The Siemens ELA 75-15 was developed as a specialised active filtering system capable of modifying the frequency response of an audio signal over a wide range. Designed for insertion into a recording console, mixing desk or audio amplification system, it provides multiple stages of frequency correction, allowing individual sections of the audio spectrum to be adjusted independently.
Unlike a conventional equaliser with a fixed collection of centre frequencies, the Siemens ELA 75-15 employs a series of cascaded filter modules, each responsible for a particular region of the frequency spectrum. The arrangement provides separate bass and treble shelving correction, together with two band-pass correction sections for shaping selected frequency regions.
An important aspect of the design is the symmetrical relationship between boost and attenuation. The circuitry allows a frequency correction introduced in one direction to be cancelled by applying the equivalent correction in the opposite direction. Siemens specified that the difference between the correction levels of two units was limited to approximately ±1 dB, with frequency accuracy of approximately 1%. This consistency also allowed two Siemens ELA 75-15 equalisers to be configured for stereo operation by applying matching settings to both channels.
Modular Construction and Circuit Design
The Siemens ELA 75-15 employs a modular construction comprising six removable filter modules and a seventh module containing the input and output amplifiers and power supply circuitry. The six filter modules are arranged as a bass corrector, a treble corrector and four band-filter modules operating in pairs to form two independently adjustable band correction sections. Each module incorporates its own printed circuit board, control assemblies and shielding arrangements.
The seventh module houses the input and output amplification stages, transformers, power supply components, fuses, indicator lamps and the overall equaliser switching facilities. This modular approach separates the individual filtering functions while allowing the complete system to operate as a cascaded signal-processing network. Electrical connections between modules are provided through seven connectors mounted within the chassis.
The input and output signal paths employ transformers for balanced operation. The input preamplifier incorporates an emitter-follower stage, while the output amplifier uses a pilot stage, phase inverter and a final push-pull transistor stage. The overall gain can be configured internally for either 0 dB or +15 dB through a solder-strap connection.
Bass and Treble Correction
The outer sections of the Siemens ELA 75-15 provide dedicated low-frequency and high-frequency correction. The bass corrector operates as an active high-pass filtering network, while the treble corrector employs a corresponding low-pass arrangement. Both sections can operate in enhancement, linear or attenuation modes, selected using mechanically interlocked push-buttons. Each section offers five selectable crossover frequencies, with the bass section covering 45 Hz to 710 Hz and the treble section covering 2.8 kHz to 12 kHz.
A rotary control determines the maximum amount of correction, providing stepped adjustment from 0 dB to ±21 dB in 3 dB increments. The filter slope can also be selected between 6 dB/octave and 12 dB/octave using a slide switch. These controls allow broad changes to the low- and high-frequency response, with independent adjustment of the turnover frequency, correction amount and filter slope.
Dual Band Correction Sections
Between the bass and treble shelving sections are four filter modules arranged in pairs to form two independent band correction sections. One section operates over the lower-frequency region, while the other covers higher frequencies. Each band corrector employs independently selectable initial and final frequencies. The two sets of frequency-selection buttons determine the boundaries of the affected frequency band, with the corresponding middle frequency indicated on the front panel. The lower band section provides selectable peak frequencies from 125 Hz to 710 Hz, while the upper band section offers peak frequencies from 1 kHz to 5.6 kHz.
Both sections use a fixed 12 dB/octave filter slope and provide correction levels from 0 dB to ±14 dB in 2 dB increments. Internally, each band correction network combines low-pass and high-pass active filtering stages with Darlington-type transistor repeaters. The selected initial and final frequencies determine the correction bandwidth, while the attenuation and feedback circuitry establishes the amount of correction. The independent selection of the band boundaries allows the user to control the region being affected rather than relying exclusively on predetermined fixed-band equalisation.
Enhancement, Attenuation and Linear Modes
Each of the principal correction sections offers three operating modes, designated E, L and A. In Enhancement mode, the selected filter network is incorporated into the amplifier feedback arrangement to produce frequency-dependent boost. Attenuation mode introduces the correction circuitry into the signal path to reduce the selected frequency region. Linear mode bypasses the correction circuitry and establishes a direct metallic connection between the input and output of the individual section.
A mechanical interlock prevents more than one operating mode from being selected simultaneously. In addition to these individual controls, the Siemens ELA 75-15 incorporates an overall IN-OUT switch, allowing the entire equalisation system to be switched into or out of the signal path. This enables a complete equalisation setting to be prepared and introduced when required.
Audio Performance
The Siemens ELA 75-15 was specified with a frequency response of ±0.6 dB between 30 Hz and 20 kHz when operating in its linear condition. Maximum input and output levels are approximately +18 dB relative to the specified 775 mV reference level, while total harmonic distortion is specified at less than 0.3% between 40 Hz and 20 kHz at a +15 dB output level.
The input impedance is greater than 10 kΩ, with an output impedance of 25 Ω or less. The output is designed to operate into loads of at least 200 Ω. The transformer-coupled input and output stages provide the signal interfacing required for integration into professional audio systems.
Power Supply and Serviceability
The Siemens ELA 75-15 uses a dedicated internal power supply designed for operation from a 220 V AC mains supply at 50 to 60 Hz. The supply employs a mains transformer, full-wave bridge rectification and transistor-based series voltage regulation. Additional circuitry provides current limiting and protection against persistent short-circuit or overload conditions.
The modular construction also facilitates servicing. Individual filter modules can be removed after the front-panel controls and retaining screws have been detached, providing access to their printed circuit boards and associated components. The original technical documentation includes individual circuit diagrams, mounting diagrams, component identification tables and calibration procedures for the amplifier and power supply stages.
Features
- Professional analogue equaliser employing cascaded active filter networks.
- Modular construction with six removable filter modules and a separate amplifier and power supply module.
- Dedicated low-frequency and high-frequency shelving correction sections.
- Two independently adjustable band correction sections.
- Selectable bass crossover frequencies from 45 Hz to 710 Hz.
- Selectable treble crossover frequencies from 2.8 kHz to 12 kHz.
- Independent selection of initial and final frequencies for each band corrector.
- Low-frequency band peak selection from 125 Hz to 710 Hz.
- High-frequency band peak selection from 1 kHz to 5.6 kHz.
- Symmetrical boost and attenuation characteristics.
- Shelving correction adjustable from 0 dB to ±21 dB in 3 dB steps.
- Band correction adjustable from 0 dB to ±14 dB in 2 dB steps.
- Selectable 6 dB/octave or 12 dB/octave shelving filter slopes.
- Fixed 12 dB/octave band-filter slopes.
- Individual Enhancement, Linear and Attenuation modes with mechanically interlocked selection buttons.
- Overall IN-OUT equaliser bypass switch.
- Transformer-balanced input and output amplification stages.
- Internally selectable 0 dB or +15 dB gain.
- Transistor-based amplifier and active filter circuitry.
- Regulated internal power supply with overload and short-circuit protection.
- Suitable for paired operation in stereo systems using matched settings.