Mission 776 User manual

MISSION OWNER'S MANUAL
MISSION 776/777 UNPACKING-INSTALLATION
MODEL 776
The followin items are included with the pre-amplifier.
- Warranty card
- Owners manual
- Pre-amplifier line cord
- Banana plu
- 1 spare line fuse
MODEL 777
The followin items are included with the power amplifier.
- Warranty card
- Owners manual .
- Power amplifier line cord
- 1 spare line fuse, 2 spare output fuses
If any of the above items are not included contact your dealer or the factory.
IMPORTANT: REFER TO OPERATING INSTRUCTIONS PRIOR TO USING SYSTEM.
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MISSION 776 OPERATING INSTRUCTIONS
(Bracketed numbers refer to panel drawin s)
FRONT PANEL
1. ON/OFF SWITCH (5) & (9)
There are two push button switches situated on the ri ht hand side of the front panel, the switch on the
extreme ri ht is desi nated "S1" (9) & beside it is "S2" (5).
With switch "S1" in the "out" position & "S2" depressed the pre-amplifier is powered from the internal
battery & all A.C. line currents are disconnected. After about 20 seconds the reen L.E.D. (7) will li ht
up indicatin the normal operatin mode of the amplifier.
When both switches are in the "out" position the amplifier is totally dead & is in its transportation mode.
With switch "S1" depressed (S2 in either position) the pre-amplifier is shut down & the battery is put on
char e.
When this is done the red L.E.D. (8) must always come on to indicate full char in mode. If the battery
requires full char in the red L.E.D. will remain on; otherwise, after a few minutes the yellow L.E.D. (6)
will illuminate indicatin trickle char e mode.
The switch over from full char e to trickle char e is done automatically.
In order to avoid the situation of havin a dischar ed battery just when you wanted to use the
amplifier, it is recommended that after normal use the pre-amplifier be switched to char e mode. The
battery can be left on char e for several days & weeks ready to be used. A fully char ed battery can
produce optimum performance for as lon as 20-30 hours of continuous use,
2. INPUT SELECTOR SWITCH (1)
The input selector switch is located at the extreme left of the front panel. The input selected is indicated by
the linin up of the indentations on the selector switch knob with the push button (2) just under it. The
middle indentation represents phono, left indentation is tape & the ri ht indentation is auxiliary input.
3. VOLUME CONTROL (3)
The volume control is the knob with pro ressively enlar ed indentations which indicate volume settin .
Its ali nment is relative to the push button (4) just under it. The volume control adjusts ain from -70db
(small indentation) up to 0dB or full ain (lar e indentation) in a stepwise fashion.
4, TAPE MONITOR SWITCH (2)
This is the push button under the input selector knob (1). Tape monitor is selected by depressin the
button.
5. MONO/STEREO SWITCH (4)
This push button is located under the volume knob (3). Mono operation is selected by depressin the
button.
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BACK PANEL
NOTE: the top row of audio connectors represents the LEFT channel & the bottom row the RIGHT channel.
1. A.C. CONNECTOR HOUSING (10) & (11)
The detachable line cord connects to the 776 at A.C. input (10). Access to the line fuse (11) is possible only
when the line cord has been disconnected & the clear plastic window is moved aside. The fuse housin (11)
also contains a small universal volta e card that selects the correct line volta e for the country in which the
amplifier is used. See precautionary measures for correct line volta e & fuse ratin .
2. GROUND (18)
Most hi h quality turntables are manufactured with a separate roundin wire. This wire does not carry
si nal current but its provided for shieldin purposes. It is very important that the tonearm metal work
& the platter be at the same potential as the pre-amplifier chassis. If a separate roundin wire is not
provided then the shield connection is automatically done throu h the turntable interconnects -
althou h this method is not as effective as the separate wire approach.
