话放电路比较，召唤“沙丁鱼”评析(资料已翻译好)

happyguy007

6 v* T% U& I) b, E  \8 U# x: D
这是做好后的样子，只有65x65mm大小。

happyguy007

这是说明（E文有困难的朋友如有需要，我可以帮翻成中文，不过不是学电子的，可能翻译的比较蹩脚:p）
8 [6 _4 m4 |8 h; |8 d
介绍
该简单的设计具有非常低的噪音，接近理论上的最小值，高交流声抑制以及通过单联电位器调节增益. 它与那些在专业级调音台中使用的很相似，可以组成适用于现场工作的分布式录音混音器的基础。

( g' q# C/ n3 |8 l1 {该设计带有接到差分复合对管发射极的共模(浮动)增益控制。与任何单管相比，2N4403和BC549复合对管线性更好。该电路是差分输入和输出，因此需要一个平衡到非平衡的缓冲器以便获得一个合适的输出，输入到混音台一个信道的信号级。它是由一个TL071或你自己选择的相似IC高性能运放差分增益级实现的。该级可获得6或15dB的增益，最大输入电平在未限幅前为1.5Vrms，这相当于一个带话筒的SPL超过150dB！
% f# L4 B4 N& t. o* U% M$ o  ^
总增益为1000倍或60dB. 失真低到几乎无法测得，因为在高增益时它低于噪音水平。CMRR (共模抑制比) 超过60dB，比现有的其它接有交流抑制的话筒电缆还好。带宽超过100kHz，并且事实证明它不需要RF抑制。话筒上的输入阻抗或负载用了两个3.3k欧电阻。这适合于几乎所有阻抗在150到600欧之间的话筒。
: i! m3 B: W. Z3 m
' Z3 l! u$ U0 Z/ H0 b8 L--------------------------------------------------------------------------------
, P- e3 h4 d& Z& h6 }. v描述
) {- K: y( W# ~  I输入级按最小噪音设计，这意味着要使用了一个非IC的方法，有些特殊的IC可以用于话筒前置放大，它们包括一个与此相似的电路，只不过是集成在一个芯片上的，比如现在已经很老的SSM2017或INA217.
! [) e" i$ _! X8 {% ?* `, j& {
7 e) [( D; j  j* W/ D除了用于增益控制的10千欧电位器外，其它元件应该很容易找到。需要一个反向的对数电位器---或在电路中用一个波段开关，每档增益为6dB，共60dB。
6 ~: f. K- m: y  N8 o* h$ B
5 J+ l/ f, J# C! H" I  o. q编者案 – 另外，也可以使用一个标准的对数电位器，但必须是反旋向的。如果把标记改成“衰减”而不是“增益”的话，那就对了。当电位器向顺时针转动时，增益量减少(增加衰减量)，因此当电位器反向到底时将对应最大增益。
8 g* `1 _5 F. @6 P
& O3 L7 v" K1 e  J7 ]- T4 z, x+/-15V 电源也是很重要的，它必须稳压和低噪。如果使用常用的稳压IC，我建议加一个由一个10欧姆电阻和一个470微法的电容组成的后级过滤以消除IC中产生的噪音，某些7815 IC就象一个噪音发生器而可调压的LM317, LM337要安静的多。可以用一个独立的整流板向多个前置放大供电，每个前置放大都带有后级滤波电路，由于使用了大量的滤波，建议本前置放大器使用后图的供电电路。

为了最小化噪音，在输入对管的集电极和发射极应该使用高质量的金属膜电阻，在高增益电路中当一个电阻上加有很重要的直流电压时通常应该选用低噪型。避免使用水泥电阻、金属陶瓷电阻和很老的碳膜电阻。同时由于球型钽电容可能会泄漏和破裂，也要避免使用。他们只是由一些易碎的成分制作的。100nF的电容(C6)应该布置在距运放供电脚尽可能近的地方---在高频的地方建议用一个陶瓷电容以确保得到最佳的旁路效果。

1000uF电容可用普通耐压10或16v的电解电容。 现代电子中， 0偏压直流通常不会有问题。其它的最小耐压为25V。根据查阅有关SSM2017噪音的出版物，本版本的前置放大器是在至少相当于200欧内阻(多数典型的动态话筒)下测量的。
2 C. I( v$ N! i6 W0 W4 kEIN = 0.27 uVrms（在20 kHz 带宽，200欧内阻下）
= 1.9 nV每 root Hz (与SSM 2017指标相等)
6 g. Z( W" x7 Y- Z  C噪音值 = 0.9 dB （200欧下）.

