The Korf Blog

The inside story: our research,
development and opinions

21 March 2023
What Causes Turntables to Hum?
After half a year of advanced theory in our turntable drive exploration, I think it's time to get down to Earth (at least for a while). Let's talk about something that virtually any analogue enthusiast can immediately use.

Two questions appear again and again in the mail that we receive:

  • How do I get rid of the hum? I've changed a cartridge, a preamp, a tonearm, a cable, I moved house, or I just moved the stereo to another side of the room—and now there's intrusive hum!
  • You refer to "quality tonearm cables". How are the "quality" ones different? Is there a way to know which is which?
As those questions are very closely related, I will try to answer them in today's post.

What Causes Hum?
For once, there is a clear consensus that isn't subject to any controversies. Everyone agrees on this: our house wiring causes the hum.

The miles of mains cabling (or kilometers, depending on where you live) act as a giant primary winding of a transformer, creating a stray low frequency magnetic field. It turns into annoying hum when these things happen in the audio signal chain:

  • Something must act as a secondary winding of a transformer
  • There must be a connection from this "winding" to a signal input
  • To convert a current in the "winding" into voltage, some resistance must be present
Sounds familiar? We have just described a typical vinyl playback front end:
Looking at this, the main question isn't "why it hums". One wonders why sometimes it doesn't.

The cartridge itself is a very effective secondary transformer winding (unless the designer took good care to shield it). The signal-carrying shield can also be an unintended secondary winding. Having two ground paths between the source and the amplifier can modulate the signal ground with the induced noise.

And, of course, there's plenty of high impedance on hand to do the hum I to V conversion.

There's a name for what we have just described. Pro audio people call it the "ground loop". They also have a set of recipes that are supposed to solve it:

  • Break a shield ("lift the ground")
  • Only connect the shield at the source
  • Put a small resistor in series with the shield
Et cetera, et cetera

If you implement any of it with a turntable, the hum will increase, not decrease. This advice has active powered sources in mind, and can hardly be applied to millivolt-level signals from a pickup cartridge.

Is there no hope?

Instrumentation Connection
Fortunately for us, the scientists have battled with interference problems much longer than audiophiles. Scientific measurements are often done with low voltage signal sources at the end of long cables that go into high impedance inputs.

And the measurement engineers have a solution. My university lab supervisor called it an instrumentation connection, and that's what I will call it.
This is often incorrectly called a "balanced connection" ("instrumentation" is quite a mouthful). In reality, it is a single-ended connection, built using a few simple rules:

  • Do not use the shield (or drain wire) of a cable as a signal ground or power ground
  • Connect the shield of a signal cable to ground at one end of the cable only. Leave the other end floating (not connected to ground)
  • There must be just one point in the system where all the shields and the signal ground meet ("star ground")
This said, a magnetic pickup cartridge with a separate ground can actually be treated as a balanced source and can be connected to a balanced input.
Early tonearm designers understood this well. Audio was state of the art entertainment back in the 1950s, and the caliber of people working on it was exceptional. This is why we have 5 wires going out of a typical stereo tonearm, not 4 or 3.

Let's draw the instrumentation connection as it applies to vinyl playback front end. Here's the right channel inside a tonearm with a typical 5-pin connector.
All the requirements for the low noise instrumentation connection are met. The shield and ground are separate. No current flows through the shield. And nowhere in the tonearm are the grounds connected—they will only come together at the preamp input (provided our tonearm cable is made right).
Warning! Most Shure cartridges came from the factory with a metal tab shorting the cartridge shield to one of the signal grounds. If the cartridge touches any metal part of the tonearm, the instrumentation connection is no more. Shure publishes a knowledge base article on how to remove this tab.

Cables Good, Bad and Ugly
From the above scheme, it's clear that a competent tonearm cable must preserve the instrumentation connection. It should keep all the signal wires safely shielded, the shield separate, and the signal grounds away from the shield and each other.

In short, it should be like this (for clarity, only the left channel is shown):
Sadly, almost all tonearm cables are actually built like this:
This ruins the instrumentation connection, passing the signal ground through a shield. Still, the ground wire remains separate. This is neither optimal nor truly terrible. We still have a measure of hum rejection. In normal use, no stray current should flow through a shield. Seeing such design in a $5000 cable is depressing, but one can do a lot worse.

Like this, for example. This is the stuff of nightmares. All the grounds are tied together at the tonearm connector, and the ground wire (if it exists at all) is purely decorative.
But you will say, "So what if the cable is poorly designed, if it doesn't hum in my system?"

In a typical household, 50 or 60 Hz mains aren't the only source of electromagnetic noise. Electric motors (HVAC, washing machines, dryers, dishwashers, refrigerators), bang-bang controlled appliances such as microwave ovens, and especially switched mode power supplies (SMPS) generate massive amounts of electromagnetic noise.

Unlike a fixed frequency of the power noise, these devices spread their emissions over a wide spectrum. You will not hear the interference from them as hum or buzz—but rather as veiling. The signal to noise ratio suffers.

This is why replacing a poorly designed cable with something better often results in "opening" of the sound.
Every single "power saving" LED light bulb has a nastiest cheapest SMPS imaginable inside (this is why they never last).

If you have to have LED light bulbs, at the very least try to keep them on a separate circuit from your stereo.

Is Cable Quality Only About the Hum?
Is hum rejection the only thing that matters in tonearm cables?

Of course not. There's a lot more. For example, very high frequency noise rejection (10 MHz+) works completely differently, often in unpredictable and surprising ways. It used to matter very little if at all, but now the mobile phones are everywhere and their interference is often dominant.

Cable capacitance can be quite important, especially if you're using high inductance fixed coil (MM or MI) cartridges. We have covered the reasons for it here:

The connectors obviously matter a lot. Not just the quality of the electrical connection itself—a poorly designed connector can ruin any attempt at shielding.

We will not descend too far down this rabbit hole. There are people who dedicate their whole lives to cables, and it's up to them to tell you their secrets if they wish. My point today is, in contrast, very simple:
Nothing matters unless the basic cable configuration is right
—Nothing matters unless the basic cable configuration is right

You can have the best triple-shielded cable terminated with the best designed connectors. But if the grounds are all tied together and the signal is routed through the shileds, it will suck.

Should We Build One?
I must confess: I really do not want to build yet another tonearm cable. There are so many of them on the market at all price points. But, helping our customers with their hum problems, we came to a conclusion. In practically every single case hum could have been cured with a well built cable.

Should we offer a simple competent cable to match our tonearms? Nothing fancy—a Mogami, Belden or Van Damme twin mic cable, and some basic gold plated connectors? Priced under €100?

What do you think?

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