Sparta AS -30B User manual

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SPARTA
AS
-30B
AUDIO
CO
SOL
TECH
!CAL
'1.AN
AL

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MIXERS:
INPUTS:
OUTPUTS :
F i
ve
AS - 308 CONSOLE
SPEC I FICATIONS
Nine stereo pairs; On e each for MLxers l through 4 and five
for Mixer 5, plus exte rnal AIR input .
Low-l eve l; - 55 db nominal from 150/250 ohms (50 ohm selected
by j umper), standard on Mixer l (optional for Mixer 2).
Hi -l evel; -10 dbm nomina l from 600 ohm source, stand a rd on
Mixers 2 thr ough 5. S ee note 1.
AI R; D epends upo n ex t erna l monitor-amp .
Program; 8 dbm into bal an ced 600 ohms at 0 vu . 22 dbm max .
Au
dition; 8 dbm nominal into 600 ohms single-ended . (from 60 ohm
so u rce . See note 2).
Monitor; lv nomin al into hi -Z l oad.
Cue; O. lv nomin al into hi-Z lo a d.
Phone s ; 1 mw nomin al into -lOK ohms.
FREQ. R ESPONSE: A ll outputs less t han 2db down , 20 Hz to 20 kHz .
NOISE:
D
ISTORTION:
C Ra3STALK:
POWER:
SIZE:
SHIPPING WT:
MUT I NG :
NOTE : 1:
NOTE 2:
Bel ow 65 db at 0 vu out with - 55 db low level input
Bel ow 75 db a t 0 vu out with -10 dbm hi -level input
A ll outputs less than 0. 5% THO at norma l operating levels .
L
ess than 1% THO a t max outputs .
Below nois e in a ll channe ls and modes
117 /234 vac , 50/60 Hz.
W , 15 1/2"; H, 6 5/8" ; D, 10".
24 lb s (incl. pwr . supply )
Switch clo s ur e to rear-panel t erm inals from !\ILxers l to 3, clos ed
in Aud . & Prog. modes for control of external DC relay s .
H
i-le ve l inp uts _ar e balanced-bridging to allow individual termination
of input lines ;.it rear panel, if required . Permits constunt lin e lo :id
pl
us u se of higµ-imp c dance sources .
I
dentic al line amplifiers for · Prog r am and Audition outputs .

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SPARTA ELECTRONIC CORPORATJON
D
ear Customer,
C are fu l attent ion to QuaJity Control is an other important element in our daily effor t to provide you
with
excellence of product and service. At SP AR TA each pi e ce of equjpm ent and sub-as se mbly
receiv es numerous in spect ions and te s ts in th e p rocess of p rod uction. Th e final result s must m easu re
within our exact ing requirements before it is shipped to you. Listed below are just a few of the major
check points and tests this particular pie ce of e qwpment has r e~eivcd be for e be ing prepared f or
shipmen t. Should you note any discr epa ncy in the appearance or operation of your SPARTA Pr oducts
or if you h av e any gene ra l comments as to how we might be of greate r service , your suggestions
will be gr ea tly appreci ated.
AS-308
Custom er : U.-.,,._ .:,v- .. liv •-~~ , u -,'<-L''<-· l:rp SERIAL NO._--'-J_______.._f_.' / ___ DATE: <r; //4 /.,,--, )
(1-r,1-
PWR . SUP. SERIAL NO: VJ j (7 CAL. BY ✓ .2- U<.....Q No. OF PREAMPS
' --~--- ----= =- ----
OUTPUT DISTORTI
ON
SIGNAL/NOISE
PHASE
CH ..: lL:
<~
, / ':i
-%
r,', q ·
+ dbm -db
.r.t. I,
CH- lR : +
,_ / ,
dbm I 7z; % C; y
db
CH-lL Freq. Resp.
-2db at
Hz
-2db at
KHz
l
KHz Ref .
--------
CH-lR Freq. Resp. -2db at
Hz -2db
at
KHz
--------
MONO -S TEREO MIC.R & L MONO-STEREO OUT CROSSTALK
CH-2L
+
/'-
.. 0
dbm
I /4
%
-r
/".
db
CH-2R +
,-,~
dbrn ' 1~ %
2' '?('
db
Fr e q. Resp. CH-2L I 1
11
-::;:- 2 ~ l(
CH2-R
11' - )
/'l{ L evel
CH-3 L +
~
d bm
~ L& % /
9' db
CH-3 R +
dbm
I I
-1-
%
/Y db
Freq. Resp . CH-3L
z.u -2
(f f{ CH3-R (
7- :; Cit (
CH-4 L + <..
""~
V
dbm
,, I 2-
% )
'-1
db
CH-4 R + <~,;- d
bm
., LI_
% ,
--
-v
db
Freq. Resp. CH-4L
I.Cf
?, r A
CH4 -R I -
,-;
--:.,.~
I
CH-SLATh .E +
..,.
~-----
£:'.'
dbm
" )7 % , D db
CH -5 R l',.. Th . E +
c..~
dbm
7'7
%
~ ~.
db,
., ,,
Fr eq, Resp . CH-SL
?-(.I - 2
':J''l[
CHS-R
/_
.. ~ -2 .. {r
I -•' t
(..,
Audition L +
,. ,~
dbm
I
I'}_
% ~
ZtJ
db
Output R + C
dbm
~ I u %
db
Monitor Air R & L L Pr og R & L t. Aud R & L
----- ----- -- -----
P HONE
CUE MUTING
---------
----~- -- -- ----------
TEP-3S
ASC-30 5B
Wow & Flutt er RTT
Calib by Ca rt.
------ - ---
LTT
3/29/71