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编者注：
- c1 ^+ I* v* ~0 v3 @8 V0 @  d我建议本电路中全部使用1%误差的金属膜电阻---不会增加多少费用的，这也确保了平衡缓冲级(U1)更为平衡。即使在输入和反馈元件中出现很小的问题都将降低共模抑制。

我还建议不要使用钽电容, 多数读者都会注意到我在任何项目中都不建议使用这种电容(尽管还有建议说如果你想用也可以用)。我曾经对一个间歇性短路故障进行分析，最后发现是一个珠状钽电容---这是很难发现的:p
8 w) d+ b9 H9 I* x  m) V8 a3 X+ H
大家可以放心按本页的电容来试验，2N4403晶体管对某些读者来说可能难于获得，可以用BC559s代用但可能会增加一些噪音。 但愿这些增加在大多数情况下能被接受，性能与所述的基本一致。
6 S1 {9 W6 `% c4 r
本前置放大设计成便携式，可以用两块9V叠层电池。

--------------------------------------------------------------------------------
( S4 S/ J4 n/ h注意：可提供本电路的PCB板。有些很小的电路改动，PCB板是一个双前置放大器---两套独立的话筒前置放大可安装于同一块PCB上。包括一个与电位器相比能提供更多线性转换增益控制的制作资料（随板附上）。

[ 本帖最后由 happyguy007 于 2005-10-22 14:56 编辑 ]

happyguy007

推荐为上述话放供电的电路
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! `/ t3 N! ^/ ^8 {/ e. nPower Supply for Preamplifiers (Revision A)
Rod Elliott - ESP

Introduction

The original version of P05 has been around for a very long time now (around 4 years), and there are some worthwhile reasons for the updates. For the latest version of this project, please see P05 Rev-B. Although the performance of the original was not lacking in any way, I decided to change to adjustable regulators. This allows greater flexibility (one can easily make a small variable lab supply with the new version), and the adjustable regulators actually have lower noise.

In addition, the PCB now has a loss of AC detector, making muting circuits much easier (the AUX output can simply drive a relay if you want to), or it provides a useful signal for any other circuits that can benefit from an AC (or loss of AC) detector.

For anyone who feels a burning desire to upgrade their original P05 to the new P05A, the board is (almost) exactly the same size, and has the same mounting hole positions. The new PCB is a couple of millimetres shorter than the original, and will fit into the same location.

Preamps may in some cases use a simple regulator. With the supplies taken from the main amp power supply, this can be a problem if the main amp is of very high power. The supply voltage will be too high for 3-terminal regulator ICs, and they will fail. This will also be a problem if the main amp is under warranty or you just don't want to fiddle with it.

For these occasions, a simple, high performance supply can be built using an external AC power pack (no mains to worry about, and you don't even need a power lead). Power packs (wall warts, wall transformers) are available in a variety of voltages, and if you can find a 16V AC version, this is ideal. With 16V, you can easily get +/-15V DC regulated, using the circuit shown below. If you cannot find a 16V unit, you can use a 12V version instead, but the regulator resistor networks will have to be changed accordingly. In fact, the supply may be configured for any voltage from ±2.5V up to ±25V (although 15V is the most practical for opamps).

Alternatively, the supply can be run from a conventional split voltage transformer (e.g. 15-0-15V AC). It is designed to be as flexible as possible, and to this end, an auxiliary supply is also provided, complete with a 'loss of AC' detector. This can be used to power a muting relay, with virtually no additional circuitry needed ... other than the relay and a suitable voltage dropping resistor for the coil. Even the diode is on the PCB.

As always, inclusion of a fuse suitable for the transformer used is highly recommended, and a thermal fuse is a good idea too, since the power transformer will be left on permanently in most installations. If a Power switch is incorporated in the preamp, this can be a simple low voltage type since no mains voltages are present, and can be in either AC input lead (if you use the single winding transformer option) - there is no need to break both leads with the switch. Naturally, if you use a standard transformer it is better to switch the mains to conserve power.

NOTES:

• This circuit will not work with external DC power supplies without modification - as shown, it must use AC.
• The GND (0 Volt) line must be connected to the ground of the equipment being powered. Serious malfunctions can be caused by leaving off this connection.

