Designing Microphone Preamplifiers

Designing Microphone PreamplifiersBy Gary K. Hebert129th AES ConventionSan Francisco CA, November 2010

The Tutorial OverviewSection 1Support CircuitrySection 2The Amplifier2Microphone Preamplifier Design129th AES Convention, Nov 2010Copyright 2010, THAT Corporation

Simple Block DiagramMicrophone inputAmplifierLine level outputMicrophone signal levels vary widely due to: Microphone sensitivity Source SPL Proximity to sourceLine level outputs are somewhat more constrained: “Standard” maximum operating levels include 24, 18, 15 dBu A/D converter input levels are approximately 8 dBu or 2 Vrmsdifferential3Microphone Preamplifier Design129th AES Convention, Nov 2010Copyright 2010, THAT Corporation

Typical 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 widerangePhantom 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, at the expense of noise May be required depending on minimum gain and supply rails 20 dB is commonResistant to common mode noise and RFIReliable4Microphone Preamplifier Design129th AES Convention, Nov 2010Copyright 2010, THAT Corporation

Preamplifier 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 today5Microphone Preamplifier Design129th AES Convention, Nov 2010Copyright 2010, THAT Corporation

Amplifier Input Bias Current IN Rg1GRg2-INR1R2Must provide a DC current path to supply the amplifier input bias current6Microphone Preamplifier Design129th AES Convention, Nov 2010Copyright 2010, THAT Corporation

Gain Control IN Rg1GRgRg2-INR1R2The amplifier is often designed to vary gain using a single variableresistor (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 available7Microphone Preamplifier Design129th AES Convention, Nov 2010Copyright 2010, THAT Corporation

Phantom Power 48VPhantom Power6k816k81C1 IN Rg1G-RgRg2 INC2R1R2– C1 and C2 required to block the 48 V from the amplifier inputs– 6.81k series resistors are specified in the standards for 48V phantompower– On/Off is available via a Simple mechanical switch in manual applications Relay or silicon switch in digitally controlled systems8Microphone Preamplifier Design129th AES Convention, Nov 2010Copyright 2010, THAT Corporation

Input Pad 48VPhantom PowerInput Pad6k816k81C1R3 IN Rg1GRgR5Rg2 R4 INC2R1R2Input pad is simply a signal attenuator prior to the amplifierThis is a differential-only pad, it does not attenuate commonmode signals9Microphone Preamplifier Design129th AES Convention, Nov 2010Copyright 2010, THAT Corporation

“Complete” Microphone Preamp 48VPhantom PowerInput Pad6k816k81C1 R3 IN Rg1GR5RgRg2 R4INC210R1R2Microphone Preamplifier Design129th AES Convention, Nov 2010Copyright 2010, THAT Corporation

It would be nice to say “that’s all there is”but there are gremlins are in the details!!11Microphone Preamplifier Design129th AES Convention, Nov 2010Copyright 2010, THAT Corporation

DC Offset Changes 48VPhantom PowerInput Pad6k816k81C1 R3 IN Rg1GRgR5Rg2 R4CgINC2R1R2 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 offsetchangesA servo-amplifier can also be effective, but we don’t have time to discuss them today12Microphone Preamplifier Design129th AES Convention, Nov 2010Copyright 2010, THAT Corporation

Trade-offs with Cg 48VPhantom PowerInput Pad6k816k81C1 R3 IN Rg1GR4CgRg2 - RgR5INC2R1R2Rg and Cg create a high-pass filter in the signal pathRg can vary from 5 to 10k ohmsCg must have a very large capacitance to avoid low frequency audioattenuation– Worst at highest gain13Microphone Preamplifier Design129th AES Convention, Nov 2010Copyright 2010, THAT Corporation

Resistor Value SelectionPhantom Power 48V6k816k81C1 IN Rg1Rg-CgRg2 INC21k211k21– Microphone are commonly specified for 2 to 3 kohm loads– Differential input impedance is (R1 ll 6.81k) (R2 ll 6.81k)– Therefore, suitable values for R1 & R2 are between 1172 and 1924ohms14Microphone Preamplifier Design129th AES Convention, Nov 2010Copyright 2010, THAT Corporation

