Martin Audio MA2.8 User manual

Martin Audio – MA2.8 Power Amplifier
CONTENTS (ENGLISH) Page No
1 UNPACKING 2
2 WARNINGS 2
3 USER RESPONSIBILITY 3
3.1 Speaker damage 3
3.2 Shock hazard 3
3.3 Radio interference 3
4 INTRODUCTION 4
4.1 Front panel 4
4.2 Rear panel 5
5 INSTALLATION 6
5.1 Mounting 6
5.2 Cooling 6
5.3 Operating voltage 6
5.4 Grounding 7
5.5 Power consumption 7
5.6 Input connections 8
5.7 Connecting speakers 11
5.8 Impedance matching (MLS switch) 13
6 OPERATION MODES 13
6.1 Stereo operation 13
6.2 Tandem mono 13
6.3 Bridged mono 14
6.4 Stereo reverse 14
7 OPERATION 15
7.1 Operation precautions 15
7.2 Powering up - Soft start 15
7.3 Input attenuators 16
7.4 Gain switch 16
7.5 Indicators 16
8 PROTECTION FEATURES 17
9 DESIGN FEATURES 19
10 SPECIFICATIONS 21
11 APPENDIX A 22
12 MAINTENANCE 23
13 WARRANTY 24
14 TECHNICAL SPECIFICATIONS 25
The Martin Experience
MARTIN AUDIO
LONDON
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1 UNPACKING
Each Martin Audio MA Series amplifier is built to the highest standard and
thoroughly inspected before it leaves the factory. After unpacking the unit,
examine it carefully for any signs of transit damage and inform your dealer
if such damage is found. It is suggested that you retain the original
packaging so that the unit can be repackaged at a future date if necessary.
Please note that Martin Audio and its distributors cannot accept
responsibility for damage to any returned product through the use of
non-approved packaging.
2 WARNINGS
• Do not use this amplifier if the mains cable is damaged.
• Always operate the amplifier with the chassis ground wire connected
to the electrical safety earth.
• Do not parallel or series connect an amplifier output with any other
amplifier output. Do not connect the amplifier output to any other
voltage source, such as battery, mains source or power supply,
regardless of whether the amplifier is turned on or off.
• Do not run the output of any amplifier back into another channel's input.
• Do not block the air intake or exhaust ports.
• Do not operate the amplifier near any source of heat.
• Do not spill water or other liquids into or on the unit.
Do not operate the amplifier if liquid ingress is suspected.
• Do not remove top or bottom covers. Removal of the covers will
expose hazardous lethal voltages and may void warranty.
There are no user serviceable parts inside the amplifier.
Refer servicing to qualified personnel only.
• Keep this manual for future reference.
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This equipment conforms to the requirements of the
EMC Directive 89/336/EEC, amended by 92/31/EEC
and 93/68/EEC and the requirements of the Low
Voltage Directive 73/23/EEC, amended by 93/68/EEC.
Standards Applied EMC Emission EN55103-1:1996
Immunity EN55103-2:1996
Electrical Safety EN60065:1993
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3 USER RESPONSIBILITY
3.1 Speaker damage
Many loudspeakers can be easily damaged or destroyed by overpowering,
especially with the high power available from a bridged amplifier. Always
check the speakers continuous and peak power capabilities. Even if the gain is
reduced by using the amplifier's front panel attenuator, it is still possible to
reach full output power, if the input signal level is high enough.
3.2 Shock hazard
Power amplifiers are capable of producing hazardous output voltages.
To avoid electrical shock, do not touch any exposed speaker wiring when
the amplifier is operational. See page 11 regarding correct connection of
outputs to loudspeakers.
3.3 Radio interference
A sample of this product has been tested and complies with the limits for the
European Electro Magnetic Compatibility (EMC) directive. These limits are
designed to provide reasonable protection against harmful interference from
electrical equipment. This product uses radio frequency energy and if not used
or installed in accordance with these operating instructions, may cause
interference to other equipment. Compliance with the (EMC) directive does not
automatically guarantee non-disturbance of susceptible equipment in close
proximity to this amplifier.
