LL7902 Lundahl Line Übertrager High Level professional Audio

Lundahl LL7902 – flexible high-level line and isolation transformer with mu-metal shielding

The Lundahl LL7902 is a large high-level line and general-purpose audio transformer for professional input, isolation and step-down applications. Four winding sections on each side provide ratios from 1:1 to 1:4 or 4:1.

Two coils, electrostatic screens, a high-permeability mu-metal core and a mu-metal enclosure support low-interference signal transfer. The selected connection determines level capability, copper resistance and turns ratio.

Suitable applications

• High-level line isolation between professional audio devices
• Galvanically isolated 1:1 line inputs
• 1:2 or 1:4 step-up from low-impedance microphone or line sources
• 2:1 or 4:1 step-down at line inputs or line outputs
• Balanced-to-unbalanced conversion with suitable grounding
• Studio, broadcast, measurement, DAC and high-quality DIY audio interfaces

Technical data

Product type

High-level line input, isolation and general-purpose audio transformer

Model

LL7902

Basic winding ratio

1+1+1+1:1+1+1+1

Documented ratios

1:1, 1:2, 1:4, 2:1 and 4:1

Core material

High-permeability mu-metal laminations

Magnetic shielding

Mu-metal enclosure

Electrostatic shielding

Internal screens between primary and secondary windings

DC resistance per primary section

Approx. 28 Ω

DC resistance per secondary section

Approx. 28 Ω

Distortion at +10 dBu

Approx. 0.1% at 50 Hz with the primary sections in series and a 600 Ω source

Distortion at +28 dBu

Approx. 1% at 50 Hz under the documented conditions

Frequency response

10 Hz to 100 kHz within ±0.5 dB under the manufacturer test conditions

Self-resonance

Above 150 kHz

Recommended termination network

5 kΩ in series with 1.3 nF for optimised square-wave behaviour with a 600 Ω source

Primary-to-secondary dielectric test

4 kV AC

Winding-to-screen dielectric test

2 kV AC

DC compatibility

Not documented; use for DC-free audio signals

Dimensions above the PCB

Approx. 66 × 33 × 21 mm

Weight

Approx. 155 g

Pin spacing

5.08 mm with a row spacing of 45.72 mm

Recommended PCB hole diameter

Approx. 1.5 mm

Documented connection alternatives

High-level 1:1 isolation

L4-R4 connection, +28 dBu maximum primary level at less than 1% distortion and 50 Hz

Reduced-copper 1:1 connection

L2-R2 connection, +22 dBu maximum primary level

Low-resistance 1:1 isolation

L1-R1 connection with approximately 14 Ω total transformer copper resistance and +16 dBu maximum level

1:2 microphone or line input

L2-R4 connection, +22 dBu maximum primary level

1:4 microphone or line input

L1-R4 connection, +16 dBu maximum primary level

2:1 step-down

L4-R2 connection, +28 dBu maximum primary level

4:1 step-down

L4-R1 connection, +28 dBu maximum primary level

Selecting ratio and level capability

The complete series connection provides the greatest number of active turns and substantial level capability. Lower-resistance parallel connections reduce copper resistance but also reduce the primary level available before reaching the documented 1% distortion point at 50 Hz.

In step-up service, the secondary load is reflected to the source by the square of the turns ratio. A 1:4 connection therefore loads the source substantially more than 1:1 and requires a sufficiently low source impedance.

Step-down operation reduces output voltage and effective output impedance. The actual load and recommended termination network must suit the selected connection.

Comparison with the LL7901

The LL7901 prioritises extreme signal-level capability and reaches +34 dBu at 50 Hz. The LL7902 instead provides lower copper resistance, self-resonance above 150 kHz and a specified response extending to 100 kHz. Selection therefore depends on signal level, source impedance, load and required winding resistance.

Grounding and installation

Connect the internal screen and mu-metal enclosure to a quiet chassis or screen point as shown in the Lundahl drawing. Do not defeat the required galvanic signal isolation with an additional direct signal-ground path between the connected devices.

Transformer isolation can reduce ground-loop problems but does not guarantee complete freedom from hum. Socket isolation, cable shielding, chassis grounding and distance from mains transformers remain important.

Questions about selection and use

Which line transformer provides high-level 1:1 isolation up to +28 dBu at 50 Hz?

Lundahl LL7902 provides 1:1 high-level isolation in its complete L4-R4 connection, with a documented primary level of +28 dBu at less than 1% distortion and 50 Hz.

Which flexible audio transformer provides 1:2, 1:4, 2:1 and 4:1 in addition to 1:1?

Lundahl LL7902 provides these ratios through its four winding sections. Select the connection from source impedance, load, required level and necessary headroom.

Can the LL7902 be used as a step-down line output transformer?

Yes, Lundahl documents 2:1 and 4:1 step-down connections for line-input or line-output applications. The source, load and termination must suit the selected winding arrangement, and DC must be excluded.

Is the LL7902 suitable for an isolated stereo line path?

Yes, the LL7902 is suitable for stereo line isolation, with one transformer required per channel. Both channels should use identical wiring, termination and physical installation.