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WSRS Technical note - Preamplifier Noise test kit usage

Tone sourceWildlife sound recordists often record at the low volume end of field recording, where recorder gains are high and noise is an issue. Manufacturers rarely specify their equipment noise performances at full gain since the figures look better at low gains, which are often those used during music recording.

We have developed a method whereby sound recordists can get an indication of the noise performance and sensitivity of their recorder mic inputs, using a low cost tone source and termination resistance. This describes how to use these items. They are not designed for field use - the measurement should be taken with all equipment having settled at room temperature (15-25ºC) for at least an hour. Please make sure there are no mobile phones, TV monitors or other equipment switched on within 5m of the device under test. Ideally your field recording gear should be battery powered using fully charged batteries. There should be no other connections made to the recorder, either to line out or the headphone socket (you can line up using headphones but please turn down the headphone gain and remove the headphones from the socket during test)

Equipment supplied

Please check that you have received the following items:

  1. The tone source - a rectangular aluminium box with an XLR connector  and others as shown above
  2. A short green XLR male to female cable.
  3. A short black 3.5mm stereo jack to jack cable
  4. A 9V PP3 battery in a plastic battery case
  5. A short PP3 to DC coaxial power cable
  6. two XLR terminating plugs, one marked 150L and the other 150R

Take the battery out of the plastic case and connect it to the power lead without the lead plugged into the tone source. The reason for doing it this way round is there is a diode in the tone source that will short the power if it is connected the wrong way round. By connecting the snap connector to the battery unconnected to the tone source, you can ensure the battery is connected right before applying power to the tone source. 

Insert the power plug into the coaxial power socket (second from the top on the left in the photo above). The green LED below should light. What you do next depends on whether you are testing equipment that has balanced XLR inputs or an unbalanced 3.5mm minijack. When recording, please set the recorder to 44.1kHz stereo uncompressed 16 bits. For the purposes of this test 24bits is not needed, though such recordings can be processed.

Balanced Inputs

If your recorder has balanced inputs, adjust the recorder to set the inputs to the highest mic sensitivity available. So if the XLR input sensitivity is selectable from Line, Mic Lo-sense, Mic Hi-sens set the inputs to Mic Hi-sens. Set phantom power off (you can repeat  the measurement with it on, as long as that recording is labelled as such, but for comparative purposes we shall use phantom off). 

Select manual gain and set the gain control to mid-range, and connect the tone source to the left-channel input using the supplied short green cable. Connect the supplied power cable to the PP3 rechargeable battery and plug the other end into the tone source. The green LED should light. If your recorder is stereo then connect the supplied 150R terminating plug to the unused input.

The output of the tone source is low, -67dBµV,, and you should be able to bring the microphone gain up to full gain without the tone reaching full scale/0dBFS. This is normal and desirable. If it does reach or go over 0dBFS then you have an unusually sensitive piece of gear and this test cannot proceed further.

Record this tone for about a minute. It is important that the equipment is still and nothing is mechanically disturbed during the test.

Now unplug the power from the tone source and remove it and the short cable from the recorder input. Replace with the 150L terminating plug. Make another 1min recording without changing any of the recorder settings like gain etc.

You are done - please send the two recordings and the checklist back with the equipment and we will process the results.

Unbalanced inputs

Adjust the recorder to set the inputs to the highest sensitivity possible. Connect the mic input to the 3.5mm stereo jack using the lead provided. Connect the supplied power cable to the PP3 rechargeable battery and plug the other end into the tone source. The green LED should light. Select manual gain and set the gain control to mid-range. Monitor the levels and bring the gain up to maximum. 

The output of the tone source is low, -67dBµV,, and you should be able to bring the microphone gain up to full gain without the tone reaching full scale/0dBFS. This is normal and desirable. If it does reach or go over 0dBFS then you have an unusually sensitive piece of gear and this test cannot proceed further.

Record this tone for about a minute. It is important that the equipment is still and nothing is mechanically disturbed during the test.

Now unplug the power from the tone source. Leave it connected to the recorder input - this is important as the unpowered tone source is now providing a 150Ω termination to the input.  Make another 1min recording without changing any of the recorder settings like gain etc.

You are done - please send the two recordings and the checklist back with the equipment and we will process the results.

If you wish you can also check the results yourself using this method.


Richard M

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