Figure 1 - Preamplifier Power Supply
If a single AC supply is connected between GND and AC2, the rectifier is a "full-wave voltage doubler" type, and with an input of 16VAC will provide about +/-20V DC at a current of up to at least 100mA - this should be enough for the most power-hungry preamp. All diodes are 1N4001 or similar (100V / 1A minimum rating for all). (Note that the supply will work, but if you use GND and AC1 for a single AC supply, the loss of AC detector cannot function, as it gets its signal from the AC2 terminal.
If a split AC supply is used (such as 15-0-15V AC), then the transformer centre tap connects to GND, and the two 15V winding ends connect to AC1 and AC2. Although virtually any transformer of 0.5A or more will work (provided the voltage is correct), there is very little to be gained by using anything more than 30VA (and even that is likely to be overkill).
The 3-terminal regulators must be TO-220 types, and unless your preamp requires lots of current, they will not require a heatsink.
0 J& o8 M# G: N. p/ ]$ ZThe diodes around the 3-terminal regulators prevent reverse voltages being applied to the regulator chips under any condition. They are not strictly necessary, but are considered a good idea. The bypass caps must be close to the IC power leads to prevent oscillation.

Photo of Completed Unit
The photo shows the completed PCB, and has no heatsinks for the regulators. These will not be needed in most cases, but using them will do no harm, either. Make sure that they are well insulated from each other, or are insulated from the heatsink.
The PCB can be wired to use a single 16V AC supply, or a 15-0-15 AC supply from a conventional power transformer. Or, if you need to, it may be powered directly from an existing source of DC - make sure that the input voltage is below +/-30V under all operating conditions - this is important. For this connection, the rectifier diodes must not be used, and the loss of AC detector won't function unless it is connected separately to a source of AC. This option is recommended for experienced hobbyists only, although the construction guide does have some additional information.
It will be noted that there are no component values shown, other than for the semiconductors. This information (plus quite a bit more) is available in the construction guide - when (and only when) you purchase the PCB.
As an added bonus, the PCB can be used to implement the little lab supply described in Project 44. The voltage pots are connected in place of R4 (A & B) and R6 (A & B). The only thing that you will need to do is add a decent sized heatsink, and in this case a suitable bracket is recommended. If you only ever plan to use the supply for preamps, the heatsink can even be omitted, although I don't recommend this.
Loss of AC Detector
' n  H% V- g, A# N# q* W7 @4 [This function needs a bit of explanation. There are quite a few circuits (both opamp based and discrete) that insist on making stupid noises, especially as the supply voltage falls away to zero. The most common are squeaks and whistles, or sometimes rather disconcerting clicks and pops.

Adding a muting relay solves this (and there are a few described in the project pages), but there are no boards available, and they can be irksome to wire up. Using just the Auxiliary output from the P05A connected to a relay, you have a muting system - note that you will almost certainly need a resistor in series with the relay coil.
Although the circuit activates very quickly when power is applied, it is still just a few milliseconds behind the main supplies. This time is just enough to prevent the majority of switch-on noises. When AC power is removed, the Aux output will fall to zero withing a few AC cycles, the relay will release, and muting will be activated. All of this happens well before the voltage has fallen far enough for the attached circuits to make a sound, so any of the silly/annoying noises you used to get will be muted, and will not get through to the power amp.
The Aux output does require a load though - any relay will be more than enough, but if it is used to power some other circuit (such as the P110 remote control), no additional load is needed. The minimum load should be about 10mA.
8 k% l! s3 e. HNote that the Aux output is not regulated! Taking any switched current from the regulated supply is not a good idea, as it is possible to induce noise into the regulated supply. This rather negates the whole idea of using a low noise regulated supply in the first place.

happyguy007

供电部分实样
) c3 N6 i, J' m8 W: Q1 A# u* U, N（不知道这个AUX是用来作什么的，是不是可以省略掉）

happyguy007

供电部分说明：
Introduction
6 }% J( N# [( \. x$ sThe original version of P05 has been around for a very long time now (around 4 years), and there are some worthwhile reasons for the updates. Although the performance of the original was not lacking in any way, I decided to change to adjustable regulators. This allows greater flexibility (one can easily make a small variable lab supply with the new version), and the adjustable regulators actually have lower noise.
0 l4 D; e7 d3 {1 `# G3 j3 L
9 _: N, R1 S5 c) Z8 ?" sIn addition, the PCB now has a loss of AC detector, making muting circuits much easier (the AUX output can simply drive a relay if you want to), or it provides a useful signal for any other circuits that can benefit from an AC (or loss of AC) detector.