If the amplifier is suspected of causing interference, this can be easily checked
by powering the unit on and off and observing the disturbance.
The user can correct the interference by one or more of the following steps:
1. Increase the distance between the equipment.
2. Connect the power cable to a socket on a different circuit from
that to which the affected unit is connected.
3. If a radio receiver is affected (normally only am receivers are
affected); re-orientate the antenna.
4. Check if the affected equipment complies with the EMC limits for
immunity, (CE-labelled). If not, address the problem with the
manufacturer or supplier. All electrical products sold in the EC must
be approved for immunity against electromagnetic fields, high voltage
flashes and radio interference.
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4 INTRODUCTION
Thank you for purchasing a MARTIN AUDIO power amplifier. This manual
contains important information on operating your amplifier correctly and
safely. Please take some time to read this manual and familiarize yourself
with the advanced features of this amplifier.
4.1 The front panel
Figure 1. Front panel
1. Carry/protection handle
Both handles can be used to carry the amplifier; they also act as protection
for the front panel and controls. In fixed installations or where rack front
covers are too shallow, they may be removed by unscrewing the retaining
bolts behind the front panel.
2. Input level attenuators
These controls are used to set the signal level entering the amplifier. They
are calibrated in dB to assist the set up of active loudspeaker systems.
(See page 16).
3. Protect indicator LED
This indicator illuminates if the amplifier attempts to function above its
maximum operating temperature (90ºC). The indicator first comes on as a
warning to either turn down the input level or check the cooling
arrangements, after which point the amplifier will mute the input signal.
When the cooling fans have returned the output heat sinks to the normal
operating temperature, the input signal is un-muted.
The Protect LED also illuminates when constant signals above 12kHz at full
power are detected at the output terminals. When this occurs the input
signal is muted until the VHF signal is no longer present. (See page 17).
4. Clip/limit indicator LED
This indicator signals when the amplifier output is clipping or limiting. It has
two different indication states:
• When the clip limiter is engaged, it has a short time constant and
illuminates briefly. (See pages 16-17).
• When the clip limiter is not engaged, it has an increased time constant
and illuminates for a longer period.
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5. Fan grille filter
A foam filter is located behind the front panel grille to prevent dust entering
the amplifier.
6. Power switch
This is used to power up the amplifier.
(See page 7 and 15)
4.2 The rear panel
Figure 2. Rear panel
1. Speakon connector
This type of speaker connector may be unfamiliar to some users. A full
description is found in the operation section. (See page 11).
2. Minimum load selector (MLS™) switch
This switch is used to select the minimum nominal speaker impedance the
amplifier is going to drive (see page 12).
3. Clip limiter switch
Turns the clip limiter on and off. (See page 17).
4. Input jack
Alternative to using input XLR or for linking inputs with other amplifiers.
(See page 9).
5. Input signal XLR (See page 8).
6. Polarity reverse switch for channel B
For reversing the input signal polarity of channel B to allow bridged
operation. (See page 13).
7. Input link switch
Allows a single input to drive both channels simultaneously. (See page 13).
8. Gain select switch
Allows amplifier gain to be switched between 32dB (1.55Vrms input
sensitivity) and 38dB gain (0.775mV input sensitivity). (See page 16).
9. AC power cable (See page 6).
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5 INSTALLATION
5.1 Mounting
The amplifier is two rack units high (2U) and will mount in a standard EIA
19'' rack. Amplifiers may be stacked directly on top of each other. There is
no need for spacing between units. If it is the intention to fill a rack with
amplifiers, we recommend racking is started from the bottom of the rack. It
is also recommended that rear supports are used for amplifiers mounted in
the middle of the rack, especially if used as part of a portable system.
5.2 Cooling
Your amplifier uses forced air cooling to maintain a low and even operating
temperature. All MARTIN AUDIO MA Series amplifiers have front to rear
cooling. Never try to reverse the air flow, as the amplifier needs a pressure
chamber between the fans and heat sink and this only operates in one
direction of the air flow (see Design features on page 19).
Should the heat sink get too hot, its sensing circuit will mute the hot
channel. If the power supply overheats, another sensing circuit will mute all
output channels until it cools down to a safe operating temperature.
