JBL 6230 User manual

BEFORE PROCEEDING WITH COMPLETE UNPACKING AND SETUP,
READ THE SECTION ON UNPACKING AND INSPECTION
UBL
model 6230/6260
POWER AMPLIFIER
JBL Incorporated
8500 BALBOA BOULEVARD
P.O. BOX 2200
NORTHRIDGE, CA 91329, U.S.A.
PHONE: [phone]
TELEX: 4720424
SERVICE ADDRESS:
8460 SAN FERNANDO ROAD
SUN VALLEY, CA 91352, U.S.A.
PHONE: [phone]
Printed In U.S.A.
©Copyright 1984, JBL Inc

PREFACE
Thank you for purchasing this JBL Power Amplifier. We have
prepared this instruction manual to enable you to achieve optimum
utility and performance from your new amplifier. We encourage
you to read it and to make use of the material contained in this
manual.
We welcome your comments or suggestions on our products and
on this manual.
This product was manufactured for JBL by UREI ,Inc.

TABLE OF CONTENTS
PAGE
SECTION I. INTRODUCTION 1
1.1
SPECIFICATIONS 2
SECTION II. INSPECTION AND INSTALLATION 5
2.1 UNPACKING AND INSPECTION 5
2.2 ENVIRONMENTAL CONSIDERATIONS 5
2.3 AC POWER 6
2.4 AMPLIFIER MODE SWITCH 6
2.5 EXTERNAL CONNECTIONS 7
2.5.1 INPUT CONNECTIONS, GENERAL 7
2.5.2 INPUT CONNECTION, DUAL MONO MODE 8
2.5.3 INPUT CONNECTION, BRIDGED MONO MODE 8
2.5.4 FIVE-WAY BINDING POST OUTPUTS 8
2.5.5 ABOUT OUTPUT POLARITY AND BRIDGED MONO
CONNECTION 8
2.6 INPUT IMPEDANCE AND TERMINATION 9
2.7 APPROPRIATELY RATED LOADS 10
2.7.1 IT IS THE USER'S RESPONSIBILITY TO AVOID
OVERPOWERING 1
0
2.7.2 MORE ABOUT LOUDSPEAKER POWER RATINGS 11
2.7.3 SOMETIMES ALARGER AMPLIFIER CAN BE SAFER 12
2.7.4 REMEMBER THE DIVIDING NETWORK (CROSSOVER) 12
2.8 SPEAKER CABLES 13
2.8.1 TYPE OF WIRE 13
2.8.2 DAMPING FACTOR 13
2.8.3 CALCULATING LOSSES TN SPEAKER CABLE 14
2.9 GROUNDING 14
2.10 SECURITY COVERS 15
FIGURE 2-1 BALANCED INPUT CONNECTIONS 17
FIGURE 2-2 UNBALANCED INPUT CONNECTIONS 17
FIGURE 2-3 BARRIER STRIP CONNECTIONS 18
FIGURE 2-4 USE OF FIVE-WAY BINDING POSTS 19
FIGURE 2-5 OUTPUT CONNECTIONS TO TWO CHANNEL
SPEAKER SYSTEM 19
FIGURE 2-6 BRIDGED OUTPUT CONNECTION TO MONAURAL
SPEAKER SYSTEM 1
9
SECTION III. OPERATING INSTRUCTIONS 20
3.1 GENERAL 20
3.2 TURN ON AND SYSTEM CHECK 20
3.2.1 ANOTE ON AMPLIFIER COOLING 20
3.3 CLIP INDICATORS 20
3.3.1 ABOUT SENSITIVITY RATINGS 21

TABLE OF CONTENTS
(Continued)
PAGE
SECTION IV. THEORY OF OPERATION 22
4.1 INPUT DIFFERENTIAL AMPLIFIER 22
4.2 MODE SWITCH 22
4.3 POWER AMPLIFIER 23
4.4 RELAY DRIVER/DC DETECTOR 24
4.5 CLIP CIRCUIT 25
4.6 POWER SUPPLY 25
SECTION V. MAINTENANCE 26
5.1 GENERAL 26
5.2 REPAIRS AND WARRANTY 26
SECTION VI. SCHEMATIC DIAGRAM
ii