Included with the pre-amplifier is a solder-less banana plu for connectin a separate round wire. Strip ¾
inch of insulation from the wire & unscrew plastic cap from banana plu . Push the wire throu h the small
hole of the plastic cap & into the centre hole of the metal plu so that the stripped portion of wire extends
out the side hole. Wrap the bare portion of wire around & under the exposed metal collar of the plu .
Ti hten securely the plastic cap to the banana plu . For a proper connection no bare wire will be visible.
3. PHONO INPUT (17)
The phono input will accept si nal from any movin ma net or hi h output movin coil cartrid e.
4., AUXILIARY INPUT (16)
This input will accept any line level connection. Most often this will be the output of a tuner, but it
could also be the output of a second tape recorder. This would then allow tape copyin if another
recorder was connected via tape input.
5. TAPE INPUT (5)
This input will accept any line level connection. Most often this will be the output of a tape recorder,
but it could also be the output of a television with stereo
6. BALANCE (14) IMPORTANT NOTICE
In spite of the special care taken in implementation of the balance control feature (use of the hi hest
quality switches & incorporatin the circuitry in the feedback loop, in order to further reduce any
adverse effects of this feature), our recent findin s show that this control is somewhat detrimental to
the hi h quality sonic reproduction of the 776.
Given our commitment to strai ht line philosophy & the type of associated equipment which would
invariably be used with this enre of equipment the usefulness of a balance control, at all, is questionable.
Considerin the pros & cons, Mission have decided to eliminate the balance control feature in the new
eneration of the 776 pre-amplifiers.
Dependin on the production batch which you will receive, you will find that the balance control
switch (back panel may or may not be installed. On models where the switch has already been installed
the balance control circuit has been disabled.
7. TAPE OUTPUT (13)
Interconnect cables run from here to the line input of a tape recorder to permit recordin from any source
as selected by the input selector switch. That is, phono, tuner, television, or another tape recorder.
8. PRE-AMPLIFIER OUTPUT (12)
Interconnect cables run from here to the main input of the power amplifier. This output can also feed
an equaliser which in turn would feed the input of the power amplifier.
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SPECIFICATIONS
MISSION 776 PRE-AMPLIFIER
Input Sensitivity:
Phono: 2mV. for 1 Volt rms output
Line: 100mV for 1 Volt rms output
Input Overload at 1 kHz: Phono sta e 150m.v.
Output Level:
Nominal 1 Volt rms
Maximum 11 Volts peak
Input Impedance: 47kΩ, 150pF.
Output Impedance: 250Ω, 4µ7F.
Si nal/Noise:
Phono input 80dB "A" wei hted 20Hz-20kHz (ref. 5mV input)
Line input 95dB "CCIR" wei hted 20Hz-20kHz (ref. 200mV input)
HUM >100dB below rated output
Volume Control: Channels track within 0.2dB from 0dB down to -70dB
Distortion:
THD: <0.5% (20Hz-20kHz)
IMD: <0.5% (60Hz & 7kHz at 4:1)
TID: immeasurable
Frequency Response:
Phono: +0.2dB 20Hz~20kHz
-3dB 10Hz~50kHz
Line: -0.2dB 20Hz & 20kHz
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MISSION 777 OPERATING INSTRUCTIONS
(Bracketed numbers refer to panel drawin s)
FRONT PANEL
1. GREEN L.E.D. (1) This li ht illuminates when the amplifier is turned on.
2. ON/OFF SWITCH (1) Self explanatory.
BACK PANEL
1. A.C. CONNECTOR HOUSING (2) & (3)
The detachable line cord connects to the 777 at A.C. input (2). Access to the line fuse (3) is possible only
when the line cord has been disconnected & the clear plastic window is moved aside. The fuse housin (3)
also contains a small universal volta e card that selects the correct line volta e for the country in which the
amplifier is used. See precautionary measures for correct line volta e & fuse ratin .
2. INPUT (6)
Interconnect cables from the pre-amplifier output connect here.
