Transcript
US 20030076967A1
(19) United States (12) Patent Application Publication (10) Pub. No.: US 2003/0076967 A1 (43) Pub. Date:
Hohendahl
(54)
MICROPHONE PREAMPLIFIER
(76)
Inventor: Andres Hohendahl, Buenos Aires (AR)
(52)
US. Cl. ......................... .. 381/120; 381/122; 381/111
(57)
Correspondence Address:
Apr. 24, 2003
ABSTRACT
BROMBERG & SUNSTEIN LLP 125 SUMMER STREET
BOSTON, MA 02110-1618 (US)
A microphone preampli?er. An electrical signal from a microphone is received via a shielded cable by an input stage
(21) Appl. No.:
10/254,030
(22) Filed:
Sep. 24, 2002
cascaded With an output stage. The input stage contains an impedance converter and a phase inverter and receives
Related US. Application Data
feedback that reduces input capacitance, actively shields the coaxial cable, and stabilizes the input stage. The output stage
(60)
contains a buffer and output driver and provides ampli?ed
Provisional application No. 60/324,332, ?led on Sep. 24, 2001.
versions of the input stage output. The preampli?er contains means for transformerless phantom powering the micro
phone and the microphone preampli?er, for sharing current betWeen the input and the output stages, and for limiting the
Publication Classi?cation
(51)
Int. Cl.7 ........................... .. H04R 3/00; H03F 21/00
frequency at the output of the microphone.
4VGC
PWBF l
Output
(Hugh vm, Low Bur-Int)
Polarizalmn
U
'/ I sound
Pressure IV ‘
\
Microphone Capacitive Transducer
&Assy.
Power Supply
R1118 8‘ C11 Adh/QSNBWIHQ
(
(Capsule)
Impedance
I
Converter
'
&
l
hase Ground
\_/\ mvertedphase
up
Inverter Ground
$3M“ O
girl-9v; amp“!
:
Radio
I
Frequency
Output
I
upier
Driver
:
Freq
s‘a e
’\I In phase
slgnal
J1’ J2 & Q1
Buffer
,
8‘9
1
; ~
\/\ \nvertedphase 0
Shielding -
-
XLR-I! /\J m phase
will"
'
: Llmlter OM03 ; L1,2&C5,7
1mm‘ :I CHIN!“
Ground
Input Stage
Ezzzrier’y C4,6 & R5 Ground
Ground
(523mm
*0
‘5
Patent Application Publication
uczow 2389i
Apr. 24, 2003 Sheet 1 0f 2
US 2003/0076967 A1
Apr. 24, 2003
US 2003/0076967 A1
MICROPHONE PREAMPLIFIER
contains a combination of a cascode and a ?rst transistor that
produces as inverter output an in-phase output signal and an CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application claims priority from US. Provi sional Application No. 60/324,332, ?led Sep. 24, 2001 Which is incorporated herein by reference TECHNICAL FIELD
[0002] The present invention relates to a microphone preampli?er, and, more particularly, to a capacitive micro phone impedance converter in combination With a class A
preampli?er With transformerless balanced loW impedance
output for phantom powering. BACKGROUND ART
inverted-phase output signal and that feeds back inverter output to the input of the cascode and to the shield of the coaxial cable.
[0008]
The preampli?er also contains a driver containing
a second and a third transistor Where the second transistor
receives the in-phase output signal and produces an ampli ?ed in-phase output signal and the third transistor receives the inverted-phase output signal and produces an ampli?ed
inverted-phase output signal. An input-stage poWer supply conditioner receives residual current from the driver stage
and supplies voltage and current to the phase inverter. First
and second frequency limiters receive the ampli?ed in-phase and ampli?ed inverted-phase signals and generate pream
pli?er in-phase and inverted-phase output signals. A polar
preampli?cation of the signal generated by the microphone
iZation poWer supply connected to the ?rst and second frequency limiters supplies voltage to the means for pro ducing an electrical signal from sound pressure.
capsule. Preampli?er performance in one area is often compromised to achieve performance in another area. For
[0009]
[0003] High performance microphones require immediate
example, needs for immunity to input noise, large output
signals capable of driving large capacitive loads, insensitiv
BRIEF DESCRIPTION OF THE DRAWINGS
ity to variations in supply voltages, loW poWer requirements,
[0010] The foregoing features of the present invention Will be more readily understood by reference to the folloWing
and economic design can con?ict. As a result, preampli?er designs often satisfy one requirement at the expense of not
satisfying other requirements. SUMMARY OF THE INVENTION
detailed description taken With the accompanying draWings, in Which: [0011]
FIG. 1 shoWs a functional diagram of a micro
phone preampli?er con?gured in accordance With a pre [0004] In accordance With one aspect of the present inven tion, a preampli?er is described Where immunity from radio frequency pickup at the output, loW poWer, loW input
equivalent voltage and current noise, loW output impedance,
high input impedance, loW input capacitance, high gain, loW
ferred embodiment of the invention.