SECTION I
INTRODUCTION
The JBL Model 6230 and 6260 power amplifiers have been
designed to meet the most critical professional sound
requirements. They are rugged and road-worthy, conservatively
rated, and can handle reactive loads with ease.
The engineering design approach stresses the optimization of
each stage, allowing high slew rate and relatively low loop gain.
Overall feedback has been held to aminimum and is employed only
to stabilize the gain and the operating point. This design
approach results in amplifiers with excellent performance under
the most demanding dynamic input and load conditions. As
evidence of the stress on dynamic rather than static or steady-
state distortion mechanisms, transient intermodulation distortion
measures less than 0.03% by the DIM 100 test. (Leinonen, Otala,
and Curl, "A Method for Measuring Transient Intermodulation
Distortion (TIM)", Journal of the Audio Engineering Society ,Vol.
25, No. 4, April, 1977, pp. 170-177.)
The front panel and heat sinks of the amplifiers are made of
heavy aluminum extrusions; the chassis is fabricated of heavy
gauge steel. All internal components are easily accessible
through removal of top and bottom panels. Front panel graphic
details are incorporated on the rear side of apolycarbonate
laminate which is virtually indestructible.
JBL amplifiers use multiple 200-watt output devices in
complementary configuration for high reliability and low distor-
tion. At rated power into 8ohms, these output devices are
operated at less than 25% of their rated power dissipation. The
benefit is high reliability and long component life.
Failsafe operation of these amplifiers is ensured through
the following protection modes: Current is limited under
improper load or drive conditions. An output relay, with front
panel LED indication, protects the loudspeaker load under cond-
itions of DC offset or large low-frequency transients. The relay
also provides power-up, power-down, and "brown out" muting to
protect loudspeakers from AC power transients generated anywhere
in the signal path. LED's on the front panel indicate the onset
of clipping and standby mode.
The JBL amplifiers may be operated in the normal stereo-
phonic mode, dual monophonic mode, or bridged monophonic mode.
Rear panel switching sets these modes, obviating the need for
patch cords, level matching, etc.
Active differential input circuitry offers the benefits of
balanced operation without the use of input transformers. Input
connections may be made via 3-pin XL-type connector, three-cond-
-1-

uctor (TRS) 6.3 mm (1/4 in.) phone jack, or barrier strip. The
barrier strip has separate terminals for audio ground and
chassis ground.
The five-way output binding posts are arranged in a19 mm
(3/4 in.) array so that bridged as well as normal connections may
be made with standard dual banana plugs, bare wire, or terminal
lugs.
1.1SPECIFICATIONS
(Common to both models except where noted)
OUTPUT POWER:
6230 6260
Rated Power
20 Hz-20 kHz Midband Power
1kHz Rated Power
20 Hz-20 kHz Midband Power
1kHz
8-ohm stereo
(per channel) 75 W110 W150 W190 W
4-ohm stereo
(per channel) 150 W175 W300 W315 W
16-ohm bridge 150 W220 W300 W380 W
8-ohm bridge 300 W350 W600 W630 W
Rated Power is minimum continuous sine wave output per channel,
with both channels driving their rated load over apower bandwidth
of 20 Hz to 20 kHz. Maximum total harmonic or intermodulation
distortion measured at any power level from 250 milliwatts to rated
power is less than 0.1% for 8ohm stereo and 16 ohm bridge, 0.2%
for 4ohm stereo and 8ohm bridge.
Midband Power is maximum output power at onset of clipping, both
channels ^driven with 1kHz sine wave, THD 1%.
TRANSIENT INTERMODULATION DISTORTION: .03% max at rated output
FREQUENCY RESPONSE: +0, -1 dB, 20 Hz to 20 kHz, at any level up to
rated output
NOISE: At least 100 dB below rated output (15.7 kHz noise band-
width)
INPUT: Balanced bridging differential amplifier
-2-

