Summary
In spite of allegations that op amps should not be used in audio
equipment, I will show you that an OPA134 is very practicable for
building a condenser/electret microphone. A bootstrapped op amp follower
turns out to be an excellent device to match electrets: they only meet a
discreet resistor of 1 GΩ || 2 pF, so that distortion at the higher
frequencies is no item. If the design shape of this resistor has been
chosen carefully, the dynamic range and the sound is to such an extent
that only an ADC with a very low jitter clock will entirely show the
excellent quality. |
Preface
Designing linear audio circuits you should be aware of junctions which
are not biased proparly. I mean, junctions which are close to pinch off,
represent a non-linear capacitance which value could double within
hundred millivolts. For instance the gate capacitance of the - in HF
applications very popular - JFET: J310, measured 6 pF at Ugs = -5 V, 9
pF at Ugs = 0 V and 32 pF at Ugs = 0.6 V, which is close to pinch off.
So the region around 0 volt should be avoided, certainly in high
impedance environments because the audio signals will phase-modulate
each other. |
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The microphone
In former days, before the electret had been invented in the late
sixties, dynamic microphones had been used in low-priced applications.
The electret microphone, with its built-in FET amp, was cheap and had
been produced in very small cartridges which enlarged the application
field. Moreover the quality of electrets became better than that of
dynamic microphones, at least for not to critical applications as public
address. |
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The line driver
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DistortionApart from the choice of the op amp, the input impedance still borders me! The capacitances between drain and gate and between gate and source of the (small) well-biased input-FETs are in the order of 5 pF or more! Even the capacitances of these well-biased junctions change with the amplitude of the input signal, the more |
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The bootstrap![]()
If the power supply voltages (U40 and U70) of A1 should follow the output signal (U60) and thus
the inputs (U20 and U30), all dynamic capacitances are eliminated.
Also partly inside the op amp circuit because the substrate has been
connected to the negative power terminal. Let us see how it works: A3
repeats the output signal U60 of A1 to U10 of A3
up till 72 kHz. The pole R3 - C3 serves the stability of the loop.
The six green LED's serve as two low noise zeners which are connected
between two low noise current sources BC368 and BC369 (small Rbb') with a
red LED at their base. For a noise arm functioning, LED's should
operate at ~2 mA. The voltage across three green LED's becomes about 6
volt.
The little bit of noise left, will be suppressed by C4 - R16 and C5 -
R15 which form low pass poles well below the high pass poles C2 - R2 and
the capacitance of the mike with R1. |
The op amp's
In the data sheet of the OPA134 I find: |
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Noise performance
Circuit noise is determined by the thermal noise of external
resistors and op amp noise. Op amp noise is described by
two parameters: noise voltage and noise current. The total
noise is quantified by the equation: |
HVF2512T1007FE: thin film chip resistor (see the label in Photo-1). I glued it on top of the OPA134 (Photo-2). This seemed to be a good choice! I never had so little noise as with this resistor. |
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The microphone cartridgesThe used microphone cartridges stem from the Sennheiser ME62 for the omnidirectional microphone (see Photo-3) and from the Audio-technica PRO 37R for the unidirectional one (Photo-4). With both cartridges it is rather simple to get rid of the built-in FET. With the Sennheiser the FET is simply soldered on the small PCB at the backside and with the PRO 37R the FET is located within the 'container' |
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Housing and cables
With my approach, the microphone needs plus and minus 12 volt from the
power supply. I do not like many cables, so that the microphones are
connected to a die cast box on the stand.
From this box, one cable goes down to the disc recorder. The system is
semi-
balanced: all connections are screened, the 'return ground' as well. The
return ground and the screening are interconnected only at the entrance
of the disk recorder.
I realise that this approach is no standard anyhow, but I did not want
any compromise here! |
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Dynamic range, noise floor
SPL measurements are not always simple. With my 'R&S ELT 2
Präzision-Schallpegelmesser' (from Marten Dijkstra) I measured the level
at which my disk-recorder shows 0 dB with the microphones in question.
As a 1 kHz auditory source I use a 10 cm loudspeaker in a small box. I
have found: |
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MeasurementsUsing the spectrum analyser of Adobe Audition with 16 bits - 44.1 kHz in the home brew disk-recorder with the RME DIGI96/8 PRO sound card from Intelligent Audio Solutions controlled by the rutgerS'Clock (jitter < 1 ps), we get a different image of the noise floor, but before looking at the noise from the microphone and its first stage (A1) we look at the noise from the next stage by taking off the mike from the die cast box (see Photo-5) and shortcut 2 and 4 of the XLR-plug. This offers the picture below.
The next picture shows the noise from the mike + A1, it is to say, the
cartridge has been replaced by a condensor of 18 pF.
As can be seen, the noise at frequencies above 1 kHz are determined by
the mike and its first stage. The noise at the low frequencies are
determined by the folowing stages, including the ADC inside the computer
that acts as the disk recorder!
Below we see a -1 dB - 1 kHz signal. For this measurement the electret
has been replaced by an 18 pF capacitor with a Walter & Golterman
PSE-11 in series. Mind that the second harmonic just peeks to -90 dB and
the noise floor rises to -98 dB, (because of the syntesizer in the
PSE-11).
Let us try to 'imitate' Henks measurement: add two signals via 9 pF
capacitors to the input of A1 and see if sidebands will arise around the
high frequency, showing Phase Modulation. There is a correlation
between jitter and PM.
After switching off the bootstrap we get the picture below. Indeed, two
sidebands arise 500 Hz from the 6 kHz only 68 dB below signal level,
but I wonder if this is phase modulation. I think of intermodulation. ![]() |
Dynamic performanceWith CLIO a number of bursts have been generated and connected to 10 Ω in series with 18 pF which replaced the microphone cartridge. Below are the plots of the recordings with 100 Hz and 4.5 kHz.
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Input impedance
The data sheet of the OPA134 shows: differential input impedance = 1013 Ω || 2 pF. |
Input levelWith the omnidirectional ME62 (gain 24.3 dB) the signal level of the cartridge is 155 mV for 0 dB on the display of the recorder. |
Used measurement equipment
Home-brew disc recorder, running Windows XP Professional SP3, with RME
DIGI96/8 PRO sound card from Intelligent Audio Solutions controlled by
the rutgerS'Clock (jitter << 1 ps),
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Conclusions
Building an electret microphone pre-amplifier with an op amp is simple.
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Last remarks
In the last fifteen years I have built many amplifiers for the ME62 and
the PRO 37R (also see the Dutch article [PAoSU]). Electret Microfoons',
on my website). The one
sounded a little bit better than the other until the OPA134 with the
bootstrap came in sight, together with the rutgerS'Clock in the ADC! |
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To get an idea of the quality of the microphones, three recordings are
presented below. All recordings have been made in a acoustical dry room
for better judgement.
The first recording was made with the omnidirectional ME62 cartridge,
the two next recordings with the TBA-165A. Mind the stereo image of
recording 3!
July 4 - 2011
Herbert Rutgers.
During my holydays in 2015 I got flowers from NYKFRY, being not just any...:
[Wurcer] Low Noise Microphone Amplifiers Scott Wurcer, in Linear Audio - Volume1 of April 2011, page 99
[PAoSU] Electret-Microfoons H.L. Rutgers, PAoSU, Eindhoven, in Electron - mei 2001, blz. 183
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Some Pictures
Photo-5. The plundered PRO 37R's at the prototype-box with electronics on a stand. |