Jul 222011
 

This is a user report of the Swiss made Nagra two-track portable tape machine, the model IV-S (sometimes referred to as the model 4 – S, the ‘S’ denoting “stereo”) as used in a domestic audio set-up.

As I don’t have another working tape machine with which to compare it, apart from an elderly Ferrograph 632 that is now no longer in use pending renewal of the idler wheels; this is not a review as such.


(Image: Likecool.com)

Introduction and history

A complete history of the Nagra Company can be found at the company’s own web site [1]. Such was the success of the Model III and later Model IV portable machines that they became the de facto standard in the film industry for location recording of dialogue. The build standard, proven reliability and the quality of recording made on them have become legendary. These qualities also led them being used by broadcasters, such as the BBC, RTF and RAI for field recordings; especially natural history programmes.

First of all here is a description of the Nagra IV-S machine. The Nagra IV-S is a portable stereo 6.35mm (¼ inch) analogue audio tape recorder. It has two 2-mm wide audio tracks and a central track for pilot or time code use, depending on the machine type. The Nagra IV-S can thus be delivered in three different versions, these are: NQS-LSP (without any synchronisation track), NQS-L (pilot), and NQS-TC (Time code). Each version has three speeds: 38, 19 and 9.5 cm/s (15, 7½, 3¾ ips), with NAB or CCIR and NAGRAMASTER (at 38 cm/s) equalization standards.

The two audio channels can be connected to two microphone preamplifiers. These can be switched between dynamic, T (+12V) or P (+48V) condenser with phase reversal, or to two current line inputs. The two level potentiometers can be mechanically ganged so that the two move together which makes for easier adjustment of the recording level.

The Nagra IV-S contains separate recording and playback heads, a built-in monitoring speaker that can be switched between source or monitoring ‘off tape’, provision for connection to an external noise reduction system, a reference generator, a recording level meter (quaintly called a “Modulometer” by Nagra) and an indicator for power supply and tape transport.

Finding a Nagra IV-S for domestic use

None of these machines are now in production, though thousands were made and they regularly appear on used equipment auction sites. When seeking out a good used sample of a Nagra IV, one must be aware that versions made for the US market are obviously adapted for 60Hz mains as well as having the necessary equalisation conforming to NAB standards. The NAB equalisation is not really a problem, unless you want to play back tapes that have been recorded using European CCIR equalisation. Conversion of a machine designed for 60Hz to one that is to be used on a 50Hz mains, requires modification that can only be done by the local Nagra distributor.

When the synchronisation track is not required, the Nagra IV can be fitted with standard two-track heads. These allow more of the tape width to be used; with a resultant 2dB improvement in signal to noise ratio at all speeds.

Sometimes modified Nagra IV-S machines appear on the used market. These are the model IV-J machines, adapted for use as instrumentation recorders and used by institutes to record for subsequent analysis vibration information (and sometimes bird and insect calls). They look attractive: the recording circuits have a frequency response that extends to 50kHz; the recording level potentiometers replaced by precision attenuators, and the microphone input sockets replaced by multi-way Lemo connectors for use with Brüel & Kjær instrumentation microphones – but they are of no real use to audio enthusiasts, because to preserve signal waveform fidelity these machines use no equalisation whatsoever to the detriment of the signal/noise ratio. Avoid!

The best version to track down is the one I’m about to discuss: the version NQS-LSP, without any film synchronisation track and fitted with ‘full width’ (2.75mm) two-track recording and playback heads. In dispensing with the central synchronisation track, more of the tape width can be used, with a subsequent 2dB improvement in signal to noise ratio.

As mentioned these machines are no longer made, though used samples regularly turn up on Internet auction sites. It is unlikely that the operating instruction manual will come with these machines, so this user report details the use of the Nagra IV-S within a domestic audio system. These notes are based on practical experience having used a Nagra IV-S machine for several years. It does not describe the use of the machine as a portable field recorder.

Connecting the Nagra IV-S to a domestic audio system


(Input and output connections of the Nagra IV)

Apart from the microphone inputs, which use XLR (F) connectors, the line-level inputs and outputs are via Tuchel connectors. These are similar to DIN connectors; in fact the size and disposition of the pins are the same in either connector of 7 ways or less. Unlike DIN connectors that usually simply push into the mating receptacle (though some have a bayonet-style “twist to lock” locking ring), Tuchel connectors are secured by a freely rotating threaded outer ring. Nagra electronics follow the convention where the (single-sided) voltage rail is –ve with respect to the chassis and the 0 volt rail. This is where the shielding of the coaxial cables is connected: the centre pin of the 7-way connector. Both input and output sockets are on the left hand side of the machine. The wiring for both the input and output Tuchel connectors on the Nagra recorder are as follows: pin 7 (centre), 0V; pin 1, left-hand (red) channel; pin3, right-hand (green) channel. The wiring convention is marked around each Tuchel socket.