For anyone who feels a burning desire to upgrade their original P05 to the new P05A, the board is (almost) exactly the same size, and has the same mounting hole positions. The new PCB is a couple of millimetres shorter than the original, and will fit into the same location.
" n$ M* R  O& t  @" I
! M  i' U8 n; Z% H" ]" J" A' ^- JPreamps may in some cases use a simple regulator. With the supplies taken from the main amp power supply, this can be a problem if the main amp is of very high power. The supply voltage will be too high for 3-terminal regulator ICs, and they will fail. This will also be a problem if the main amp is under warranty or you just don't want to fiddle with it.
9 ^5 o5 @7 _. t: j
" M  n9 l' u% C' K7 x. V" UFor these occasions, a simple, high performance supply can be built using an external AC power pack (no mains to worry about, and you don't even need a power lead). Power packs (wall warts, wall transformers) are available in a variety of voltages, and if you can find a 16V AC version, this is ideal. With 16V, you can easily get +/-15V DC regulated, using the circuit shown below. If you cannot find a 16V unit, you can use a 12V version instead, but the regulator resistor networks will have to be changed accordingly. In fact, the supply may be configured for any voltage from ±2.5V up to ±25V (although 15V is the most practical for opamps).

Alternatively, the supply can be run from a conventional split voltage transformer (e.g. 15-0-15V AC). It is designed to be as flexible as possible, and to this end, an auxiliary supply is also provided, complete with a 'loss of AC' detector. This can be used to power a muting relay, with virtually no additional circuitry needed ... other than the relay and a suitable voltage dropping resistor for the coil. Even the diode is on the PCB.
5 p% t/ d4 r8 ]
  E$ M' p9 L) {& `  VAs always, inclusion of a fuse suitable for the transformer used is highly recommended, and a thermal fuse is a good idea too, since the power transformer will be left on permanently in most installations. If a Power switch is incorporated in the preamp, this can be a simple low voltage type since no mains voltages are present, and can be in either AC input lead (if you use the single winding transformer option) - there is no need to break both leads with the switch. Naturally, if you use a standard transformer it is better to switch the mains to conserve power.
8 Q8 @- [  l! a1 O
NOTES:
This circuit will not work with external DC power supplies without modification - as shown, it must use AC.
The GND (0 Volt) line must be connected to the ground of the equipment being powered. Serious malfunctions can be caused by leaving off this connection.

; g' S6 ~; B. s. e+ m9 `If a single AC supply is connected between GND and AC2, the rectifier is a "full-wave voltage doubler" type, and with an input of 16VAC will provide about +/-20V DC at a current of up to at least 100mA - this should be enough for the most power-hungry preamp. All diodes are 1N4001 or similar (100V / 1A minimum rating for all). (Note that the supply will work, but if you use GND and AC1 for a single AC supply, the loss of AC detector cannot function, as it gets its signal from the AC2 terminal.
4 [' _! S& r; j
# t! r3 ]8 L1 p$ @0 i8 L' {4 v5 ZIf a split AC supply is used (such as 15-0-15V AC), then the transformer centre tap connects to GND, and the two 15V winding ends connect to AC1 and AC2. Although virtually any transformer of 0.5A or more will work (provided the voltage is correct), there is very little to be gained by using anything more than 30VA (and even that is likely to be overkill).
  p4 C& U3 g$ Q! g  [4 T
! I: \8 j; D" L, w& ~The 3-terminal regulators must be TO-220 types, and unless your preamp requires lots of current, they will not require a heatsink.

The diodes around the 3-terminal regulators prevent reverse voltages being applied to the regulator chips under any condition. They are not strictly necessary, but are considered a good idea. The bypass caps must be close to the IC power leads to prevent oscillation.
/ p! z# |9 Z+ f% P
The photo shows the completed PCB, and has no heatsinks for the regulators. These will not be needed in most cases, but using them will do no harm, either. Make sure that they are well insulated from each other, or are insulated from the heatsink.
% Z  a; O  Z0 I
; a+ _* }, z0 l/ x# y. Z3 d% e, vThe PCB can be wired to use a single 16V AC supply, or a 15-0-15 AC supply from a conventional power transformer. Or, if you need to, it may be powered directly from an existing source of DC - make sure that the input voltage is below +/-30V under all operating conditions - this is important. For this connection, the rectifier diodes must not be used, and the loss of AC detector won't function unless it is connected separately to a source of AC. This option is recommended for experienced hobbyists only, although the construction guide does have some additional information.

