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ANALOG DEVICES AN-242 APPLICATION NOTE Manual

1. and Ciy2 decouple the dc level and pass the audio signal to U1 When the phantom power source is switched on off or the microphone cable is plugged in out potentially de structive transients of up to 48 V can be coupled through Cin and Cina and will appear across Rpg and or Reo If these spikes are not properly controlled and safely dissipated they can cause destruction of the amplifier U1 Note that this is true for virtually any amplifier input stage not just the SSM 2017 as the stored energy in the coupling capacitors can develop peak discharge currents of several amperes if not limited by some means In this circuit CM voltage limiting is used on each of the differ ential input lines This is done with pairs of back to back low voltage Zeners Z1 Z2 and Z3 Z4 These are stan dard 1N752 400 mW V 5 6 V units from the 1N75x series equivalent to BZX79 series in Europe and are effective in safely limiting the peak voltage of the dis charge to 10 V or less In addition peak current limiting for transient discharge is provided by the series protec tion resistors Rp and Rp In dissipating the transient safely the peak current must be limited to a level which will not cause failure of the protective Zeners thus this resistance is necessary l While the series resistances do add some noise over that of the amplifier used without them it is mitigated in the fact that it is necessary only with phantom powered ca
2. ANALOG DEVICES AN 242 APPLICATION NOTE ONE TECHNOLOGY WAY e P O BOX 9106 e NORWOOD MASSACHUSETTS 02062 9106 617 329 4700 A Low Noise Microphone Preamp with a Phantom Power Option by Walt Jung and Adolfo Garcia INTRODUCTION A preferred method of amplifying low level audio sig nals from balanced sources is by the use of an instru mentation amplifier preamp stage The instrumentation amplifier in IC or other form is configured for gain with just one user adjustable resistor Allowing transformer less gain it features maximum rejection of common mode CM noises such as hum and has low operating noise with commonly used microphones by virtue of a low noise input stage In performance terms this trans lates to an amplifier input referred noise of less than 1 nV Hz from the circuit or device used over a working range of gains of 2 to 2000 times 6 dB to 66 dB while rejecting CM noise signals 90 dB or more With design Cm1 Rpt 2200F 10V 47uF 49 92 _63V_ l TO MICROPHONE V l a m 2200uF 10V C2 Re2i 3 z4 i 47uF 49 92 NOTES 1 Z1 24 1N752 SEE TEXT 2 Cinx Cgx LOW LEAKAGE ELECTROLYTIC TYPES SEE TEXT 3 GAIN G 2 x 10k Rg 1 SEE TEXT 4 ALL RESISTORS 1 METAL FILM SSM 2017P i L Cpe O TRR 7 Rg 3 5 options exercised such amplifiers can also provide gains of 70 dB or greater But while low noise and distortion are very
3. er Zeners from the 1N75x series can work for Z1 Z4 but they should be restricted to lower volt ages 7 5 V or less From a view of simply adequate protection higher wattage types such as 1N47xx 1W and 1N53xx 5W families also absorb the transients but do not appear to be fundamentally necessary The higher wattage Zener families have excessive non linear capacitance which can produce distortion and thus for this reason they are not recommended For example the 1N75x series diodes measured about 160 pF of capacitance while the highest distortion mea surements not shown on all three diode families show below 0 01 with a 1 V rms level to the preamp with the higher capacitance devices showing slightly higher dis tortion 0 0097 versus 0 0075 The complete preamp with protection was exercised with a pair of 0 1 Q power MOSFET switches driving the input lines with a higher value of coupling capacitance 100 uF in lieu of 47 pF It survived this torture test still measuring THD N below 0 008 From this it would appear that there is some latitude for variation of cou pling capacitor size and the protection resistance While slightly smaller protection resistances down to about 30 Q can likely be used large value changes here should be reverified from a protection aspect RFI While not directly related to the dc transient problem another source of potential problems with high gain preamps is radio frequency i
4. g In addition the gain of stage two is fixed at 2 so the overall preamp gain G is then G 2 x 1o 1 1 BE G In practice Rg can be either a pot or a switch controlled resistance used as a gain control for the entire circuit with a minimum gain of 2 as _ 20k 5 Rg 6 2 2 For convenience a table of Ro values for various gains is provided using the closest standard 1 values Since the circuit is likely to be used with Ro either switch selected or a reverse log taper pot electrolytic decou pling capacitors Cg and Cg are used with Rg This operates the U1 stage at a dc gain of unity and prevents noise injection or thumps due to operating gain changes the gain Pins 1 and 8 of the SSM 2017 bias about 0 6 V below common which provides a bias to these capacitors Table I Gain Table R is rounded to closest 1 value where noted If overall gains greater than 66 dB are needed the sec ond stage gain can be increased by lowering R1 A 5k value for example provides a maximum gain of 72 dB and generally 6 dB more over the gain ranges of Rg PHANTOM POWER AND PROTECTION With a phantom powered microphone in use the IN and IN pins of the mic connector will see a large CM dc voltage Dependent upon the particular mic in use and the power drain this voltage can be in the range of 10 48 V dc and upon it is superimposed the balanced audio signal In normal operation capacitors Cin
