Operating Instructions for the REVOX A77
equipped with DOLBY B Noise Reduction System

A77 MK III Dolby

A77 MK IV Dolby

Compared with the Standard version, the A77 tape recorder with DOLBY B electronics features a few additional operating controls. The following instructions are with reference to the additional operations required when utilizing the DOLBY noise reduction system. (Dolby is a registrated trade mark)

Before switching your recorder on, please read carefully the instructions ( brief operating instructions ) for the standard model, while also paying attention to the designations used for the operating controls as shown in the illustrations an the following pages. All other index numbers make reference to the operating instructions for the standard version except for the following changes:

Standard Recorder

DOLBY Version





Monitor selector (14):


- Position NAB

corresponds to TAPE

- Position IEC

replaced by Calibrate ( new function )


Index Numbers
The index numbers 50 to 53 are used for identifying the additional operating controls an the DOLBY equipped REVOX A77 tape recorder.


Switch to activate or to deactivate the DOLBY noise reduction circuits.



51 Record Calibration for Channel I

52 Record Calibration for Channel II


Switch for turning the multiplex filter an or off.

1. Operation without DOLBY- Electronics
When it is desired to record or to playback without the DOLBY Noise Reduction System, place the DOLBY switch 50 into position OUT. The recorder will then function like any standard REVOX A77 tape recorder.

2. Recording DOLBY Encoded tapes
It is important that the following conditions are being met in order to obtain the full benefit of the DOLBY B noise reduction process.

Signal to Noise Ratio of the Sound Source
The DOLBY System reduces only those noise components which originate between the record and playback process; in other words, it affects tape hiss only. Noise which is already present at the input to the recorder cannot be eliminated. It is therefore, necessary that the original sound event provides an exceptionally wide dynamic range (signal to noise ratio of at least 65 to 70 dB).

Equal Level Requirement for Record and Playback
The DOLBY noise reduction System operates with a dynamic filter whose characteristics are determined by the constantly varying sound levels. To ensure a playback performance which is an exact mirror image of the record process, it is absolutely essential that the record and playback levels are equal. This is achieved by a special gain calibrating circuit ( record calibration ) which enables the Operator to compensate for differences in sensitivity between different tape oxides ( or even between batch differences of the same oxide).

3. Record Calibration (prior to recording)
The following calibration procedure is to be performed prior to each recording.

1. Switch the recorder on by selecting tape speed and reel size. Load recorder with tape.
2. Put DOLBY electronics into circuit by flicking switch 50 into position IN.
3. Close record level controls 15 and 17 by turning them fully anticlockwise.
4. Press channel selector buttons 32 and/or 35.
5. Turn monitor selector 14 to Position CAL.
6. Start machine in the record mode.
7. The VU meters 33 and 34 will deflect to an undefined reading. Now adjust the CALIBRATION CONTROLS 51 and 52 until a reading of exactly 0 VU (100 %) is obtained an the channel which is to be used.
Control 51 = Adjustment for Channel I VU meter 33
Control 52 = Adjustment for Channel II VU meter 34
8. With the calibration procedure completed, turn monitor selector 14 to Position TAPE (off tape monitoring).

The calibration controls 51 and 52 must not be touched alter they have been adjusted to suit a particular tape's sensitivity.
The machine is now ready to produce DOLBY encoded recordings.

4. Recording
Check calibration prior to each recording. If necessary, recalibrate by performing the steps as outlined under section 3. Record calibration.
Attention! While recording, do not turn switch 14 to Position CAL, as this would cause the test-signal to become recorded on tape.

Record Level
DOLBY encoded recordings are to be modulated in the same manner as any standard recording by adjusting the record levels with the level controls 15 and 17.

Special Effects
All special effects such as echo, multiplay etc. are possible on the DOLBY version just as an any other standard A77 recorder, because each record and playback channel is equipped with a completely independent DOLBY processor.
A DOLBY equipped recorder is especially suitable for multi playback recordings since the reduced tape hiss will permit an increased number of transfers without any deterioration in quality ( your attention is drawn to the signal to noise ratio of the sound source).

Multiplex Filter
When recording stereo broadcasts from a FM tuner, place switch 53 filter in position IN.
This connects a multiplex filter into each record channel to protect the DOLBY electronics from interferences which would otherwise be caused by pilot frequency and sub-carrier residuals.
The multiplex Filters are also effective when not working with the DOLBY noise reduction circuits. In that case they protect the recording from possible interference sounds which may occur with the bias frequency.

5. Playback
1. To playback a DOLBY encoded tape, place the DOLBY switch 50 into position IN.
2. Turn monitor selector 14 to Position TAPE.
3. The switch filter 53 and the calibration controls 51 and 52 are of no significance for the playback process.

Normal playback of a DOLBY encoded Tape
If the switch DOLBY 50 accidentally remains in position OUT while playing a DOLBY encoded recording, the treble sounds of low level passages will become overemphasized and the natural balance will be disturbed.

Playback of a Normal Recording through the DOLBY Playback-Processor
When playing back a normal recording with the DOLBY switch 50 in Position IN, the quality of the reproduced sound will become dull during low level passages; mid-high frequencies are being attenuated.
Normal recordings with a somewhat high hiss level may benefit from the dynamic filter action.

The A77 Dolby has been produced as A77-version MK III and version MK IV. To identify a A77 Dolby is very easy because of 4 remarkable points :

  • two additional little switches
  • the two calibration controls
  • the Dolby-signet on the front

(Photographes taken from a A77 MK III Dolby)

Revox A77 with DOLBY Noise Reduction System, Additional Data
All Measurements taken with Revox 601 tape, DOLBY electronics switched on.
2-Track Version (values for the 4-Track Version are shown in parenthesis)

Signal To Noise Ratio, off tape, weighted as per CCIT:
At 7 1/2 ips. better than 67 dB (65 dB)
At 3 3/4 ips. better than 64 dB (62 dB)

Weighted as per ASA A:
At 7 1/2 ips. better than 70 dB (67 dB)
At 3 3/4 ips. better than 67 dB (65 dB)

Distortion measured off tape, 500 Hz
Peak Level (assumed to be + 6 VU)

At 7 1/2 ips. better than 1% (1,5%)
At 3 3/4 ips. better than 1,5% (2%)

Operating Level 0 dB (100%)
At 7 1/2 ips. better than 0,5% (0,5%)
At 3 3/4 ips. better than 0,5% (1%)

All other performance data are identical with the data for the standard version of the A77 recorder.

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