It will be noted that there are no component values shown, other than for the semiconductors. This information (plus quite a bit more) is available in the construction guide - when (and only when) you purchase the PCB.
: d) K+ x9 N$ [6 z7 B9 z) l
. @3 a& m' y, sAs an added bonus, the PCB can be used to implement the little lab supply described in Project 44. The voltage pots are connected in place of R4 (A & B，) and R6 (A & B，). The only thing that you will need to do is add a decent sized heatsink, and in this case a suitable bracket is recommended. If you only ever plan to use the supply for preamps, the heatsink can even be omitted, although I don't recommend this.

--------------------------------------------------------------------------------
Loss of AC Detector
! c2 W/ ]9 Q! z8 w/ [: n# vThis function needs a bit of explanation. There are quite a few circuits (both opamp based and discrete) that insist on making stupid noises, especially as the supply voltage falls away to zero. The most common are squeaks and whistles, or sometimes rather disconcerting clicks and pops.

Adding a muting relay solves this (and there are a few described in the project pages), but there are no boards available, and they can be irksome to wire up. Using just the Auxiliary output from the P05A connected to a relay, you have a muting system - note that you will almost certainly need a resistor in series with the relay coil.

4 k' i) w; `& t' TAlthough the circuit activates very quickly when power is applied, it is still just a few milliseconds behind the main supplies. This time is just enough to prevent the majority of switch-on noises. When AC power is removed, the Aux output will fall to zero withing a few AC cycles, the relay will release, and muting will be activated. All of this happens well before the voltage has fallen far enough for the attached circuits to make a sound, so any of the silly/annoying noises you used to get will be muted, and will not get through to the power amp.
  s% v: d- s$ U+ }* M0 q2 L& a+ B
  E" V" H9 E7 o! w! ]3 g' mThe Aux output does require a load though - any relay will be more than enough, but if it is used to power some other circuit (such as the P110 remote control), no additional load is needed. The minimum load should be about 10mA.
" w7 f2 [  Z# t2 D0 [
Note that the Aux output is not regulated! Taking any switched current from the regulated supply is not a good idea, as it is possible to induce noise into the regulated supply. This rather negates the whole idea of using a low noise regulated supply in the first place.
& |7 E& ]. _  g2 V
! z4 \* ?) O: b! L0 @% K[ 本帖最后由 happyguy007 于 2005-10-21 11:04 编辑 ]

happyguy007

翻译好一篇，沙丁鱼看看入法眼不？
, U. l( G1 p: D; T( Z0 R. s这样看看，好象适用于阻抗600欧以下的话筒，而且性能也相当不错（准专业级的哦）不知道驻极体咪头适不适用？如果能改造一下咪头就好了:crying:，沙丁鱼一定要好好研究一下啊
  p8 D/ H7 N$ Y9 h供电部分我先不翻了，反正可以用2块叠层电池的。

[ 本帖最后由 happyguy007 于 2005-10-22 16:20 编辑 ]

CZQ83

不明白。帮顶！！！！！！！！！

CZQ83

帮技术帖子顶了。！！！！！！！

So_Snake

十个字十个字十个字十个字十个字十个字十个字十个字十个字十个字十个字十个字十个字十个字十个字十个字十个字十个字十个字十个字十个字十个字十个字十个字十个字:wacko:

