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MiniDisc - Mic Preamplifier Peak Level Measurements

Aim

To establish signal levels into the mic preamp of a Sony MZ NH700 minidisc recorder to achieve 0dBFS, and at what level input stage clipping occurs. The available gain also has a bearing on the usefulness of the device in wildlife sound recording in that there needs to be sufficient gain to lift weak sounds high enough to give a properly modulated recording. Some devices aimed at the musician can lack preamp gain as sound levels are normally much higher in music recording than nature recording.

Minidisc amplifiers can be susceptible to early clipping - knowing the capabilities of the recorder enables the recordist to configure the machine optimally for the particular sounds he is trying to record.

Maximum signal levels are one half of the equation, noise levels are the other part and are described separately

Summary results

The highest gain level available is high - the lowest input level capable of driving the MD to a fully modulated 0dBFS recording is -66dBu. The NH700 MD preamplifiers clip at -31dBu and -11dBu on the Mic-Hi and Mic-Lo sensitivity settings respectively.

All you need to know in the field is, using a MZ-NH700 on the mic inputs, you can record signals without preamp clipping up to a level where you have to reduce the record level control to 11 or below. If that is the case, if you are on Mic Hi sens you should change to Mic Low sens. If you are already on the Mic Low setting, you should change to line in, provided you don't need plug-in-power.

Users of different models of MD should determine the clipping levels for your particular model. Clipping can be easily seen on the waveform and the Rightmark[1] audio analyzer is free and can be used to determine these parameters and more using your sound card as test generator.

Experimental approach

test setup

Fig.1: Test setup - the DC blocking capacitor is 4.7μF 35V + to D.U.T. to prevent excessive loading of the plug-in-power system.

A 1kHz tone was set up on the oscillator, a Farnell LFM-4 Wien-bridge analogue sinewave oscillator. An AVO M2007 rms reading digital multimeter was used to control signal levels. For signals greater than 27dBu the input resistive attenuator was not used, but the capacitor was retained.

Results

Gain Mic Hi Mic Lo Line in max input levels graph
10 clip clip
11 -31 -11
12 -33 -13
13 -35 -15
14 -38 -18 13
15 -40 -20 11
16 -42 -22 9
17 -44 -24 7
18 -46 -26 5
19 -48 -27 3
20 -50 -29 3
21 -51 -30 1
22 -53 -32 -1
23 -54 -33 -3
24 -56 -35 -4
25 -57 -36 -5
26 -59 -38 -7
27 -60 -39 -9
28 -62 -41 -9
29 -64 -42 -11
30 -66 -44 -13

Fig.2: Maximum input levels to the Sony MZ-NH-600 HiMD recorder before the onset of peak clipping on the various ranges and gain settings

The test generator was not capable of generating more than 3.5Vrms/13dBu hence the absence of values for the line in above that level. There is a certain degree of overlap between the ranges, and my gut feeling was that it would be better to use the mic low sensitivity range at a gain of 30, instead of the high sensitivity range at 17, for a -44dBu signal level. This is not actually the case -

spectrum plot of tone and noise floor

Fig.3: -44dBu tone spectrum with the 1kHz tone -
recorded using Mic Sens H at 17

The experiment was repeated with the Sens Low setting and a gain of 30

spectrum plot

Fig.4: -44dBu tone spectrum with the 1kHz tone -
recorded using Mic Sens L at 30

It is better to stay on the high sensitivity range than to switch early to using the lower sensitivity range at a higer recording setting. A further measurement was taken with a zero input to make sure that the noise floor of the generator was not limiting the results.

These results are measured on this particular example of a MD (Sony MZ-NH700) at 1kHz and may vary from model to model and with frequency - it needs to be determined for each design.

Richard M

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  1. http://audio.rightmark.org/

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