Model 2800
Kilowatt Mosfet Audio
Amplifier
Table Of Contents
Audio Amplifier
Section
Introduction
Amplifier Overview
Amplifier Features
Amplifier Topology
Getting Started
|
Protection Section
Amplifier & Speaker Protection Overview
Amplifier Protection Controller
Amplifier External Load Detector
Amplifier Rail Detector
Amplifier DC Offset Detector
Amplifier Short Circuit Detector
Amplifier Temperature Detector Input
|
Construction Notes
Component Reference Designators
Component Assembly Order
Component Orientation
Component Installation
Preliminary Test
Finally Assembly
Final Test
Matching Mosfet Transistors
Audio Amplifier Section Parts List
Protection Section Parts List
Schematics
|
Introduction
The goals
for the DIY amplifier project was to design a low noise class ab mosfet
amplifier that is able to deliver
high
power into an 8 ohm load, be 2 ohm stable and drive 1 ohm speakers playing
music not continuous test tones.
The amplifier
will use discrete circuits with no signal processing and will include a
differential input stage, large bank of output stage transistors and have the
ability to bridge two channels to create a single even more powerful monoblock
amplifier.
Instead of
designing an amplifier from scratch, one DIY amplifier design chosen that met
the requirements has
already
been designed and published by Anthony E. Holton. He designed many amplifiers and the AV800
design
was
chosen for this project. You can find
his designs on his website at the url at the bottom of
this page. The
AV800
amplifier is rated for 800 watts continuous driving a 4 ohm load. He also designed a 1000 watts version of this
design by increasing the rail voltage and increasing the output stage
transistor bank. Both these versions are
able to
drive 2 ohm loads short term providing anywhere from 1200 watts to 1600 watts.
This
document describes the “Model 2800 – Kilowatt Mosfet Audio Amplifier” circuit
board which is a single channel
AV800 – 1KW design with extra features and modifications.
The output power of these amplifiers is determined by
the power
supply rail voltage and output stage redundancy. Using rail voltages between 95 volts and 100
volts
can
achieve approximately 1600 watts into 2 ohms short term, but overall nominal
rail voltage of 90 to 95 volts will
work nice
to create a cushion of safe operating area for the amplifier with a small
penalty to output power.
Anthony E.
Holton wrote the original documentation to describe the AV800 amplifier. Some of his documentation has been included
here, but it may have been altered to reflect the Model 2800 printed circuit
board and schematic. You can refer to
the AV800 construction manual for reference but the component reference
designators on his documentation will not match the Model 2800 printed circuit
board. Use the schematic in this document.
Anthony E. Holton’s website
http://www.aussieamplifiers.com/
Amplifier Overview
AV800 Amplifier
The AV800
is rated for 800 watts and has been tested by Anthony Holton to output
continuous power of 450 watts
into 8
ohms, 820 watts into 4 ohms, and short term power of 1200 watts into 2 ohms
using 90 volt rails. It’s a four
stage
amplifier consisting of a differential input stage, voltage amplification
stage, bias/buffer stage, and output stage.
The output stage consists of 7 output stage transistors per voltage rail
(14 transistors per channel).
AV800 - 1KW Amplifier
The AV800 -
1KW is the same amplifier as the AV800 except it uses more output stage
transistors and higher
voltage
rails to achieve a power rating of 1000 watts.
Anthony Holton has tested the continuous output power of
550 watts
into 8 ohms, 1050 watts into 4 ohms, and short term power of 1600 watts into 2
ohms using 96 volt rails
loaded. The output stage consists of 10 output stage
transistors per voltage rail (20 transistors per channel).
Model 2800 – Kilowatt Mosfet Audio Amplifier
Refer to
the Amplifier Features section below for additional information. To summarize,
the Model 2800 printed
circuit
board includes a SINGLE CHANNEL
AV800 – 1KW design plus the following;
1. More output stage transistors, up to 28 per
channel.