happyguy007

这是另一个话放的电路图，这个应该是能用于驻极体的吧，请再给分析一下吧。谢谢。

happyguy007

E文说明：
6 c3 J. L) y4 l0 J7 y; S6 Z: kCircuit Description
The circuit amplifies and buffers the voltage regulator with one of the op-amp sections. It also has a clipping indicator, and a +20dB gain selector incorporated into the power switching function.
REF1 is a temperature compensated voltage reference biased via R1, the output of which is amplified and buffered by the first op-amp section configured in a non-inverting topology. Note that the supply for this reference is itself derrived from the actual regulated output of the op-amp, which increases the regulation effect. That really isn't necessary for the high-quality reference used, but hey, its essentially free. The voltage at the output of this opamp section should be 4.17V +/- whatever the precision of the resistors used for R2 and R3 is.
- j. y3 V1 D+ f2 LR4 and C1 form a low-pass filter that reduces the noise of the voltage reference section.
* D# b8 G/ g  V* b9 ~  mR5 and the microphone capsule plugged into J1 form a voltage divider across which the audio signal is manifest. R5 provides line powering to the capsule.
C2 filters out any RF that may wander in on the microphone cable.
R6, R7, and R8 bias the op-amp section to 1/2 the supply voltage. C9 allows this bias to happen.
R9 and R10 form a beta section for the second op-amp section, producing a gain of approximately 20. This gain is reduced to approximately 2 when R11 / C5 are switched in-circuit by SW1A. The ratio of these two gains is about 10, or 20dB. R9 and C4 form a high-pass filter with a cutoff frequency of 1Hz. R11 and C5 form a low-pass filter with a cutoff frequency of 200kHz.
' b& M& B+ C6 }+ I# q; V* c/ zR12 supresses any oscillation the second op-amp section might otherwise experience driving the output cable. C6 and R13 restore the output DC level to ground, and also form a high-pass filter with the load. For this reason, loads lower that 10k or so should be avoided unless some low frequency rolloff can be tolerated.
R14, R15, and R16 form a resistive divider tree that provide voltage references to the window comparator formed by the third and fourth op-amp sections. If the voltage at point 'X' goes outside of these references, than either D4 or D5 conducts, which charges the pulse-stretching capacitor C7, and turns on Q1, thus lighting LED1.
Switch Section SW1B provides power switching, and C8 filters the battery power.

happyguy007

完成后的图是这样的。

磐石

专业话筒有带阻抗变换的用平衡放大(正反相输入)能够起到降噪的作用,民用的就用一般的放大器就好.

happyguy007

8 d4 m! ^3 N* C: A" y7 ?2 a/ U
+ o, }  E! N) X$ `4 f' i0 s; A原来发的丢失了，现在再补发一下，请沙丁鱼来评析一下，准备自己也做一个。
) S( ^. j% y, M# C. @! {0 t% a
; d0 E4 g6 C" }5 t/ t5 s0 e* X[ 本帖最后由 happyguy007 于 2005-10-22 16:19 编辑 ]
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& J# z0 C0 q* N; k! a0 P. F应网友要求，再次放上来：
3 y- G* u' N" W2 X% q                             Low Noise Balanced Microphone Preamp
                                                   By Phil Allison
) ^4 L: r; }: U3 G                                        (Edited by Rod Elliott - ESP)
                                               Updated 17 May 2008

Introduction
This simple design has very low noise, close to the theoretical minimum, high hum rejection and variable gain with a single rotary pot. It is similar to that used in many professional grade mixing desks and can form the basis of a no compromise recording mixer for live work.
. Q6 H; ]  B8 d, @The design consists of differential compound pairs of transistors with a common mode (floating) gain control connecting the emitters of the pair. The compound pairs of 2N4403 and BC549s are far more linear than any single transistor. The circuit is differential in and out and therefore requires a balanced to unbalanced buffer to give suitable output for the next signal stages of a channel in a mixing desk. This is provided by a high performance op-amp differential gain stage, which can be a TL071 or similar IC of your choice. The stage has a gain of six or 15 dB and that sets the maximum input level at about 1.5 volts rms before clipping. This equals an SPL of over 150dB with a typical microphone!
Full gain is 1000 times or 60dB. Distortion is low to unmeasurable because it is below the noise level at high gains. The CMRR (Common Mode Rejection Ratio) is well over 60 dB and better than any available mic cable as far as hum rejection is concerned. The bandwidth extends beyond 100kHz, and no RF suppression is shown as it has proved unnecessary in practice. The input impedance or load on the mic is set by the two 3.3k ohm resistors. This will suit almost any mic with a nominal impedance of 150 to 600 ohms.
Description
The input stage is configured for least noise and this has meant a non IC approach. There are some special ICs that can be used for mic pre-amps, they contain a circuit like this one except fabricated on one chip. Examples include the SSM2017 (now obsolete) or the replacement INA217.
Components should all be readily available except for the 10 k ohm pot for the gain control. This needs to be a reverse log taper - or else use a multi-position switch with 6 dB gain steps covering the 60 dB range of the circuit. Make sure it is make before break.
# @0 J/ w/ I4 YEditor's Note - Alternatively, a standard log pot can be used, but wired "backwards". This will work fine if it is labelled "Attenuation" instead of "Gain". As the pot is advanced clockwise, the gain is reduced (attenuation is increased). Maximum gain will therefore be applied when the pot is fully anti-clockwise.
! F% k& i0 f! Z$ \The +/-15 Volt power supply is important too, it must be regulated and low noise. If the usual voltage regulator ICs are used I recommend fitting a post filter consisting of a 10 ohm resistor and a 470 uF capacitor to remove any noise generated in the ICs (as shown in Figure 1). Some 7815 ICs could be sold as noise generators, the adjustable voltage ones (LM317, LM337) are very much quieter. A single regulator board may be used to power multiple preamps, with each preamp having its own post filter circuits. Because of the extensive filtering applied, the P05 (Rev-A) power supply is recommended for this preamp.


