How to catch noisy preamps and scratchy pots on a used mixer

Seen a good deal on a used mixer but worried it might hiss or crackle? You should check the preamps and pots before you buy. A little testing saves a lot of headaches.

What’s going on

Preamps boost a mic or line signal. If they are noisy, you’ll hear hiss, hum, or bursts of static. Pots (pots are knobs that change volume or tone) can get dirty or worn and make scratchy sounds when you turn them.

Why it matters

Noisy preamps can ruin quiet takes and make live sound unreliable. Scratchy pots can pop during a show or while tracking. Fixes exist, but repair cost and downtime matter more than a discount.

What to check before you buy

Start with a quick visual look. Smell for burnt electronics. Check for missing knobs or corrosion around jacks. Then run these basic tests in order:

• Power on the mixer and let it warm for 30–60 seconds to settle the electronics.

• Plug in a good pair of headphones to the mixer’s headphone jack; this isolates the board and keeps things simple.

• Use a known-good microphone and cable; a cheap cable can mimic noise problems.

• Try a steady signal (talk, sing, or play a looping source) while you raise each channel’s gain slowly.

• Turn each channel’s pan, volume, and EQ knobs while listening for scratch, static, or jumps in level.

• Engage phantom power if you plan to use condenser mics and listen for new noises.

• If the mixer has insert points, test them by routing a small effect or headphone amp and checking for noise.

Quick hands-on checks to spot trouble

Gains: Set channel gain low and raise it to a normal level. A clean preamp stays quiet at low gain and stays smooth as you add gain. If hiss or gross distortion pops up early, that channel’s preamp is suspect.

Pots: Rotate each knob slowly from stop to stop while you listen on headphones. Scratchy or intermittent sound while turning is a sign the pot is dirty or failing. Light taps on the potentiometer with your knuckle can reveal loose parts that click or shift.

Micro-moment: You meet the seller in a bright spot and plug in your own mic and headphones. You talk, twist every knob slowly, and watch for hiss or pops. If a channel fails those simple moves, you walk away or knock the price down.

Red flags and how to score them

If you hear any of the following, treat them as red flags:

Constant hiss at normal gain levels. Loud pops when switching phantom power on or off. Scratchy pots that don’t clean up with a gentle twist. Channels that suddenly drop in level or cut out. Corrosion around XLR/TRS jacks or loose mounting screws.

If noise is subtle and only at very high gain, the mixer might still be usable for live PA work but not for quiet studio tracking. If pots are scratchy but the board otherwise works, a good contact cleaner can fix it. If the preamp is noisy across multiple channels, plan on repair costs that often exceed what you saved buying used.

What to negotiate or ask about

Ask about the mixer’s history: smoke, spilled drinks, heavy gig use, or recent repairs. If the seller offers a short return window or a modest warranty, that reduces your risk. For a fair price drop, point to any persistent noise you found and estimate repair cost: a pot cleaning is cheap, a preamp board swap is not.

Bottom line

You don’t need lab gear to find the big problems. Bring headphones, a good cable, and a known mic or phone player. Check every channel and every knob slowly. Walk away if a channel hisses badly or cuts out, or factor repair into your offer.

One slow pass across every fret can save you from a repair bill that kills the deal.

Today’s takeaway: Test every channel and knob with headphones and a real mic so noisy preamps and scratchy pots don’t surprise you later.