"Putting MUSIC FIRST"

Step-up Transformer

Introduction

The transformer is a high performance Moving Coil Cartridge step-up Transformer. The primary application is for connecting low output moving coil cartridges to phono equalizer amplifier correction preamplifiers with conventional input sensitivity matched to moving magnet cartridges. Secondary applications include the use as Microphone Input transformer of exceptional performance.

The transformer was developed purely based on maximizing performance; cost or other economic considerations did not enter the equation. The transformer uses an incredible mixture of sections of both clockwise and anti-clockwise windings that takes multi section construction towards the limit, to maximize bandwidth. The 80% Nickel core used in this transformer is to our knowledge of the largest size ever applied to low level transformers, thus ensuring an exceptional dynamic range with up to +10dbu Input level and at the same time usability with nominal operating levels of -80dbu or less.

This transformer offers a performance level at the very boundaries of what is possible. It’s extremely wide bandwidth; low distortion and high level handling ensure that it is ready for applications in connection with modern wide bandwidth and high dynamic range recording systems such as 196kHz/24Bit DVD Audio Standard digital audio or SACD.

Technical details

The transformer is housed in a Mu-metal shielding can that measures 60.25mm (2^3/8in) in diameter x 72mm (2^13/16in) in height, excluding the connecting leads. No inbuilt mounting hardware or mounting provisions are included, due the substantial size and weight of the transformer which would make the conventional methods unreliable.

The input and output connections are made via silver coated 0.6mm dia. (22gauge) copper wire. The transformer has the primary winding split into four identical, separately accessible sections. The secondary (output) winding is accessible in one whole section. This allows winding and voltage ratios of 1:5, 1:10 and 1:20. The Common mode rejection ratio exceeds 78db at 10KHz. The resulting step-up ratios input and suggested source impedances as well the maximum level @ 40Hz are detailed in the table below. Please also consult the graph showing the amount of distortion for a given level and step-up ratio.

Below is a table of loading values (in ohms) that a phono cartridge sees when selecting various dummy load and transformer winding ratio combinations.

Note: O/C is open circuit. The cartridge will see the transformer’s load divided by the transformer's ratio squared (i.e. in the 1:10 ratio the cart will see the input impedance of the phono stage (often 47k) divided by 100 (10 squared) giving a load to the cartridge in this example of 470ohms. n>

The full primary (all primary sections in series) has around 16 Ohm DC Winding resistance. The secondary winding DCR is around 130 Ohm. The primary inductive reactance at 20 Hz is in excess of 1.7kOhm (14 H Primary Inductance) and thus provides an input impedance of more than 1.7kOhm across the audio band if the secondary loading is infinite. The table shown above details the Input Impedances for various secondary loads and the various step-up ratios. 47kOhm is the typhical input impedance of a phono equalizer amplifier moving magnet cartridge input.

With all transformers the ultimately realised output bandwidth is very much system and application dependent, but the bandwidth limited transformer when driven from a sufficiently low impedance (see table) will provide a usable bandwidth of at least 10Hz – 100kHz (+/-1db) or better (see graph).

Connection considerations

The transformer has distinctly separate primary and secondary windings allowing full galvanic insulation between the input and output circuit. The fully symmetrical primary and secondary winding structure allows the connection of the primary and secondary in balanced or unbalanced mode. An electrostatic screen is provided and should be connected to ground. Unlike all the connecting wires for the individual windings, the wire for the electrostatic screen is unmarked and of different color.

The connection of the primary sections for the various step-up ratios is illustrated below. The input wiring is moderately insensitive to the pickup of stray electrical fields (hum/noise) and may omit shielding. It is however recommended that the input wires are tightly twisted. While the heavy duty shielding can provides more than 70db rejection of external magnetic fields, such fields should be kept to a minimum by suitable mechanical arrangements.

The output wiring is relatively sensitive to noise pickup and should be shielded. Also, to minimize any pickup of external fields and/or loss of performance due to excessive load capacitance the length of the output wiring should be minimized. Low Capacitance shielded cable should be utilized for external connections.

Normally the input load of the following phono equalizer amplifier should suffice to provide the required secondary load. Should it be desired to reduce the input impedance, for example to control electromechanical resonances in the pickup cartridge, this can be achieved by connecting a suitable value resistor in parallel to the secondary winding of the transformer. If a 13kOhm resistor is paralleled with the transformers secondary winding, this resistance combined with the phono equalizer amplifier input impedance of 47Kohm will give a secondary load of appx. 10Kohm, a 5K6 resistor in parallel to the secondary will give around 5Kohm secondary load. See the table detailing the input impedances for the different step-up ratios for a detailed listing of the various achievable input impedances.

Other applications

As mentioned, other applications for the transformer include the use as super premium grade Microphone input Transformer.

If the transformer is used as microphone input transformer in conjunction with a phantom power supply care should be taken that NO DC offset is presented to the primary sections. Alternatively a suitable value capacitor may be connected in the central junction between the primary sections as shown in the application schematic. With a 22uF capacitor the -3db point is set to 16Hz. For best performance a Rubicon SWX107K 100uF/35V encased Mylar Capacitor is recommended in this position, giving around 3.5Hz as -3db point.

The frequency response for this application is shown below, a range of <10Hz to >90KHz is covered with less than 1db attenuation, making the transformer ideal for application in advanced digital recording systems requiring a wide bandwidth and a wide dynamic range, such as 96KHz sample rate DVD-Audio recordings.

The equivalent Johnson noise from the resistive components of the Transformer equals around 0.8nV Hz input referred. This, for the given 10Hz – 90KHz bandwidth equals 240nV RMS noise, equivalent to -132dbV (-130dbu). With the maximum level at +10dbu this gives a usable dynamic range for the Transformer in excess of 140db. Combined with the wide frequency response the transformer clearly provides state of the art performance for Microphone inputs. The flexible input connection also allows the transformer to be used with 26db step-up in conjunction with ribbon microphones. Here an RMS noisefloor level of 16nV (-156dbV/-154dbu) allows the highest possible dynamic range of 152db.