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GENERAL:
AS-30B CONSOLE
DESCRIPTION
The AS-30B is a desk-type Stereo Audio Conso le featuring five mixing channels with
push-but ton-selected multiple inputs for Mixer 5. As normally supplied, the first
mix er inclu des low-l evel preamplificrs for 50 to 150 ohm microphones and the
rem aining four arc supplied with high - level balanced inp ut transformers . The inp ut
transform ers for ML"'<.er l are located on cards tb allow replacement with another pair
of low-l evel preamplifiers.
The hi gh -l evel i nputs are balanced bridging, wh ich avoids the source l oading of th e
typical 600 ohm i nput and greatly increases the flexibi~~ty of the console. In the very-
rare in stance where a 600 ohm termination is required, it is a simple matter to add a
pair of 620 ohm resistors across the line at either the source or lo ad end . Some
sources, such as Ampex and TEAt, provide for this int ernally. When operating from
sources intended for single - ended, high-impedance lo ads ,t he minus input terminals of
th e consoles are connected to the shield and the conso le now appears as a single-ended
h igh -i mpedance (l0K) load .
T
he Audition and Program Amplifiers in the AS-30B are id entica l and interchangeable .
In
the Audition side , the li ne-amplifier gain is de~ermined by internal resistors and is
set to provide approximately the same l eve l output as the Program side with normal
· g ain control settings . The audition output terminals are fed directly from the audition
lin e amplifiers from a single-ended source impedance of 60 ohms. The audition out -
put is then capable of driving multiple l oads with little, if any attenuation.
The input-so urce selection for Mixer 1 is via a push-button assembly which allows
more than one source to be selected simultaneously . This feature , if used judiciously,
will permit more d1an one source to be mixed simultaneous ly on the one mixer pot.
If this is attempted, the individu al source levels must be adjus t ed externally but .
th e console will not lo.id either source ·. It must be considered , however, that each
-1-