3. OUTPUT (5)
These five way heavy duty bindin posts provide different methods of connectin the speaker cable to
the amplifier. For example, when usin multi-strand heavy au e wire, divide the conductors into two
parts, pass one half throu h the hole in the terminal & wrap the remainder around the terminal post.
Ti hten plastic cap to secure wire. These terminals are also equipped to accept standard banana plu s.
4. OUTPUT FUSE (4)
Individual 3.5 ampere fast blow fuses are installed at the factory. These fuses protect the loudspeakers
from overload conditions & the amplifier from shorted speaker wires. See precautionary measures for
correct fuse ratin .
5. SOFT CLIP (7) IMPORTANT NOTICE
Due to inherently excellent clippin characteristics & ample headroom of the 777 power amplifier,
reports from the field & recommendations made by many reviewers indicate that the soft clip feature
incorporated in the amplifier is redundant.
In order to simplify & at the same time eliminate any de radation of the sonic quality which may arise
from intentional or inadvertent usa e of this feature, the soft clip switch has been completely disabled.
SPECIFICATIONS
MISSION 777 POWER AMPLIFIER
Input Sensitivity: 775 m.v. for 28.4Vrms out (100W into 8Ω)
Power:(continuous power
outputwith both channels
driven from20Hz-20kHz)
>100watts/ch. (8Ω)
>175watts/ch. (4Ω)
Slew Rate: 150 Volts/microsecond
Rise Time: 0.32 microsecond
Dampin Factor: 60 (DC-40kHz)
Open Loop Bandwidth: 230kHz
Closed Loop Bandwidth: 1.15MHz
Ne ative Feedback: 14 dB
Current Delivery: 10 amps RMS continuous 40 amps peak-to-peak instantaneous
THD: <0.2% (DC-40kHz)
Difference Frequency
Distortion: 0.05% (10 watts into 8Ω, 300Hz apart, swept from 0-200kHz)
IMD 0.05% (10 watts into 8Ω, 70Hz, fixed & swept from 0-200kHz, 4:1 ratio)
Si nal/Noise: >100dB
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PRECAUTIONARY MEASURES
1. When disconnectin any piece of equipment such as a tape deck, tuner, etc. be sure to turn off
system power.
2. When chan in records, re-dialin tuner or rewindin tapes, it is recommended that the volume
control be turned down.
3. The 777 amplifier can be positioned wherever convenient, provided that some unrestricted airflow is
permitted at the top & bottom of the case. Do not place the amplifier on carpetin or any other material
that would restrict airflow. Under normal conditions the 777 will et moderately warm.
4. If it is necessary to replace a fuse, be sure to maintain the proper fuse ratin .
(A) LINE FUSE (refer to back panel drawin s (3) & (11).
JAPAN (100 Volt): ¾ ampere, slow blow, Model 776
5 ampere, slow blow, Model 777
NORTH AMERICA (120 Volt): ½ ampere, slow blow, Model 776
4 ampere, slow blow, Model 777
U.K. & EUROPE (220-240 Volt): ¼ ampere, slow blow, Model 776
2 ampere, slow blow, Model 777
When selectin a different line volta e, remove & rotate volta e card to desired volta e, then reinsert
card such that the printed volta e is clearly visible throu h the window. Chan e the fuse accordin to
the above specifications.
4. (B) OUTPUT (PROTECTION) FUSE, MODEL 777
(refer to back panel drawin (4).)
USING 8Ω SPEAKERS: 3½ or 4 ampere, fast blow
USING 4Ω SPEAKERS: 5 ampere, fast blow
5. For best possible si nal to noise fi ures Model 776 & 777 amplifiers must be connected to a properly
rounded outlet. Do not remove the third wire roundin pin on power cord plu .
6. Do not use hi h au e hi h capacitance audio cable.
SPEAKER CABLE should be 14 au e for runs of under 20 feet, otherwise use 12 au e or lower. In
either case the cable must be multi strand of pure copper. It is recommended that positive & ne ative
wires to the same speaker be separated a couple of inches for best results.
