Designing Microphone Preamplifiers - THAT …

1 By Gary K. Hebert129thAES ConventionSan Francisco CA, November 2010 Designing Microphone Preamplifiers2 Microphone preamplifier Design129thAES Convention, Nov 2010 Copyright 2010, THAT CorporationThe Tutorial OverviewSection 1 Support CircuitrySection 2 The Amplifier3 Microphone preamplifier Design129thAES Convention, Nov 2010 Copyright 2010, THAT CorporationSimple Block DiagramMicrophone signal levels vary widely due to: Microphone sensitivity Source SPL Proximity to sourceLine level outputs are somewhat more constrained.

2 Standard maximum operating levels include 24, 18, 15 dBu A/D converter input levels are approximately 8 dBu or 2 Vrms differentialMicrophone inputLine level outputAmplifier4 Microphone preamplifier Design129thAES Convention, Nov 2010 Copyright 2010, THAT CorporationTypical RequirementsGain Up to 40 dB covers the majority of close-mic d applications Some situations require more than 70 dB Variability of input levels requires adjustable gain over a very wide rangePhantom Power Required for many microphones Standardized in IEC EN 6193848 Volts +/- 4V at up to 10 mA per Microphone On / off controlInput Pad Can allow higher input signal levels.

3 At the expense of noise May be required depending on minimum gain and supply rails 20 dB is commonResistant to common mode noise and RFIR eliable5 Microphone preamplifier Design129thAES Convention, Nov 2010 Copyright 2010, THAT CorporationPreamplifier TechnologiesTransformer-Coupled Vacuum Tube Robust Colorful CostlyTransformer-Coupled Solid State Also Robust Performance can be excellent Cost can be highTransformerless Solid State More vulnerable Performance can be excellent Cost ranges from very low to highTransformerless solid state designs are the focus today6 Microphone preamplifier Design129thAES Convention, Nov 2010 Copyright 2010.

4 THAT CorporationAmplifier Input Bias CurrentMust provide a DC current path to supply the amplifier input bias currentIN+IN-Rg1Rg2R1R2+G-7 Microphone preamplifier Design129thAES Convention, Nov 2010 Copyright 2010, THAT CorporationGain ControlThe amplifier is often designed to vary gain using a single variable resistor (Rg) Manually controlled options Potentiometer with continuous control over a defined range Switched resistor network with a fixed number of steps and gain settingsDigitally controlled options Digitally switched resistor network with a predetermined number of steps Switches are either relays or silicon devices Both discrete and integrated circuit solutions are availableIN+IN-RgRg1Rg2R1R2+G-8 Microphone preamplifier Design129thAES Convention, Nov 2010 Copyright 2010.

5 THAT CorporationPhantom Power C1 and C2 required to block the 48 V from the amplifier inputs series resistors are specified in the standards for 48V phantom power On/Off is available via a Simple mechanical switch in manual applications Relay or silicon switch in digitally controlled systems IN+IN-RgRg1Rg26k816k81++C1C2R1R2+48V+G-P hantom Power9 Microphone preamplifier Design129thAES Convention, Nov 2010 Copyright 2010, THAT CorporationInput Pad Input pad is simply a signal attenuator prior to the amplifier This is a differential-only pad, it does not attenuate common-mode signalsIN+IN-RgRg1Rg26k81++C1C2R1R2+48V+ G-Phantom Power6k81R3R4R5 Input Pad10 Microphone preamplifier Design129thAES Convention, Nov 2010 Copyright 2010, THAT Corporation Complete Microphone PreampIN+IN-RgRg1Rg26k816k81++C1C2R1R2+4 8VR3R4R5+G-Input PadPhantom Power11 Microphone preamplifier Design129thAES Convention, Nov 2010 Copyright 2010.

