Altec M-500 ([1955])

Name: Altec M-500
Brand: Altec

The Altec Lansing M-500 is a historically significant piece of professional audio test equipment, introduced in the mid-1950s as a high-precision audio oscillator. Designed for use in broadcast studios, recording facilities, and audio engineering laboratories, the M-500 was engineered to deliver stable, low-distortion sine wave signals across the audible frequency spectrum. It played a critical role in the alignment, calibration, and maintenance of audio equipment during the analog era, particularly in tape recorder setup, equalization verification, and system frequency response analysis.

Manufactured by Altec Lansing, a company renowned for its contributions to professional audio, cinema sound systems, and high-fidelity equipment, the M-500 reflects the company’s commitment to precision and reliability. Altec Lansing, formed in 1941 from the merger of Altec Service Corporation and Lansing Manufacturing Company, quickly became a leader in audio technology. The M-500 was part of their broader line of test and measurement instruments aimed at audio professionals who demanded accuracy and repeatability in their work.

Design and Build Quality

The Altec Lansing M-500 is housed in a rugged, all-metal chassis with a black crackle-finish paint, typical of professional gear from the 1950s. The front panel features a large, clearly marked rotary frequency control, a coarse/fine tuning knob system, and a high-contrast dial with frequency graduations. The instrument includes output level controls, multiple output jacks, and a built-in vacuum tube voltmeter (VTVM) for monitoring signal levels. The meter, often labeled as a "zero-center" type, allowed users to balance levels during alignment procedures, particularly when working with stereo or multi-track tape machines.

Internally, the M-500 employs vacuum tube circuitry, utilizing high-quality components such as precision resistors, stable capacitors, and hand-wired point-to-point construction. The oscillator circuit is based on a Wien bridge design, known for its low harmonic distortion and excellent frequency stability. Typical total harmonic distortion (THD) is reported to be less than 0.1%, an impressive figure for its time. The unit operates on standard line voltage (115V AC, 60Hz) and includes a power switch and fuse protection on the rear panel.

Technical Specifications

Frequency Range: 20 Hz to 20 kHz, continuously variable
Frequency Accuracy: ±1% across the range
Distortion: Less than 0.1% THD
Output Levels: Adjustable from 0.1V to 10V RMS
Output Impedance: 600 ohms, balanced
Outputs: Front panel binding posts and phone jacks for high and low-level signals
Tubes Used: Typically includes 6SN7, 6SL7, and 6X4 or equivalent dual-diode rectifier
Power Requirements: 115V AC, 60Hz, ~50 watts
Dimensions: Approximately 14" W × 7" H × 10" D
Weight: Around 25 lbs (11.3 kg)

The M-500 features both coarse and fine frequency controls, allowing engineers to dial in exact frequencies for alignment tones—commonly 100 Hz, 400 Hz, 1 kHz, 10 kHz—for NAB (National Association of Broadcasters) or IEC (International Electrotechnical Commission) calibration standards. The inclusion of a calibrated output attenuator enables precise signal level setting, essential when driving tape recorder inputs or feeding test signals into mixing consoles.

Applications in Audio Engineering

The primary use of the Altec Lansing M-500 was in the alignment of analog tape recorders. Before a tape machine could be used for recording or playback, its electronics and heads needed to be calibrated using reference tones. The M-500 would generate these tones at known frequencies and levels, which were then recorded onto tape. During playback, the machine’s response was measured and adjusted to ensure flat frequency response, proper azimuth, and correct equalization settings.

Broadcast engineers relied on the M-500 for system checks, line balancing, and distortion measurements. Its low-noise output made it ideal for testing microphone preamps, filters, and equalizers. In recording studios, the oscillator was used to verify the performance of monitoring systems and to troubleshoot audio paths.

The M-500 was often used in conjunction with other test equipment such as the Altec 214A Vacuum Tube Voltmeter, the 1567A Distortion Analyzer, or the 1558A Level Oscillator. Together, these instruments formed a complete test bench for professional audio maintenance.

Legacy and Collectibility

While largely obsolete in the digital age, the Altec Lansing M-500 remains a sought-after item among vintage audio enthusiasts, restorers, and museum curators. Its robust construction, accurate performance, and historical significance make it a prized addition to any collection of classic test gear. Many units still function after more than 60 years, a testament to their build quality.

Restoration of the M-500 typically involves replacing aging capacitors (especially paper and electrolytic types), cleaning potentiometers and switches, and verifying tube performance. Due to the high voltages present in tube equipment, servicing should only be performed by qualified technicians.

Today, the M-500 is appreciated not only for its utility but also as a symbol of the golden age of analog audio. It represents a time when audio engineering was a hands-on craft, requiring deep technical knowledge and precision tools. In educational settings, restored units are sometimes used to teach students about analog signal generation and measurement principles.

Conclusion

The Altec Lansing M-500 stands as a milestone in the history of professional audio test equipment. More than just a signal generator, it was an essential tool that ensured the fidelity and consistency of audio recordings and broadcasts during a transformative period in sound technology. Its design, accuracy, and durability reflect the high standards of mid-century American engineering.

Though modern digital audio workstations and USB-connected test gear have rendered the M-500 functionally obsolete, its legacy endures. For historians, engineers, and collectors, the M-500 is not merely a relic—it is a working artifact of a bygone era, embodying the precision and craftsmanship that defined professional audio in the analog age. Whether on display or in active use, the Altec Lansing M-500 continues to command respect as a cornerstone of audio measurement history.