from a reprint in AUDIO RECORD REVIEW

THE REVOX A77
Manufactured by Willi Studer, Switzerland.

Models:
1102 (2T) 1104 (4T) Chassis in wood case, less output amplifiers
1122 (2T) 1124 (4T) Chassis in wood case with10 + 10 watt power amplifiers
1222 (2T) 1224 (4T) Suitcase model with amplifiers and speakers
1302 (2T) 1304 (4T) Chassis in metal case for rack mounting
1322 (2T) 1324 (4T) Chassis in metal case for rack mounting with amplifiers

Manufacturer’s specification:
Push-button controls. Drive system: three motors, with servo controlled capstan motor. Speeds: 7½ and 3¾ i.p.s. +/-2%. Wow and flutter: equal to or less than 0.08% at 7½ i.p.s. and  0.1% at 3¾ i.p.s. Spool max. Capacity: 10½ in. Amplifier: silicon planar transistors; printed circuits. Non-linear (harmonic) distortion: equal to or less than: 2% at 7½ i.p.s. and 3% for 3¾ i.p.s. Equalization: recording NAB. Reproduction: NAB and IEC, by switching. S/N ratio: equal to or better than 54 db. at 7½ i.p.s. and 52 db. at 3¾ i.p.s. Crosstalk: Mono: 60 db, Stereo: 45 db. Bias oscillator frequency: 120 kHz. Inputs: microphone (switched) low and high impedance: Low: 50-600 ohms 0.2 mV. High: 100 kOhms. 2 mV. 5 pin DIN for Radio: 2 mV/33 kOhms. Cinch for auxiliary: 40 mV/1 Megohm. Outputs: Cinch for output max. 2.5 V/Ri 600 ohms. 5-pin DIN for Radio max.: 1.2 V/Ri 2.5 kOhms. Jack: 200-600 ohms. These input/output figures apply to each channel. Remote control by impulse system for all functions, if required. Output wattage (8 ohms): Music Power rating 20 watts (10 watts per channel); r.m.s. rating: 16 watts (8 watts per channel). Output impedances: 4 to16 ohms. Electronically-regulated power supply. Circuitry Includes 54 transistors and 32 diodes. Mains voltages: 110 130, 150, 220, 240, 250 Volts. 50/60 Hz.

It really does not seem long ago since Mr. Colin Hammond first brought me a Revox tape recorder for examination, but l find it was back in 1962, when he had just formed a company to bring these remarkable Swiss machines into Britain. I can readily recall my mounting enthusiasm as the then model E36 passed test after test with flying colours and the realization was born that I had in my hands a machine which was head and shoulders above its competitors. Since then the 36 series has gone from strength to strength following a continuing process of evolution, which has involved one major mechanical re-design, plus a number of detail improvements. It is now properly established as the recorder for those who place quality first: and, a praiseworthy point, through all this progress the price has been held down so that the sheer value for money was itself a challenge.

One of the tasks of a technical journalist, even a part-time one, is to combine the attributes of Sherlock Holmes with those of a bloodhound and in this way to anticipate the arrival of anything of interest to his public. This Sherlock reasoned sometime ago that the 36 could not be developed much further and that, therefore, a complete re-design was probably under way. The bloodhound bit set about uncovering the details and then harassed the long-suffering Colin Hammond (several thousand Revoxes later) until the poor fellow was persuaded to part with his sample of this new model 77 only a few days after its arrival in this country.

ARR readers then are fortunate in being the first to have details of this exciting new machine: even more fortunate are those who find themselves in a position to acquire one. At the time of writing I have had the machine only a few weeks; a second sample was lent to me for several days so that I might check the consistency, for I normally like to use a product for sometime before writing about it; but I have no hesitation about this one.

The new Revox is so advanced an example of brilliant technical thinking and is executed to such superb standards of engineering and electronic skill that any existing competitor who has not got his head stuck firmly in the sand might well feel like giving up. Indeed I might hazard a guess that as the external appearance has obviously been deliberately given a Trans-Atlantic slant, there could be some future wailing and gnashing of teeth in Japan, where so many of the machines on the American market originate – with or without acknowledgement.

All Revox domestic recorders clearly show their close association with the world famous Studer professional machines, which are used by broadcasting and recording companies for everything from Gotterdammerung to The Dales. All are three-motor designs and the high-torque spooling motors have the necessary power and are positioned well out to the corners so that 10½ inch reels can be used. Control of tape motion is by push switches and it is with these that we start on the features of the new 77 where light action push buttons are now used to trigger transistor circuits and interlocked relays. It therefore now becomes possible to perform all functions of recording, playback and fast winding in both directions remotely, by impulse switching and a multipole socket is installed for this purpose. As in the G-36, a photosensitive cell cut-off switches off the motor if the tape breaks.

