MC Step-up Transformer

McStep

From MC to MM with a gain of 10

Last modified: 06-Jun-2004

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Intro

This mini project did result from the Tweety project. When I finished Tweety I wanted to use Tweety with my Clearaudio turntable which has a MC cartridge of Dynavector with a very low output of 0.23V. Therefore I needed to amplify the signal of the cartridge about 10 times before Tweety could handle it.

Modern phono amps do not need such a head-amp or step-up transformers. These amps often use OpAmps that can easily provide the enormous gain needed for MC cartridge amplification (unfortunately sometimes with a penalty in noise). Anyway, for my tube amp there were only two alternatives:

• Use a MC Head Amp, such as the Head-Eek project
• Make use of a Step-up transformer (high-quality)

Although an active headamp such as the Head-Eek works perfectly, I'm very impressed with the sound quality of a pair of good step-up transformers. There are at least two manufacturers of top-quality transformers: Sowter in UK and Lundahl in Sweden. Prices are similar and good stuff is expensive. A pair of Lundahl 9206 transformers will set you back for € 110,00

What's inside?

In the little metal box are two Lundahl 9206 transformers which are internally connected to the cinch chassis connectors.

The connections that need to be made on the transformers themselves is dependent on the application. As I needed a gain of about 10 times and my cartridge would probably like an input impedance of around 100 Ohms I selected schematic "C" for my application. All useful connections are found on the datasheet on the Lundahl site, but for your eyes only I copied the schematic I used below.

Attention Points

Something I did but do not recommend to my readers is using a magnetic housing for this project. Rather use one of those aluminum boxes I order from Conrad since in my experience these are less prone to humm. After all, we are talking about very low signal levels and everything that could act as an antenna should be avoided in this device.

I used massive silverwire (0.8 mm) with PTFE sleeving for making all connections. I do think however that 0.4 mm makes an easier connection on the little transformers.

Especially making the ground connection is critical. I did experiment with the housing and found the spot I'm using now but you might find a better place to make the chassis ground in your own design.

But not always do you have to connect the ground wire of the turntable to the step-up ground point itself. In some comfigurations I used to ground the turntable to the preamp itself.

© Maarten&Annemarie, 2002, 2003, 2004, 2006