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AS -3013 D escription (con t'd)
of t:wo sources will then be loaded by the other , and suitable isol atio n mu st be pr ovi ded,
if requi red .
INSTALLATION:
Input and Output Connections, wit:h the exception of the Mixer 1 XLR
co nnectors , are made vi a rear-panel b::irri er strips . Spade lu gs or fanning-strips are
n ot required s in ce the barrier st rips arc d es igned with captiv e plates to easily
accommodate severa l str ip ped wires per termin al, either solid or stranded . Rear
p ane l connections aJ:c clearly id en tified to facil i tate installation without resorting to th e
manual or to a numbered diagram .
Audio
connecti ons are norm a lly made with twisted-pair , shie lded cab le such as Be l den
8737 ( strand ed) or 8739 ( solid ), Either may be us e d, although it is generally more
sa ti sfactory to use solid wire in a permanent i ns t allation ·where flexibility is not
r equired since solid wi re is eas i er to han dle and l es s apt to i nadvertently short to an
adjacent terminal. S ingle-ended outputs and in puts can use the same type cable by
simp ly clippin g off d1e unused conductor . Th e input t ermina ls _ include a ground
con nec tio n for the cable shield and the transformer input windings are ungrou nde d or
"floating" . This prev ents the possibility of setting up a severe ground- Ioo.p and al so
J:)2rm it s one side of the transformer i nput to be grounded for single -ended operat ion
if desired. In like manner , th e program line out :lt1ts are al so isolated .
A central ground lug is provided on the r ear pan el for connection to the system
m as ter ground, and as heavy a guage strap or braid as pract ic al sh ould be u se d.

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CIRCUIT DI:SCRIPTlON , SPARTA AUDIO CONSOLES
GENERAL . The description to fo ll ow is generally applicable to all
SPART A consoles , m ono or stereo, and out lin es the b as ic syst em of
selecting , mixing and amplifying the usu a1 program- sources p lus th e
me
th ods used to process auxilliary functions such as cue and monit or
signals. Features applicable to specific consol es are t hen discussed
elsewher e in th e appropria te manu al section s .
Microphon e Preamplifiers . With a nomina l gain of SSdb . the output
l eve l from the preamp to the mixer potentiometer is ty pically -1 0
to Jf dbm . depending , of course, on th e sound l eve l and microphon e
u
sed. In a norma l s ituation, then , th e preamplifier output is at
essentia ll y the same l eve l as th e high- l evel input signals , resultin g
in approximate ly th e same mixe r-l eve l settings for normal operation .
Pr eamp gain can be changed to accommodat~ unusua l situations by
ref erring to the preamplifier circuit description.
As n orma lly supplied by SPARTA , t he ·i nput tran s former is wired for u se
with 150/ 250 ohm microphones . A transformer tap is _ provided for
easy conv e rsion to use with 30/50 ohm mi cropho n es .
All SPARTA Aud io Consoles are supplied as standard with a microphone
preamplifier in th e firs t mixer position . Th e preamplifiers are int e rch ange -
abl e with high- l eve l input cards , how e ver, so additional microphone
l.

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CIRCUIT DESCRIPTION, GENERAL
pre amp lifi ers may be inc orporated , or alternately , all inputs may be high -
l
evel .
High - Level Inputs are brought to the primar~es of input-i so l ation
transform ers which are suitably terminated and connected directly to
t he proper l 111ix er potentiometers . The primaries are floating (unground ed)
to a ll ov1
1 s
in gle-ended connection and also to prev e nt the possibility
of setting up undesired ground-loops or common - mode si gnals via th e
inp ut cables.
Mixing ta kes place by feeding the output of each mixer pot e ntiometer
t hrough a high-valu e series resistor (l0K or 27K) to a common mixing
bus, which in turn l eads to a mixing amp lifi er . Th e mixing amplifier,
which is usually lo ca ted with the line amplifier, is specifically designed
to have a v ery l ow input imp eda nce , typica ll y 100 ohms .
Since the Mixer feed resistors are so much higher in value , the mixer
amp becom es a "curr en t sink" and responds to the current in the feed
resistors , which in turn is de termined by the voltage available from th e
Mix er pots . The output of th e mixer amp , then, is propoi;-tional to
th e sum of th e currents at it s input.
2.