2. More on-board power supply capacitance,
typically 13,000uF – 28,000uF depending on capacitors chosen.
3. More on-board “VAS” capacitance, typically
3,000uF – 6,000uF depending on capacitors chosen.
4. Protection circuits including a programmable
controller with soft start that drives a high current speaker relay.
5. PCB footprints allow the installation of high
quality polypropylene capacitors.
6. On-board beryllium copper fuse holders
(clips) that support 30 amperes for each rail.
7. Input signal terminal block (3 pin) for (+)
input, (-) input and also has a center ground pin for esoteric needs.
8. Molex connectors power input (6 pin) and
speaker output (2 pin). Extra parallel pins for redundancy.
9. Four layer printed circuit board layout with
power and ground planes and power redundant tracks.
The Model
2800 printed circuit board is designed to allow up to 14 output stage
transistors per voltage rail
(28 transistors per channel). This is 8 more
output stage transistors than the AV800 – 1KW amplifier and it should
allow 2
ohm continuous operation and perhaps 1 ohm music if adequate heatsinking is
provided. The output impedance of the
buffer stage may need adjustment to drive the extra Hexfet transistors, it’s unknown at this time.
Single Channel
Configuration
Refer to the
“Model 2800 - Single Channel and Bridging Configurations” schematic for the
wiring diagram. Single
channel
configuration is the most common where one Model 2800 printed circuit is one
amplification channel.
Connect the
audio signal positive (+) to the positive (+) input on the terminal block.
Connect the
audio signal negative (-) to the negative (-) input on the terminal block.
The
terminal block center pin is grounded to the plane and may be used for esoteric
needs such as shielding, etc.
The speaker output Molex connector connects to the
(+) speaker terminal.
The (-) speaker terminal connects to the DC power
supply “star” ground.
Bridged Mode
Configuration
Refer to the “Model 2800 - Single Channel and
Bridging Configurations” schematic for the wiring diagram.
Bridge Mode is when you use two amplifier channels
connected together to form a single channel of greater power than a single channel, theoretically 4x more power,
but typically 2x more power often referred to as a monoblock.
Connect the audio signal positive (+) to channel 1
positive (+) input and connect it to channel 2 negative (-) input.
Connect the audio signal negative (-) to channel 1
negative (-) input and connect it to channel 2 positive (+) input.
Connect the speaker across both positive outputs of
both channels only. Do not connect the speaker the negative
or ground speaker terminals.
Channel 1 + speaker output connects
to the + speaker terminal.
Channel 2 + speaker output connects
to the – speaker terminal.
Amplifier Features
AV800 Specifications
Specifications
|
AV800 -
800 Watt
|
AV800 -
1000 Watt
|
8 Ohm Power Ratings
|
450 W RMS continuous
|
550 W RMS continuous
|
4 Ohm Power Ratings
|
820 W RMS continuous
|
1050 W RMS continuous
|
2 Ohm Power Ratings
|
1200 W RMS short term
|
1600 W RMS short term
|
Frequency Response
|
10HZ – 100KHZ
|
10HZ – 100KHZ
|
Distortion @ 1KHZ (100W into 8
Ohms)
|
0.01% THD
|
0.01% THD
|
Damping Factor
|
400
|
400
|
Voltage Rails
|
+90V, -90V
|
+110V, -110V (Loaded = +96V, -96V)
|
Output Stage Transistors per Rail
|
7
|
10
|
Power Supply Capacitance
|
10,000 uF
|
10,000 uF
|
Model 2800 Minor Electrical Modifications
Modifications
|
Model
2800
|
AV800 –
800W
|
AV800 –
1KW
|
Output Transistors
|
14 per voltage rail
28 per channel
|
7 per voltage rail
14 per channel
|
10 per voltage rail
20 per channel
|
On-Board Power Supply Capacitance
|
9, 520 uF per voltage rail
19,040 uF per channel
|
329 uF per voltage rail
658 uF per channel
|
470 uF per voltage rail
940 uF per channel
|
VAS Capacitance
|
2,040 uF per voltage rail
4,080 uF per channel
|
220 uF per voltage rail
440 uF per channel
|
220 uF per voltage rail
440 uF per channel
|
Model 2800 Protection Circuits
Circuit
Name
|
Description
|
Author
|
Protection Controller
|
Main
controller that receives input from the protection circuits. It also
functions as an eight second turn-on delay to drive the speaker relay.
