LL1630 5mA Lundahl Übertrager Interstage Output transformer

Lundahl LL1630/5mA – direct plate-fed 7.2:1 line output transformer

The Lundahl LL1630/5mA is a single-ended line output transformer for direct plate-fed valve stages. Its primary can carry the documented plate current, while the divided secondary provides a balanced, galvanically isolated output.

With both sides connected in series, the overall ratio is approximately 7.2:1. The transformer is therefore suitable for level and impedance conversion from a valve stage to professional 600 Ω line inputs.

Suitable applications

• Balanced line outputs from valve preamplifiers
• Direct plate-fed single-ended output stages operating at approximately 5 mA
• Galvanic isolation between a valve stage and professional audio input
• Level and impedance step-down to a 600 Ω output
• XLR line outputs in preamplifiers, valve DAC stages and studio DIY equipment
• Mono or stereo applications using one transformer per channel

Technical data

Product type

Single-ended valve line output transformer

Winding ratio

7.2+7.2:1+1

Overall ratio in series-series connection

Approx. 7.2:1

Rated magnetic operating point

5 mA DC through the series-connected primary at approximately 0.9 T

DC current before core saturation

Approx. 9 mA according to the manufacturer

Thermal current per primary section

Up to approximately 40 mA as a winding limit; not a recommended magnetic operating current

Primary inductance

Approx. 130 H

Primary DC resistance

Approx. 480 Ω per primary half

Secondary DC resistance

Approx. 14 Ω per secondary half

Frequency response

10 Hz to 40 kHz within ±0.5 dB at 0 dBu output, with a 2 kΩ source and 600 Ω load

Maximum output voltage

Approx. 18 V RMS at 30 Hz with the secondaries connected in series

Calculated power at 18 V RMS into 600 Ω

Approx. 0.54 W; not a general continuous-power rating at all frequencies

Calculated reflected 600 Ω load

Ideally approximately 31 kΩ across the complete primary before termination and winding losses

Recommended primary termination

Approximately 10 kΩ across each primary half according to the manufacturer drawing

Winding-to-winding dielectric test

4 kV AC

Winding-to-core dielectric test

2 kV AC

Core

Lundahl audio C-core

Dimensions

Approx. 70 × 61 × 39 mm

Mounting pattern

Approx. 24 × 52 mm with four M3 mounting points

Weight

Approx. 0.5 kg

Direct plate feed

The plate quiescent current can flow directly through the primary. The documented magnetic operating point is 5 mA, while approximately 9 mA is stated as the saturation limit. The thermal current capability of an individual winding section is not a substitute for this magnetic limit.

Include the supply voltage, the DC resistance of both primary halves and the transformer voltage drop in the driver-valve operating point. Suitable stages use relatively low-impedance, current-capable triodes whose plate current and voltage swing fit the LL1630.

Balanced 600 Ω output

Connect the two secondary halves in phase and in series to form a floating balanced output. Apply any centre or screen reference only as required by the complete grounding and interface design.

One side of the secondary can be grounded for an unbalanced following stage. Do not defeat the galvanic isolation with an additional direct signal-ground path between the two devices.

Comparison with the LL1630/PP and interstage use

LL1630/5mA

Direct plate-fed single-ended line output with 130 H and approximately 18 V RMS at 30 Hz

LL1630/PP

Push-pull version with greater primary inductance and greater documented signal capability

Interstage or phase-splitter operation

Not documented in the LL1630/5mA data sheet as a general SE-to-PP application; secondary loading, grid bias and phase function require separate design

Safety notes

The primary terminals can carry high DC plate voltage. Install the open transformer inside a closed, touch-safe enclosure with adequate insulation clearances.

Reference the secondary output correctly for the ground or grid-bias requirements of the following stage. Avoid an uncontrolled floating grid bias or unintended DC voltage at the line output.

Questions about selection and use

Which line output transformer couples a 5 mA valve stage to a balanced 600 ohm input?

Lundahl LL1630/5mA is designed for direct plate feed at approximately 5 mA and provides an approximately 7.2:1 series-series step-down to a balanced 600 Ω line output.

Which Lundahl transformer provides an approximately 7.2:1 step-down from a valve plate to a line output?

Lundahl LL1630/5mA provides an overall ratio of approximately 7.2:1 using its divided primary and secondary windings. A 600 Ω output load reflects ideally as approximately 31 kΩ before additional termination and winding losses.

Can the plate quiescent current flow directly through the LL1630/5mA primary?

Yes, the plate quiescent current can flow directly through the primary. The documented magnetic operating point is 5 mA and the stated saturation limit is approximately 9 mA, so the valve operating point must be designed accordingly.

Is the Lundahl LL1630/5mA suitable for a stereo line output?

Yes, the Lundahl LL1630/5mA is suitable for stereo line outputs, with one transformer required per channel. Both channels should use identical wiring, termination and physical installation.