INTERCONNECT CABLE should be low capacitance with a braided shield.
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IN CASE OF TROUBLE
SYMPTOM PROBABLE CAUSE & CURE
No sound from one or both speakers
A) Check for blown or missin . fuses in speakers & at (3)
of model 777.
B) insure that all cable connections are correct & secure to
(2), (5), (6), of 777 & (12), (13), (15), (16), (17) of 776.
C) insure that 776 batteries are fully char ed. Refer to
operatin instructions.
Low volume sound from one or both
speakers.
Check for blown fuses at (4) of model 777. These
protection fuses are wired in the feedback loop - if the
fuse blows some sound will be audible.
Hum or cracklin from one or both
speakers. This is usually caused by a faulty
round connection of a shielded
interconnectin cable.
A) Insure that there are no faulty interconnect cables.
problem
B) Insure that all interconnect cable plu s are secure
(PUSHED IN) to (12), (13), (15), (16), (17), of 776 & (6) of
777.
C) Attach all available chassis roundin wires from
turntable, tuner, tape deck, etc. to (18) of 776. Use banana
plu supplied.
D) Lon cable runs includin speaker wire can act as
antennas causin R.F.I. problems. if the above remedies do
not work then ferrite rin s must be installed on the
offendin cable. These rin s will remove the R.F.I. without
affectin sound re production. Contact your audio dealer
or the factory for further information.
Radio frequency pick-up audible from one
or both speakers.
The Mission 776/777 amplifiers have been desi ned to
reject most R.F. interference. However, R.F.I. can be a
problem dependin on where you live & the quality of
associated equipment used with the 776 or 777 amplifiers.
Music reproduction sounds hollow, certain
instruments lack definition.
Speaker POSITIVE & NEGATIVE terminals are reversed
causin frequency cancellation. Insure that the speakers
are connected properly: red (+) terminal of 777 LEFT
channel oes to red (+) terminal of LEFT speaker & black
(-) terminal of 777 LEFT channel oes to black (-) terminal
of LEFT speaker. Repeat for the RIGHT channel.
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Mission 776 Pre-Amplifier
Mission 777 Power Amplifier
Con ratulations on your excellent choice! Your Mission pre-amplifier &/or power amplifier are precision
manufactured products & have been tested to the hi hest standards. A few minutes spent in readin
your instruction manual will ensure that you et optimum results from your system & your investment
will prove lon term.
IMPORTANT NOTES
1. Before switchin the equipment on, please ensure that it is correct for your local power supply. The
pre-amplifier & power amplifier are made in different versions for different parts of the world. Please
refer to the detailed instructions for each unit for more information.
2. Mission equipment is manufactured to the hi hest quality standards. All Mission pre-amplifiers &
amplifiers are warranted to be free of defects in materials & workmanship for a period of three years
from date of purchase.
3. Please retain packin materials. You may need them a ain for safe transport of the equipment.
INTRODUCTION
Traditionally, amplifiers are desi ned as mathematical models their characteristics measured under
steady-state conditions, & their quality jud ed by a simple set of specifications. This is inadequate for
an amplifier dynamically operatin to reproduce music. It is the dynamic behaviour of an audio
amplifier with complex interface (input/output) transducers when drivin reactive loads that hi hli hts
many obscure problems. Many "theoretically-perfect" desi ns suffer serious clippin distortions durin
musical transients; are unable to drive complex speaker loads; create poor interface with cartrid e
transducers; have inadequate power-supplies; use IC's in place of discrete components; & i nore the
detrimental effect of excessive switches & ad ets.