Make sure that there is an adequate air supply in front of the amplifier and
that the rear of the amplifier has sufficient space to allow the exhaust to
escape. If the amplifier is rack mounted, do not use covers or doors on the
front or rear of the rack.
For fixed installations with a central cooling system, usually found in fixed
installations with a dedicated rack room, it may be necessary to calculate
the maximum heat emission. Refer to Power consumption on page 8.
5.3 Operating voltage
WARNING!
A label below the mains cable on the rear of the amplifier indicates the
selected AC mains operating voltage.
Connect the power cable only to the AC source referred to on the
label. The warranty will not cover damage caused by connecting to
the wrong type of AC mains.
For converting a 230 volt amplifier to 115 volt or vice-versa, see Appendix A.
MARTIN AUDIO switch mode amplifiers use primary switching, i.e. the
mains is rectified directly before the transformer, which means that the
power supply is insensitive to the mains frequency and will operate from
DC to 400Hz.
The amplifier is supplied with an approved European AC line connector. If
this connector is not appropriate for your country, it can be cut off and wired
to a suitable connector in the following way:
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BROWN - LIVE
BLUE - NEUTRAL
GREEN/YELLOW - EARTH
Once connected to a suitable AC supply, the amplifier can be started with
the power switch. When you power up the amplifier it takes a couple of
seconds to self check (this is known as the "soft start" or "slow start"
sequence). The fans then blow at high speed before going into "idle". The 2
bottom green LED's then illuminate to show the amplifier is operational.
5.4 Grounding
There is no ground lift switch or terminal on this amplifier. The signal
ground is always floating via a resistor to chassis and the grounding system
is automatic. If a potential above 0.6V presents itself between signal ground
and chassis ground, a short circuit is introduced between the two, thereby
enabling electrical protection. If a unit in the system is faulty, its mains fuse
will blow, due to this automatic ground system.
If however, you wish to tie the signal ground to chassis, connect the XLR-
connector's shell lug to pin 1. In the interest of safety never disconnect the
earth pin on the mains connector.
For compliance with the EMC directive (conducted emissions), there is an
AC mains filter. This filter needs the chassis ground for reference, otherwise
a current loop is formed via the signal ground. Use the balanced input to
avoid hum and interference.
5.5 Power consumption
There are three ways to define the power/current consumption of the
amplifier:
First, the peak current draw at full power output. Under this condition the
amplifier will operate for less than 2 minutes before thermally limiting.
Secondly,the maximum expected average current under worst case
program material, which corresponds to 1/3 of full power according to the
FTC-standard. At this level the music will be in a state of constant clip and
is therefore the highest power level one can obtain without completely
obliterating the program.
Lastly,the "normal operating power", as defined by EN60065 as a
measurement level for approval and testing to this standard. The normal
operating power is measured using pink noise, with an average output
power equal to 1/8 of full power. The one-eighth power is the maximum
volume music can be re-played without continuous amplifier clipping.
It also corresponds to headroom of 9dB, which is very low for normal
program material.
If operating the amplifier into a sustained 2 ohm load, protection circuits
within the amplifier will limit the long-term current draw, and thermal
protection may occur.
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Martin Audio – MA2.8 Power Amplifier
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MAX OUTPUT POWER MAINS INPUT POWER
Power Full Power 1/3 Power 1/8 Power Idle
sine wave note 1 note 2
MA2.8 8 ohms 2X 1100 3100 1300 700 130
4 ohms 2X 1400 4300 1700 900 130
2 ohms 2X 1400 4700 1700 900 130
note 1 Average power with music as program note 2 Normal music power with 9dB headroom,
source. The amplifier driven to clip level. IEC standard power rating.
Table 1.
The current draw can be calculated by dividing the mains input power by
the mains voltage. We recommend that you design the power distribution
for at least the current at 1/8 power or 1/3 power for heavy duty demand
like discos etc.
The heat power can be calculated as the following example:
If we consider a headroom of at least 9dB and a 4 ohm load on an amplifier
producing 1400 watts per channel. The 1/8 power per channel is then;
1400 ÷ 8 = 175 watts, total output; 2 x 175 = 350 watts.
