Turntables and Hum

With the wider introduction of the magnetic phono cartridge from around 1968, along with the requirement to mains-safety earth non-double insulated turntables, came hum! Lots of it.

But people into hi-fi at that time tended to also be into hobby electronics and were quite well genned-up on a thing called "hum-loops", and knew how to solve them - or soon found out.

Because of the mains safety earth on the turntable as well as the mains safety earth on the amplifier, as soon as one connected the little black "earth" (or "grounding") wire between turntable and amplifier - which was supposed to make it all hum-free - it hummed!

The reason? Being safety-earthed at both ends it set-up what's known (or what was known then) as the hum-loop.

The only way to solve it was to disconnect the earth wire in the turntable mains plug! This made the turntable unsafe as far as the regulations were concerned, but any fault current would flow in the little black "earth" wire to the amplifier chassis, and via the amplifier to its mains-earth. Not ideal protection and against regulations, but if the owner accepted the risks, he/she got hum-free sound.

During the '70s the Japanese introduced the Technics and Pioneer hi-fi turntables which, intended for the world-wide market, could not rely on a mains-safety earth, and so had to be double-insulated, and were. The hum problem was solved!

Over the following years fewer people engaged in hobby electronics, and the "art of controlling hum" got forgotten about thanks to these new-fangled foreign turntables.

But then, by the (very) late '70s, a new breed of turntables came along, aimed at "better hi-fi". One of them made by Rega featured the Acos Lustre (Acos-Rega) tone-arm. This too worked hum-free. The reason being that like on the Technics and Pioneer decks, the Acos used a "5th" cartridge wire which thoroughly grounded (earthed) the entire arm tube and arm base.


(image courtessy of vinylengine.com)

However, the Acos tone-arm was the "elephant in the room" being a "brought-in" part. The new breed of hi-fi turntable makers wanted a hi-fi arm (snobbery? The Acos was a great tone-arm). The trouble being that most hi-fi tone-arms don't bother with the "5th" cartridge wire, and totally rely on the arm-bearings to ground out the arm tube.

Is that a good idea? Absolutely not! How can a lubricated bearing conduct the electrical pick-up noise (which causes hum) from the arm tube to the arm base, where the little black "earth" wire is connected to?

The answer is: very badly!

But that is often the case with "new-age" hi-fi turntables over the past 20-30 years to this day. You are totally reliant on bearing conduction to be hum-free, and it simply doesn't always work. Therefore because of the resistance between arm tube and arm base, hum can develop. It might not amount to very much, and often it varies between one tone-arm and the next because of variances in the "tightness" of the bearings, but we'd be better off without it.

You would think that arm manufacturers would have been rushing to solve the problem - after all it isn't hi-fi to have hum? Think again!

To be truly "hip" in hi-fi you use a moving coil cartridge. And if you do it actually lessens your chances of hum.

The hum noise in the poorly earthed arm-tube gets onto your music signal by it being induced into the cartridge. So if you are using a moving magnet cartridge which has lots of a thing called inductance, that hum is easily induced into it.

The moving coil cartridge doesn't have much inductance, and therefore much less hum is induced into it - it is very much quieter if not silent.

Therefore the "hip" hi-fi user doesn't have the problem and the arm manufacturer doesn't have a worry?? It's only the minority who use moving magnet cartridges who are aware.

This is such a shame as because we make (and love) moving magnet phono preamps which make moving magnet cartridges sound so good, the modern-age tone-arms without proper grounding introduce hum... and so often the lowly phono preamp gets the blame.

----------------------

Footnote: A number of other tone-arms of the '70s era made use of the "5th" cartridge wire grounding technique, and the "tell-tale" is usually the sighting of a small screw head (or two) under the arm tube just to the rear of the headshell, which is where the wire was attached.



Edited by Graham Slee - 02 Apr 2021 at 8:20am

A few years ago when my Rega deck was pressed back into service after a prolonged hiatus given over to digital replay there was a pronounced and distracting hum that I'd not been so aware of before. This was maybe because I'd forgotten or perhaps as all the electronics had been changed in the interim the problem was now amplified. The arm/cartridge combination then was RB250/G1042, anyway I changed to moving coil and the problem disappeared.  

Thanks for that explanation Graham, it makes sense of some past experiences I had with my and other people's turntables. I can also see now why disconnecting the tonearm ground generates far less hum than I would expect from a high gain MC system.

Dave, I think we've been on nearly parallel voyages of discovery with our Gorbes. I linked my armboard (yes that Michell OL version that took ages to arrive!) to the chassis, taking a decent copper wire from the armboard tag to the ground terminal on the back of my Elevator EXP. There's little hum if I disconnect it, minimal compared to instead disconnecting the tonearm's ground, which confirmed there is no electrical connection between arm and arm board.

Luckily my Zyx cartridge is not grounded through the casing so it doesn't create an ground loop when I connect the arm's earth to the terminal on my Elevator EXP. I do this rather than connecting the arm ground to the arm board as linking them would mean I'd get a ground loop if the arm ground lead came into contact with any connectors or metal hi-fi casing.

The connection between the EXP and the subsequent stage after (now an Accession, formerly a Revelation M) is more challenging as I run both units off the same PSU1. The Cusat50's give such a good ground connection I was able to cut the 0V/ground wire in my power supply cable. There was a faint hum before I cut that wire.


Edited by Fatmangolf - 24 Jan 2016 at 11:09pm

ICL1P wrote: I sold my Acos Lustre GST-1 on eBay recently. It ended up in the Netherlands and was destined for a place on a Soulines TT. I heard recently it didn't fit the Soulines despite being an SME fit, which the Soulines was.

From memory, the arm hole to spindle measurement was shorter with the Acos.