INPUT IMPEDANCE: 40k ohms used as balanced input; 20k ohms used as
unbalanced (single-ended) input
MAXIMUM INPUT LEVEL: +20 dB* (7.75 Vrms)
INPUT SENSITIVITY: 1.1 Vfor rated output into 8-ohm load
VOLTAGE AMPLIFICATION: Variable; maximum 27 dB (6230), 30 dB (6260)
RISE TIME: Less than 7microseconds
SLEW RATE: 40 V/microsecond into 8-ohm load
DAMPING FACTOR: With 8-ohm load, greater than 200 at any frequency
from 20 Hz to 1kHz
CHANNEL SEPARATION: Greater than 60 dB at 1kHz
POLARITY: Output signal is in phase with pin 3of XL-type connector,
tip of 6.3 mm (1/4 in.) phone jack and the barrier strip
"+" terminal
AC POWER: Typical AC power consumption
6230 6260
At idle (approx.) 50 W50 W
At rated output
Both channels 8-ohms 420 W600 W
At rated output
Both channels 4-ohms 720 W1180 W
DC OUTPUT OFFSET: +10 millivolts maximum
6230 6260
DIMENSIONS: 133 X483 mm 178 X483 mm
(5-1/4 X19in.
)
(7 X19 in.
DEPTH:** 280 mm 280 mm
(11in.
)
(11 in.)
NET WEIGHT: 11.9 kg 20.2 kg
(26.25 lbs) (44.5 lbs)
*In these specifications, where dB refers to aspecific level, the 0
dB reference is 0.775 volts RMS unless otherwise noted.
**Allow aminimum of 51 mm (2 in.) behind amplifier for connections
and cooling air flow.
-3-

6230 6260
SHIPPING WEIGHT: 15.9 kg 24 kg
(35 lbs) (53 lbs)
OPTIONAL ACCESSORIES: Attenuator security cover, 70 Vand 100 Vout-
put transformer panels, input transformers
PROTECTION CIRCUITRY: The amplifier output is protected against short
circuit and is stable into reactive loads.
Short circuit protection is clean with no odd
behavioral characteristics. The input amplif-
iers are protected against excess input curr-
ent. The loudspeakers are protected against
any failure in the amplifier by an internal
relay which disconnects the load from the amp-
lifier output and connects it to ground. The
circuit is activated at turn on and turn off
to prevent thumps in the loudspeaker during
system power up/down. The relay also disconn-
ects the load if excess DC voltage is detected
at the output or upon failure of any of the
amplifier power supplies. If the amplifier
overheats due to alack of ventilation the
relay opens until the amplifier has begun to
cool down.
CONTROLS: Independent channel level controls are detented. Illum-
inated rocker-type power switch. Rear panel recessed
stereo/dual mono/bridged mono mode switch.
INDICATORS: Individual channel clip indicator LEDs. STANDBY LED
indicates action of output protection circuit.
CONNECTORS
:
Input: 3-pin XL-type 3conductor 6.3 mm (1/4 in.) phone jacks and
barrier strip. May be wired balanced or unbalanced.
Ground: Chassis ground and audio ground connected with removable
shorting strap on rear panel barrier strip
Output: Color coded 5-way binding posts on 19 mm (3/4 in.) centers
Power: 1.5 m(5-foot) 3-wire AC power cord with U-ground male
connector
-4-

SECTION II
INSPECTION AND INSTALLATION
2.1UNPACKING AND INSPECTION
Your JBL Power Amplifier was carefully packed at the factory,
and the container was designed to protect the unit from rough
handling. Nevertheless, we recommend careful examination of the
shipping carton and its contents for any sign of physical damage
which could have occurred in transit.
Save the carton and packing in the unlikely event the unit
must be returned for service; if you do not have acarton, call
the factory for one before shipping the unit. If damage is
evident, do not destroy any of the packing material or the carton,
and immediately notify the carrier of apossible claim for damage.
Shipping claims must be made by the consignee.
The shipment should include the Power Amplifier and the
Instruction Manual (this book)
.
2.2 ENVIRONMENTAL CONSIDERATIONS
This amplifier has been constructed to operate satisfactorily
with program material at normal room temperature and humidity.
This presumes afree, unrestricted flow of cooling air to the rear
mounted heatsink. When the amplifier is mounted in arack it is
important to ensure that cool air is allowed to reach the heatsink,
and that heated air is allowed to flow away from the amplifier. In
most cases this will only require ventilation grills to be provided
so cool air can enter the rack at the bottom and hot air can
exhaust at the top through natural convection.
In some installations it may be necessary to provide forced
air cooling to the amplifier and the space in which it is mounted.
These amplifiers have thermo-protective circuits that will operate
if the amplifier overheats. This will happen for one or more of
the following reasons:
1. Insufficient natural air flow.
2. Average power and duty cycle of the program material too
high.
3. High ambient air temperature in which the amplifier is oper-
ating.
It is not really possible to state exact requirements for air flow
because of the number of variables, but in most cases fans with
70-120 CFM will provide sufficient air flow.
-5-