The power consumption according to the chart above is then 900 watts.
The heat power produced is the difference between the power
consumption and power output; 900 - 350 = 550 watts per amplifier.
5.6 Input connections
Both inputs are balanced on 3 pin XLR type connectors and wired pin 2
hot, pin 3 cold, pin 1 ground/shield:
PIN 1 - SIG. GROUND/SHIELD
PIN 2 - HOT (+)
PIN 3 - COLD (-)
Figure 3. XLR input connector pin-out
1
23
SIG. GND.
–
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There are also TRS jacks for linking etc. They are wired as follows:
TIP - HOT (+)
RING - COLD (-)
SLEEVE - SHIELD/SIG. GROUND
Figure 4. TRS phone plug
The input impedance is sufficiently high (20kohms balanced) to allow
"daisy-chaining", or multiple parallel input connections. To daisy-chain, use
the TRS jacks provided on each channel. The input circuits also have high
enough headroom to accept the maximum output level from virtually any
low-level signal source.
Figure 5. Rear panel connectors
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SIG.GND –+
INPUT
CH.B CH.A
Normal Pol.B
Reverse Pol.B
Stereo
Link A+B
Gain 38 dB
Gain 32 dB
Mono Bridge
XLR 1/4"
Pin 1 Gnd Sleeve
2 Pos Tip
3 Neg Ring
Must be grounded 230V AC 40-440Hz
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Do not use XLR and TRS jacks on the same channel simultaneously
for mixing or other purposes.
Figure 6. Balanced line
To use an unbalanced source, connect pin 3 (ring on TRS jack) to the
shield of the connector. If you leave one pin disconnected, you will lose
6dB in gain.
Figure 7. Unbalanced line connection
A better method for using unbalanced sources is shown in Figure 8. This is
similar to the connection for balanced lines, but pin 3 is connected to the
shield at the source. The hum and noise rejection for the cable is
equivalent to that for a balanced line. To minimize hum in the audio, use
balanced inputs whenever possible.
Figure 8. Balanced line with unbalanced equipment
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Balanced Output
Balanced Output
Amplifier Input
Amplifier Input
Signal
Grounded
Shield
2
13
Chassis
Grounded
Shield
1
23
1
23
2
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Unbalanced Output
Unbalanced Output
Amplifier Input
Amplifier Input
+
+–
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1
23
1
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2
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Unbalanced Output
Unbalanced Output
Signal
Grounded
Shield
Amplifier Input
Amplifier Input
+
+–
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5.7 Connecting speakers
Speaker connections are made via the two Neutrik NL4FC Speakon
connectors (1). These connectors meet the EC safety requirements. They
are wired as follows:
Pin -1 Speaker ground.
Pin +1 Speaker positive.
Pin -2 No connection.
Pin +2 No connection.
Figure 9. Speakon connector
Please note that this is the standard wiring convention for Speakon
connectors adopted worldwide.
Never connect either output terminal to ground or to some other
output or input terminal (see warning on page 2). For normal two-
channel operation, connect each speaker load across the outputs positive
and ground terminals. Pay attention to speaker polarity; loudspeakers
connected out of polarity can degrade sound quality.
Keep the speaker cables as short as possible and use a good quality
stranded speaker cable. Do not use shielded wire, such as microphone or
guitar cable. Remember that the speaker cable robs the power of the
amplifiers in two ways: It increases the load impedance and introduces
resistive power losses.
5.8 Impedance matching (MLS switch)
The MLS™ switches (2) are located on the rear panel. The MLS (Minimum
load select) switches offer impedance matching, so you can drive the
MA2.8 into 2 ohms without increased heat loss.
As stated earlier the MA2.8 can produce at least 1100 watts into any
impedance between 1.5 and 8 ohms. Utilise higher MLS™ positions when
connecting to lower impedances as shown in Table 2.
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[1] Component tolerance dependent. Table 2.
[2] Continuous power, one channel driven or peak power both channels driven.
(Thermal protection may occur at high continuous power).
Power in watts (EIA 1kHz,1% THD).
Table 3.
MLS™ positions can be different for each channel and a variety of
combinations can be achieved see Table 3.