As for specifications, historically too much emphasis was placed on parameters such as THD or
bandwidth. This resulted in excessive use of overall feedback - enhancin specifications by reducin the
amplitude of low-order harmonics but eneratin hi h-order harmonics & listenin fati ue. A ain much
was made of bandwidth & speed of amplifiers. Fi ures of 1MHz were considered desirable. But such
fi ures do not reveal the critical ratio of power supply bandwidth to audio bandwidth; the amount of
feedback used to achieve such specifications; linearity with varyin power levels; & the respective
relationship between the closed & open-loop curves. Also, steady-state measurements i nore time
domain problems such as time & phase errors resultin in incoherent musical information.
The Mission desi ns are the result of a new look at the fundamental parameters & basic operatin
functions of an audio amplifier. Our research has been painstakin in careful evaluation & correlation of
subjective results with advanced & novel measurements.
Considerable effort was made to create inherently stable circuit desi ns capable of drivin all complex
loads & eliminatin conventional protection circuits, Zobel networks couplin capacitors, etc. The 777
achieves the unprecedented open-loop bandwidth of 230kHz resultin in 1.15MHz bandwidth with a
mere 14dB overall feedback:
Discrete components are used throu hout, switches & connectors kept to a bare minimum.
Exceptionally low output impedance power supply is utilised with some 70,000 filterin capacity.
A totally linear harmonic distortion pattern is achieved with rapidly diminishin hi h order harmonics.
Complex distortions measured over a lar e frequency band are ne li ible. The slewin rate of the 777 is
a sta erin 180 Volts per micro second, thereby eliminatin all dynamically induced distortions such as
TIM, with special attention iven to slewin behaviour at maximum output swin s.
The 777 is DC coupled throu h out, desi ned as a perfectly symmetrical class "A" amplifier, except for
the hi h technolo y H-FET output devices which operate in class "AB" mode.
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The sonic superiority of this product is immediately audible in its clean & excitin dynamic ran e
permittin amplification of musical transients without clippin ! To rate the 777 as a 200W amplifier,
therefore, does not do justice to its transient capabilities - more suitably rated in kilowatts.
The revolutionary battery powered 776 strai ht line pre-amplifier is strictly for the purist, payin particular
attention to the all important role of power supply rejections. A true RIAA equalisation curve is followed to
within 0.2dB accuracy (as a ainst arbitrary roll-offs) to ensure phase accuracy at low frequencies.
The phono sta e overall feedback is kept to less than 10dB at 30Hz to minimise dynamically induced
distortions while maintainin the noise level at below -80db! Special attention has been iven to the
quality of the components, the effect of switches & connectors, etc. & their behavioural chan es with
temperature & a ein . Therefore, near military rade components have been used throu hout, & both
amplifiers are manufactured in an extrava ant sculptured solid aluminium casin .
DESIGN PHILOSOPHY
A. General:
Mission stron ly believes in strai ht line amplifier desi n, eliminatin all unnecessary, fancy & harmful
features offered by most desi ns on the market.
The tone control in pre-amplifiers, for instance, is believed to be absolutely unnecessary, in as much as
no simple tone control confi uration can equalise a room acoustically, nor will it compensate for bad
recordin s. At the same time, tone controls add false spices to the pro ram material which are totally
undesirable. We su est that better results can be achieved by careful positionin of the loudspeakers
in the room, or in extreme cases the use of very hi h quality equalisers in the chain.
We have observed the problems & distortions associated with switch contacts & connectors, & stron ly
believe that if & when necessary, only the hi hest quality, old plated contact switches & connectors should
be used in the si nal path. It’s quite unfortunate, however, that in practical desi ns many such devices must
be used; the secret therefore, is to keep them to a minimum number of only the hi hest quality
The use of protection circuitry in power amplifiers, as an example, very often detracts from strai ht line
philosophy & one way or another causes aberrations in the behaviour of the amplifier in deliverin hi h
currents to the load.
B. Power Supply:
The power supply, perhaps, plays as important a role as the actual circuitry of the amplifier in the
reproduction of the audio si nal.
