03 January 2008

Lundahl MM Phono Step-Up Transformers

After getting back into DIY audio, I felt it was time to re-introduce my analog gear. I had no phono preamp and though I looked at several schematics, I just went out and bought a Pro-Ject Phono Box SE (which is an opamp based unit). When everyone had stopped buying turntables to embrace the new digital age about 20 years ago, I bought a Rega Plana 3. The latest cartridge I had in it was a $AU60 Shure. I bought the cartridge believing I would never seriously listen to vinyl again. I have a few hundred records all in top condition but when I played them through the cheap Shure, they sounded so flat and dull I knew I had to upgrade. I like wood and the Grado timbre body Platinum Reference looked a good choice. With the cartridge in place the sound was definitely improved and encouraged me to buy some new vinyl.

With the Grado Platinum Reference cartridge, the Pro-Ject Phono Box SE only puts out about 1V. Though this is passed through a valve preamp with a gain of four, I found that I was turning the volume right up to obtain good listening levels. I read an excellent write-up in Stereophile magazine about a Lundahl transformer based MC step-up kit from K & K Audio. K & K Audio also supplies a MM equivalent, the Lundahl LL1550 which is what I went with.

Moving Magnet Phono Step-Up Transformers
The Lundahl LL1550 is made using amorphous core material as this type of core does not store energy (unlike typical mu-metal cores). Not only will the step-up transformer increase the output level, they will also provide electrical dampening back to the cartridge to "tighten" performance. This can also be done with electronics, but I like the whole "passive" idea.

The inside of the box was first lined with flashing (adhesive bitumen and soft aluminum) and then speaker dampening vinyl. Velcro is used to attach the transformers and to allow easy removal for maintenance. For feet I used a low profile synthetic felt stick-ons.

I had recently finished building a wooden boxed CMoy / Grado RA1 headphone amp and inspired by the Grado wood cartridge, I thought it proper and fitting to build this step-up transformer in a nice wooden case. The Grado Platinum Reference cartridge came in a beautifully crafted mahogany box. Perfect!

Grado Mahogany Box
Mahogany Box for Grado Platinum Reference Cartridge
Two aluminum strips (3 mm thick and 25 mm wide) were used to hole the gold plated RCA jacks. The surface of the strips was brushed with a plumbers wire brush. The step-up transformers were wired for a 6 dB (4 X) gain. The load resistor was calculated to reflect the correct load back to the cartridge. Don't forget you must earth the cores. Every earth must be joined together otherwise hum will occur. Silver wire was used for all connections.

Lundahl LL1550 Transformers
Lundahl MM Cartridge Step-Up Transformers
Results - Moving Magnet Phono Step-Up Transformers
The step-up transformers appear to add nothing but gain and take nothing away from the sound. The transformers do take time to break in so things may only get better.

The small wooden case was hard to work with, but it looks goods. Feel free to build this in a metal case or whatever, but keep in mind that step-up transformers are microphonic. If you are not using wood, use a heavy solid case and dampen it.

Mark Houston

Moving Magnet Step-Up Transformer
Finished Enclosure for Lundahl MM Cartridge Step-Up Transformers

MM Step-Up Transformer and Gainclone
Rega P3-24 with Lundahl MM Cartridge Step-Up Transformers

More Phono Projects

1 comment:

  1. So can you use wood instead of aluminium for the case? no hum problems?