LT700

The LT700 is an Audio Transformer, originally manufactured by Eagle International in Taiwan. It is typically used on the output stage of an electronic circuit to drive a loudspeaker.

The primary side of this transformer has three terminals due to the centre tapping, and the impedance is typically 1.2 kΩ between each coil, with the centre tap as the common. The secondary side has two terminals, which directly connects to a loudspeaker. The impedance of the secondary coil is typically 3.2 Ω. The standard dimensions of this transformer are 16 mm × 20 mm × 16 mm, and mass approximately 13 g. This transformer usually follows a colour code convention and is usually red in colour.

LT700 Specifications

The two coils on the primary side consist of 420 turns each. Therefore, there are 840 turns in total with a centre tapping. The total inductance is 550 mH, and DC resistance 63 Ω.

On the secondary side, the coil consists of 45 turns. The inductance is 2.25 mH, and the DC resistance is 0.38 Ω.

Purpose for Using

Transformer coupling is a useful method of transferring an AC signal, whilst at the same time blocking the DC offset. It therefore blocks DC and isolates it. The transformer works very well for AC signals in the audio frequency range.

Pinouts

This transformer is available in a PCB mounting version, with short terminals designed to fit into a prototyping board. There is also a chassis-mounting version with long wired leads.

On the older chassis-mounting versions, the red and green wires are for the primary coil. The white wire is the centre tapping, which is often not used. The Black and white wires are on the secondary coil.

Typical Cost

The average online price is one pound for the transformer and one pound for the postage. To avoid minimum order requirements, eBay might be a good source. Currently, this transformer is used in my designs because the price is affordable. However, with the emergence of efficient silicon transistors, capable of driving a speaker load directly, engineers may switch to alternative circuits, if the cost increases further. Let us hope the price remains affordable.

LT44 Transformer

The same company also manufactures a similar looking transformer model LT44. This transformer is typically yellow in colour, and sometimes found in blue. It has two terminals on the primary side and three on the secondary. This transformer will not work in place of the LT400, as it has different electrical specifications.