Some prominent people in the industry have addressed the subject & we have shown in our research
work that the power supply is an inte ral part of the reproduction chain.
It is important to conceive of the power supply not as a peripheral component, merely necessary to
provide DC volta e for the amplifier, but as a char e stora e device controlled by the amplifier to
deliver current as required for replicatin the si nal input, across the load, readily & without any
hindrance. To achieve this requires a power supply with extremely low output impedance & a
bandwidth in excess of that of the amplifier.
C. Amplifier:
The amplifier section is, of course, most responsible for quality of music reproduction. Parameters
normally measured & referred to for the evaluation & comparison of amplifiers enerally tell the truth,
but not the whole truth. Total harmonic distortion, for instance has been iven a lot of false wei ht for
too lon , whereas until recent years not much attention was iven to dynamically induced distortions,
stability & realistic load handlin capabilities of the power amplifier.
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We believe that the complete & true yardstick for measurement & evaluation has not yet been
discovered. However, there are certain parameters which best relate subjective evaluation to the
objective measurements. These are briefly as follows;
1. Measure of stability of the amplifier to be tolerant to all types of realistic but awkward reactive loads.
2. Utilisation of a minimum amount of overall feedback - just enou h to keep distortions down to
acceptable levels (which means that the open loop distortions in the amplifier should be inherently
low.)
3. Bandwidth; it is important that this be lar e enou h in its open loop value. We believe that the open
loop bandwidth should be several times lar er than the si nal bandwidth at the input of the amplifier in
order to prevent any type of dynamically induced distortions due to band limitin of the amplifier
sta es.
4. Linearity of distortion, that is:
a. constancy of percenta e distortion with increase of power (mainly in the power amplifier).
b. comb type distortion characteristics with rapidly diminishin hi h order harmonics.
5. Slewin rate, but more important is slewin behaviour at maximum output swin s.
6. Complex distortion measurements over a wide frequency band (that is, swept intermodulation &
different frequency distortions), payin special attention to hi h order distortion components in the
former, & low frequency distortion components of the latter.
7. Clippin behaviour of the amplifier; that is, whether or not the undesirable arba e on the DC
volta e rails is passed on to the load at clippin , & speed of recovery from clipped output.
8. Inherently low output impedance (open loop), to ether with low overall feedback in order to
minimise interface distortions caused by the speaker's back emf appearin at the output of the power
amplifier & thus re eneratin throu h the feedback loop.
9. Distortions under dynamic conditions which are lar ely a function of the above parameters in eneral.
776 PRE-AMPLIFIER
1. Construction:
a. Mechanical
The amplifier case is the same castin as used in the 777 power amplifier; the lo o in the front panel
actin , in this case, as a recess for knobs & switches.
The inside of the amplifier is completely exposed when the top plate is removed which facilitates
servicin of the unit.
Generally, it should not be necessary to remove the chassis in order to replace any major components.
b. General
The 776 has been desi ned to be modular in construction, while keepin the number of connectors &
contacts in the si nal path to an absolute minimum. Switches & connectors used are of the best quality
with old plated contacts.
The layout of the modules & components has been done in such a way as to minimise interconnectin
tracks & si nal path len ths, while most tracks on the printed circuit board are doubled up on both
sides in order to reduce interconnectin resistance in the si nal path.
All printed circuit boards have 2oz of copper tracks on each side.
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c. Components
1. All active components used in the amplifier section of the 776 are of the low noise discrete type, with
hi h ain x bandwidth fi ures. This is because no inte rated circuit (operational amplifier) has yet been
made available with a wide enou h open loop bandwidth for true hi h fidelity reproduction.
2. Use of couplin capacitors, however, has been unavoidable in the 776. When this has been necessary,
the best polyester capacitors have been mixed & doubled up (from 5 top quality manufacturers) in order to
reduce the sonic si nature of one type or make. We have shown in our research work that each make of
capacitors has its own structural si nature which can be added to the pro ram material as coloration.