Furthermore, remember that the amplifier is heavy. The
rack, especially its mounting rails, should be capable of supp-
orting the amplifier. When arack is to be transported with a
portable sound system, the amplifier also should be supported
from below; afew pieces of angle iron secured to the sides of
the rack will suffice. It is further recommended that the amp-
lifier be placed low in the rack to keep the center of gravity
low and thus avoid any tendency for the rack to tip over.
2.3 AC POWER
All JBL Power Amplifiers shipped to destinations in the USA
and Canada are wired for 120 VAC 60 Hz operation. Amplifiers
wired for operation on any other voltage are identified as such
with asticker on the rear panel and atag attached to the
amplifier power cord.
WARNING
BE SURE TO VERIFY BOTH THE ACTUAL LINE VOLTAGE AND THE
VOLTAGE FOR WHICH THE AMPLIFIER HAS BEEN WIRED BEFORE
CONNECTING AC POWER. APPLICATION OF EXCESSIVE VOLTAGE TO
THE POWER SUPPLY MAY RESULT IN EXTENSIVE DAMAGE WHICH IS
NOT COVERED BY THE WARRANTY.
To comply with most electrical codes this amplifier is
supplied with athree-conductor AC power cable, the grounding pin
of which is connected to the chassis. In some installations this
may create ground loop problems when an AC potential exists
between conduit ground and audio ground. This will be evidenced
by hum or buzz in the amplifier output. If this should occur
please refer to Section 2.9 for suggestions. Proper grounding of
the amplifier is important for both noise and safety reasons. Be
aware that unless the amplifier is properly grounded, asafety
hazard can existT JBL~aiccepts no "responsibility for legal
actions or for direct, indirect or consequential damages that may
result from violation of electrical codes.
2.4 AMPLIFIER MODE SWITCH
A recessed switch on the rear panel provides for convenient
change of amplifier mode from stereo to dual mono and bridged
mono output. The switch may be actuated with asmall screw-
driver. The functions are as follows;
Stereo;
Input to Channel A. Output is on Channel Aand level
is controlled by Channel Alevel control.
Input to Channel B. Output is on Channel Band level
is controlled by Channel Blevel control.
-6-

Dual Mono:
If both channels of the amplifier are driving the same
signal to different loudspeakers the dual mono mode saves a
patch cord or Y-cord. Input is to Channel A. Output on
Channel Ais controlled by the Channel ALevel control.
Output on Channel Bis controlled by the Channel BLevel
control. Input Bis not used.
Bridged Mono:
This mode makes the stereo amplifier into asingle mono
amplifier with the power of both channels combined. Input
is to Channel A. Level control is by Channel ALevel control
and output is taken from the red binding posts of Channels A
and B as described in Section 2.5.5.
2.5 EXTERNAL CONNECTIONS
2.5.1 INPUT CONNECTIONS, GENERAL
Input signal wires should be shielded cable, and connected
in accordance with standard wiring practice to either the three-
conductor 6.3 mm (1/4 in) phone jacks, the XL-type connectors or
the barrier strip on the back of the chassis. See Figures 2-1,
2-2 and 2—3.
NOTE: The JBL Amplifier input connectors are wired as
follows:
PHONE XL-TYPE BARRIER STRIP
JACK CONNECTOR CONNECTION
Tip Pin #3 +or HIGH
Ring Pin #2 -or LOW
Sleeve Pin #1AUDIO GROUND
For agiven channel, either the XL-type connector, the phone
jack or the barrier strip may be used. Since all three connect-
ors are wired in parallel, however, only one should be used at a
given time (unless it is specifically desired to loop asignal
through the amplifier input).
The amplifier will not unbalance floating or balanced input
sources since the input circuits consist of balanced diff-
erential amplifiers. To use an unbalanced source, wire the
signal carrying conductor of the cable from that source to XL-
type pin 3(phone plug tip) ,and wire the shield to XL-type pin 1
(phone plug sleeve). The unused connector terminal, pin 2
(ring) ,should also be connected to shield ground. Unbalanced
connections are simplified by using two-conductor standard phone
plugs because they automatically short the ring and sleeve
together when inserted in the input jacks.
-7-

