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Saturday, April 14, 2018

The New Field Coil Power Supplies


I have built various field coil power supplies in the past. Some in the landscape chassis style, some with tube rectifiers, and one in the tower chassis style. Here are my latest power supplies for field coil speakers built in the portrait chassis.

These are meant to power a pair of speakers with field coil bass and mid/high drivers.

Each unit contains two independent supplies for left and right speaker.

The one on the left is meant to provide the field coil current for drivers like the famous Western Electric 555 or others with similar requirements.

It provides a voltage of about 7V at 1.5A.

A set of switches allows variation around these values in  small discrete steps.

Voltage and current can be monitored through the digital meters on the top.

The bass unit is meant for drivers like the Supravox EXC series.

Also with the same switch arrangement to allow for different settings.

With these the current can be altered over a certain  range to change the parameters of the drivers or to adapt for drivers with different current requirement.

The bass unit uses larger power transformers, hence the slightly taller covers on top.

Here a series of photos showing the adjustment range of the mid/high unit, starting with the lowest setting.

The power supplies are purely passive without regulation and use a LCL filter approach with two chokes for each supply.

The switches on the bottom select between different taps on the secondary side of the transformers. This way the AC voltage into the rectifiers can be altered in 1V steps.

The actual DC output voltage does not follow in 1V steps, since it is a choke input supply and since the resulting voltage also depends on the DC resistance of the field coil.

Typically one would start with the lowest setting and increase step by step until the correct current as specified for the field coil is drawn.

This allows to adjust through a number of discrete steps.

Adjustments can be made without turning the units off.

In addition fine adjustment can be done through the two switches at the top.

These select different primary taps on the power transformers which allows for roughly +/- 5% adjustment.

Next some settings on the bass unit, starting with the minimal voltage.

The maximum setting:

Boosting another 5% through the primary switches:

This would be the typical setting:

The backside showing the connections, primary fuse and separate fuses for the output voltages.

Such units can also be customised for other voltage/current ranges.

Best regards


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