*Serviced* (c.1965) Dolby Laboratories London | Audio Noise Reduction System A301 | S / N Stretcher Unit | SER No 453 | Made in England

Ray Dolby’s first product, the model A301, used a circuit design that reduced tape noise by compressing and expanding audio signals. Recording on magnetic tape introduced unwanted noise or “hiss” to the sound; the noise only accumulated as the original production tapes were duplicated, then played. In 1965 Ray Dolby invented electronic circuitry that removed unwanted noise by processing the audio signal. Recording studios quickly adopted Dolby’s system, and makers of cassette tape players soon followed.

Source: National Museum of American History

Condition:

Serviced and fitted with a 2m IEC C13 power cord for ease. This can easily be removed and fitted with an original Belden cable (if you can find one). VU meters function, signal tests passed for both recording and play modes. Electrical safety test passed.

Product Information from Back-in-the-Day

The A301 Audio Noise Reduction System has been designed to reduce noises commonly encountered in audio signal recording and transmission systems. These noises may take the form of rumble, hum, crosstalk, clicks, pops, buzzing, and hiss, as well as discrete frequency interference such as television synchronising pulse crosstalk, All of these noises are reduced by the A301 without affecting the overall frequency response or dynamics of the signal itself.

In the particular field of magnetic tape recording, the system will reduce tape hiss and also alleviate other problems such as print-through and high-frequency modulation noise. The print-through reduction is of special significance, as it allows the long-term storage of high quality master tapes with minimal degradation.

The system is suitable for use in any situation in which the signal is available for processing at both ends of the recording or transmission chain. The processing operations can be separated by any distance or any time duration, since once correctly adjusted, the A301 parameters are extremely accurate and stable. Furthermore, the system is tolerant of gain-errors in the recording or transmission channel. An incorrect level to the playback unit of ± 3 dB does not result in any perceptible alteration of the restored signal.

It should be appreciated that when recording or transmission noise is reduced, other noises masked by it naturally become more apparent. Full use of the increased dynamic range provided by the noise reduction system may therefore sometimes require a tightening of standards in the rest of the chain - i.e. in connection with noise from microphone amplifiers, mixers, and monitor amplifiers, as well as noise from wholly acoustic sources such as buzzing fluorescent lights, creaking chairs, and the movement of persons in the studio. (On the other hand, it can be argued that acoustic noises, having purely natural origins, contribute to a feeling of realism and immediacy.)

The A301 unit is fully self-contained and is normally supplied for mounting in a 19'' rack.

Specifications

• Layout: Two independent signal processors per A301 unit; each may be switched into either record mode or playback mode.
• Signal connections: One XLR input and output for each processor (Tuchel connectors optional).
• Inputs: 10k ohm balanced bridging transformers, accepting all normal line levels. Provision made for addition of termination resistors, if required, to match line impedance.
• Outputs: Balanced - levels and impedances to order. Normal options are:
• 0 VU (+4dBm), 600 ohms (U. S, standard). 1.55v (+6dB), 30 ohms (Continental standard), +8dBm peak, 600 ohms (U. K, broadcasting standard).
• Output termination switches provided on 600 ohm units.
• Overall (record-playback) frequency response: Better than ‡ 1dB, 20Hz to 20kHz.
• Overall total harmonic distortion: At + 8dBm (600 ohms),less than 0.1% at 1kHz; less than 0, 2% from 40Hz to 20kHz.
• Output clipping point: Better than + 18dBm (600 ohms); better than + 16dB (30 ohms).
• Noise reduction: 10dB from 20Hz to 5kHz, rising to 15dB at 15kHz; with noise reduction switched off, system becomes unity-gain line amplifier.
• Overall noise level: Better than 80dB (unweighted) below + 8dBm (600 ohms) or +6dB (30 ohms).
• Crosstalk: Better than 80dB, processor to processor, 20Hz to 20kHz.
• Matching between units: Better than 1dB at any level and any frequency.
• Stability: System is highly stable - does not require routine alignment.
• Operating temperature: Up to 450 C.
• Operating controls: None.
• Panel meters: NAB and DIN level setting meters for recorder gain calibration.
• Construction: Modular, eight plug-in circuit modules together with plug-in chassis module for power supply and audio transformers. Fibreglass printed circuits, solid state devices throughout (103 silicon transistors including stabilized power supply).
• Finish: Anodized aluminum case, front panels dark grey with white characters.
• Size: 8 3/4"x 19"' rack mounting, 10 5/8" deep (225 x 483 x 270 mm).
• Weight: 28 lbs, (13 kgs).
• Power requirements: 100-125 v and 200-250v, voltage selector, 50-60 Hz single phase, 18 VA.

Source: Dolby A301 Operating Manual

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