I bought this Audio Technica AT-MCA20a microphone preamplifier on Yahoo.jp auctions for 1600 yen (about $12 US). As a curiosity and learning tool, to see how it was made.
Here, for your viewing pleasure, are photos on the inside.
It's discontinued but recently sold for about $200. It's billed as being for security/surveillance rather than music/recording but it seems to be built to commercial broadcasting standards. The THD of 0.1% or less at1kHz, +4dBm seems respectable, but it's hard to judge.
■ Microphone amplifier
■ 1ch
■ Input impedance 2kΩ
■ Reference input level MIC -50, -40, -30dBV switching
■ Output impedance: Rated 600Ω (effective 200Ω)
■ Standard output level: +4dBm (LINE) -26dBm (MIC)
■ Maximum output level: +14 dBm (LINE) -16 dBm (MIC)
■ Output ATT: 30dB
■ Frequency response: 40-25,000Hz
■ Low cut: 200Hz, -6dB/oct
■ Total harmonic distortion rate: 0.1% or less at1kHz, +4dBm
■ Noise level: Input conversion (150Ω termination)-115 dB or less (JIS-A)
■ Phantom power supply: 12V
■ Power supply: AC100V (50/60Hz)
■ Power consumption: 0.8W
■ Weight: 730g
I realized after the fact that the phantom power supply is only 12V so it won't work with my AT4047SV even to test. Drat.
As for the circuit, the linear power supply comprises a small transformer, bridge rectifier, filter cap, and a 24 V voltage regulator. A set of Zener diodes creates +12 V and -12 V supplies and a virtual ground for running the three op amps, a NEC C842C, JRC 2068DD, and JRC 4556AD. The 4556 is a dual hifi op amp, it's the output stage, the 2068 is a low noise dual op amp, used as the input stage and buffer. The C842 is a high speed, general purpose op amp so it's probably part of the power supply. Interestingly, there are four very small transistors near the C842... I don't know their function.
Here, for your viewing pleasure, are photos on the inside.
It's discontinued but recently sold for about $200. It's billed as being for security/surveillance rather than music/recording but it seems to be built to commercial broadcasting standards. The THD of 0.1% or less at1kHz, +4dBm seems respectable, but it's hard to judge.
■ Microphone amplifier
■ 1ch
■ Input impedance 2kΩ
■ Reference input level MIC -50, -40, -30dBV switching
■ Output impedance: Rated 600Ω (effective 200Ω)
■ Standard output level: +4dBm (LINE) -26dBm (MIC)
■ Maximum output level: +14 dBm (LINE) -16 dBm (MIC)
■ Output ATT: 30dB
■ Frequency response: 40-25,000Hz
■ Low cut: 200Hz, -6dB/oct
■ Total harmonic distortion rate: 0.1% or less at1kHz, +4dBm
■ Noise level: Input conversion (150Ω termination)-115 dB or less (JIS-A)
■ Phantom power supply: 12V
■ Power supply: AC100V (50/60Hz)
■ Power consumption: 0.8W
■ Weight: 730g
I realized after the fact that the phantom power supply is only 12V so it won't work with my AT4047SV even to test. Drat.
As for the circuit, the linear power supply comprises a small transformer, bridge rectifier, filter cap, and a 24 V voltage regulator. A set of Zener diodes creates +12 V and -12 V supplies and a virtual ground for running the three op amps, a NEC C842C, JRC 2068DD, and JRC 4556AD. The 4556 is a dual hifi op amp, it's the output stage, the 2068 is a low noise dual op amp, used as the input stage and buffer. The C842 is a high speed, general purpose op amp so it's probably part of the power supply. Interestingly, there are four very small transistors near the C842... I don't know their function.
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