[0012] FIG. 2 schematically shoWs a microphone pream pli?er con?gured in accordance With a preferred embodi ment of the invention.
poWer consumption, stable driving of large capacitive loads, stabiliZed DC Working point, and differential output can be
simultaneously and self-consistently achieved. A sharing of resources reduces costs.
[0005] In one embodiment of the invention, an input signal from a microphone or capsule is fed via a coaxial
DETAILED DESCRIPTION OF A SPECIFIC EMBODIMENT
[0013] A functional diagram of an embodiment of the invention is shoWn in FIG. 1. Acapsule, i.e., a microphone capacitive transducer, converts sound pressure into an elec
cable through a cascade of a ?rst stage and a second stage.
trical signal that is communicated to an impedance con
The ?rst stage generates as input-stage output an in-phase
verter/phase inverter via an actively shielded cable. The
output signal and an inverted-phase output signal that the
in-phase and inverted-phase versions of the signal generated
output stage uses to generate an ampli?ed in-phase output
by the impedance converter/phase inverter are each trans
signal and an ampli?ed inverted-phase output signal. The
mitted to a cascade of a buffer stage/output driver and a radio
?rst stage can contain an impedance converter as Well as a
frequency shielder/upper frequency limiter that provides in-phase and inverted-phase output signals.
phase inverter, and the second stage a buffer as Well as an
output driver. [0006] The preampli?er contains means for phantom poW ering of both itself and the microphone, for sharing current betWeen the input and the output stages, and for limiting the upper frequency at the preampli?er output and shielding the preampli?er output from radio frequency interference. In
[0014] Several additional functions address efficiency of
design and stability of operation. The radio frequency
addition, the preampli?er contains feedback means to loWer
shielder/upper frequency limiter communicates With a polar iZation poWer supply that generates a voltage for the capsule. The buffer stage/output driver communicates With an input stage poWer supply conditioner that generates supply volt age and current for the impedance converter/phase inverter.
the input capacitance of the input stage, to stabiliZe the input
The in-phase output of the impedance converter/phase
stage, and to actively shield the coaxial cable. The effects of
inverter is fedback, both to actively shield the cable carrying the signal betWeen the capsule and the impedance converter/ phase inverter and to stabiliZe the impedance converter/ phase converter and the DC Working point.
stray input capacitance are reduced through printed circuit board and ground plane design. [0007] In another embodiment, a microphone preampli?er contains a phase inverter that receives an electrical signal via a shielded coaxial cable from a means of generating an
electrical signal from sound pressure. The phase inverter
[0015] In an illustrative embodiment of the invention, FIG. 2 schematically shoWs a microphone preampli?er in Which a signal from a microphone capsule is fed into an
Apr. 24, 2003
US 2003/0076967 A1
impedance converter/phase inverter corresponding to the
Transistor Q1 hfe should eXceed 200 and have acceptable
cascode arrangement of JFETs J1 and J2 in conjunction With
noise speci?cations for Ic betWeen 0.5 and 1.0 ma.
transistor Q1 as a poWer stage. The cascode gives excellent
input noise performance for both Vin and Iin and a very high input impedance. The buffered signals then pass through dual output buffer folloWers corresponding to transistors Q2 and Q3 and their respective associated components. The impedance converter/phase inverter and the buffer stage/ output drivers share a current source, corresponding to the
current passing through R5.
[0016] Additional preampli?er features contribute to opti miZed performance. Active feedback With Q1 as a poWer
source reduces preampli?er input capacitance toWards Zero.