The most stimulating development concerns the capstan motor. Tape drive is still achieved by the operation of a solenoid, which causes the tape to be sandwiched between a hard rubber capstan roller and a small diameter capstan shaft rotating at motor speed. This speed in past models was synchronous with the electricity supply mains and fluctuations were ironed out by loosely coupling to the motor a massive flywheel requiring a substantial bearing situated in a die cast housing above the motor. All this heavy engineering has gone, and in its stead one can notice an even grooving around the periphery of the outer rotating bell housing of the capstan motor and adjacent to it the pole piece of a small pick-up coil. Electronics have taken over; a few transistors, inductors, resistors and capacitors now tell the motor to revolve at the correct speed for 3¾ or 7½ i.p.s. tape travel. The pick-up coil and the grooving are there to keep a check on it and apply remedial action at the slightest defection: no matter if the tape is at the beginning or end of a reel: no matter if the power supply is 50 Hz or 60 Hz – or even wandering in between! So good is this servo control that it has proved extremely difficult in practice to measure any waver of wow or flutter for quite long intervals. The meter drops below the point at which reliable readings are possible.

New tape heads of large radius (and therefore slow wear) have been designed for this machine and these on the sample I have been testing, although not yet properly lapped in, have enabled me to demonstrate a frequency response which is only a dB, or so down at 20kHz at a tape speed of 3¾ i.p.s. The first time this has ever happened I think, and answers any criticisms made about the heads on earlier Revox machines, with regard to performance and wear.

Extensive use has been made in the construction of girder section die castings; these with the omission of the flywheel etc. have brought about a welcome reduction in weight. The motors apart, a major heavy item is the substantial low field mains transformer. This chassis takes supplies between 110 and 250 volts and has five secondaries to feed motors, relays, early transistor stages (via an electronically regulated power supply) and two power amplifiers. The latter, formed of a heat sink for the output transistor pair and a printed circuit board for the rectifier smoothing and driver components, plug edgewise into sockets and are held by a locking screw. They can thus be added to the “Less Power Amplifier” versions at a later date if required.

For such a complex piece of equipment the degree of accessibility is remarkable. Small plug-in printed panels are used for the recording and playback amplifiers (two of each), input amplifiers, 120 kHz push-pull bias and erase oscillator, and the recording relay circuits. With nothing more than a screwdriver you can be holding any one of these in your hand within a minute of approaching the machine. Similarly above deck, only a couple of screws need to be removed and the rest is held in spring clips.

Controls are logically grouped: at the left the push buttons directing tape motion are placed above the volume and balance knobs; the latter are surrounded concentrically by the Mono/Stereo mode switch and the Monitor and playback characteristic selector switch. Over to the right is a pair of recording level VU meters, each flanked by a red push/push switch selecting that channel for recording (in the mono condition the unused input stages are available for mixing inputs). Below the meters are the channel input gain controls, each surrounded by its input selector switch. Five positions are provided for high or low impedance microphone, radio, auxiliary and channel interchange.

Connections are to a recessed panel at the rear: phono sockets are provided except for radio where a five pin DIN socket is used carrying both the recording and playback feeds. The microphone sockets are repeated as jack sockets on the front panel where there is in addition a jack socket for stereo headphones. All the shafts of the controls pass through a top plate and originate on a glass fibre printed circuit board; in fact the print is extended in gold-layered sections to form the actual switch contacts. Also on the board are the multiple sockets, which accept perpendicularly the printed boards already mentioned. The latter are all locked into place by a screening plate, punched at various places to give screwdriver access to the numerous pre-set adjustments – each one labelled. Inter-unit wiring is neatly disposed and terminates in push-on terminal tags so that any section can be disconnected without recourse to a soldering iron. Needless to say all individual components are high-grade products of close tolerance.

A red indicator lamp and an On/Off switch, having two ‘On’ positions at each speed, one for large and one for small reels, complete the panel controls. This power switch is an ingenious assembly of micro-switches operated by a nylon cam, another fascinating touch; there is also an interlock disconnecting the supply when the case is removed. A four-digit tape position indicator, with cancel button, is fitted between the reels; the latter have spring retainers for the machine can be used in any position – upside down if you wish it!

