The New Step Up Transformer Kit from Andy Grove Support Information from Andy Grove

Inside the Step-up Kit

The AudioNote Kits step-up transformer is configurable as 1:10 or 1:20 ratio, offering gains of 20dB or 26dB respectively. These two ratios are suitable for medium to high output moving coil cartridges, with output from around 0.25mV up to 1mV. The load a transformer reflects is proportional to the square of the turns ratio, so 1:10 gives an impedance transformation of 1:100, and the 1:20 gives a transformation ratio of 1:400. When loaded with the 47k of a standard phono stage the reflected impedances will be 470 ohms, and 117.5 ohms, usually the lower output cartridges require a lower impedance load, in the hundred-ohm range. If the 470 ohms or 117.5 ohms loads are way outside the specified load for your cartridge then extra loading resistors can be added at the secondary to correct the impedance if necessary, but generally medium output cartridges are fairly load tolerant.

The input sensitivity of a phono stage is usually around 2mV to 5mV for full output, and the 1:10, or 1:20 ratio increases the sensitivity to 0.2mV to 0.5mV to 0.1mV to 0.25mV, don’t worry too much if the output is somewhat over that rated for your phono stage because, especially with a valve phono stage, there will be considerable headroom before overload. What may be more of a problem is that you won’t be able to crack the volume control open without it becoming too loud, experiment to see.

A simple electronic model of a cartridge would show a parallel and a series inductance, how these inductances are loaded will affect the tonal balance of the cartridge. A smaller load impedance, for example, generally leads to less treble and deeper bass, and the reverse is also true, however there are other parasitic elements in the circuit so when the load becomes too high resonance will result and you’ll get peaky treble. Lowering the load impedance also means less output voltage, how much this reduces depends upon the internal generator impedance of the cartridge and varies from model to model.

The core of the transformer is high quality mumetal, a 78% nickel alloy with iron, and some trace metals. The material is rolled and punched into laminations and then the lams are heat treated to maximise permeability and minimise losses. This is very important for small signal transformers because they operate at very, very low flux levels.

One characteristic of mumetal is that it’s physically extremely soft, not only does this mean that care must be taken during assembly, but as many audiophiles have found the physical characteristics of a material have a bearing upon the sound. Mumetal really doesn’t possess any of the hard steely colourations of lesser irons, this is generally true of nickel iron core materials, but especially so for mumetal, unfortunately it’s also very expensive.

The MCT also possesses an interwinding screen, and any decent low level transformer should have one. It’s important not only to prevent RF ingress, but to prevent hum coupling from the primary to the secondary, really it’s an essential feature of any small signal transformer, and many large signal transformers too.

The transformer is mounted in a mumetal can; this is to screen it from external AC magnetic fields that would otherwise induce considerable hum. Even within the screening can care must be taken over the positioning of the transformer to avoid hum pickup.

Andy Grove - Audio Note design engineer