Line output is quite straightforward: for full modulation of the tape it is a useful 1V into any load > 500Ω (for 0dB displayed on the Modulometer) (≡ +2.2dBm). So for example with the Quad 44 preamplifier, set the ‘DIP’ switches to “0dBm”. Nagra have their own cable for this: the QCSS or QCSSC [2], the former terminating in three 4mm ‘banana’ plugs, the latter terminated in two RCA ‘phono’ plugs. This latter is the one most users will need. The line output can also be obtained using the appropriate 4mm sockets on the right-hand side of the machine; though it is unlikely this alternative option will be much used.

The input or recording circuits on the Nagra IV are current driven. This is very important, for if any source is connected to the Nagra without a series current-limiting resistor fitted, permanent damage could occur! The correct lead to use is either the Nagra QCSE terminating in three ‘banana’ plugs (some versions are terminated with XLR connectors) or the QCSSD, terminated with a DIN plug. Both of these leads are fitted with 56kΩ current limiting resistors.


(Nagra QCSE recording lead)

Often one will come across an Internet vendor selling suitable recording leads that are reluctant to state the value of the series resistor. There is no mystery here. For those able to use a soldering iron and who wish to make their own recording cables, then all you need to know is that full modulation (0dB on the Modulometer) is achieved with a current of 8μA. Thus for an output from the preamplifier of U (volts) the value of the current limiting resistor, R is:

R(kΩ) = 125.U(V)

Thus for a preamplifier output of 0.5V (the minimum voltage level as recommended by Nagra), R = 62kΩ, whereas for an output of say 1V, R = 120kΩ, and for 0dBm (≡ 0.775V) the resistor value is 100kΩ. The leads I use have either a 47kΩ current limiting resistor (this is the minimum value permitted) or one of 56kΩ. There is no need to have to pay the ridiculous prices some of these vendors want to charge.

When fed from a Quad 44 preamp, set the ‘DIP’ switches to 0dBm and use a 100kΩ series resistor. If a Quad 33 preamp is used, choose the high output (‘H’) position on the tape board (~ 300mV) and fit a 47kΩ resistor. For other preamplifiers, consult the operating handbook.

Powering the Nagra IV


(Nagra ATN-2 power supply)

Being designed as a portable recorder, the Nagra IV can either be powered by 12 ‘D’ cells or a custom-made power supply (18 – 30V) can be used. It is most likely that this latter arrangement will be used for those owners who intend using the Nagra with a domestic audio system. For this they need either the Nagra ATN-2 or ATN-3 power supply [2], or one of the many (cheaper) alternatives available. These power supplies are connected to the Nagra IV via a 6 pin Tuchel plug to the appropriate socket on the right-hand side of the machine.

Problems may be encountered when using an alternative power supply. Sometimes these supplies will have the 0V line connected to the mains earth pin. Dependent on the amplifier connected to the Nagra machine, severe ‘motor boating’ can occur (a regular beating sound reminiscent of an idling outboard motor). This can be eliminated by disconnecting the mains earth from the power supply; but only do this if you are sure the signal screens are connected to the mains earth within the preamplifier.

Selection between battery power and an external mains power supply is made using the lowest of the switches on the extreme right-hand side of the front panel marked POWER. If using an external power supply the switch toggle is ‘up’ and ‘down’ if batteries are used.

Maximum spool size for use with the Nagra IV

Normally the Nagra IV comes with a hinged top cover, which when closed can only accommodate spool sizes up to 5” diameter.


(Nagra fitted with 5” diameter tape spools)

Some versions are available with the larger lid option: the QSET-2 for 7” diameter spools. Regardless of the lid fitted, when open and hinged back, the maximum spool size of 7” can be used. There is the rare and very expensive 10.5” reel adaptor: the Nagra QGB, however to use this adaptor the lid must be removed. If you want to use 10.5” reels you would be much better off with one of the semi-pro machines such as, say, the Revox PR99 or one of the professional Studer machines.

Speed and equalisation selector

This is located between the two spools and is only practically accessible if 5” spools are used.


(Nagra fitted with 5” spools allowing access to the equalisation selector switch situated between the two spools)

If 7” spools are fitted, then the rims of the supply and take-up spool almost touch and overlap the selector switch. For this reason, if using 7” diameter spools the speed selector must be set before fitting the tape spools.

