ANALOG DEVICES |. AN-115 APPLICATION NOTE ONE TECHNOLOGY WAY e P.O. BOX 9106 NORWOOD, MASSACHUSETTS 02062-9106 e 617/329-4700 Balanced Low Noise Microphone Preamplifier Design The SSM-2015 differential amplifier is utilized in a transformer- less, active-balanced input amplifier. The circuit shown in Figure 1 provides a microphone preamplifier design with excel- lent performance and low noise. The design features a trans- formerless preamplifier circuit with true-balanced input, 1500 input loading, phantom microphone powering, and high .com- mon-mode rejection. The design shown also includes a twelve position gain selector, or for fixed gain usage, component value calculations. The design includes microphone input loading of 1500, but the load resistor can be changed to accommodate other appli- cations. Input loading is capacitive reactive at higher frequen- cies to attenuate unwanted RF and ultrasonic voltages at the input terminals. The phantom microphone powering circuit provides power for condenser microphones that require 24 to 48 volts DC. The zener diodes CR,, CR,, CR,, and CR, protect the input transis- tors of the SSM-2015 when connecting the microphone to the preamplifier circuit. The common-mode voltage range is +5.5 volts. Its common- mode rejection is optimized for most applications by the true- balanced and differential input topology of the SSM-2015. A balanced single pole low-pass filter at the input terminals provides protection for the circuit from radio frequency interfer- ence and prevents slewing of the SSM-2015 amplifier. The output circuit topology is complementary bipolar producing 6V/ us slew rate, and able to drive a 2kQ unbalanced load. The circuit design incorporates a gain switch with twelve (12) calculated gain settings. For an output voltage of -10dBu, the microphone amplifier circuit has an input sensitivity range of 65dBu to -27.5dBu, and an output headroom of 33dB. The overall circuit gain is adjustable from 27.5dB to 55dB in 2.5dB steps. sw Gas e,,,(4B) R, VALUE (2) 1 27.5 -37.5 Ri. 1.00k 2 30 40 Rig 715 3 32.5 42.5 Ri, 511 4 35 45 Ria 374 5 37.5 47.5 Rig 280 6 40 -50 Roo 205 7 42.5 -52.5 R, 154 8 45 -55 Roo 115 9 47.5 ~57.5 Ros 86.6 10 50 ~60 Rog 63.4 11 52.5 62.5 Pos 47.5 12 55 -65 Ros 35.7 CS 33uF 50V . i HIO K 1000pF ste, m } s arka oc ska c : 33uF SOV > tooopr S=N2] I (_e Loo nd K CR, CR, Guo O__9 Z R, $A, SR, re Ka 6a1Ka 5.6V cr, > Icp, L 100nF |~ S0Vpc \ FIGURE 1 AUDIO PRODUCTS| SSM-2015 input circuitry utilizes two identical low noise bipolar transistors, with access to the emitters that provide the gain adjustment. R,, (R,, through R.,,) sets the amplifier's gain using the equation: 3 Gain = 3.5 + (20x10) for Ro, & R13 = 10.0kQ Unspecified gain can be calculated from the equation: : 3 Gains = 20 log [3.5 + (20 10 * Ra The frequency response amplitude is +0.1dB from 20 to 20,000Hz, and THD + noise of better than 0.03% over the gain range described with a typical EIN (Equivalent Input Noise) of 124dBu. The transformerless microphone preamplifier circuit described above demonstrates real-world usage refinements and in- cludes most operational features required by equipment de- signers. 4-12 AUDIO PRODUCTS TABLE 1: Circuit Performance Specifications Frequency Response (20Hz to 20kHz) +0.1dB THD + Noise (@ +23dBu, 20Hz to 20kHz) 0.03% IMD (@ +23dBu, SMPTE 60Hz & 4kHz, 4:1) 0.05% EIN (Equivalent Input Noise, 150Q source) ~124dB CMR (Common-Mode Rejection at 1kHz) 105dB Slew Rate 6V/ps Output Voltage (2kQ load) +23dBu or 11Vi 4. Output Headroom (2kQ load, 10dBu nominal) 33dB