sounds that only happen under certain movements. If everything is quiet and smooth, the mixer likely needs little work. If you heard intermittent crackles, ask for a lower price or factor in a cleaning/repair cost.

A straight neck and even frets matter more than shiny hardware — price the setup before you fall for the look.

Today’s takeaway: Test every channel slowly with good headphones and spare cables; scratchy pots often clean up, but noisy preamps usually cost more to fix.