Capacitor Value SelectionPhantom Power 48V6k816k8133 uF IN Rg1Rg-Rg2 33 uFCgIN1k211k21– High-pass filter corner frequency is set by the blocking capacitor and bias resistorand is equal to 1 / (2 x pi x R x C)– 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 Hz15Microphone Preamplifier Design129th AES Convention, Nov 2010Copyright 2010, THAT Corporation

Alternative Resistor-Capacitor Value SelectionPhantom Power 48V6k816k81C1 IN Rg1Rt-RgRg2 INC2––––Cg10k10kC1 and C2 can be made smaller if bias resistors are made largerRin is defined by RtHowever, C1 and C2 convert 1/f noise to 1/f 2 noise10k resistors contribute thermal noise and current noise*R16Microphone Preamplifier Design129th AES Convention, Nov 2010Copyright 2010, THAT Corporation

Common Mode Rejection (CMRR)Phantom Power 48V6k816k8133 uF IN Rg1Rg-Rg2 33 uFCgIN1k211k21– Common-mode to differential conversion results frommismatches in: 6.81 k resistors 1.21 k resistors– Low frequency CMRR affected by capacitor mismatch17Microphone Preamplifier Design129th AES Convention, Nov 2010Copyright 2010, THAT Corporation

U-Pad Attenuator 48VPhantom Power6k816k81R3 IN Rg1GInput PadRgR5CgRg2-INR41k21 1k21ZIN with and without pad can be closely matchedCan be designed for any attenuation– 20dB is typical Noise performance is degradedBetter noise, less headroom with less attenuation18Microphone Preamplifier Design129th AES Convention, Nov 2010Copyright 2010, THAT Corporation

Example -20 dB Input Pad 48VPhantom Power20 dB Pad6k816k8133 uF IN 1k1Rg1GRg2671k1-Rg2 IN-33 uF Cg1k211k21ZIN with and without pad is approximately 2k20 dB AttenuationPad output impedance is approximately 240 ohmsSee THAT Design Note DN-140 for details andalternatives19Microphone Preamplifier Design129th AES Convention, Nov 2010Copyright 2010, THAT Corporation

48VRFI ProtectionPhantom PowerInput Pad6k816k8133 uF 100 pfGIN C1Rg1267100 pf-1k11k1Rg470 pf CgRg2C2IN-33 uF1k211k21RFI 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 bothinternal and external sources20Microphone Preamplifier Design129th AES Convention, Nov 2010Copyright 2010, THAT Corporation

Phantom Power Faults Shorting input pins to ground with phantomturned on– 33uF coupling caps C1 & C2 start charged to 48V– Positive end of C1, C2 connect to ground– Negative end of C1, C2 driven to -48V! The shorting sequence can vary––––“Single-ended”: One input to ground“Common-mode”: both inputs to ground simultaneously“Differential”: One input to ground, then the otherDifferential is worst Big currents flow as C1, C2 discharge– Currents over 3 amperes flow in the capacitors21Microphone Preamplifier Design129th AES Convention, Nov 2010Copyright 2010, THAT Corporation

Phantom Fault ProtectionPhantom Power 48VVCCVEE 6k8133 uF 100 pfGIN 1k1-1k1C1Rg1470 pfC2RgCgRg2 33 uF 10R267100 pf 6k81-ProtectionBridge20 dB PadIN10R1k211k21Limit the current with small value resistorsDirect fault currents away from the amplifier inputs– Input diodes provide a conduction path which bypassesthe amplifier– This current varies with gain setting Diode bridge directs fault current to rails– Consider impact on supply rails– Minimize supply transient with capacitance22Microphone Preamplifier Design129th AES Convention, Nov 2010Copyright 2010, THAT Corporation

Complete Microphone Preamp 48VPhantom PowerVCCVEE 6k833 uF1k1 6k8-ProtectionBridge20 dB Pad 10RIN 100pfRg1G267100pf470 pf1k133 uFCgRg2 -RgIN10R1k21231k21Microphone Preamplifier Design129th AES Convention, Nov 2010Copyright 2010, THAT Corporation