There are three speeds available: 3 ¾ ips (= 9.5 cps), 7 ½ ips (= 19 cps) and 15 ips (= 38 cps). Practically the highest speed usable in a domestic environment is 7 ½ ips. With ‘Long Play’ tape this corresponds to 1,800’ of tape or a total recording time of 48 minutes. It is not advisable to use ‘double play’ tape (2,400’) as it is of necessity thinner and thus subject to tape stretch and ‘print through’ when stored.

Though not related to equalisation, the replay frequency response can be tailored with the switch situated on the front panel to the lower right of the Modulometer. Despite the obvious MUSIC setting: placing the -3dB low frequency point at about 35Hz, I prefer to use the FLAT setting where the low frequency -3dB point is about 15Hz.

Loading the Nagra with tape

Regardless of spool size (either 5” or 7”), the tape path is cleared by pulling the tape pinch-roller lever fully forward. On all machines this disengages the tape pinch roller from the tape capstan. On later machines this action will also cause the mu-metal head guard on the playback head (and on the synchronising head, if fitted) to drop down.


(Loading the Nagra with tape)

The tape spools are fitted over the three stainless steel pins of the tape hubs. These pins must be correctly aligned with the appropriate notches of the tape spool for proper stability. Once the spools are seated correctly they are secured with the treaded ‘keepers’ or hub locks that screw down on the treaded hub spigots.

These keepers are easily lost and whilst the machine can be used without them, true alignment of the tape with the heads cannot be guaranteed. A useful tip is to drop the two keepers over the handle of the tape pinch-roller lever, until the tape is fully treaded from supply spool around the tape tension rollers across the heads and on to the take-up spool. Then the keepers can be fitted. I, on the other hand, prefer to fit the keepers once the spools are in place, but before the tape is threaded.


(The Nagra IV loaded with tape)

Once the tape is threaded, the tape pinch roller lever is pushed back allowing the tape to be ‘pinched’ between the tape capstan and the pinch roller.


(Front panel of the Nagra IV, showing the Modulometer, recording level potentiometers and other controls)

Setting up recording levels and recording

The ‘Modulometer’ of the Nagra is a two-channel peak-reading meter. Although appearing to be a single channel meter with one needle, the meter fitted to the Nagra is a twin mechanism device with separate needles: coloured red for the left-hand channel and green for the right-hand channel. The recording levels are adjusted with the two potentiometers (also coloured red and green) on the front panel. If the source has identical output paths, each having a similar level, as is likely in a domestic set up, then the two potentiometers may be ‘ganged’ together by moving the clutch, situated on the left (red) potentiometer knob clockwise. When this is done, when you move one potentiometer the other will move in a precise linked manner. Moving the clutch anti-clockwise will decouple the two potentiometers, so each can operate independently.

In a domestic situation, recordings will be made through the line level sockets, and this input is selected by setting the small switch situated to the upper left of the left-hand level potentiometer (coloured red) to LINE. At the same time, in the absence of any external noise reduction processor (such as Dolby or dbx), the lower switch should be set to NORMAL.

Just to the right and between the level and low-frequency filter selector switches, (themselves situated to the right of the Modulometer), is a small screwdriver adjustable switch. This is the Mono-Stereo selector for the recording configuration. For most situations this will be set to STEREO. In the MONO position the two input channels are connected together and fed equally to the two recording channels. In the STEREO position the two inputs are separate and individually feed the two recording channels. There is a third position, marked ST.HS (Stereo High Sensitivity). With this setting the sensitivity of the input channels are increased by +6dB. There should be little need for this increased sensitivity when used with the tape output levels of most preamplifiers. However if you find that to achieve the correct recording level, as described in the next paragraph, you need to advance the recording level potentiometers too high and thereby increase the bias floor hiss, the extra sensitivity will help. In the great majority of situations the STEREO position is sufficient.

The upper scale of the Modulometer corresponds to the recording level, and is displayed when the upper switch, just to the right of the meter is set to LEVEL. Unlike the typical VU meter fitted to most tape machines the Modulometer fitted to a Nagra is a peak level meter that shows the exact peak of the loudest level of the signal. Compared to VU meter readings, -8dB on the Modulometer corresponds to 0dB on a VU meter. It follows that to prevent tape saturation and subsequent distortion, the recording level potentiometers should be adjusted so the needles on the Modulometer peak between –10dB and –6dB; that is around 12 o’ clock on the meter scale, where the needles are vertical. If the correct value of current limiting resistor has been fitted, it will be found that the potentiometers too will be close to the vertical position, around 11 o’ clock.