2.5.2 INPUT CONNECTION, DUAL MONO MODE
When operating the amplifier as two independent amplifiers,
but with the same program signal, only the Channel Ainput need
be used. Set the mode switch to Dual Mono. Do not apply signal
to the Channel Binput.
2.5.3 INPUT CONNECTION, BRIDGED MONO MODE
When operating the amplifier as ahigh powered single-
channel amplifier, only the Channel Ainput is used. That signal
is applied "in phase" to Channel A, and, with inverted polarity,
to Channel B. No signal should be applied to the Channel B
input, and the Channel Blevel control should be turned all the
way down (fully counterclockwise).
2.5.4 FIVE-WAY BINDING POST OUTPUTS
Five-way binding post outputs have been chosen because they
allow connections to be made quickly, they facilitate polarity
reversals for speaker "phasing," and they can handle high current
with agreater margin of safety than phone jacks. See Figure 2-4.
The preferred connection method is to use adual banana plug
for each speaker cable. Simply insert each plug into the corres-
ponding channel's red and black binding posts. See Figure 2-5.
In the absence of adual banana plug (or two single banana
plugs), there are other alternatives. To connect stranded
speaker cable, loosen the plastic terminal nut, wrap the stripped
and twisted wire end clockwise around the terminal, and secure it
by tightening the nut.
NOTE: It is preferable to tin the wire ends with solder to
prevent unraveling; avoid excess solder as it can promote
cable breakage. Smaller speaker cable could be pushed
through the hole in the binding post shaft, but we recommend
using heavier gauge cables that ought to be wrapped around
the shaft.
If alug is installed on the cable, loosen the terminal nut,
push one "leg" of the lug through the hole in the shaft, and
tighten the nut.
2.5.5 ABOUT OUTPUT POLARITY AND BRIDGED MONO CONNECTION
In normal stereo operation (or dual mono) ,apositive-going
signal applied to an input's pin #3, the phone jack tip, or the +
terminal of the barrier strip will cause apositive-going signal
to appear at the corresponding channel's red output binding post.
-8-

In bridged mono operation the two amplifier channels are
driven with the same signal rbut with Channel Breversed in
polarity. The speaker cable is then connected to the two
channels' red binding posts (the black posts are not used in
bridged mono mode) •In this case, apositive-going signal
applied to the Channel Ainput appears as apositive-going signal
at the Channel Ared binding post and as anegative-going signal
at the Bred binding post. See Figure 2-6.
NOTE; The two channels* binding posts are clustered to fac-
ilitate mono connection across the red posts with astandard
dual banana plug. DO NOT CONNECT THE TWO RED BINDING POSTS
TO EACH OTHER, and DO NOT GROUND EITHER SIDE OF THE OUTPUT
IN BRIDGED MONO MODE.
2.6 INPUT IMPEDANCE AND TERMINATION
Audio engineering has its roots in the telephone industry,
and "600 ohm circuits" (together with their predecessors, "500
ohm circuits") are carry-overs from telephone transmission
practices. Long audio transmission lines, like their video
counterparts, must be properly sourced from and terminated in
equipment which matches their characteristic impedance if optimum
frequency response and noise rejection are to be achieved.
However, transmission line theory and techniques are not
only unnecessary but impractical within modern recording studios,
broadcast studios and other local audio systems where transmiss-
ion circuits are seldom more than several hundred feet in length.
The advent of negative feedback circuitry and solid-state elect-
ronics has spawned modern audio amplifiers and other signal pro-
cessing devices having source impedances of only afew ohms.
They are essentially indifferent to load impedances and, by
varying their output current inversely to changes in load
impedance, maintain the same output voltage into any load
impedance above arated minimum, with no change in frequency
response.
Modern audio systems, therefore, utilize amplifiers and
other active devices which have very low output impedances and
high (lOK to 50K ohm) input impedances. These products may thus
be cascaded (operated in series), or many inputs may be connected
to asingle output of apreceding device, without regard to
impedance matching. Switching and patching is simplified because
double loads and unterminated bugaboos are essentially elimin-
ated. Floating (ungrounded) transformer outputs minimize ground
loop problems, and differential transformerless input circuitry
(or input transformers) minimize common mode noise or interfer-
ence which may be induced into the interconnecting wires or
cables.
-9-

