Measurement Microphone Preamplifier
Model 411

This preamplifier was especially designed for sound measurements and to inter-compare microphones*. It supplies +5 V phantom to unbalanced mics such as the TDL® models 817A and 818. It also supplies +48 V phantom to balanced mics such as the Behringer ECM-8000 and Dayton EMM-6. The 411 has very low output noise because the power supply (including the 48 V DC-to-DC converter) is housed in its own cast aluminum box inside the outer cast aluminum enclosure. For convenience, all the input and output connectors are located on the rear side of the enclosure (see Figure 1).

INPUT: Designed for microphones needing an input impedance of 150 to 600 ohms. Female RCA for an unbalanced electret capacitor mic (+5 V phantom). Female XLR for a balanced electret condenser mic (+48 V phantom). NOTE: Both microphone inputs cannot be used at the same time.

GAIN: Zero dB (unity gain) to 40 dB, 10 to 50 dB or 20 to 60 dB in 4 dB steps using an 11-position rotary switch. The gain range is selected by removing the top cover plate and changing the position of a slide-on jumper.

RESPONSE: Flat (±0.5 dB) 10 Hz to at least 50 kHz.

NOISE and DYNAMIC RANGE: Figure 2 shows the output noise spectrum at 40 dB gain. At 1000 Hz the noise floor is -107 dBu (3.5 uV RMS). The maximum 411 output is about 3 V RMS. This gives a dynamic range of 118.7 dB at a zero signal-to- noise ratio, S/N. For an S/N of 20 dB, the dynamic range is nearly 100 dB. At 30 dB gain, the noise floor will be 10 dB lower. At 50 dB gain it will be 10 dB higher so it is useful to use no more gain than is needed.

POWER: Operation from a 24 VDC wall power supply (furnished). A red LED indicates power on. (No power on-off switch as operation from a power strip is assumed.)

CONNECTORS: Microphone connectors as describe in INPUT. Female RCA for output. 2.5 mm male connector for the DC power input.

PHYSICAL: Cast aluminum enclosure measuring 220 x 170 x 63 mm (8.8 x 6.7 x 2.5 inches) overall including connectors and knobs. Weight is 1077 grams (2.4 pounds). There are rubber bumpers on the four corners of the bottom of the enclosure.

PRICE: See price list. PRICES

* For example, see the article "DIY Microphone Calibration" by Ron Tipton. Published in audioXpress magazine, April 2009. A pdf copy of the article can be downloaded here.