3. Throu hout the pre-amplifier only the best quality 1% metal film resistors have been used to ensure
accuracy of ain & channel balance throu hout the audio band. These are, in most cases, over-rated in
order to reduce thermal noise.
4. An instrument type line connector, incorporatin line fuse & line volta e selector card is used for
safety & ease of transportation/handlin .
2. Power Supply
The power supply for the 776 pre-amplifier modules is derived from two beefy batteries (rechar eable &
sealed) providin 25 Volts DC at 1.5 amperes/hour capacity. These are followed by two sta es of
filterin throu h 10,000 microfarads & then separatin two channels by the second sta e of filterin
throu h 2200 microfarad capacitors decoupled by 4.7 microfarad polyester capacitors.
This confi uration ensures:
(i) zero line induced hum.
(ii) very low impedance as seen by the amplifier throu hout the audio spectrum.
(iii) an excellent inter-channel power supply rejection.
When the amplifier is in its play mode, all line AC comin into the amplifier case is cut off, ensurin no
line hum pick up within the pre-amplifier unit.
When the amplifier is not in use & the battery is put on full char e; the circuit will chan e over to
trickle char e mode automatically when the battery is within 97% of its final char e. The amplifier can
remain in this mode of char e indefinitely. The battery can produce optimum performance for as lon
as 20-30 hours of continuous use. It is recommended, however, that the battery be put on char e after
each lon listenin session.
3. Pre-amplifier.
Two identical modules make up the two channels of the pre-amplifier, each consistin of a phono & a
line section.
a. Phono Sta e
This sta e uses a low noise differential amplifier followed by a current pumpin volta e amplifier sta e.
The active RIAA equalisation network in the feedback loop utilises precision components in order to
ensure ±0.2dB accuracy of the frequency response. A true RIAA equalisation curve has been followed
(as a ainst RIAA/IEC standards or arbitrary rollin off <20Hz) in order to ensure phase accuracy in the
bottom end.
This, naturally, puts more constraint on the quality of the turntable used in the chain. The overall
feedback is <10dB at 20Hz; Ensurin minimal dynamically induced distortions in the phono sta e
while maintainin the noise level down to -80 dB (A-wei hted) reference 5mV. input.
It is interestin to note that much superior RIAA accuracy with better noise fi ures can be achieved by
employin hi her feedback values. This, however, would be at the expense of inferior sonic quality &
would detract from purist performance of the pre-amplifier.
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b. Line Sta e
This is a simple CE sta e followed by a current pumpin volta e amplifier. The overall feedback of this
sta e is <20dB with an output impedance of 200Ω.
c. The Pre-Amplifier modules
These have a ain of 54 dB which is suitable for all movin ma net & hi h output movin coil
cartrid es.
d. Facilities
Only necessary controls & facilities have been provided where these will not detract from the purist
quality of the pre-amplifier.
Two push buttons provide tape monitor & mono/stereo switchin . An extremely hi h quality rotary
switch is used to select the input (phono, tape & auxiliary).
A precision, low noise stepped potentiometer has been used for the volume control. Channel balance
accuracy at each step from 0dB down to -70dB is >0.2 dB.
777 POWER AMPLIFIER
1. Construction
a. Mechanical:
The amplifier case is cast aluminium bearin the company lo o in it's front panel. The lo o acts lar ely
as a heat sink, acceleratin heat dissipation from the main heat sink block (which is formed as an
extension of the ISSI into the case). In effect the whole castin works as a iant heat sink.
The inside of the amplifier is totally exposed by removin the top plate, which facilitates servicin of
the unit. In eneral, there should not be any requirement to remove the chassis in order to chan e any
major parts.
b. General:
The 777 has been desi ned to be totally modular in construction, while keepin the number of
connectors to a minimum. This lar ely facilitates replacement of any defective parts with relative ease.
The connectors used for this purpose are of the hi hest quality available.