Where audio must be transmitted through cables or wire pairs
of more than several hundred feet in length, however, transmiss-
ion line termination practices should still be observed.
This amplifier has input impedances of 40,000 ohms when used
in abalanced, differential input configuration, and 20,000 ohms
when used unbalanced (one side grounded). This makes the amp-
lifier suitable for use with any normally encountered source
impedance, low or high. Therefore, there are only two situations
which will require an input load at the amplifier:
1) when the source requires a600 ohm load, such as a
passive equalizer, older vacuum tube equipment, etc,
2) when the source is atransmission line such as atele-
phone line.
In some instances it may prove beneficial to treat the
input feed to the power amplifier as atransmission line
to lower its impedance and its susceptibility to noise
pickup.
Input load resistors, if required, may most conveniently be
attached to the barrier strip connector of the amplifier,
2,7 APPROPRIATELY RATED LOADS
2,7,1 IT IS THE USER’S RESPONSIBILITY TO AVOID OVERPOWERING
It is essential that the amplifier be used with loudspeakers
of suitable impedance that can handle the amplifier's power out-
put, We realize this is not always easy to determine, especially
since speaker power ratings have not been standardized. Nonethe-
less, JBL IS NOT RESPONSIBLE FOR DAMAGE TO LOUDSPEAKERS RESULT-
ING FROM OVERPOWERING,
Fuses may be inserted in series with the loudspeaker to
protect against overpowering. The fuse value must be chosen with
some care. Ideally, the value will be high enough that the fuse
does not excessively reduce the capability of the loudspeaker to
handle peak transients which are above its continuous power
rating. On the other hand, the fuse value must be low enough
that the fuse can actually do its job. It takes some period of
time to heat the fuse element enough to cause it to melt and
break the circuit. If it takes too long, the loudspeaker may go
first. Obviously, delayed action (slo-blo) fuses are not accept-
able for use here. Also, if fuses are used, consideration must
be given to their location. Put them where they are accessible
for ease of replacement and provide clear labelling of the
replacement fuse value. Place spare fuses nearby for ease of
replacement with the correct type.
-10-

2.7.2 MORE ABOUT LOUDSPEAKER POWER RATINGS
While there is no cut-and-dried method to establish an
appropriate amplifier power for agiven speaker system, certain
guidelines do exist. If aloudspeaker manufacturer specifies "to
be used with amplifiers rated at no more than 'x' watts," then
neither speaker nor amplifier warranty is likely to cover damage
if alarger amplifier is used. If the amplifier power is only
recommended, or if apower rating is given without mentioning the
amplifier, then the question as to whether the amplifier is
"safe" becomes more difficult to answer.
Loudspeakers usually fail due to one of two factors: thermal
or mechanical overload. Thermal overload means overheating, and
is almost always caused by applying too high alevel of
sustained, average power; the voice coil insulation may burn and
short circuit, the coil may deform due to the heating and scrape
in the gap, and, in some cases, speaker cones have been known to
burst into flame. Mechanical overload is another way of describ-
ing excessive diaphragm/voice coil travel. Asingle very high
power transient, especially at lower frequencies, can literally
tear aloudspeaker apart. Sometimes mechanical overload is more
gradual, with the voice coil "bottoming" against the magnetic
assembly until it is deformed; acompression driver diaphragm can
strike the phase plug and shatter, or the suspension can be over-
extended and simply tear apart.
The frequency and waveform of asignal have alot to do with
the destructive potential of agiven power output. Band limited
pink noise is somewhat akin to "average program" power, although
the meaning of such arating depends heavily on crest factor
(peak to average noise voltage) and specific frequency limits.
Unfiltered or unweighted white noise stresses the tweeters more
than the woofers. Swept sine waves may cause less thermal heat-
ing, but can cause larger excursions at low frequencies. With
compression drivers, the mechanical power handling capability is
approximately quadrupled simply by raising the minimum crossover
frequency by an octave. If any conclusion can be drawn, it is
this:
NO SINGLE POWER RATING REALLY DESCRIBES THE POWER HANDLING
CAPABILITY OF ANY LOUDSPEAKER OR LOUDSPEAKER SYSTEM. ALSO,
WHEN ALOUDSPEAKER SYSTEM IS MULTI-AMPLIFIED, i.e. AHIGH
FREQUENCY DRIVER IS CONNECTED DIRECTLY TO THE POWER AMP-
LIFIER OUTPUT, ASERIES PROTECTION CAPACITOR IS STRONGLY
RECOMMENDED TO REDUCE THE POSSIBILITY OF ACCIDENTAL DAMAGE
TO THE DRIVER DIAPHRAGM. CONSULT THE MANUFACTURER OF THE
DRIVER AND HORN FOR RECOMMENDATIONS ON PROTECTION CAPACITOR
TYPES AND VALUES.