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CIRCUIT DESCRIPTION , GENERAL
Mixer isolation is excellent with th is system since the current from
each fe ed resistor will fol.low the easiest path - - obvious ly into the
mix er amp rath er than to another Mixer pot. Changing one mixer-pot ,
t
hen , will have relative ly littl e effect upon the program level coming
from another mixer - pot.
There is obviously a significant loss of program power in the mixer
feed re. sisters , but th e power l eve ls are so low that it is a simple matter to
recover the l oss in th e mixer amplifier. Mixer amp. ga±n is set to restore
the program to substanUally the same level as originally fed to the
mixer pots , with the mixer-pots at their usual operating positions ,
about one o'clock.
Since a "current" mixer operates from a rel a tiv e ly high source imp eda nce,
it allows the use of high i mpedance mixer-pots . And since it is not
critical of t he precise source impedanc e , thete is no need to resort to
the bulk and cost of precision ladder attenuators .
Output of the mixer system is t hen fed to the Master gain control at
essenti a lly the same l eve l as the origina l high -l eve l program material.
To t est the entire mixer system , a - 10 or~· dbm signul is provid ed at
a high-l evel input , th e appropriate mixer pot is set at th e normal
operating position and the signa l is viewed or measured across t he
Master guin contro l.
3,

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CIRC UIT DI:SCRIPrION , GENERAL
The Linc Amplifier provides th e final amplification between th e Master gain
control and the li ne output terminals . It must therefore recover t he signa l
l
oss of th e Master gain control , compensate for any losses in matching
or isolation pads , and provide power -gain for driving the output li ne at
rat ed l eve l.
F or a rated output of 8dbm into a 600 ohm li ne at zero VU , several factors
must be considered : A minimum of 10 dbm of add ition al gain , without
distortion, mu st be available to handle program peaks ( headroom ). Also ,
th e output transformer must be isolated from the line with a pad , typic a lly
4 · db , to prevent int eract i on with line reactances . Fin ally, the li ne
amplifie r output - imp e dance must properly match th e lin e t ransformer primary
so the output te rmin als a re a tru e 600 ohm source.
The lin e transformer must th en handle a nominal l evel of l 2dbm, and 22 dbm
on program peaks . The lin e amplifier must be capable of supplying 18dbm
to th e line plus the power -l osses in the li ne pad , insertion loss in the
t ransform er and internal l oss necessary to obtai n a proper imp edance match .
This is considered in detail in the line-amplifier section of the manua l.
VU Meter . The standard VU m eter contains rectifier diodes and consequently
appears as a non - lin ear imp edance to its signal source . To avoid introducin g
4.

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CIRCUIT DESCRIPTION , GENEHAL
dist ortion on th e p rog r am -lin e , it is necessary to provide isolation be tw ee n
th e meter and th e line.
It is a l so n ecessar y to a ttenw::ite the l evel to the mete r so it can indic ate
~ vu when the line is a t 8dbm and finally, the attenuator must be so
desig ned as to appear to the meter as a 3 . 9K ohm source to p reserve th e
.
lin earity of th e me ter in dication .
This l atter requirem e nt accounts for the meter p ad being a "T" configuration
r ather th an a simple se ri es r esistor . In SPARTA consoles, th e metering source
is th e lin e transformer output, ah ea d of thG line pad , which al.lows an added
4 db of attenuation in the met er pad for increased i sola t ion. A balanced
"H" pad is not required since t he met er n eed not be ba l anced to ground.
Cu e . Common to a ll SPARTA consoles is a " Cue" position on each Mixer
pot entiome t er. In t he simpl es t case, program materi ol from each mixer ,
thr ough th e cue switches and isolat i on resistor's , i s delivered t o a rear - pa nel
te rm ainal for feed to a n e xt erna l cue amplifjer . In more elaborate consoles , a
cue l eve l control, i nterna l ampli.fier and optional-u se internal speaker are .
p
rovid ed .
Monitor. Although provis ion s for program-line monitoring vary in different
SP ARTA conso l es , in a ll cases th e source of program mc1 t erial is th e output
5.

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CIRCUIT Dr-:SCJUPrION, GENERAL
stage of the progrum line amplifier. Since there are no active or non - linear
components to follow , the monitor provides constant assurance of both level
and quality, whereas the VU meter alone only indicates level.
6.