|
DreadLordpk
|
Rail Detector
|
Monitors
rail voltage imbalance.
|
DreadLordpk
|
External Load Detector
|
Monitors
the speaker terminals for abnormal resistive load during the initial power on
sequence, then the circuit is disconnected when the speaker relay engages.
This detector
has user
programmable load settings and it’s used to detect wiring errors or if an
incorrect
load was connected to the amplifier.
|
DreadLordpk
|
DC Offset Detector
|
This is
an ordinary common dc offset detector.
|
Public Domain
|
Output Short Circuit Detector
|
This is
an ordinary common output short circuit detector.
|
Public Domain
|
Clipping Detector
|
This is
the Rob Elliot clipping detector. http://sound.westhost.com/index.html
|
Rob Elliot
|
Rail Fuses
|
The
printed circuit board accommodates AGC solder mount fuse clips. Beryllium
copper recommended over standard tin type fuse clips.
|
|
Temperature Detector Input
|
This
input accommodates a ground triggered temperature switch configuration.
|
|
Model 2800 Printed Circuit Board Information
Item
|
Description
|
4 Layer Printed Circuit Board
|
Top Layer
1 - Component side, power and ground tracks, alternative routing.
Inner
Layer 2 - Speaker output plane, protection circuit power supply plane.
Inner
Layer 3 - primary power & ground plane, ground barriers to steer ground
currents.
Bottom
Layer 4 - Primary amplifier signal routing, power and ground tracks.
|
Printed Circuit Board Properties
|
1. Board
size: 6.5”W x 10.75”L
2.
Material: 125 mils thick epoxy-fiberglass, grade FR-4.
3. Finish:
Gold-over-nickel
4.
Soldermask both sides, LPI or equivalent, color = red
5. Top
layer silkscreen using white non-conductive CAT-L-Link or equivalent.
6. 25
mils minimum separation between layers.
|
Power & Ground Redundancy
|
Top &
bottom layer have extra power & ground tracks in parallel with layer 3
power & ground plane.
|
On-Board Transistor Mounting
|
There are
three output stage transistor mounting schemes. Vertical, horizontal, and
underneath the board.
There are
surface mount pads on the top and bottom of the board and two sets of holes
for soldering.
|
Power Supply Capacitors
|
Supports
up to 750 mil diameter capacitors using a 300 mil lead space. There are 14
capacitor footprints per voltage rail and also supports 3 capacitors for the
VAS per rail.
|
Molex
Power & Speaker Connectors
|
Designed
for Molex “Mini-Fit Sr.” connectors. The speaker output uses a 2 pin
connector and the power input uses a 6 pin connector. The contacts are rated
for 50 amperes. The design uses two pins per function for
reliability, i.e. both pins on the speaker output are shorted on the
PCB. A similar setup for the power input is used.
|
DPDT Speaker Relay
|
Paralleled
contacts for increased current capability and reliability. Relay N.O. contact
ratings are;
Resistive
Load: 120/277 VAC 30 ampere per contact. 28 VDC 20 ampere
per contact.
|
Input Signal Terminal Block
|
Supports
a standard terminal block of 650 mil x 475 mil, 200 mil centers.
|
Black Gate Capacitor
|
Optional
support for a single Black Gate 300uF 100V N series capacitor for the
feedback loop. 18mm x 36mm
|
Polypropylene Capacitor
|
Large
capacitor PCB footprints to allow the use of signal path polypropylene
capacitors where applicable.
|
Radial Source Resistors
|
Supports
radial source resistors with 200 mil lead space to save PCB real estate.
