Eagle LT700 LT44 C703 Radio Output Or Inter-stage Tapped Audio Matching Transformer

Audio Matching Transformer, Output and Inter-Stage Types LT44 LT700 250mW

This is one item of new audio output matching or transistor inter-stage matching transformer. These used to have the Eagle type numbers LT700 and LT44. The nominal power rating is 250mW. Four different varieties are available, those being LT700 RED, LT44 BLUE, LT44 YELLOW, and C703 YELLOW. Please note the difference between D.C. resistances measured with a multimeter, and impedances specified or measured at 1 kHz. audio frequency. Full details are provided in the measurements and pin-out sketches shown in the secondary pictures. These are not Hi-Fi transformers, but they do still have their uses in electronic projects. Note that the original paper information sheets, and some data available from current manufacturers confuses the half or full impedances of the center tapped windings. In all of the following information I only ever refer to the whole of any center tapped or combined series winding, and it is correct.

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LT700 RED. This part is currently manufactured by Eagle - Altai and is sometimes sold under part number P631T.

The whole of the primary side has a nominal impedance of 1.2K Ohms and a tapping point in the middle of this. The primary input side was originally designed to be driven by two PNP germanium transistors in a phase split class B output stage. The output side is designed to connect to a nominal loudspeaker impedance of 3.2 Ohms, such as a typical portable radio speaker or low impedance headphones. Using an 8 Ohm speaker will work just fine, and that higher impedance will be reflected back into the input side to a large extent.

The D.C. resistance of the whole primary is approximately 90 Ohms, and that of the secondary is 0.4 Ohms. The approximate turns and voltage ratio is 20:1, and something close to this is seen when testing with the appropriate resistive input and output impedances connected. The nominal impedance ratio is 1.2K to 3.2 Ohms.

This is the part which can be successfully used in the 1971 Ladybird transistor radio book design. These are sometimes driven in reverse to make low-level electrical stimulation generators, or to generate a higher voltage drive for miniature piezo speakers. A copy of the old style paper information sheet is shown in one of the pictures. The measured -3dB frequency response is approximately 100Hz. to 20kHz.

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LT44 BLUE. This part currently is manufactured by Eagle - Altai and is sometimes sold under part number P631M

These were originally used to match a germanium transistor single ended voltage amplifier stage to a phase split 2 X PNP class B output stage in small radios. The primary side is a single untapped winding, and the secondary side has a center tap. The primary side would traditionally form the collector load for the voltage amplifier transistor stage. The D.C. resistance of the primary side is approximately 1.2K Ohms. The D.C. resistance of the whole secondary side is approximately 100 Ohms. The approximate voltage and turns ratio is 3.2, and something close to this is seen when testing at 1kHz with the appropriate resistive input and output impedances connected. The nominal impedance ratio is 20K to 2K Ohms using the whole of the secondary winding. Note that this contradicts a straightforward interpretation of the old paper data sheet. The measured -3dB frequency response is approximately 100Hz. to 20kHz.

These parts are still useful to match crystal radio detectors to higher impedance 1K magnetic or 27nF capacitive piezo earpieces. This allows the detector diode to work at a higher impedance section of the circuit where the higher voltages overcome the forward diode drop more easily, leading to higher efficiency. The transformer also provides a D.C. path to ground when using a piezo earpiece, doing away with the need for a lossy bleed resistor across the piezo element. They are also useful to make traditional transformer based ring modulator or octave frequency doubler cores.

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LT44 YELLOW

This is a similar device to the LT44 BLUE, but it has different colored bobbin tape and a somewhat lower primary D.C. resistance. This will result in lower losses in some applications. The primary resistance is 480 Ohms. The whole secondary D.C. resistance is 104 Ohms.

The measured -3dB frequency response is approximately 100 Hz. to 20 kHz.

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C703 YELLOW

This is a somewhat similar device to the LT700 RED output transformer with some significant differences. The D.C. resistance of the primaries is somewhat lower offering reduced losses in some applications. The pin-out wiring is completely different. The primary side windings are not connected at the center tap, so they must be connected together externally if being used in the usual circuit configuration. The separate primary windings offer the possibility of various electronic phase splitting, combining, and D.C. isolating applications.

The output secondary has a D.C. resistance of 0.4 Ohms and is suitable for driving 8 Ohm loudspeaker loads. The combined series primaries have a D.C. resistance of 246 Ohms. The impedance of the combined series primaries at 1kHz. is 7K Ohms. The nominal impedance ratio is therefore 7K : 8 Ohms using the two primaries connected in series. The two primaries could be used in parallel to achieve 1.75K : 8 Ohms. The voltage and turns ratio is approximately 36:1.

The measured -3dB frequency response is approximately 120 Hz. to 20 kHz. The C703 type does not have an overall steel cover with soldering tabs.

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Other uses of the LT700 type includes the creation of Hartley oscillators driving a good level of output into a loudspeaker using just a single transistor and a 3V supply.

Dimensions, all units:
Height above the board 16mm
Length across the core section 20mm
Width across the plastic bobbin 16mm
Lead Pitch approx: 4mm input side and 8mm output side.

250mW power handling

Miniature size
LT700 red, LT44 blue, LT44 yellow, and separate primaries C703 yellow types available
Traditionally used in small transistor radios
Used in various applications including oscillators, ring modulators, radio kits and voltage generators
Center tapped windings fo phase splitter applications

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