The way to find the best MLS™ position for your application is by
experimentation using table 3 for initial starting positions. The amplifier is
very well protected (even down to 0.3 ohms).
The MA2.8 amplifier is a power converter and when you select MLS™
positions you are allocating a portion of power from 3000 watts.
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MA2.8 MLS Switch Position
LOAD CONFIGURATION -5dB -4dB -2dB 0dB
16 ohms Stereo (2 channel) 160W 180W 340W 520W
8 ohms Stereo (2 channel) 300W 350W 650W 1100W
4 ohms Stereo (2 channel) 570W 680W 1100W 1400W
1900W[2]
2 ohms Stereo (2 channel) 1040W 1200W 1200W 1400W[1]
1400W[2] 2900W[2]
16 ohms Bridged mono 600W 700W 1300W 2000W
8 ohms Bridged mono 1200W 1400W 2200W 2800W
4 ohms Bridged mono 2100W 2400W 2400W 2800W
Channel Impedance MLS Power Comments
A 8 -2dB 650W Power
B 8 -2dB 650W reduction
A 4 0dB 1900W High peak
B 4 0dB 1900W power
A 8 0dB 1100W Power
B 2 -4dB 1200W matching
A 8 -2dB 650W Power
B 4 0dB 1900W sharing
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Figure 10. MLS switches on rear panel
6 OPERATION MODES
6.1 Stereo operation
For stereo (dual channel) operation, leave the Link and Phase reverse
switches in the un-depressed position. In this mode, both channels operate
independently of each other, with their level attenuators controlling their
respective levels. Never connect either output terminal to ground or in
parallel. The recommended minimum nominal impedance for stereo or
tandem operation, is 2 ohms per channel.
6.2 Tandem mono
For tandem (dual channel-single input) operation, depress the Link switch.
Both channels can now be driven by a signal at either input connector. The
output connection is the same as in stereo mode. You can use either TRS
connectors for linking out etc. Do not use the remaining XLR and TRS
connectors for mixing or other purposes. Both level attenuators are
active, allowing you to set different levels for each channel.
Never connect either output terminal to ground or in parallel.
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1–CH.B–
OUTPUT CH.B
MLS Switch
dB
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6.3 Bridged mono
To bridge the amplifier, depress the Link switch (7) and Phase reverse switch
(6). Both channels are then driven by a single signal at either input. You can
use any remaining input connectors for linking etc. Do not use the remaining
XLR and TRS as input jacks simultaneously for mixing or other purposes.
To obtain an output, connect the speaker leads to pin +1 on channel A
Speakon to speaker positive terminal and pin +1 on channel B
Speakon to speaker negative terminal. Do not connect either of the -1
(negative) pins of the Speakons. Do not connect speakers to channel
A or B in the normal manner in bridge mode, as this can cause
serious damage.
The recommended minimum nominal impedance for bridged mono is
4 ohms (equivalent to driving both channels at 2 ohms). Driving bridged
loads of less than 4 ohms may cause a thermal overload.
Figure 11. Bridge mono connection
Both level attenuators must be at the same position. We recommend that
you put them in the 0dB (full) position.
6.4 Stereo reverse
This mode is similar to the Tandem mono mode. Apart from depressing the
Link switch, you also depress the Phase reverse switch, as in the Bridge
mono. Channel B is now phase reversed. To compensate for that, connect
pin +1 on channel B Speakon to speaker negative terminal and pin -1 on
channel B to speaker positive terminal.
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CHA +
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I/P
LINK
PHASE
INVERT
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Figure 12. Stereo reverse mode
Channel A output is connected as in the normal stereo mode. By having
channel A and B operating in opposite polarity, the energy storage in the
power supply is more efficient. This means that the amplifier can deliver up
to 10% more power than in tandem mono mode. This is significant for
signals below 100Hz (sub bass etc.).
7 OPERATION
7.1 Operation precautions
• Make sure that the power switch is off before making any input or
output connections or operating the switches on rear panel. See pages
6-11 about installation.
• Make sure that the AC mains is correct and the same as that printed on
the rear panel of the amplifier. See pages 6-8 about operating voltage
and power consumption.
