Microphone Preamplifier Measurements

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PREAMPLIFIER CHARACTERISTICS
The preamp was measured in 1/3 octave steps from 2.0Hz to 500kHz under both the high gain and low gain switch settings.  The input waveform was supplied by a Hewlett Packard #33120A 15MHz function generator, and the input and output waveforms monitored simultaneously on an HP #54600A 100MHz oscilloscope.  These tests were conducted on the upgraded version of the mic preamp (LM6134 op-amp and LM4040 voltage reference).  Below are the measured amplitude and phase response plots of the preamplifier.  Please note that these measurements apply only to the electronics and not the microphone/preamp combination, which of course will vary across different microphone models and unit-to-unit tolerances.


Figure 1.  Amplitude and phase of mic preamp; high gain position


Figure 2.  Amplitude and phase of mic preamp; low gain position
 

Table 1.  Amplitude and phase at select points

 
-3dB
 lower 
 -3dB 
upper
-1dB
  lower 
-1dB
upper
 Phase 
20 Hz
Phase
20 kHz
High Gain
2.8 Hz
250 kHz
5.2 Hz
131 kHz
0.0o
-4.0o
Low Gain
2.7 Hz
260 kHz
4.9 Hz
105 kHz
2.0o
-2.0o

For the purposes of comparison, here's how the total current draw stacks up between the LM6134 and TL074 versions of the preamp:

LM6134:  IT = 3.7mA   TL074:  IT = 6.7mA



MICROPHONE CHARACTERISTICS

Frequency Response
Following are the calibration results of ten Panasonic WM-61A microphone capsules.  The data was obtained for samples from a single order, so it is assumed the mics are from a common manufacturing run.  Indeed, the frequency response curves indicate a similar "trend".  On average, the mics maintain a remarkably flat response which deviates less than 0.5dB up to 6kHz, transitioning to a gradual rise peaking at +1.9dB at 12.8kHz, and finally settling back to the 0dB reference slightly above 20kHz.  It is likely that those taken from a another manufacturing batch will vary somewhat.  Therefore, these curves are intended only as an indication of the typical frequency response of this particular microphone element, and as such, should not be used to derive the frequency response of an arbitrary mic of the same type.


Figure 3.  Frequency response of ten microphone samples



Averaging the response of the ten samples we obtain the following curve:


Figure 4.  Mean frequency response of ten microphone samples

Clearly, the Panasonic capsule delivers a respectably linear frequency response for a mic of any price, and appears even more remarkable when one considers its price. It is important to note that, for the purposes of acoustic testing, a known frequency response is the most important factor.  Flat response, while perhaps desirable, is by no means necessary.  By applying a correction curve to the measured response, the resultant data is the same as if the device under test (DUT) had been measured with a microphone of perfectly flat response characteristics.

Actually, this contention is only true up to a point.  If the microphone exhibits severe aberrations, it is possible that the data will suffer from time-domain errors.  Barring a defective sample, there would appear to be no such concerns with this particular microphone.  It may even represent a wise choice for a recording microphone.  Musicians often prefer a microphone with certain tonal characteristics--typically referred to in general terms as "aggressive", "laid-back", "open", etc.  In this case, one might expect a rather neutral tonal balance and "sparkling" top end.  OK, so I'm not so good at these subjective characterizations. . .sue me.  A battery powered preamp and microphone combo has its advantages though.  Portability and low noise immediately come to mind.  Remember, however, that battery drain increases when driving long cables.  If you use this combination in a recording setup, please email me with your impressions.

Back to the objective analysis:  comparing the response of each mic with its brethren, we can determine the maximum deviation at any frequency.  This comparison is an indication of manufacturing tolerances--i.e. how well matched are devices taken from a particular manufacturing run.  Here we see that the frequency response within our test group tracks exceedingly well (within 0.5dB) up to at least 8kHz, whereafter there is a divergence in response of the top octave.  The maximum difference exhibited was approximately 3dB at 21kHz.


Figure 5.  Maximum deviation in frequency response of ten microphone samples



Phase Response
Phase response plots are derived from the Hilbert transform of the amplitude response, and are provided below.  Here, however, two assembled mics (in the fashion described here) have been overlaid with the ten "raw" calibrated capsules.  As you can see, the phase response of the assembled mics tracks very closely with that of the raw mics until around 15kHz.  This behavior can be attributed to the slightly larger diameter of the assembled mic's "baffle" formed by the brass wand (9/32" vs. 15/64").  Incidentally, there is no appreciable difference in the amplitude response between the assembled and unassembled microphones.


Figure 6.  Phase response of ten calibrated capsules and two assembled microphones



Sensitivity
With regard to conducting absolute SPL measurements, we use a sensitivity specification which relates the capsule's output voltage to a particular reference SPL.  The most common reference is 1 Pascal of sound pressure which corresponds to 94dB SPL.  We know that the preamplifier exhibits a gain of 19.5 absolute or 25.8dB in its high gain setting, and 1.91 absolute or 5.66dB in its low gain setting.  Given the microphone's sensitivity and the voltage gain of the preamplifier, obtaining absolute SPL measurements is a simple matter involving only rudimentary algebra.  How either you or your software chooses to process this data will vary, so it is presented in both absolute form (volts) and dB relative to 1V per Pascal.  Here are the cumulative results of our test batch:

Table 2.  Mic sensitivity results

 
Average
Max / Min
Units
Output voltage @ 94dB
25.60
28.69 / 22.17
mV / Pa
dB referenced to 1V/Pa
-31.9
-30.8 / -33.1
dB

Calibrated microphones are available for inclusion with the preamplifier kits.  Please see the order form for details.  The service includes a Panasonic microphone capsule with flexible leads, calibration file on 3.5" diskette (including the mic sensitivity specification @ 1kHz), and a graphical printout of the frequency response.  Alternatively, if you have already constructed a microphone wand, you may send it in for calibration to:

Kim Girardin
Suite 2
1400 Homer Road
Winona, MN 55987
Voice/Fax 507-454-8844
kmgrdn@luminet.net


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Revised:  January 27, 2006
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