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Instrumentation for acoustical measurement usually starts with a transducer which converts the audio aound pressure into electrical quantities. The transducers or microphones are based on different principles of operation. One would invariably choose a condenser microphone when the whole frequency band has to be taken into consideration and when tolerances of flatness of response are tight. However, a condenser microphone needs an impedance converter (converting the high impedance at the input into a low impedance at the output) directly connected next to it, and vacuum tube preamplifiers have been used for this purpose for a long time. Their replacement by solid state amplifiers was felt to be a rather difficult problem. Some of the most important properties expected of a preamplifier are: high input impedance; low input capacitance; low output impedance; and low noise. A vacuum tube used as a cathode follower fulfills these reuqirements to a large extent. In order to avoid disturbances in the acoustical field to be measured-especially at higher frequencies-the dimensions of the whole microphone assembly have to be small. Efforts have been made to reduce the size of the condenser microphone cartridge, however the size of the preamplifier is practically limited by the size of the vacuum tube. Thus the replacement of the vacuum tube by solid state circuitry is urgently required, and the advantages one would gain from this are: more flexible cables; better gain (practically unity); and less microphonics.
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 Scott Dorsey |
Comment posted March 13, 2019 @ 17:33:39 UTC
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HP had just introduced the first FFT analyzer and they had already in their line some swept-filter spectrum analysis gear for audio, from the Lake Stevens Instrument Division. To be a real player in the audio analysis world, though, they needed some laboratory-grade microphones and accelerometers to use as front ends for those. For the microphones, they contacted a German division of HP which came up with this design. The German HP division used some measurement capsules from Neumann which were derived from the classic Western Electric 640AA, through some B&K lineage, but they designed their own electronics. The microphone body had plenty of gain, producing an unbalanced line level output with a nice low output impedance able to drive a spectrum analyzer directly through a long cable. Powering the mike off a 200V supply allowed the use of a shielded two-wire cable, plenty of headroom in the front end, and 200V polarization of the capsule. This is one of the first microphones that I ever seriously analyzed the circuit of, and I highly recommend the detailed discussion of how the circuit works. (Respond to this comment)
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