2.7.3 SOMETIMES ALARGER AMPLIFIER CAN BE SAFER
If the desired sound level is high enough that it requires a
low powered amplifier to be operated at afairly high distortion
level (e.g., well into clipping), it may be safer to use alarger
amplifier that will be free of distortion. It is true that in a
small amp which is clipping, peak power is restricted by the size
of the amplifier's power. However, the average power output
rises due to the increased signal density caused by distortion
components. Not only does this increase the thermal stress, it
also increases mechanical stress because the squared waveforms
place greater "G" loads on the moving parts, and abnormally high
proportions of high frequency harmonics are generated which can
fry the tweeter voice coils.
Amore powerful amplifier which is able to generate the
desired peak sound level without clipping avoids the large
increase in average power (thermal) and the high acceleration
(mechanical) stresses caused by the overdriven small amplifier.
The major drawback of the larger amplifier is that it can produce
higher peak outputs that may instantly destroy aloudspeaker.
Therefore, great care must be exercised to ensure that the ampl-
ifier will not be driven at too high alevel, certainly never at
aclip. This can be accomplished either by knowing the program
material (if recorded) and setting levels accordingly, or by
using peak limiters (with live or unknown program material).
2.7.4 REMEMBER THE DIVIDING NETWORK (CROSSOVER)
If the amplifier is connected to amulti-way loudspeaker
system that includes apassive, high-level dividing network
(crossover), be sure to consider its presence. The network
should be rated to handle the amplifier's power. Pre-packaged
speaker system power ratings already take the crossover into
account.
If you obtain separate components and assemble asystem, the
chances are that the higher frequency drivers will be rated at
less power than the woofer(s). This works out overall because
the higher frequency drivers tend to be more efficient (more
sound per watt). In fact, higher sensitivity in the high
frequency components usually requires some attenuation to be
applied in the dividing network (remember, a3dB more sensitive
driver must be driven with half the power to generate the same
sound level).
For example, consider atypical two-way system with a200
watt woofer and 50 watt high frequency compression driver. The
woofer. 1meter from its enclosure, generates 100 dB SPL with 1
watt at its input, and the compression driver, 1meter from its
horn, generates 106 dB SPL at 1watt input. If both components
are driven by a200 watt program signal, the compression driver
-12-

will be 6dB too loud (four times the level of the woofer).
Therefore, the crossover network must provide 6dB of attenuation
for the compression driver, lowering its input power to 50 watts.
The 200 watt amplifier turns out to be perfectly suited to driv-
ing this system, even though one driver is rated at just 50
watts.
2.8 SPEAKER CABLES
2.8.1 TYPE OF WIRE
Anumber of companies offer various special "speaker cables"
which are claimed to vastly improve the signal delivered from
amplifier to loudspeaker system. Without going into great
details, suffice it to say that some special cables have merit,
others may actually be detrimental, and overall the value will
have to be determined by the user. In our opinion, for most
applications just two factors need to be considered: DC resist-
ance and durability.
Generally, the larger the wire gauge, the better. DC resis-
tance is lower with larger wire, and hence more of the amplifier
power gets to the loudspeaker (and damping factor is not degrad-
ed; see Section 2.8.2). More strands of wire in agiven wire
gauge are beneficial because they let the cable handle more
flexing without fatigue.
2.8.2 DAMPING FACTOR
The higher the damping factor of an amplifier, the greater
its ability to control unwanted speaker cone movements —espec-
ially at low frequencies. Damping factor is easy to calculate;
divide the speaker's rated load impedance by the amplifier's out-
put source impedance. For example, an amplifier with 0.04 ohms
output impedance at 1kHz presents adamping factor of 200 to an
8ohm speaker (8 /0.04 *200). To understand how ahigh damping
factor improves sound quality, one must first understand the
underlying principles.
When an amplifier drives awoofer, current flowing through
the voice coil creates amagnetic field which interacts with the
permanent magnetic field in the gap and forces the diaphragm/
voice coil assembly to move. Consider what happens when the
resting cone is accelerated to amaximum velocity by asignal
pulse; the maximum signal builds up, and the voice coil/diaphragm
assembly tracks the current, moving outward proportionately.
When the current returns to zero, the suspension and the air mass
loading the diaphragm pull it back toward its original resting
position, and momentum tends to cause overshoot past that point.
In the absence of an applied signal, the voice coil is
moving through amagnetic field, and according to basic physical
-13-