6 THAT CorporationIt would be nice to say that s all there is are gremlins are in the details!!12 Microphone preamplifier Design129thAES Convention, Nov 2010 Copyright 2010, THAT CorporationDC Offset Changes Changes in gain can result in the DC offset changes at the output of the amplifier 2 solutions are available Adding a capacitor (Cg) sets the DC gain to a fixed value and avoids these offset changes A servo-amplifier can also be effective, but we don t have time to discuss them todayIN+IN-Rg1Rg26k816k81++C1C2R1R2+48VR 3R4R5+G-Input PadPhantom PowerRgCg13 Microphone preamplifier Design129thAES Convention, Nov 2010 Copyright 2010.

7 THAT CorporationTrade-offs with CgIN+IN-Rg1Rg26k816k81++C1C2R1R2+48VR3R4 R5+G-Input PadPhantom PowerRgCg Rg and Cg create a high-pass filter in the signal path Rg can vary from <5 to >10k ohms Cg must have a very large capacitance to avoid low frequency audio attenuation Worst at highest gain14 Microphone preamplifier Design129thAES Convention, Nov 2010 Copyright 2010, THAT CorporationResistor Value Selection Microphone are commonly specified for 2 to 3 kohm loads Differential input impedance is (R1 ll ) + (R2 ll ) Therefore, suitable values for R1 & R2 are between 1172 and 1924ohmsIN+IN-Rg1Rg26k816k81++C1C21k211k 21+48V+-Phantom PowerRgCg15 Microphone preamplifier Design129thAES Convention, Nov 2010 Copyright 2010, THAT CorporationCapacitor Value Selection High-pass filter corner frequency is set by the blocking capacitor and bias resistor and is equal to 1 / (2 x pi x R x C)

8 For a 5 Hz corner frequency, the minimum values for C1 & C2 are 26 uF The next largest standard value is 33 uF Results in a nominal corner frequency of about 4 HzIN+IN-Rg1Rg26k816k81++33 uF33 uF1k211k21+48V+-Phantom PowerRgCg16 Microphone preamplifier Design129thAES Convention, Nov 2010 Copyright 2010, THAT CorporationAlternative Resistor-Capacitor Value SelectionIN+IN-Rg1Rg26k816k81++C1C210k10 k+48V+-Phantom PowerRgCgRt C1 and C2 can be made smaller if bias resistors are made larger Rin is defined by Rt However, C1 and C2 convert 1/f noise to 1/f^2 noise 10k resistors contribute thermal noise and current noise*R17 Microphone preamplifier Design129thAES Convention, Nov 2010 Copyright 2010, THAT CorporationCommon Mode Rejection (CMRR) Common-mode to differential conversion results from mismatches in.

9 K resistors k resistors Low frequency CMRR affected by capacitor mismatchIN+IN-Rg1Rg26k816k81++33 uF33 uF1k211k21+48V+-Phantom PowerRgCg18 Microphone preamplifier Design129thAES Convention, Nov 2010 Copyright 2010, THAT CorporationU-Pad Attenuator ZINwith and without pad can be closely matched Can be designed for any attenuation 20dB is typical Noise performance is degraded Better noise, less headroom with less attenuationIN+IN-Rg1Rg26k816k811k211k21+ 48VR3R4R5+G-Input PadPhantom PowerRgCg19 Microphone preamplifier Design129thAES Convention, Nov 2010 Copyright 2010.

10 THAT CorporationExample -20 dB Input Pad ZINwith and without pad is approximately 2k 20 dB Attenuation Pad output impedance is approximately 240 ohms See THAT Design Note DN-140 for details and alternativesIN+IN-Rg1Rg26k816k81++33 uF33 uF1k211k21+48V1k11k1267+G-20 dB PadPhantom PowerRgCg20 Microphone preamplifier Design129thAES Convention, Nov 2010 Copyright 2010, THAT CorporationRFI ProtectionRFI protection is required in any practical design100 pf caps at the input connector attenuate differential and common-mode RFI470 pf cap at amplifier input pins reduces differential high frequencies from both internal and external sources470 pf100 pf100 pfIN+IN-Rg1Rg26k816k81++33 uF33 uF1k211k21+48V1k11k1267+G-Input PadPhantom PowerRgCgC1C221 Microphone preamplifier Design129thAES Convention, Nov 2010 Copyright 2010.