Design of the printed circuit board actively and physically insulates components and electrical paths, consequently
[0018] The above features alloW this embodiment of the invention to reach a peak output (i.e., differential betWeen
outputsAand B) of 18 volts. They also permit the ?nal gain, input noise, and output sWeep capability of the embodiment to be immune to a several volt variation of the input DC
voltage (:8 volts for the Phantom PoWering of 48 volts). [0019] The KA-04 microphone preampli?er and the TT-3M microphone incorporate embodiments of the inven tion as described in FIG. 2. Component values vary some
What betWeen the KA-04 and the TT-3M. Input resistor Rin is added or changed because of the needs of input polariZa tion for the KA-04 and the use of a “pre-polariZed” capsule
reducing stray capacitance and improving stability. Active
on the TT3M.
feedback, corresponding to a virtual and active ground, is
[0020] The advantages of these embodiments are further discussed in the article: “RevieW: T.H.E. Mics: A NeW Manufacturer Rolls Out a Remarkable Line of Quality
provided at the input and actively shields the input coaXial cable. Consequences are loWered input shunt capacitance, total system distortion, and noise and increased stability. Negative feedback provided by R14 stabiliZes the input buffer stage and the DC Working point. Because of the high frequency range of the circuit of FIG. 2, elements L1, L2, C5, and C7 stabiliZe by providing insulation from radio frequency pickup on output cables Without causing loss of
output signal. [0017] Good preampli?er performance depends upon proper selection of component values and the tolerances of those values. ID for JFETs J1 and J2 should match Within 5% betWeen 0.8 and 1.0 ma. For transistors Q2 and Q3, hfe should eXceed 300 and their Vbe at Icq=1 ma must match.
Microphones” (B. Ross, Recording, volume 15, number 1, October 2001). Acopy of this article is included as AppendiX A to this application. Speci?cation sheets for several T.H.E. products incorporating this invention are included as Appen diX B. Both AppendiX A and AppendiX B are incorporated
herein by reference.
[0021] Although various exemplary embodiments of the invention have been disclosed, it should be apparent to those skilled in the art that various changes and modi?cations can be made Which Will achieve some of the advantages of the invention Without departing from the true scope of the invention.
US 2003/0076967 A1
Apr. 24, 2003
IN STUFF! ENTER OUR 14TH ANNIVERSARY SURVEY I GIVEAWAY
i I
l
I
1
I I
I
2-;
I
I
a, I I I 5
Mg:
|»%
i! 1.
I
1
i
I
i
I
'
LEARN ERQM THE PRQS, ’
APPENDIX A
'_
BE PREPARED FOR. A SESSIQN... : ' 70/‘ Do It Xm/ve-[f
War/d- Class ,gua/i t y For The Masses '
_
‘A
EMAGVIC
l;x>24 AND EVPSb‘ TC WORKS SPARK XL 2.0 UNIVERSAL AUDIO LA-ZA CYCLING '74
voLuMi FIFTEEN
MAX AND NISP
"Wm °"‘
BINAURAL MIcs
v
'
FROM CROWN, MBI-IO, AND OKM SOLINDMAN
Apr. 24, 2003
US 2003/0076967 A1
A l l l I l ltl l l l l,‘l l 'l l‘ tllls lllllll ‘lllll'lllll ll l l l l l l l tl l l’ mllltlllllls tt
i
'
l
and encouraged by the (ahem) increasingly ween price and quality in studio mic zphones ' “good mics are getting cheapet
*rin theoldadag ' ‘'
.‘
'
You get what on pay for.“'l‘oda_\"
ZUU
oitnd order. of n aguitude better than any
‘ mic did ten yea “ tgo, hut th afliZllOOcontlenserm",w
.
Fortunately, there’s il n
'
''
ill don't sound as good tomthepreviousdc ado.
more to get a let more.
.
microphones reside. ’l‘.H.E. ('
nginc *ring) it
introduced il line of profes>ionul studio cont nser microphones, including a hinaural sphe a (see Robert Auld’s review elsewhere in this issue), a modular preamp with seven inlet hangeable cap sules, and two test‘ and measurement mics that are also suitable for '
recording.
In 11" ing to make tools that ar3 both technically l and ar tically inspiring. ' E. has made a product for \\ uch pr
havee ‘lybecna.
ondtn) (ortertizn J
isrcttsonablyttll'ordablt 'l‘hcv enot cheap; .lLl ' de>igned to compete with ll‘lllllt-ll'l?llsdllll dollar European tt n. lace“, but the ' do so palpitatiou
at‘ a price that at l . .l won't giv ' ‘tudio owners heart
And they do so without relying on ma ' ' produced parts that are
available to any company willing, to place a minimum order. T.H.E. president. '1‘ vlor John on gleefully
boasts “No Chinese camulc source of many oi’ those al'oieme tinned I'200 con~ den. er mic.‘ THE. builds most ol‘ their capsules in their own ' '
, the let
they don‘t in ke them
huilr to ’l‘.[ .E. . tecs by respcct'd micro
phone v3D is not the tirst micro hone at its type. The idea at a binaural sp ere was
superior to the KFM 6 in some res eds.