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CIRCUIT DESCRI PTION/MAINTENJ\NCE
.PREAMPLIFI ER , 1 018
The 1018 Preamp li fie r i .s supplied standard for opera ti on with 1 50/250 ohm mic-
rophon es and pro vi des a nom in al gain of SSdb into a 600 ohm or hi gher load. /1.
ju mper is pro vided on the circuit bo ard , eith er a wire or a low-valu e resis t or , to
accommod a te 30/50 ohm mi crophones .
The 101 8 Preamplifier differs from its predecessor , the 1008A, in several resp ects .
Th e microphone transformer is a min i atu riz ed PC-mounting type with extended hi gh
and tow frequency re spo nse . The 1018 circuitry t akes full • advanta9e of th e tran s-
former ' s BOK secondary imp edance to provide improv ed noise performance , expand-
ed frequency response and excellent overload churacteristics .
With normal mic r ophone in put l evels , the output level to th e mixer will be approx-
0 dbm. The maximum output l evel of the 1 018 before clipping is in excess of 14
dbm, wh ich assures more than adequate h ead-room in normal operation. In th e
event th at microphon es must be u sed, which prov i de unu sua lly high output , a pad
may be added in ser i es with th e mi crophone or a resistor in the 101 8 may be
ch anged in value to l owe r th e preamp ga in, as described l ate r.
T he i nput signa l is amplified approximately 20 db by t he inpu t tr ansfonner and then
amplified a fur t her 3Sdb by transistors Ql and Q2 . Th e ou t pu t signal from Q2 th e n
passes t hrough emHte r -follower Q3 to'provid e the - necessary l ow output Imp edance .
Th e em itt er of Q2 is heavily bypa ssed to ground by capacitor CB; t herefore th e
base imp cdnnce of Q2 is q ui te lo w , being essentia lly a forward - biased diode;
l.

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Most o[ the signc1l current from Ql posses to the base of Q2, then , ra t her than
throu gh the much hi<]her impedance of the collector resistor R3 . This assures
maximum current gain from Ql, a lthou gh the voltage gain is relatively small.
The emitter of Q3 is not bypassed, however , so the : base impedance of Q3 is
re lati vely high compared to Q2 's collector resistor R4. The signal current from
Q2 must therefore cause a significant signal voltage across R4. This assur es a
maximu m of voltug e gain from Q2 , and since Q3 is an emitter-follower this signal
appears at the low impedance emitter of Q3 .
A
portion of the output signal determined by fe edback attenuator R8/R9 app ea rs at
the emitter of Q1. Sinc e Q1 and Q2 both invert the signals through them, the
feed-b a ck to the emitter of Ql is in-ph a se with th e signal input to the base of Ql .
This consitutcs ne gative feed-back , sinc e the feed-back attempts to cancel the
input signal. Th e open- l oop gain ( with o ut feed-back) is extr emely high, so the
normal closed-l o op gain is de termined by the turns- ratio of th e input transformer
plus th e ratio of RS to R9. It can now be seen that changing th e value of either
RS or R9 will change the gain of the amplifier proportionally. For exampl e , if R9
were to be incre a sed to 360 ohms, the voltage gain of the amplifier would de crease
by Gdb, since th e output of th e amplifi er would only need to swing half as far to
provid e the sam e feed-back voltage to t he emitter of Ql . Cc , ms e quently , reason-
ab le sel.cct~on of ga i.n cun be obtained simply by se lecting the w1 lue of R9 .
The use of negative feedback to control gain via the input emitter also has the
effect of raising th e input imp e dance to Ql . Thjs pe rmits the use of a high-ratio
2.
.,

