Introduction
In this post I’m taking a close look at the Line Magnetic LM-555 compression driver on the Auditorium A23 mid-frequency horn. This driver is Line Magnetic’s modern reproduction of the legendary Western Electric 555. It’s a field-coil unit built for horn loading, intended to cover the midrange through the lower treble—just like the original.
Be sure to check out the 555 mounted on the ES-450 Biradial here.
According to the published specs, the LM-555 is designed for a working range of about 100 Hz to 6 kHz when used with the right horn. It presents a nominal impedance of around 12.8 Ω, and it’s really meant for modest wattages in the single-digit range. In other words, this isn’t a “PA” driver but a classic low-power, high-sensitivity midrange transducer for serious horn systems.
For this evaluation I mounted the LM-555 on the Auditorium A23 horn and ran a series of tests: raw frequency response, off-axis behaviour, distortion at two drive levels, burst decay and CSD. All the plots you’ll see below are from my own measurements.
The field coil is powered by the NT1 power supply by Acoustic Plan. Voltage was set to 7V.
Frequency Response + Impedance
Raw frequency response of the LM-555 on the A23 Auditorium horn at 1m mic distance.
The LM-555 starts to produce meaningful output above roughly 200 Hz where it has a rising response up to 2kHz. Above this region there is a 12dB/octave falling response which remains quite smooth in terms of peaks and dips.
Off-Axis Behaviour
Off-axis response at 0°, 15°, 30°, 45° for the LM-555 on the A23 horn.
It's a little difficult to decipher the response off-axis so I've separated the responses below for each off-axis angle.
15° Off-Axis Only
30° Off-Axis Only
45° Off-Axis Only
Distortion
Harmonic distortion is impressively low especially at elevated SPL. Second order harmonic distortion is dominant over the higher order products, but even then, it is extremely low at only 0.19% for H2 and below 0.05% for H3 and H4.
Harmonic distortion of the LM-555 at 85 dB SPL on the A23 horn.
Harmonic distortion of the LM-555 at 95 dB SPL on the A23 horn.
The IMD tests (500 Hz/3 kHz 1:4 power ratio) show the same pattern: low artefact energy at moderate levels, with more sidebands appearing when pushed hard.
IMD test at 85 dB SPL. ARTA only shows SMPTE Distortion Figures when a 10kHz signal is used. Please ignore the 68% distortion figure in red at the bottom of the graph.
IMD test at 95 dB SPL.
Looking at the burst decay and cumulative spectral decay plots, I can see a few resonant modes in the upper midrange and lower treble (2–5 kHz) that hang on a bit longer than ideal.
Burst decay of the LM-555 on the A23 horn.
Cumulative spectral decay (CSD) of the LM-555 on the A23 horn.
Conclusion
My impression from the test data is that the LM-555 on the Auditorium A23 horn is a very capable classic midrange solution. Within its intended band (≈100 Hz – 6kHz) it delivers the kind of immediacy and presence that made the original Western Electric systems famous.
Strengths I'm seeing in the plots:
-
Clear, dynamic midrange with real presence.
-
Very low distortion even at elevated listening levels.
Trade-offs to keep in mind:
-
Output above 6 kHz rolls off quickly; you’ll want a tweeter or small HF horn to cover the top octave.
-
Off-axis becomes more ragged
For me, though, that’s part of the charm. In the right system, crossed over and augmented properly, the LM-555 offers a taste of the classic horn sound in a modern, available package.