To establish the correct levels before recording, the function selector control is moved clockwise to the first position from horizontal, marked TEST. Allow a few moments (~ 10 seconds) for the recording circuits to ‘warm up’ and settle down, before adjusting the potentiometers to set the correct recording levels. Once these levels have been established, the recording is made by moving the switch to the next position marked RECORD, or RECORD (NO LIMITER). In either of these positions the tape transport mechanism is activated. Provided you are not recording material with enormous dynamic range, and have correctly set up the recording levels, the preferred RECORD position is NO LIMITER, which bypasses the compressing limiter option.

To stop recording the function selector control is turned anti-clockwise so the bar is in the horizontal position.

The “NAGRAMASTER” 15 ips recording equalisation curve

Owing to the limitation on tape spool size (a maximum of 7” with the lid open), and the consequential limitation in tape length and thus recording time, most domestic users are unlikely to use speeds higher than 7.5 ips. With a maximum tape length of 1800’, that translates to a maximum of 48 minutes. Users who can fit the Nagra QRS large-spool adapter and thus use 10.5” diameter spools will be able to use the higher speed of 15 ips.

There are two equalisations available at the recording speed of 15ips: one with the usual CCIR or NAB equalisation and a second labelled NAGRAMASTER. NAGRAMASTER is a special equalisation, which gives a greatly improved signal-to-noise ratio at 15ips compared with CCIR or NAB standards. With current high-level tape formulations it is said to compare favourably with DAT recording.

Recording at the standard speed of 7.5ips requires substantial treble boost (pre-emphasis) to compensate for losses in the recording process. This treble boost reduces the high frequency headroom. It is possible to record all the way up to “ +4” modulation at the lower frequencies, but up at 10kHz, the boost has hit the tape saturation ceiling, so though the frequency response is flat at low and moderate levels, high treble levels will saturate.

The maximum recording level at 10kHz with today’s tapes is 5 – 10dB lower than the low frequency maximum. This is not a problem with voice or classical music because the peak energy spectrum of these sources, also roll off at higher frequencies. Also at 7.5ips there is no complementary roll-off in the playback, as there is in the RIAA disc equalisation, so the overall phase shift with frequency, called ‘group delay’ is an inherent sonic limitation of 7.5ips recording.

At 15ips, no treble boost is needed. Indeed, with today’s tapes, a small amount of treble roll-off is required. This makes the standard 15ips record equalisation good for recording cymbals or other high treble instruments, which would require turning down to avoid treble saturation at 7.5ips. Not having the pre-emphasis at l5ips also means much less group delay.

The NAGRAMASTER equalisation curve uses the 7.5ips treble boost for recording at 15ips with a complementary treble roll-off during playback to restore the flat frequency response. This leads to a dramatic decrease in the tape noise level, and furthermore, shapes the noise spectrum to a velvety ‘hush’ rather than an intrusive ‘hiss’.

There can however be a disadvantage in using the NAGRAMASTER recording characteristic: that is the possibility of high frequency saturation due to the pre-emphasis. This is no problem if you are used to recording at 7.5ips. The pre-emphasis is the same, so the same recording level regime will suffice.

The greater dynamic range of NAGRAMASTER allows one to under-record a little, providing greater headroom for sudden level changes. With the NAGRAMASTER equalisation quieter recordings can be made: with about a 5dB improvement in signal-to-noise ratio over that of 15ips (NAB). Due to the frequency spectrum of the tape noise, it has a smoother or ‘velvet-like’ quality that is less irritable or intrusive to the recording.

To summarise then, NAGRAMASTER is the best recording equalisation for ¼-inch tape that is available on all Nagra IV-S machines; provided of course you can cope with the high speed and short recording time.

Rewind and Fast Forward

Having made a recording, the tape can be rewound back onto the left-hand spool by first opening up the tape path through pulling the tape pinch roller lever forward, then move the function selector control to TEST.

Next keep your right index finger near, but not touching, the right-hand side of the take-up spool. With your left hand, switch the Rewind/Fast Forward switch from the centre position to the left. This switch is situated at the lower left on the top plate.

When the tape has almost been rewound, click the Rewind/Fast Forward switch back to the centre position and use your right index finger as gentle brake. If this is not done, because the Nagra does not have brakes, if the tape has not left the take up spool, it will spill out, and if it has left the take up spool, the loose end will flay about.

Similarly, the Fast Forward facility is not quite the same as that found on other professional or semi-pro machines. When selected the tape runs at twice the normal speed selected. When in playback mode, switching the Rewind/Fast Forwards switch to the right advances the speed to double normal speed. (The loudspeaker symbol next to the Fast Forward label is a reminder that to allow this facility the machine must be in playback mode.)