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input
transformer
for
added
voltage-guin
without
added
noise.
Distortion
is
also
lowered
with
negative
feed-back
since
any
difference
between
the
input
and
output
wave-forms
appears
as
an
error
signal.
Distortion
in
the
1018
is
held
well
below
0.1%.
A
stable
operating-point,
or
"Q",
js
one
of
the
most
important
characteristics
of
an
amplifier
because
the
maximum
signal
output
without
clipping
and
therefore
the
headroom
is
dctcrmim~d
by
the
average
DC
voltage
at
the
emitter
of
Q3.
The
emitter
can
move
no
higher
than
the
supply
voltage
and
no
lower
than
ground,
so
the
idea
1
operuting-point,
permitting
maximum
dynamic
range,
is
logically
some-
where
near
to
one-half
the
supply
voltage.
The
emitter
voltage
of
Q3
is
set
by
the
collect
cir
voltage
of
Q2
and
since
the
collector
current
of
Q2
is
also
the
emitter
current
of
Q2,
any
change
in
the
coJlect-
or
voltage
of
Q2
also
~ppears
as
an
inverted
change
at
the
emitter
of
Q2
..
The
bias
current
for
Ql
is
obtained
through
RG
from
the
emitter
of
Q2;
therefore,
any
change
of
emitter
volta9e
at
Q2
results
in
a
corrective
change
of
bias
current
to
Ql.
This
not
only
provides
normal
operating
bius
for
Ql,
but
any
change
at
Q2
is
also
fed-
back
to
Ql,
as
an
error
signal
to
maintain
the
proper
opNaUng-point.
Maintenance:
from
the
foregoing
circuH
description,
it
can
be
surmised
that
failure
of
any
component
involved
in
the
DC
biasing
of
the
circuit,
including
leaky
capacitors,
will
cause
a
shift
in
the
opernting-·point
of
Q3.
Stated
in
c1
n1'xc
use~ul
manner,
if
the
emitter
of
()3
is
at
a
retisohable
voltage,
then
all
of
the
components
involved
must
be
functioning
normally.
3.

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During troub le - shooting most components , i nc ludin g th e tran sis tor s , can
be remov ed from ques ti on simp ly by ma king two DC voltage mca suremcnts :
Fir s t, the supp ly voltage at t ermi na l 3, and second , th e volt age at th e
emitter of Q3 . When we consider that res i sto rs may va ry 5% or 10% and
i ndividu a l tr ans istor gains may va ry by a factor of 2 or more , it is
r easonable to expect a possible va riation of±. 20% or so in th e operating
po int. Thi s , th en , wou ld i ndica te a reasonab le voltage range for the
emitt er of Q3 of from approximate ly 10 . 5 to 15 . 5 volts. In the event of
cata s trophic fa ilur e of an y of th e r es istors or tr a nsistors, or of excessive
le akage or shorts in any of the capacitors , it wi ll c!lmost inv a riubly
result in a gross s hift of th e operating-point t owards t he supply or t owards
ground . If th e operating-point is beyond th e limit given bu t still cap a ble
of moving further in e ith er direction , t he amplifier w ill still be operative ;
it simp ly wil l not have as much dynamic r ange or h ead _ room .
If th e operating point is found to be correct , but th e amplifiG r gain is
abnorma ll y high or l ow , ( which usuully would be accompanied by high
di st ortion ) it would most li ke ly be due to either an open capacitor or u
def ecti ve input transformer . Tran s istor failure wo uld norm a ll y not be a factor
· s imply becc::iusc , at audio frequencies , a transistor cannot te ll the difference
between AC and DC , so we would l ook for a component wh i ch could alter
th e sign.:11 gain without affecting th e DC opera t ing-pojnt . Excessive n oise ·
can be due to almos t any compon e nt . The most likely susp e cts would be
t he input tr a n sis t or , Ql , fo ll owed b>' the resistors and capac it ors a ssoc iatcd
with th,: first stage since noise generated th e re would be subject to th e
m
ost amplification .
4.