5. g the net output dc offset will be essentially the sum of the voltage offset of U2B and the offset current errors and will be independent of the output dc offset of U1 With 1 values used for R4 and R5 the circuit s overall dc offset should typically be about 2 mV For lowest integration errors film capacitors should be used for C1 and C2 such as polycarbonate or polyester types Diodes D1 and D2 provide protective clamping for U2B PERFORMANCE This amplifier s performance is quite good over pro grammed gain ranges of 2 2000 For a typical audio load of 600 Q THD N at various gains and an output level of 10 V rms is shown in Figure 2 these tests results do not show the effects of the protection components but as noted these produced relatively small distortion degra dations For all but the very highest gain the THD N is consistent and well below 0 01 while the gain of 2000 becomes more limited by noise THD N dB 20 100 1k FREQUENCY Hz 10k 20k Figure 2 Low Noise Microphone Preamplifier THD N Performance Various Gain Vour 10 Vrms R 600 Q Noise performance of the preamp circuit is exceptional and is illustrated in Figure 3 This photo is a linear sweep spectrum analyzer plot over a range of 0 kHz 25 kHz The operating gain of the preamp is 1000 so a displayed vertical scale factor of 1uV Hz is equivalent to 1 nV Hz referred to the preamp input In this photo the noise level is displayed at abou
6. her voltage ratings will aid in controlling leakage i If desired these can also be bypassed with small film capacitors to control high frequency resonances Metal film types are recommended for all signal path resistors Physical construction is important also that is a neat compact layout for the circuit board should be used In particular attention to lead dress and layout around the inputs to U1 will be important for minimum noise pickup Some types of input coupling capacitors neces sary for phantom power operation are physically large and so are susceptible to noise pickup Guarding and screening around these and the other sensitive nodes at the input will pay off with good performance Finally well regulated power supplies should be used well by passed with large electrolytics returned to the output common point ACKNOWLEDGMENTS In preparing this article the authors appreciated the sup port of James Wong and Dan Parks and comments from SSM 2017 designer Derek Bowers all of the Analog De vices PMI division Helpful comments also came from Chas Brooke of BSS Audio Ltd Ben Duncan of Ben Duncan Research David Josephson of Josephson Engi neering and Gordon Kapes of Studio Technologies REFERENCES ANSI Standard 268 15 Revision 1987 amendments 1989 1990 1991 American National Standards Insti tute 11 W 42nd St New York NY 10036 Other Considerations RFI Protection AD625 Instru mentation Ampl
7. ifier Data Sheet 3W Kester 1993 System Applications Guide Analog Devices 1993 Chapter 1 pp 37 55 Panasonic 91 92 Electrolytic Capacitor Catalog Pana sonic Industrial Co 2 Panasonic Way Secaucus NJ 07094 E1747a 2 11 93 PRINTED IN U S A
8. important they are by no means the entire requirement For exam ple with the use of microphone phantom powering where CM 48 V dc power is fed to a remote microphone capsule the received audio signal from the microphone must be amplified cleanly without side effects from the dc In the steady state this is no great problem but switching transients from the dc feed can wreak havoc with amplifier input stages if care is not taken Conse quently amplifiers must be protected against power surges while still operating with little or no sonic degradation Vs eE TR 21V C6 c3 PHANTOM POWER SUPPLY CONNECTIONS 0 1uF TS 100pF 25V INTERLOCKED WITH V SEE NOTE 5 c7 tca 0 1uF 100uF 25V Ns 21V R1 10k Vs c 1uF FILM U2B OP 275GP 5 DOTTED PHANTOM POWER RELATED COMPONENTS OPTIONAL SEE TEXT C2 tuF FILM Figure 1 Low Noise Microphone Preamp The Analog Devices SSM 2017 differential input audio IC preamp is suitable for use in mic preamps and has the requisite low input voltage noise plus high CM rejection and low distortion Gain of this 8 pin IC is set via one external resistor Rg and is basically adjustable over a range of 1 1000 times 0 dB to 60 dB Differential inputs at Pins 2 3 allow balanced mode input signals and a single ended output signal appears between the ICs out put 6 and reference 5 terminals The SSM 2017 archi tecture is similar to family