The 777 utilises two identical mono amplifiers, each fed from its own transformer & power supply module.
The layout of the two channels allows minimum len ths of wirin , while input/output connections are
made throu h less than 2 inches of wire harness len ths.
c. Components:
1. All active components used in the 777 amplifier are discrete. Low power transistors are low noise,
hi h ain x bandwidth & extremely reliable. The output devices employed are hi h volta e H-FETs, with
their Ft ≈100MHz. This is to ensure that the bandwidth of the last sta e is reater than that of the
drivin sta es.
2. All resistors used in the power amplifier section are hi hest quality metal film, 1% tolerance & low
noise. They are enerally over-rated to ensure low thermal noise & biasin variations.
3. Power supply filterin is done by hi h ripple current, structurally sound & extremely reliable
electrolytic capacitors. All decouplin capacitors used in the 777 are best quality polyester, to minimise
hi h frequency roll-up effects.
4. Heavy duty connectors are used to connect DC to the power amplifier module, while old plated
connectors are employed for si nal line connections to the back panel of the amplifier.
5. An instrument type line connector, incorporatin a line fuse & line volta e selector card is used for
safety & ease of transportation & handlin .
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2, Power Supply:
Each of the two mono power supplies comprises a specially desi ned, extremely low re ulation
transformer (200 Volt-amperes at.3.5% re ulation), feedin directly into the power supply module with
extra heavy duty copper tracks &, typically, 35,000 microfarads of filterin capacity, decoupled by
hi hest quality polyester capacitors for hi h frequency performance. This havin very lar e bandwidth
presents extremely low output impedance to the power amplifier, which accounts for the 777's superb
reproduction of the full frequency spectrum of pro ram material,
It is interestin to note that most desi ns tend to i nore the importance of the power supply in the
reproduction of music; utilisin under-rated transformers with hi h re ulation, use of low capacity &
structurally weak electrolytic capacitors, not usin small decouplin capacitors &/or use of low au e
wires & interconnectin cables which will in turn add to the output impedance of the power supply. :
3. Power Amplifier:
The 777 amplifier module is a perfectly symmetrical class "A" amplifier, except for output devices which
operate in class "AB" mode. Symmetry has been achieved throu h cascadin sta es of the amplifier in
alternate PNP/NPN complementary transistors, while drawin symmetrical currents from positive &
ne ative rails.
The amplifier is DC coupled throu hout, utilisin the same amount of feedback for both si nal & DC.
DC bias stability is achieved by the application of very hi h local feedback loops. This is also
responsible for the lar e open loop bandwidth of the amplifier (230kHz) .
The source & load impedances of each sta e have been matched carefully for optimum frequency
response & minimum intersta e distortion. Basically, two sta es of volta e amplifier are used, these
bein separated by a unity ain cascade sta e as a buffer.
Zobel or half Zobel networks are normally used in all desi ns of power amplifiers to reduce the THD
measured at the hi her end of the spectrum &, while renderin the amplifier more stable under hi hly
reactive loads, limits the power bandwidth available to the load, detractin from purist quality of
reproduction. Due to the inherently stable desi n of the 777, it was not necessary to use such networks
or any type of output filterin as such. This affords an actual hi h frequency roll-off behaviour of
6dB/octave to amplifier/loudspeaker combination.
The 777 is an invertin amplifier. This has been done to ensure a much lower common mode slewin
difference between positive & ne ative swin s of the output.
Overall feedback employed in the 777 is a mere 14dB, which is very low by any standards. This
provides THD fi ures of <0.2% from DC up to 40kHz, & a closed loop bandwidth of 1.15MHz.
protection is provided for both the power amplifier & speakers throu h an output fuse which has been
incorporated in the feedback loop in order to reduce its adverse effects by a factor of 5:1. Considerin
that the 777 is an extremely robust & reliable amplifier, it was envisa ed that this type of protection is
ample for 99% of the situation where some type of protection is required.
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