At first one cannot see how to thread the tape, but a little investigation shows that the section above the controls folds away out of sight exposing two more push/push switches, for loudspeaker cut-out (on versions with amplifiers) and reel motor On/Of (so that threading can be done with the electrically operated brakes relaxed. Tape threading is now absolutely unimpeded and one can see that a tape jointing groove has been provided in the tape head cover; the latter slips off for easy head cleaning.

The layout and styling of the control panel have been chosen with forethought and care for the format is to be repeated in a matching 2×50 watt amplifier and a Stereo FM tuner to come. These may well be subjects for a future report.

There is so much more that should be told about this exemplary machine, but I must leave room for the test figures, which are so very good. However, it must be put on record that the model A77 handles like a thoroughbred (which it is) and used in conjunction with an extremely good amplifier and a pair of good loudspeakers it was able to fool the best ears. Even at 3¾ i.p.s., given a good tape and a little care in setting up the levels, one can operate the tape monitoring switch several times in rapid succession and then challenge any listener to tell one whether the switch was left in the ‘TAPE’ or ‘DIRECT’ condition – a most severe test.

Now for test figures.

Speed: at 3¾ i.p.s, and 7½ i.p.s. was exactly ‘spot-on’.

Wow and flutter performance was below that stated in specification for long periods, but rose to the limit for a few seconds at random intervals. This was more noticeable at 3¾ i.p.s. and was seen to consist of two components: flutter at capstan speed and wow at capstan idler speed. The second machine tested did not exhibit the latter variation, but in any case we are considering very small degrees of change, much below those existing in the majority of tape recorders. We found that nothing was distorted aurally on piano recording.

Frequency response: from standard test tapes the playback response was within +/-1 db. over the range covered (40-15,000 Hz.). The record and replay response as received showed a rise of some 7 db in the treble when using Agfa PE.31 tape (previous Revox machines were set up for this tape). It was ascertained that this early machine was probably set up on a works’ test tape and that the bias will be adjusted for a specified tape by the British distributor. Accordingly the bias was adjusted on both tracks and both speeds to suit PE.31.

This then yielded a most excellent result: if one reads the measuring meter to the nearest 0.5 db., which is usual with tape recorders, then we found the response at 3¾ i.p.s. was typically as follows: – 2 db. at 20 Hz.; – 1 5 db. at 30 Hz.; – 0 5 db. at 40 Hz., then level from 50 Hz. to 17 kHz., falling to – 1 db. At 18 kHz.; – 3 db. At 19 kHz., – 6 db. at 20 kHz. At 7½ i.p.s., the level range is extended to 20 kHz. and the 6 db. point occurs as high as 26 kHz. In addition, a small trough of less than 1 db. occurs in the range 3 to 5 kHz.

Signal-to-noise ratio: from Ampex standard test tape, 53 db. on both channels; but from 1kHz. recordings made to the stated distortion levels of 3% at 3¾ i.p.s. and 2% at 7½ i.p.s. (which required the VU meter to be down off the scale in both cases!), over 60 db. was possible.

Crosstalk: between stereo channels, 42 db. under worst conditions. 56 db. from unwanted track in the mono mode.

Power Output: at clipping level was found to be 10 watts per channel, continuous r.m.s. between 20 Hz. and 10 kHz., falling to 8 watts at 20 kHz. Distortion at the onset of clipping: 0.2%, between 40 and 10 kHz. All figures measured at 8ohms resistive load. Stability was good under reactive load conditions and the square wave response was excellent.

SUMMARY:
No digest of the performance of the new A77 series Revox can do justice to the sonic results and I have to ask the tape enthusiast to study this review in detail, if you wish, comparing it with the test report of the earlier G36 Revox (March, 1967 ARR by Peter Cox and Donald Aldous). As you will soon appreciate, both machines are outstanding. The 77 series offers such features in addition, to standard Revox facilities, as: electronically controlled capstan motor; functions controlled by inter-locked relays and transistor circuitry; DC braking; Plug-in transistor panels; new type metal heads; no mods. required to work on 60 Hz mains; DIN phono and standard jack inputs; special editing feature; four-digit counter; and fast start time.

Herr Willi Studer, making one of his too rare visits to this country and knowing that I had this early model on test, found time to call on me and discuss my findings. With the thoroughness typical of his nation, he inspected the stripped out chassis on which I had been doing some tests, bent his head to listen to the faint noises of the motor and pressed me for details of any faults I might have discovered. I mentioned a few extremely trivial things and immediately regretted my remarks, for I felt I had spoiled his day! And yet at the end of my rigorous tests the only real complaint I have is the whine produced by the gears of the tape counter when fast winding. Either I must be slipping or Herr Studer and his team have truly produced a tape recorder landmark.
GEOFFREY HORN