References and Additional InformationTHAT Corp “THAT 1510/1512” data sheetTHAT Corp “THAT 1570 & 5171” data sheets,THAT Corp “Design Note 140”THAT Corp “Design Note 138”THAT Corp “Analog Secrets Your Mother Never Told You”THAT Corp “More Analog Secrets Your Mother Never Told You”“The 48 Volt Phantom Menace Returns” Audio Engineering Society Preprint from the127th AES Convention, Oct 2009– “The 48 Volt Phantom Menace” Audio Engineering Society Preprint from the 110thAES Convention, May 2001–––––––All THAT Corp references are available at thatcorp.com24Microphone Preamplifier Design129th AES Convention, Nov 2010Copyright 2010, THAT Corporation

Amplifier TopologiesWhat’s inside the triangle?25Microphone Preamplifier Design129th AES Convention, Nov 2010Copyright 2010, THAT Corporation

Scope We will concentrate on topologies thatallow a wide range of gain with a singlecontrol. The goal is to balance the requirementsfor low distortion and low noise at bothends of the gain range.26Microphone Preamplifier Design129th AES Convention, Nov 2010Copyright 2010, THAT Corporation

Simple Mic Preamp Schematic27Microphone Preamplifier Design129th AES Convention, Nov 2010Copyright 2010, THAT Corporation

Simple Mic Preamp Ic 1 mA per inputtransistor, set by ( VEE VBE)/14.3k Diff Gain 22k/(re Rg/2 14.3k) where re 1/gm KT/qIC 26 ohms But – “re” is currentdependent! Minimum gain 22k/14.3k 3.7 dB28Microphone Preamplifier Design129th AES Convention, Nov 2010Copyright 2010, THAT Corporation

Simple Mic Pre THD PerformanceTHD vs. Gain, 1 kHz, 20 dBu OutTHD vs. Gain, 20 dBu Out1THD (%)0.1Simple MP0.010.0010102030405060Gain (dB)29Microphone Preamplifier Design129th AES Convention, Nov 2010Copyright 2010, THAT Corporation

High-Gain Noise Sources of Simple Mic Preamp Input noise at highgains dominated by: Q1, Q2 IC Shot Noise(RTI) 2 gqI C 4kTrem Q1, Q2 rb ThermalNoise 8kTrb R1, R2, Rg ThermalNoise 4kT(R1 R2 Rg )30Microphone Preamplifier Design129th AES Convention, Nov 2010Copyright 2010, THAT Corporation

Low-Gain Noise Sources of Simple Mic Preamp Input noise at lowgains dominated by: Thermal Noise of R5and R6 Thermal Noise of:R (R R )4g 3 Q1,Q2 IB shot noiseacross R g (R 3 R 4 )2 EIN of U131Microphone Preamplifier Design129th AES Convention, Nov 2010Copyright 2010, THAT Corporation

Noise Performance of Simple Mic PreampEIN (dBu, 20 Hz - 20 kHz, Rs 150) vs. Gain(dB)-90.0EIN (dBu)-100.0-110.0Simple MP-120.0-130.00102030405060Gain (dB)32Microphone Preamplifier Design129th AES Convention, Nov 2010Copyright 2010, THAT Corporation

CMRR Performance of Simple Mic Preamp CM to differentialconversion canoccur due tomismatches in: R3 and R4 R5 and R6 R7 and R8 R9 and R10 Q1 and Q233Microphone Preamplifier Design129th AES Convention, Nov 2010Copyright 2010, THAT Corporation

Simple Mic Preamp 2 Transistors and 1 Opamp Very Low Cost Marginal Performance34Microphone Preamplifier Design129th AES Convention, Nov 2010Copyright 2010, THAT Corporation

Complementary Feedback Pair Mic Preamp35Microphone Preamplifier Design129th AES Convention, Nov 2010Copyright 2010, THAT Corporation