Why Does My Turntable Hum?

A phono input is the most sensitive input in a hi-fi system by a wide margin. A moving-magnet cartridge puts out something like 4 or 5 millivolts and a phono stage multiplies it by a factor of around a hundred, so anything else that gets into that signal path gets multiplied by a hundred too. That is why turntables hum and CD players do not, and it is why the diagnosis has a definite order.

First: what pitch is it?

This single question splits the problem in half, and it is arithmetic rather than opinion. Mains power in the United States alternates at 60 Hz. A full-wave rectifier — which is what sits inside essentially every audio power supply — produces a ripple at twicethe line frequency, because it flips the negative half of each cycle into a positive one: 2 × 60 = 120 Hz.

• A low, smooth 60 Hz hum— roughly the pitch of the lowest B on a bass guitar — is induced or grounding-related. The signal path is picking up the mains field directly, or there is a difference in ground potential between two connected boxes.
• A buzzier 120 Hz hum, an octave higher and harsher, points at a power supply: its own rectifier ripple leaking into the audio, or a nearby transformer radiating into the cartridge or the tonearm wiring.
• A steady rush or hiss with no pitchis not hum at all. That is broadband noise, and it is almost always a gain problem — see below.

You do not need test equipment to tell them apart; the two are an octave apart and the difference is obvious once you know to listen for it. If it changes when you turn the volume up, the source is before the volume control. If it does not, the source is after it, which puts the power amplifier or its supply in the frame and takes the turntable out of it entirely.

Second: the ground wire, which is not optional

Nearly every turntable has a thin separate wire, usually with a spade or bare end, that is not one of the two RCA plugs. It connects the metal of the chassis, platter bearing and tonearm to the ground of whatever the turntable is plugged into. Its entire job is preventing a 60 Hz hum, and it is left disconnected constantly because it looks like something optional.

It goes to the ground terminal on the phono stage or the amplifier — the one marked GND or with the earth symbol — and not to a mains earth, a radiator or a screw on the back of the case. If the amplifier has no ground terminal, which some do not, the usual fix is a chassis screw on the same unit the RCAs go into. If the hum gets worsewhen you connect it, you have a ground loop rather than a missing ground: two paths to ground between the same two boxes. In that case the cure is removing one path, not adding another — our connection guide covers the orders in which to try it.

Third: the gain setting, and the figures that prove it

This is the most useful section on the page, because the symptom is usually described as hum and the cause is published in black and white by every maker who bothers to document their product.

A phono stage has to amplify much harder for a moving-coil cartridge (output measured in tenths of a millivolt) than for a moving magnet (around 4–5 mV). More gain means more of the stage’s own noise. Here is what the makers publish:

• Schiit Mani 2:> 108 dB signal-to-noise at 33 dB gain, > 98 dB at 42 dB, > 96 dB at 48 dB, and > 87 dB at 60 dB gain— all A-weighted, referenced to 2 V RMS. That is a 21 dB range across one box.
• Pro-Ject Phono Box DC: 94 dBA noise floor in MM, 75 dBA in MC— a 19 dB penalty for flipping one switch.
• iFi ZEN Phono 3: 91 dB(A) in MM, 81 dB(A) in MC High, 89.6 dB(A) in MC Low, 78.6 dB(A) in MC V-Low. Note that this one is not in order— iFi’s published MC Low figure is better than its MC High figure. We publish it as published; a maker’s own sequence is not something to tidy up by guessing.

Fourth: double amplification, which is the other switch problem

Several turntables we cover have a phono stage inside them with a PHONO/LINE switch — the Audio-Technica AT-LP120XUSB (36 dB nominal gain), the AT-LP70X (36 dB), the Fluance RT81. If that switch is on LINE and the output is plugged into an amplifier’s PHONO input, the signal gets amplified and RIAA-equalised twice. The result is loud, bass-heavy, distorted and usually hums, because the second stage is amplifying the first stage’s noise floor by another factor of a hundred.

The rule is one stage, ever. Deck set to PHONO into a phono input, or deck set to LINE into a line input — never LINE into PHONO. Our phono stage guide has the 30-second test for working out which you have.

Fifth: position, and what a transformer radiates

A cartridge is a coil of wire moving in a magnetic field, which is also a perfectly good receiver for any other magnetic field nearby. Mains transformers radiate one. So does a dimmer, a fluorescent ballast, a laptop charger and the power supply of the phono stage itself.

Every stage in our top three uses an externalsupply, which is a design choice made for exactly this reason: Schiit specifies a 16 VAC transformer feeding dual filtered regulated ± 16 V rails, Pro-Ject an outboard 18 V DC unit, Fluance a 24 V 500 mA adapter, and iFi a 5 V DC supply. The point of putting the transformer in a separate lump is that you can move the lump. If there is a 120 Hz buzz, move the wall wart as far from the turntable and the phono stage as its cable allows, then move the turntable away from the amplifier, then try a different socket. Each of those is free and any one of them can be the whole fix.

Sixth: what a subsonic filter does, and does not, do

A rumble you can feel more than hear — cones visibly moving, bass thickening on every record — is usually below the audio band rather than in it, coming from bearing noise, a warped record or footfall. That is what a subsonic filter is for, and it is a specification worth checking:

• Schiit Mani 2: yes, selectable 6 or 12 dB/octave at 15 Hz, and fully passive— the most completely specified filter among the stages we cover.
• Fluance PA10: yes, selectable high-pass rolling off below 20 Hz.
• iFi ZEN Phono 3:yes, described as an “Intelligent Subsonic Filter” — iFi publishes no corner frequency or slope for it.
• Pro-Ject Phono Box DC: not published.

What it will not do is fix a 60 Hz hum, because 60 Hz is well inside the audio band and a filter at 15 or 20 Hz does not touch it. If a subsonic filter cures your problem, the problem was rumble; if it does not, work back up this page.

The short diagnostic list

1. Is the hum an octave above or below the one your neighbour’s fridge makes? 60 Hz means grounding or induction; 120 Hz means a power supply.
2. Is the ground wire connected to the GND terminal on the box the RCAs go into?
3. Is the phono stage set for the cartridge you own — MM for a moving magnet?
4. Is anything amplifying the signal twice: a deck on LINE into a PHONO input?
5. How far is the nearest transformer, and does the hum change if you move it?
6. Does it change with the volume control? If not, the turntable is not the cause.

Work down that list before buying anything. If the answer turns out to be the phono stage — and it does when the stage is a cheap one with no published noise figure at all — our roundup ranks seven of them on exactly the specifications this page uses.