laws, it generates acurrent opposite to that of the original
driving signal. This current induces avoltage or "back EMF" at
the amplifier's output terminals.
The back EMF travels through the amplifier's output source
impedance to ground. The lower that impedance, the better the
"braking" action on the voice coil; adirect short across the
coil (zero ohms) would allow minimal overshoot. Low impedance
equals high damping factor, so the benefits of high damping
factor become obvious —tighter control of the loudspeaker.
The theoretical damping factor of an amplifier may not be
realized at the speaker because cables always have afinite
resistance or impedance; the back EMF must also travel through
the cable, so the damping becomes less. Hence, larger gauge
cables not only reduce signal losses, they improve performance by
affecting damping factor less than smaller gauge cables.
2.8.3 CALCULATING LOSSES IN SPEAKER CABLE
WIRE
DIAM.
(mm)
SOLID
A.W.G.
WIRE
GAUGE
DC RESISTANCE
PER 30 m(100 ft)
OF TWO-CONDUCTOR
CABLE (OHMS)
CABLE LENGTH
PRODUCE AONEWHICH WILL
dB POWER LOSS
0POUR
meters ohms
feet 0EIGHT
meters ohms
feet
4.115 60.08 366 1200 740 2425
3.264 80.13 244 800 488 1600
2.588 10 0.20 145 475 290 950
2.05 12 0.32 91 300 183 600
1.63 14 0.52 58 190 114 375
1.29 16 0.82 37 120 73 240
1.02 18 1.32 23 75 46 150
0.813 20 2.08 15 50 30 100
0.643 22 3.30 930 18 60
POWER LOSSES IN SPEAKER CABLES
This chart may be consulted to establish the approximate
power loss (and damping factor degradation) for various wire
gauges.
2.9GROUNDING
For safe operation the amplifier must be connected to agood
mechanical ground. This provides for acurrent path for any
voltage which should appear on the chassis due to afault in the
amplifier. This current path will result in blowing the main
power supply fuse. Without this current path, the amplifier
-14-

would present ashock hazard. In addition, agood quality ground
on the chassis provides shielding from external fields and minim-
izes radiation of internal fields to the outside world. To comply
with safety regulations in many localities and to protect our
customers, we provide this product with aground connection through
the 3-wire electrical cord. In many situations this will present
no problem. But there are instances where ahum or buzz will be
noticed in the amplifier output due to aphenomenon known as a
ground loop. This results when there is asignificant potential
between the audio ground from the previous piece of equipment and
the mechanical ground to which the amplifier has been connected.
If this is the case, the first attempt at asolution should
be to remove the strap on the rear panel barrier strip which conn-
ects audio ground and chassis. Audio ground will then be refer-
enced from the signal source and the chassis ground will be
separate and still connected to mechanical ground. In some
instances, the voltage difference between the grounds will be so
great that adirect connection to mechanical ground is not possible
without hum in the amplifier output. Check for this using a3
prong to 2prong AC adaptor between the power cord and the power
outlet, temporarily ungrounding the amplifier. Try the amplifier
both with and without the ground strap on the barrier strip.
Determine which connection works best. Remember, however, that for
safety you must still have aconnection to chassis ground. This is
normally made through aproperly grounded third pin connection.
2.10 SECURITY COVERS
In some installations it may be necessary to safeguard the
amplifier gain control settings from deliberate or accidental mis-
adjustment. For this purpose the Model 6200SC Security Cover is
made available. The 6200SC contains enough individual covers for
six amplifiers. Installation is simple;
1) Remove the two Level control knobs. If they are on too tight
to be removed by fingers, use apair of long nose pliers to
grasp the bar on the knob and pull outwards. Wrap the jaws of
the pliers with masking tape to prevent scratching of the knob.
2) If necessary, adjust the gain controls using asmall screw-
driver.
3) Press each of the security covers into place to cover the holes
in the front panel. Make sure they snap firmly into place.
To remove the cover, slip your fingernail under the edge of the
cover and pry it up and off. Alternately, asmall screwdriver or
knife blade may be used, giving due care not to scratch the front
panel.
-15-

IMPORTANT; DO NOT POKE YOUR FINGERS OR METAL TOOLS INTO THE
AMPLIFIER WHEN THE KNOBS ARE OFF. THERE IS THE POSSIBILITY OF
SEVERE SHOCK HAZARD DUE TO THE HIGH VOLTAGE/HIGH CURRENT DC USED
TO POWER THIS DEVICE. THE AMPLIFIER SHOULD NOT BE OPERATED
WITHOUT EITHER THE KNOBS OR SECURITY COVERS IN PLACE.
-16-