Nagra PORTABLE ANALOGUE AUDIO TAPE RECORDER Instruction Manual - page 34

used, not having a sufficiently quiet playback chain, it may be desirable to increase the 

input sensitivi ty above 80 dB. On the other hand, the problem is completely different if 

the tape produced should be used without reworking. In this case, it is necessary to 

compress according to the needs even if the noise level of the microphone 

considerably exceeds that of the tape. For these applications, there is a special range 

of high gain amplifiers available (see section 5). 

5.3. INTERFERENCE 

To obtain a good signal-to-noise ratio, it is not sufficient to place the microphone well, it is equally 

important that no interference can be introduced into the system. An explanation showing how this 

interference occurs will be given and also the means to eliminate it. 

ELECTROSTATIC INDUCTION 

The microphone and the cable, which connects it to the Nagra and the plugs, should all be well 

shielded. If these conditions are respected, no electrostatic induction can occur. 

However, certain parts of some microphones are grounded by a simple contact, which is not 

protected against corrosion. Often the plug shielding is similar, and certain cables have only a 

symbolic shielding. 

In these cases any electrostatic field can induce interference voltages into the system, in particular if 

the Nagra is not grounded but is connected to a camera, which is not interference suppressed and 

the entire system is at a voltage above ground. Hence, the ground itself, as seen by a cable, is a 

potential source of interference. In other words, interference is introduced at the slightest defect of 

the shielding. The induced interference can be at an audio frequency or a high frequency, which 

can be detected within the recorder. 

ACTION TO BE TAKEN AGAINST ELECTROSTATIC INDUCTION 

1. 

Good shielding. Above all, check the plugs. 

2. 

Avoid the conditions where the Nagra is floating above ground with an interference 

voltage. One possibility is to use a photoelectric coupler between the camera and the 

Nagra. Obviously, no trouble can arise if quartz crystal synchronization is used. 

3. 

If, for any reason, it is not possible to follow the advice given, it is possible to reduce 

the interference level by: 

a) 

Using shielded input transformers, which attenuate the passage of indirect 

interference.  

b) 

Using symmetrical input (with the middle point grounded, which will 

attenuate the passage of direct interference. 

c) 

In the most hopeless cases (such as with a radio transmitter close by) add 

external filters. 

It should be noted that the microphone preamplifiers of the Nagra 4.2 are already fitted with filters, 

but their action only begins at around 500 kHz, because they are, above all, designed to reduce 

very high frequency interference for which the usual shielding is ineffective.