This is about the only design feature of the Nagra that is poor, however it must be realised that the Nagra was designed only to record with high resolution, it was not designed for editing; for that task the tapes would be shipped to the editing suite with the reels “tails out”. It also explains why most Nagra IV machines have no form of tape counter, though some later samples were fitted with a rudimentary counter replacing the right-hand tape tension roller (shown below).

Playback

To playback a recording the Tape/Direct switch, the uppermost switch situated at the extreme right-hand side of the front panel, is switched to the left-hand position: TAPE. Then the function selector control is rotated anti-clockwise to the first position: PLAYBACK, powering both the playback electronics and the tape transport. The second position, marked with the loudspeaker symbol, activates the internal speaker. This would not be used when playing back through an audio system.

Should a set of headphones be plugged into the ¼” jack socket, situated at the extreme lower left on the front panel, this will override the line output. There are two other controls nearby that relate to the use of headphones. Details of these controls depend on the age of the machine. Early machines, of which mine is one, had a headphone level control positioned immediately above the jack socket. This control has 5 level (or volume) positions. Between the headphone volume control and the jack socket is a push button that mixes the two output channels together providing a mono signal to the headphones.


(Later model of the Nagra IV-S)

On later models (shown above), the push switch is replaced by a small screwdriver-set volume control, and the earlier 5-position volume control now selects between ‘left-hand channel’ only, ‘right-hand channel’ only, ‘stereo’, ‘mono’ or ‘off’.

With recording level adjusted as described above, with the Modulometer peaking round –8dB, the output level will be ~ 400mV. The input impedance of the preamp used must be > 500Ω. So for example, with a Quad 44 preamp, set the tape replay switches to ‘0dB’ and for the Quad 33, use the ‘M’ setting on the tape replay board.

When the machine is not being used the tape pinch roller lever needs to pushed back, so that the cover can be closed. It must not be pushed fully home but enough so the handle is just within the border of the top plate, leaving about a 0.5mm gap between the tape roller and the capstan. This is most important. If the two are left in static contact for a period of time, a ‘flat’ or indentation will develop on the tape roller with disastrous consequences for tape wow.

Other controls

As can be seen from the front panel there are other controls, however these are only used when making field recordings. Some of these are the internal reference oscillator for level set-up and calibration, the Modulometer meter illumination, and the accommodation of dynamic, T-power and phantom-power microphones.

None of these will be employed in a domestic installation, so have not been discussed in detail.

Sound quality

It almost goes without saying that the Nagra is capable of recording and playback of the highest quality. Ideally it is best to both record on the Nagra and to playback on the same machine, however tapes made on good but more venerable machines such as the Series 6 Ferrographs also sound extremely good.

Especially pleasing is the capture of the human voice: radio plays and poetry recitation sound natural, clear and open. Recordings made of symphonic and other large-scale works have a good sense of dynamics and the frequency range of the machine (30Hz – 20kHz ±2dB at 15 ips) is only limited by that of the speakers.

Notwithstanding the enormous improvement in the performance of cassette machines, it is only when you hear what recording using double the tape width and at least 4x the speed can achieve, do you appreciate the advantage of reel-to-reel machines over that of cassette; despite the convenience of the latter.

Barry Hunt

References

[1] http://www.nagraaudio.com/pro/pages/…ionHistory.php.

[2] Numerous accessories were made for the Nagra IV. Many of these are for field recording, however some of them are useful for machines used in a static domestic audio setup. Some of them have already been mentioned, others are:

QSEF Balanced input preamplifier
The line inputs on the Nagra IV are unbalanced. The QSEF is an active ‘balun’ for use with balanced-line record signal sources.

QSSF Balance output amplifier
Converts the unbalanced line output of the Nagra IV to a balanced-line output.

QSNES Accessory for connecting a noise reduction system to the Nagra IV.

QCSE Line input cable with Tuchel 7-pin male connector at one end and 3, 4mm banana plugs at the other.

QCSS Line output cable with Tuchel 7-pin male connector at one end and 3, 4mm banana plugs at the other.

QCSSC As QCSS but with the 3 banana plugs replaced by two RCA ‘phono’ (Cinch) plugs.

QGB Adapter for 10.5” reels.

QCGB Powering cable for the QGB.

QGBN NAB-type hub adapter.

QSET-2 Large lid for 7” reels.

ATN-2 Mains power supply (also later versions, the ATN-3 and ATN-4).

© Text Copyright and photos 2011 Barry Hunt, except where cited .

NB No part or portion of this article may be reproduced or quoted without written permission.

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