• Make sure that the switches on the rear panel for operation modes,
clip limiters and the MLS™ switches are in the correct position.
See pages 13 and 14 about operation modes, page 16, about clip
limiters and page 12 for the MLS switches.
• It is always a good idea to turn down the gain controls during power up
as this prevents speaker damage, especially if there is a high level
signal at the input.
7.2 Powering up - Soft start
When you power up the amplifier it takes a couple of seconds to self check.
This is known as the "soft start" or "slow start" sequence. The fans then
blow at high speed before going into "idle" and the two bottom green LED's
illuminate to show the amplifier is operational.
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1
23––
–
+++
CHA +
1
23–––
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+
CH B +
I/P
LINK
PHASE
INVERT
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7.3 Input attenuators
The two input level attenuators on the front panel adjust the signal level for
their respective amplifier channel in all modes. They are calibrated in dB to
assist setting up active loudspeaker systems.
In bridged mode, both controls must be in the same position, so that the
speaker load will be shared equally between the channels.
7.4 Gain switch
The gain switch located on the rear panel is for changing the input
sensitivity of the amplifier. Most professional mixing consoles operate at a
nominal level of +4dBu. Therefore, use the 32dB position (depressed
switch), to give you plenty of fader movement. On the other hand, for a
disco mixer that operates at a nominal level of 0dBu, use the upper position
setting, which has a sensitivity of 0dBu (0.775Vrms) for full power in 4
ohms.
7.5 Indicators
Figure 13. Front panel indicators
The two bottom green "ON" LED's indicate when the power supply and
output stage is operating correctly.
The "-40 dB" LED's illuminate when the output signal is greater than -40dB.
0dB is referenced to full output power. These LED's also act as signal
present indicators.
The ''Clip/limit'' LED indicates when the amplifier output is clipping or
limiting. It has two different indication states:
* When the clip limiter is engaged it has a short time constant and
illuminates briefly.
* When the clip limiter is not engaged it has an increased time constant and
illuminates for a longer period.
The remaining yellow "Protect" LED's indicate if the temperature or VHF
protection circuits are activated. These are described on page 17.
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8 PROTECTION FEATURES
Each MARTIN AUDIO MA Series amplifier has many advanced protection
features that will protect both the amplifier and the speakers connected to
it, should a fault condition arise. Under normal use these features are
inaudible. All protection circuits are independent.
Output limiter
An output limiter is included to avoid amplifier clipping. When an amplifier is
severely overdriven, its output waveform is clipped (its peaks are squared
off) - reducing the crest factor. In extreme cases, the waveform can
approach that of a square wave. An amplifier is normally capable of
producing far more power under these conditions than its normal
undistorted rated power output. The limiter works by monitoring the output
and comparing the distortion produced between the input and output of the
amplifier. If the distortion exceeds 1%THD for any reason (voltage or
current clipping), the limiter reduces the input signal proportionally. Note
that, if the signal is distorted or clipped before it reaches the amplifier, the
clip limiter will not be activated.
Under normal operation the clip limiting is inaudible. The limiter can be
turned On or Off by depressing the relevant clip limit switch (2). Some
manufacturers of loudspeaker processors do not recommend the use of clip
limiters in amplifiers, as they tend to upset the tracking of the processor's
limiters.
Apart from this one exception, MARTIN AUDIO recommend leaving the clip
limiters switched "on" (button depressed).
As a by-product, when the amplifier comes out of a protect condition, the
output level has a slow rise time - the effect is like turning the gain up
slowly.
Thermal protection
When the amplifier is driven very hard into a low impedance load, the
cooling fans will run at high speed. If these operating conditions continue,
the Protect indicator(s) will illuminate indicating that the amplifier is about to
go into thermal shutdown.
After five seconds the amplifier will go into thermal protect, by muting the
input signal. After 15-20 seconds the amplifier will have cooled down
enough for the amplifier to come out of shutdown and operate as normal. If
the load conditions remain unchanged the thermal protection will be re-
initiated. Thermal protection starts when the heatsink reaches a
temperature above 90ºC.