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CIRCUITRY DJ~ SCRI PrION/M/\lNTENANCE
Mixer & Line Amps 1020
The 1020 board consists of two independent amplifiers; a mixer amp and
a progr am line amp . The mixer amp is characterized by very low input
and moderate output impedances and the program line amp by hi gh input
and l ow output imp edances .
Mix er Amplifier : Th e input signals to the mixer amplifier are obtained
from a group of mixer potentiometers and each one mus t be capable of
being switched or adjusted in level without affecting th e level coming
from the others. The mixer amplif ier is specifically designed for very
l ow input Z (1 00 ohms ) and is supplied signal currents from th e mi xe rs
t
hrough high value resistors of lOK ohms or more. Each input current ,
t
hen, follows the path of le ast resistan ce into th e mixer amplifier ,
ind epe nd en t of the condition of the other mixing ch an · nels , th ereb y p ro viding
exc e ll ent isol ation be tween mixer channels .
With a very low input Z and high source 3, the input signals t ake th e form
of a current which is a lin ear function of the source voltage. The unusu a lly
l ow distortion characteristics acc ru e in part b ecause small changes of a n
alr eady low in pu t 3 ha ve virtually no effect on the signa l input curren ts .
Obviou s ly , the v oltage attenuation of th e mixer system is quite high -- it
is the input current which determines the output volt age of the mixer amplifier .
Th e output imp e dance is sufficiently low to serve as a voltag e -sourc e for
th e Mastc'r gain control. The mixer amplifjer th e refo re operat es in th e
1.

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Mixer Amplifier ( cont'd )
"tr
ans-impedan · ce" mode ; that is, the output voltage is a line a r function
of the input current, not the input voltage. P: measure of voltage gc1in is not
appropriate unless the input volt age is appU.ed through a series feed
r esistor and the measured gain wi ll then depend on the value of this
r esistor as well as the amplifier gain.
Circuit Operation : The first stage of the mixer amplifier Ql, is a con-
v enUonal grounded - emitter amplifier with operating bias determined by
cu~rent from the collector via R3 . I\ decrease of collector voltage will
cause a decrca se of base current, r esulting in a deqrea se of collector
current which tends to raise th e collector vo_ltage . R3 then, forms a
n
egative feedback path which assures that Q l's operating-point remains
within a reasonable r ange . Norma l operation of Ql will be obtained at
any collector voltage between approximately 5 and. 20 volts .
Bias resistor R3 a l so performs a secondary function : Since signal volta·ge
from Ql ' s collector is also fed back to the base , it is a negative-feedback
path to the signal, as well, l\n input signal current to the base of Ql is met
by an opposing signal current from R3 , with th e result that R3 appears to the
input signal as a shunt impedance much l ower than the schematic valu e .
This contributes t owards l owering the input impedance of th e amplifier.
Th e second stage Q2, is biased in the same manner as Ql by RS . Although
RS does have the effect of lowering the input imp e dance of Q2 , it is n ot
n ea rly so effoctive since the voltage gain of Q2 is limited by the un-bypa ssed
2.

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Circuit Operation : ( cont 'd)
emitter resis t or RB; th e refore, the relative feedback signa l current is much
l ower.
The v oltage gain of Q2 is determined by t he ratio of r esistor RB to th e
collector l oad impedance R7 . Conside r Q2 as having a ty pica l beta of 100,
and no ex ternal l oad on the co ll ector . The em itt er r esistor RB carries the
coll ector current p lu s the base cu rr e nt, but since th e base c urr en t is
only 1% of th e collector ctn-rent , we can assume the emitter and base
curr ents to be t he same for a ll practicc:il purposes .
Th e emitter is not bypassed, so a change of voltage at the base r es ults in an
equal change at th e emitter . This in t urn c hanges both emitter and collector
current s by the same amount. But the co ll ec tor current is flowing throug h
a r
es i stor , R7 , which is t en tim es l arger th an RB , so it must cause ten
~im es the voltag e -change. The vo l tage-gain of Q2, then, must simply be
t he r atio of R7 to RS, or ten . In n ormal application, Q2 i s loaded by the
Mast er ga in control (SK ohms) so th e AC collector load imp edance is 1 . 75 K ohms ,
in st ead of 2. 7K resulting in a n orma l stage gain of approximate ly 6 . 5,or l 6db .
The unbypa ssed RS , th e n, forms a third .n ega tive-fe edback path - - this time
to fix th e ac gain - - because the co ll ector current through RB produces an
e m itter volt age change which t en ds to cance l th e input base volt age , t hereby
li miting the gain .
Th e co ll ec tor volt age of Q2 , lik e Ql , is not critic a l. Th e nominal signa l
.
le ve l at the collector of Q2 is Jr dbm , or about 1 volt r ms . If we a ll ow for
l 2db · of headroom , or 4 volts rms, t he co ll ec tor must be free to sw in g
12 volts peak-to-peak or 6 vol ts in either direction . So an operating-
p oint between approximate ly 8 and 18 volts will assure normal operation .
3.