9. nterference RFI In this circuit the input capacitor C filters high frequencies above 135 kHz before they reach the preamp input In addition further filtering is provided in the second stage by R2 C5 at 241 kHz Additional RFI filtering can take on several forms sepa rately or in combination Separate low DCR in line RF chokes can be added in series with the two inputs or a single common mode choke or low pass filter available as packaged assemblies Consideration should be given in such cases to nonlinear effects in the inductor cores as well as the nonlinear C V characteristics of the RF quality bypass capacitors used For example high K disc ceramics are excellent for RF applications but are non linear with applied voltage In this regard a better choice would be either NPO ceramic or stacked film polyester types RF bypassing of the SSM 2017 input transistors can also be used from Pins 1 2 and 3 8 as described on the AD625 data sheet This option is shown on the schematic as Cae and Capo Other input interface schemes to U1 can be used for connections to high level sources in which case the dotted phantom power circuitry and the mentioned redundant circuitry can be deleted OUTPUT CONTROL In the output stage dc servo stage U2B senses the dc from U2A and compares it to a common reference point U2B is an inverting integrator with an overall low fre quency rolloff of about 0 12 Hz With this servo loop Operatin
10. pacitive microphones and can be switched out when not needed Since these microphone types tend to be higher in output this minimizes the degree of S N deg radation More importantly from a reliability point a safe upper bound on the fault current is established 1A or less in the case of a direct short to ground on a input line with a full 48 V supply worst case As noted the preamp circuit can be operated with or without the phantom power so it is logical to optimize input connections so that those portions not absolutely essential for phantom power are not in the signal path when it is not in use as well as the 48 V dc power itself being switched off This would provide a means of switching Rp and Rp out when operating without phan tom power so that lowest noise performance is retained for other sources A further step which will aid in controlling transients is to interlock the 48 V supply with the bipolar power sup ply feeding the amplifier This prevents switching on phantom power with the amplifier circuitry powered down which would otherwise definitely invite trouble However with dc power applied to the amplifier the protection is such that the phantom power supply can be switched on off at will These protection part values and types have been lab tested and represent a conservative balance between adequate fault protection and minimal degradation of the basic audio signal handling when phantom power is used Oth
11. predecessors SSM 2015 SSM 2016 but without access to the internal gain resis tors a constraint of the 8 pin package COMBINED IC AMPLIFIER IS SYNERGISTIC Figure 1 is an example of a low noise microphone preamp along these lines using the best advantages of a SSM 2017P IC combined with an OP 275GP dual bipolar JFET low noise op amp As shown the circuit is a variable gain microphone preamp with an optional phantom power feature but the circuit flexibility can also allow it to operate more generally The SSM 2017 s strengths lie in low noise and distortion and gain flexibility simplicity Rated only for 2k or higher loads however this makes driving 600 Q loads somewhat lim ited with the basic IC The circuit used here works by combining the virtues of two ICs into one single useful structure The SSM 2017 is used here to best advantage in a pro grammable gain input stage which is then is combined with a fixed gain high current output buffer and dc servo stage using the OP 275 The OP 275 output buffer provides low distortion high level drive into 600 0 loads with the second half of the dual IC employed as a servo to control output dc offset The buffer stage U2A is operated at a modest gain of 2 times keeping the re quired output swing of the SSM 2017 to a minimum and the overall distortion low DESIGN FACTORS The circuit uses the SSM 2017 first stage U1 as a gain programmable preamp block with gain set by resis tance R
12. t 12 5 uV which when divided by 1 150 to account for the 150 Hz analyzer bandwidth is a noise level of about 1 nV Hz to refer the 12 5 nV displayed noise to the preamp input divide by 1 150 x 1000 or 12 250 CH A 40 0 uV FS MKR 1 02 uV Hz 5 00 uV DIV 12 500 Hz BW 150 MHz CONDITIONS SSM 2017 GAIN 500 OP 275 GAIN 2 TOTAL GAIN 1000 Vg 20V R C 1MQ lt 60pF Figure 3 Noise Performance of Two IC Preamp Maximum output drive will be a function of the power supplies and can be as high as 10 V rms with higher voltage supplies Note that output resistor R3 will limit the swing available when driving 600 Q but should be retained for short circuit protection Supply voltages on the order of 20 V are appropriate for highest output into 600 Q but the circuit can also be operated on lower supply voltages with less output 4 SOME PRACTICAL NOTES To get the most from this circuit appropriate quality components should be used This is most critical for the electrolytic coupling capacitors which preferably should have a low leakage specification such as 0 002CV or less One type identified with this characteristic was the Panasonic HF 4 with which the circuit was tested This type is currently being phased out so alternates should be considered for example types HFQ or HFE Val ues should be in the range of 22 uF 56 pF with a volt age rating of 63 V 100 V or more hig

ANALOG DEVICES AN-242 APPLICATION NOTE Manual

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