VHF protection
If a signal of more than 12kHz, at full power, is detected at the amplifier
outputs and lasts for more than five seconds, the VHF protection mutes the
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input signal. (This is indicated on the front panel (3) labelled “Protect”).
After five seconds the outputs will un-mute and return to normal operation,
unless the output signal has remained unchanged in which case the VHF
protection will re-initiate. This circuit is inaudible in normal use, however it
can be turned off. If the VHF protection is required to be turned off i.e. for
studio monitors, please consult your supplier, as this is a non-user
adjustment.
Short circuit protection
All MARTIN AUDIO MA Series amplifiers are completely short circuit
protected. The protection circuit permits very high peak currents, but still holds
the output devices within their safe operating area. If a short circuit is
maintained, the channel affected will eventually go into a thermal protect cycle
until the short circuit is removed.
AC mains voltage protection
If the AC mains voltage is outside the operational window (over or under
voltage), the power supply will automatically shut down. When the mains
voltage is above the minimum start voltage and below its maximum operating
voltage the amplifier will restart.
It takes a couple of seconds to self check (this is known as the "soft start" or
"slow start" sequence), the gain will gradually be increased back to the
previous set level and the fans will blow at high speed before going into "idle".
The two green "ON" LED's then indicate that the power supply and output
stage is operating correctly.
D.C. protection
There are two types of DC protection:
• Fuses on the supply branches of each channel.
• A DC crowbar protection circuit that shorts the output if more than 10 volts is
detected on the outputs. Both these circuits come into effect once a DC
level is detected.
Martin Audio – MA2.8 Power Amplifier
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9 DESIGN FEATURES
Cooling
MARTIN AUDIO MA Series amplifiers run very cool due to a patented heat
exchanger. The bi-polar output devices are mounted directly onto a copper
heat sink thereby avoiding thermal losses normally found when using mica
washers. The heat sink is mounted horizontally in front of a pressure
chamber and the air flow is constant along the whole heat sink. When cool
air hits the geometric fins of the heatsink, turbulence is produced thereby
enabling efficient cooling. (In a conventional tunnel design the end of the
heat sink away from the fan invariably gets hotter than that directly in front of
the fan).
Light weight
The MA 2.8 switch mode amplifier has a very good power to weight ratio,
(280w/Kg) making it one of the most powerful, lightweight 2u amplifiers
available. This is achieved not only by the design of the power supply and
heatsink but also by the use of an alloy chassis making rear rack supports
unnecessary in most applications.
Switch mode power supply (SMPS)
Switch mode power supplies are the modern solution to the problems of size
and weight. However, early examples of SMPS used in power amplification
were not ground breaking and literally adopted the type of supply found in
many computers today. The MARTIN AUDIO SMPS is different in that it is
regulated.
Whilst the SMPS may sound complex it is actually very straightforward with
high reliability. The net result is good bass and fast response. Because the
supply is regulated, the power amplifier will produce the same power output
even if the AC line voltage falls by more than 20%!
Efficiency is also assured, and in the case of the MA2.8 this is especially
apparent; its power supply draws only 4kW and yet it produces 2 x 1400W.
(See figure 14).
Martin Audio – MA2.8 Power Amplifier
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Figure 14. Current-voltage characteristic of different power supply topologies.
1) Conventional power supply shows voltage drop due to internal resistance.
2) Regulated power supply used by MARTIN AUDIO.
3) Unregulated switch mode power supply with current limiting.
Traditional PSU's can behave erratically at low frequencies, particularly
under clip conditions with the rise in their output impedance producing
uncontrollable speaker cone movement. MARTIN AUDIO SMPS produces
its rated power output at all frequencies regardless of speaker demands and
AC line conditions.
Minimum Load Switch (MLS™)
Thanks to the regulated SMPS, the maximum power available for the output
stages can be adjusted without increased heat dissipation or efficiency loss.
This allows users to match the output power with the loudspeaker
impedance.
Because the amount of power delivered into a load is dictated by its
impedance (at 2 ohms you use the least voltage but use the most current),
the MLS switches can be used to advantage to enable powers in excess of
the normal power rating to be produced.
Martin Audio – MA2.8 Power Amplifier
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Voltage [V]
Current [A]
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