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Cii:cuit.QQ_cration ( con t 'd)
The signa l voltuge at the emitter of Q2 is the same as at the base of
Q2 und so is a second source of shunt n ega ti ve feedbac k to the base of
Ql . On ly this time , th e source is a l ow i mpedance (RS) permitting the
f eedb ac k resistor R6 to be low , al so . R6 , then , is a fourth n cgn tive-feed bac k
path und has th e most significant effect in low e ring the input Z of Q1.
In addition , the source for R6 is a voltage which is a fixed portion of
the output volt age . Therefore , R6 also serves to set the gain of Q l.
Wh en an input signal current is appli ed to Ql , th e signal v oltage at the
emitter of Q2 can only ris e to the point where the combined foedba ck
currents from R3 and R6 approach ca nce ll a tion of the input curr en t , with
b thc curr ent through R6 being dominant.
V ery low distortion result s , typically O. 035% , since any dif fe rence of
amplitude or phase between input and feedback currents appc~ars as an
error signa l . The output volt age is th erefore a lin ea r reflection of the input
current , which in turn is a linear fun ction of volt age at the source end of t he
m ix er feed re sistors .
Gai.!!
of the mixer amplifier may be verifi ed by applying an input signal
cun- ent of l O microamps through a mini.mum resistance of 2K ohms . Sign al
l
ev el at the co ll ecto r of Q1 should be c1bout 130 mv, and at the co ll ector
of Q2 ·n ea rly 850 mv with the output connected to the SK Master gain pot .
With no external lo ad , th e output should be near er to 1•3v .
4 .

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Circuit Operation (cont'd)
Do not be concerned by l O or 20% gain variations, since many l 0% r esistors
a re used and are easily compensat ed for by the normal control settings .
Line l\mplifi er : _ . The i nput stage , Q3 and Q4 , is a differential comparator
.. ~.: ~
which performs three separate functions : Fi r st is s i gnal amplification wherein
03 operates in the transconductanc e mode; t hat is, t he collector current is a
function of b ase vo l tage . Sccond, a l arge proportion of th e average de
voltage at the emitters of Q6 and Q7 , which appears at the base of Q4, is
compar~d to th e ba sc volt age of Q3 to stabilize th e operating-points of the
output transis to rs. And third, a small proportion of the output signal , which
is fed back to th e base of Q4, is compared to the input signal at the base of
Q3 , th ereb y s e ttin g th e ac gai n .
Wh en power is first a ppli ed , Cl3 must be charged to one-half of the suppl y
volt age through Q6 , and to protect Q6 the charging - rate must be li mited .
Q6 cannot turn on until QS and he nc e Q3 b eg in to conduct. But Q3 cannot
conduct any faster than C6 can charge , so C6 not only filters th e bias curren t
to 03, but a l so controls the charge-rate of Cl3 .
T he i deal de operc:iting-Qoint for th e output transistors Q6 and Q7 is one ha lf
of the s upply voltage , because this is the point at which th e greatest peak-
to-pC?ak output-volt a ge swing, or headroom, is available . When first turn ed
on, C6 charges and the bosc of Q'3 is brought to 40 % of th e supply volt age .
As C13 charges , th e volt age common to th e emHt e rs of QG and Q7 ris es
tow urds th e supply. Up on reaching ono-half of th e supply vo lt age , 80% of this (or
4 0% of the s upply voltag e) r ea ches th e base of Q4 via de divid er Rl 6 , Rl 5,
5.