The Korf Blog

The inside story: our research,
development and opinions

27 April 2023
Moving Coil Transformer Hum—How to Fix
Last time, we discussed hum in the turntable when the cartridge is directly connected to the preamp. Immediately, I got dozens of requests: "Please do the same for the moving coil transformers!"

Happy to oblige!

Ideal MC Transformer Connection
The conclusion from our previous post was simple. Use an instrumental connection, and you will be fine. To remind, we need to keep the signal grounds separate, shields separate, and have a separate ground wire too. All those should only meet once, ideally at the phonostage.

What would it look like if we apply those rules to a moving coil transformer?
Really, the only thing we need from a transformer is not to mess with the signal grounds. Keep left away from right, the source away from the output, and the chassis ground away from all of them. We actually have a bonus of galvanic isolation, the tonearm signal and signal ground end up in the transformer primary and don't go any further.

Easy, right?

Real Life MC Transformer Design
Well, this is how virtually all MC transformers are actually built.
The signal grounds and the tonearm ground are usually connected to the same ground plane inside the transformer housing. Why?!

No idea. One can save a few pennies by having a common ground plane, but can it really be the reason? I refuse to think that those designers are both incredibly stingy and dumb. If you know why they do this, can you please tell me in the comments or send me an email?

Such wiring forces the transformer to be the center of your signal "star ground", whether you want it or not. Because the phono stages are also designed to be the center of the "star ground" in respect to the incoming signal, we have not one but two ground loops.

The first one is formed by the signal grounds that are connected both on the preamp side and on the transformer side. The second, by the signal grounds and the ground wire—again, there is connection on both sides of the cable. It takes a miracle for this not to hum.
Separate grounds is the reason why those tiny inline step-up transformers sound much better than they have any right to.

Just about everybody made them in the late 1970s, from Sony to Dual. They are inexpensive and common. If you live in a high EMI area, it can't hurt having a couple just in case. You can use them to test whether the hum comes from the MC transformer or from somewhere else.

How Do We Fix This?
It's easy if you are building your own enclosure for the MC transformers you bought from Sowter, Lundahl, Cinemag etc. You do the smart thing and keep all the grounds separate.

But what if you already have a box with the transformers inside, and it's impractical to rewire/rebuild it?

Here's what you can do.
The hard part is the ground loop formed by the signal grounds. We can't break it, but we can render it ineffective. Use the shortest cable you can find, and take the one with a separate shield that's only connected at one end.

Now our ground loop becomes a really poor inductor. First, it's short, second, it's fully shielded. Hard to induce any noise in that. Good bye hum, hope to never hear from you again.

This leaves the ground loop formed by the signal grounds and the ground wire. It should not give you any grief now, but if it does, just disconnect that wire.
Will it always help, 100% of the time?

Unfortunately not. Not all hum is due to ground loops, and not all possible ground loops are described above. But usually, keeping the grounds apart inside of a transformer enclosure makes a lot more sense than tying them together.
One last thing—does anybody actually build the MC transformers right?

In my experience, Quadratic Audio does, There is a jumper to lift the chassis ground if you need to. It also sounds really good. They are a part of Cinemag, but it is a lot more than just a box for something from the Cinemag catalogue. The transformers are custom wound. Check out the manual for their transformer, it has a lot of non-obvious information.

If you are in the market for an advanced MC transformer, you'll do well to consider Quadratic Audio.

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