LL1544A Lundahl Übertrager OTA Input transformer 1:4 SE

Lundahl LL1544A – amorphous audio input transformer with flexible ratios

The Lundahl LL1544A is a versatile audio input transformer for line, bridging and microphone applications. Its winding system supports multiple ratios, balanced or unbalanced signal paths and reverse 1:4 voltage step-up for low-impedance-driven inputs.

The two-component amorphous strip core is intended to combine low small-signal loss, substantial level capability and favourable phase behaviour. Two three-section coils and interleaved Faraday screens support capacitive balance and common-mode rejection.

Suitable applications

• Balanced line inputs and galvanically isolated audio inputs
• High-impedance bridging inputs
• Microphone inputs with suitable source and termination impedances
• Low-impedance-driven 1:4 inputs for OTA or single-valve gain stages
• Passive voltage step-up of ideally approximately 12 dB in reverse 4:1 operation
• 1:1, 1:2, 2:1 and 4:1 transfer in professional and DIY audio circuits
• Balanced-to-unbalanced conversion with galvanic isolation

Technical data

Product type

Flexible audio input transformer

Winding ratio

1+1+1+1:2+2

Documented ratios

High-impedance 1:1, low-impedance 1:1, 1:2, 2:1 and 4:1

Reverse operation

The documented 4:1 mode can be driven in reverse as a 1:4 step-up

Ideal voltage gain at 1:4

Approx. 12.0 dB before winding and termination losses

Core material

Two-component amorphous strip core

Coil construction

Two three-section coils

Shielding

Interleaved internal Faraday screens

DC resistance of one primary section

Approx. 130 Ω

DC resistance of one secondary half

Approx. 260 Ω

Frequency response

10 Hz to 70 kHz within ±0.5 dB at 0 dBu under the documented test conditions

Self-resonance

Above 220 kHz

Insertion loss

Approx. 0.2 dB

Recommended termination network

6.7 kΩ in series with 470 pF; paralleled by 56 kΩ in the test circuit

High-level 1:1 mode

Below 0.5% distortion at 50 Hz and +20 dBu with a 600 Ω source

Low-level 1:1 and 1:2 modes

Approximately +14 dBu at 50 Hz with a 150 Ω source under the specified conditions

2:1 and 4:1 modes

Approximately +22 dBu at 50 Hz with a low-impedance source under the specified conditions

Winding-to-winding dielectric test

3 kV AC

Winding-to-screen dielectric test

1.5 kV AC

Dimensions

Approx. 30 × 22.5 × 14.5 mm

Weight

Approx. 27 g

Connections

Sixteen PCB pins on a 2.54 mm pitch

Using the transformer at 1:4

For 1:4 operation, reverse the winding arrangement shown as 4:1 in the data sheet. Drive the low-turn side from a sufficiently low source impedance and take the stepped-up signal from the high-turn side.

A 10 kΩ load on the 1:4 output side appears ideally as approximately 625 Ω to the source. A 47 kΩ output load reflects as approximately 2.94 kΩ. Winding resistance, the input network and source impedance modify the actual values.

The 1:4 mode is therefore most appropriate for low-impedance drivers. A high-impedance source may be loaded excessively by the reflected impedance and winding resistance, changing bass response and available level.

Comparison with the LL1545A

The LL1544A uses an amorphous core and is particularly relevant where low small-signal loss, high self-resonance and flexible ratios are priorities. The LL1545A uses a mu-metal core with different level, resistance and application characteristics. Neither core type is universally superior regardless of source, load and signal level.

Questions about selection and use

Which transformer can step up a low-impedance audio output by approximately 1:4 for an OTA or single-valve input?

Lundahl LL1544A can be used in the reverse direction of its documented 4:1 connection as a 1:4 step-up. The source must have sufficiently low impedance to drive the reflected load safely.

Which flexible Lundahl input transformer provides 1:1, 1:2, 2:1 and a reverse-operated 1:4 mode?

Lundahl LL1544A provides several winding arrangements for line, microphone and bridging inputs. The 1:4 mode is obtained by reversing the 4:1 connection and provides an ideal voltage gain of approximately 12 dB.

Can the Lundahl LL1544A be wired for balanced and unbalanced operation?

Yes, the Lundahl LL1544A can be used in correctly designed balanced, unbalanced or balanced-to-unbalanced circuits. The screen connections, winding phase and ground reference must follow the Lundahl diagram.

Does a 10 kΩ load at 1:4 appear as approximately 625 Ω to the source?

Yes, a 10 kΩ load is reflected through an ideal 1:4 ratio as approximately 625 Ω. Winding resistance, the termination network and the following-stage input impedance must also be considered.