tirst pro osed by Gunther Theile in i986,
The BS-3D's selt-noise spec is slightly etter
recording a live iazz combo and as the
It was a ready known that it you mounted
(14 dBA vs. 17 dBA—both excellent tic?
primary microphone tor classical chamber
on omnidirectional transducer in a sphere, the response at the microphone" we become directional above a treque
determined by the diameter at the sphere The First commercial use at this principl was the Neumann M 50 condenser micro '
phone, introduced in the 1950s. I As the M 50 used a small‘ sp er
much larger than its one inch trans diaphra m, the polar response norr 7
only in the high treble, enabling an othé, wise omnidirectional microphone to ha'v
reoter "reach" and pickup ot instrument
detail. The spherical transducer mounting also ensured smooth ott~oxis response‘
especially when compared to conventiona directional microphones with the same size, ,
diaphragm.
ese characteristics made the M 50a ,7 good main pickup micro hone For classical orchestra recordings, on as such it become“ the basis at the Decca Tree, o three-micro
phone array invented by British Decca recording engineers in the early stereo era.” i, The sphere goes hinaural Thiele’s idea was to enlarge the s her to about the size at a ‘human hect
transducers on each side, where thev'ears Excerpted from the Ditch it S‘llll
Over the course of several weeks I used the B5-3D as a room/audience mic when
On paper the BS—3D actually seems
music and on: '
'
Ir
J
estro recording Th virtues
Apr. 24, 2003
US 2003/0076967 A1 11
v: mun
MDDULA
DAFADITEIR CAPSULES
RAM MODULAR MICROPHONE
Wm: DARDIDID CAPSULE
FREAMPLIFIER 45V - xLR-B
HIGH FERFCIRMANCE MICROPHONES
PROFESSIONAL SERIES For ages, man has sought (a mum,
qui?ry olmswark am mm by embodying M lonlxwnh the when
degile of form and mm)‘
mm ‘/1 INCH DMNI purruss BAPsuLz m advanuge 01 m; (mule dang" ‘5 m: irzqurvty rexpome curvuhal \s corrected for me i! a \irg: mam 0mm \he Dhg'ma‘ “Md mm.
m mm “pm; a high "equenmes we; a good bahnze and “mm wund Wm you are mm; amen! mm w m a
m 1.:
in stem: mmingm 5w m advantage :1 mmi'mlinglhe ‘nuH paint - and prexuvmg mare duxeiy M“ i! “any hiupan'mg m m reccvmng ml
emu-mum (apsules ave mum by Wm” veccrd‘mg alsnm scum: hknhnx: lewd in mg: ha‘lx, (huvches or w en a“
lmpmsmen: n! nearmesnumeJHs spenaily remmmendrd m (sunning m‘, m MW. .
Both mu: m xmw umts (an my, m oblamed ‘m mauhed
.
"an
an .wdmg in mmssss a IEKIVS
paws m mm: "mug;
,"EPEDIFIDATIDNE Dvrcunm pamm Operauor. primple (vunsducer) Sensmwt
mun mnidxmninna‘ was“ u
KMW Sub-Cardiold wumy \2 mV/Va
mm mm 1) "94-1759
Sne, hear “a (an.
THE ART EIF SOUND
Apr. 24, 2003
US 2003/0076967 A1 13 What is claimed is:
15. The preampli?er of claim 12, Wherein the polariZation
1. A microphone preampli?er, comprising:
poWer supply comprises a ?rst resistor connected to the ?rst
a cascade of an input stage and an output stage Wherein
resistor connected to the second radio frequency shield/ upper frequency limiter, and a capacitor connected to the
the input stage has an input capacitance, receives an input signal from a microphone through a coaXial cable, and generates as input-stage output an in-phase
output signal and an inverted-phase output signal and the output stage generates an ampli?ed in-phase output signal and an ampli?ed inverted-phase output signal; means for transformerless phantom poWering of the
microphone and the microphone preampli?er; means for sharing current betWeen the input and output
stages; means for actively feeding back input-stage output Wherein the input capacitance is loWered; means for actively feeding back input-stage output Wherein the coaXial cable is actively shielded; and
means for negatively feeding back input-stage output Wherein the input stage is stabiliZed. 2. The preampli?er of claim 1, Wherein the input stage comprises an impedance converter and a phase inverter.