: d- ?& o$ N: `$ E5 M9 F" n+ @Figure 1 - Complete Microphone Preamp
$ ?3 X8 f& L# p+ y4 J. fGood quality components should be used with metal film resistors in the collectors and emitters of the input pairs for least noise. Where a resistor has significant DC voltage imposed on it in high gain circuits always use low noise types. Metal film resistors are about the best only bettered by wire wound which is a bit impractical. Avoid cermet, metal glaze, and very old carbon composition types. Also avoid bead tantalum capacitors, as they go leaky and crackle. They are just about the most fragile electronic components made. The 100nF capacitor (C6) should be mounted as close as possible to the opamp supply pins - a ceramic cap is recommended for best bypass performance at high frequencies.
: p( ^$ n0 U& n3 KThe 1000uF capacitor can be a normal electrolytic of 10 or 16 volts rating. There is usually no problem with zero DC bias on modern electros. All other electros should be 25V rating as a minimum.
Upon checking the published specs for the SSM2017 in regards to noise, my workshop version of the preamp measures at least as good with a 200 ohm source resistance (typical of most dynamic microphones).

EIN = 0.27 uV rms in 20 kHz bandwidth with 200 ohm source.
) c+ x: @4 _1 {. w: \7 R$ ]$ i; e= 1.9 nV per root Hz (equal to spec for SSM 2017)
: L1 A$ _  ]+ F' D" _! PNoise Figure = 0.9 dB rel 200 ohms.

Editor's Comment
I would suggest that 1% metal film resistors should be used throughout this circuit - the additional cost is negligible, and this will also ensure that the balanced buffer stage (U1) is properly balanced. Even a small error in the input and feedback components will degrade the common mode rejection.
1 \7 ~5 Z* E/ M7 ELike Phil, I also recommend against the use of tantalum capacitors, and regular readers will notice that I have not suggested them for any project (although there was one suggestion that you could use them if you wanted to). The only capacitor fault I have ever had to track down with an intermittent short circuit was a tantalum bead type - it was neither fun, nor easy to find :-(
0 `$ Q. ~  p% M% l0 [As with all circuits presented on these pages, feel free to experiment. The 2N4403 transistors may prove difficult for some readers to obtain, and BC559s can be substituted with some possible increase in noise. I would expect that any increase will be acceptable for most applications. Performance should otherwise be much the same as described.
The preamp is ideal for portable use, and can be operated from a pair of 9V batteries.
( U/ N$ ]: X2 s" q! |$ PNote: The Revision-A PCB is now available for this preamp. There are a couple of very minor changes to the circuit, and the board is a dual preamplifier - two completely independent microphone preamps on one PCB. Included with the construction data (available when you purchase the PCB) is a circuit for a switched gain control, which provides much more linear control than you will get from a pot. The new PCB is double sided, and includes a full-sized ground-plane to help minimise noise.
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* ^3 {3 |' j3 e( \, A1 _- a0 PFigure 2 - Photo of the Completed Revision-A PCB
' G% n3 y$ |$ c3 SIn all, this preamp is highly recommended for professional or semi-professional use, wildlife recording or just experimenting. As you can see from the photo, the board is very compact, and I have described a phantom feed supply and distribution board elsewhere in the project section, along with a phantom powered microphone amplifier and a series of microphone projects.