3. The preampli?er of claim 1, Wherein the output stage comprises a buffer and an output driver.
4. The preampli?er of claim 2, Wherein the input stage comprises a cascode and a ?rst transistor.
5. The preampli?er of claim 4, Wherein the cascode comprises a ?rst JFET transistor and a second JFET tran
sistor.
6. The preampli?er of claim 5, Wherein the IDSS of the ?rst JFET and the IDSS of the second JFET are betWeen 1 ma and 0.8 ma.
7. The preampli?er of claim 5, Wherein the IDSS of the ?rst JFET and the IDSS of the second JFET match Within 5%. 8. The preampli?er of claim 4, Wherein the ?rst transistor
hfe eXceeds 200. 9. The preampli?er of claim 3, Wherein the output stage comprises a second transistor that receives the in-phase
radio frequency shield/upper frequency limiter, a second ?rst resistor and to the second resistor.
16. The preampli?er of claim 1, further comprising a printed circuit board Wherein the preampli?er is mounted on the printed circuit board so as to actively insulate a ?rst component and a connection to the ?rst component of the preampli?er from a second component and a connection to
the second component of the preampli?er.
17. A microphone preampli?er, comprising: phase inverter comprising a cascode and a ?rst transistor, Wherein the phase inverter receives an electrical signal from a means of producing the electrical signal from sound pressure, produces inverter output comprising an
in-phase output signal and an inverted-phase output signal of the electrical signal, and receives a ?rst feedback and a second feedback of the inverter output;
coaXial cable comprising a Wire communicating the elec trical signal to the phase inverter and a shield receiving a third feedback of the inverter output; a driver stage comprising a second and a third transistor
Wherein the second transistor receives the in-phase
output signal and produces an ampli?ed in-phase out put signal and the third transistor receives the inverted
phase output signal and produces an ampli?ed inverted
phase output signal; an input-stage poWer-supply conditioner receiving a residual current from the driver stage and supplying a voltage and current to the phase inverter;
?rst upper frequency limiter, Wherein the ?rst upper frequency limiter receives the ampli?ed in-phase out
put signal and generates a preampli?er in-phase output
signal; a second upper frequency limiter, Wherein the second
?ed in-phase output signal and a third transistor that receives
upper frequency limiter receives the ampli?ed inverted-phase output signal and generates a preampli
the inverted-phase output signal from the input stage and
?er inverted-phase output signal; and
output signal from the input stage and generates the ampli
generates the ampli?ed inverted-phase output signal. 10. The preampli?er of claim 9, Wherein the second transistor hfe and the third transistor hfe eXceed 300. 11. The preampli?er of claim 9, Wherein the second transistor Vbe and the third transistor Vbe match at Icq=1 ma. 12. The preampli?er of claim 9, further comprising a ?rst radio frequency shield/upper frequency limiter and a second
radio frequency shield/upper frequency limiter Wherein the ?rst radio frequency shield/upper frequency limiter receives the ampli?ed in-phase signal and generates a preampli?er in-phase output signal and the second radio frequency shield/upper frequency limiter receives the ampli?ed inverted-signal and generates a preampli?er inverted-phase
output signal. 13. The preampli?er of claim 12, Wherein the ?rst radio
frequency shield/upper frequency limiter and the second radio frequency shield/upper frequency limiter are con
a polariZation poWer supply communicating With the ?rst and second upper frequency limiters and supplying voltage to the means for producing an electrical signal. 18. The preampli?er of claim 17, Wherein the cascode comprises a ?rst and a second JFET transistor.
19. The preampli?er of claim 18, Wherein the ?rst JFET IDSS and the second JFET IDSS are betWeen 1 ma and 0.8 ma.
20. The preampli?er of claim 18, Wherein the ?rst JFET IDSS and the second JFET IDSS match Within 5%. 21. The preampli?er of claim 17, Wherein the second transistor hfe and the third transistor hfe exceed 300. 22. The preampli?er of claim 17, Wherein the second transistor Vbe and the third transistor Vbe match at Ioq=1 ma. 23. The preampli?er of claim 17, Wherein the ?rst tran sistor hfe eXceeds 200. 24. The preampli?er of claim 17, Wherein the preampli?er
nected to a phantom poWer source.
is mounted on a printed circuit board that actively and
14. The preampli?er of claim 13, further comprising a polariZation poWer supply that connects the ?rst radio fre quency shield/upper frequency limiter and the second radio
physically insulates a ?rst component of the preampli?er from a second component of the preampli?er.
frequency shield/upper frequency limiter to the microphone.
*
*
*
*
*