|
Protection Circuit SMD
|
The
protection circuits are 100% surface mount devices to save PCB real estate.
|
Signal Interface Connector
|
This
connector is used to provide inputs/outputs for interfacing such as
controller status output, clipping detector output, external load detector
output, temperature
sensor input, muting input, interrupt input.
|
Amplifier Topology
by Anthony E. Holton
Modified by DreadLordpk
The Error Amp Stage
(Differential)
The first stage is what I call an asymmetrical
balance input error amplifier. It is a
design which allows only one single differential stage and yet has the ability
to accept a balanced input source. An
unbalanced source can be used if either the inverting or non-inverting input is
tied to signal ground. Now I will
explain how each device in the stage works together. Q1, Q2, R6, R7, R8, R9 form the main
differential error amplifier which then has its collectors connected to a
cascode load. Q3, Q4, R10 and ZD1 form
the cascode stage which provides a constant 14.4 volts on the collectors of Q1,
Q2. Q5, R11, R12, ZD2 and C6 form a
constant current source which supplies 1.5 milliamps to the first differential
stage. These modules form the first
stage of the amplifier and basically set up how the whole amplifier is biased
from front to back.
The Voltage
Amplification Stage
This next stage provides most of the voltage
amplification that the next stage needs to drive the output stage to full
power. Q6, Q7, Q8, Q9, R16, R17, R18,
R19, C10, C15, C16, C2901, C3401 for the second differential voltage
amplification stage. Q6 and Q7 form what
is known as a current mirror load for the second differential stage and
basically force this stage to share the current supplied from R36, which is
about 8 milliamps. The remaining
components, namely the capacitors C11, C12 provide local frequency compensation
for this stage.
The Bias and Buffer
Stage
As the name suggest Q10, Q11, Q12, R27, R25, R26,
R22, R20, R21, C14, ZD3, ZD4, form the bias and buffer stages. Its main purpose is to provide the mosfet
gates with a stable and compensated supply voltage and buffer the voltage amp
stage from the high gate source capacitance.
Which would without this stage cause the
frequency response and slew rate to be very poor indeed. The down side of this is the extra stage does
introduce and extra dominant pole into the amplifiers feedback loop.
The Output Stage or
Current Amplification Stage
Once again as the name suggest this stage converts
the voltage developed in the VAS and provides all the amperes needed to drive 8
or 4 Ohm loads. 2 Ohm loads are possible
for several minutes at a time. In fact,
I have tested the 1KW amplifier to over 1600 watts RMS into 2 Ohms. But this would not be recommended as a
long-term load at all. As it does exceed
the SOA figures of the output stage.
Note: Model 2800 with
28 output transistors and robust heatsinking should be able to drive a 2 Ohm
load continuous tone testing. It may
even drive a 1 Ohm load playing music, not continuous test tones.
Power Supply
Requirements
The power supply components for this amplifier are as
follows and are expressed for one channel or one power module only. 1 x toroidal transformer
with a core rating of 1KVA.
Primary windings are made to suit your local mains supply, e.g.: for Australia one
single primary winding with a 240 VAC rating.
For USA, 110 VAC, 115
VAC and I believe there is a 220 volt AC mains supply in some areas of the United States. For the UK it would be 220 VAC to 240 VAC.
The secondary windings are as follows.
2 x 65 volts AC at full load.
One 400 volt 35 ampere bridge
rectifier.
2 x 4.7K 5 watt ceramic resistors.
Minimum filter capacitor requirements would be 2 x
10,000 uF 100 volt electrolytic
Ideal capacity would be 40,000 uF per voltage rail.
For stereo or dual mono operation the following power
supply will be required.
1 x 2KVA toroidal transformer with a
core rating of 2KVA.
2 x 400 volt 35 ampere bridge
rectifier.
4 x 10,000 uF 100 volt electrolytic capacitors.
2 x 4.7K 5 watt ceramic resistors.
Getting Started
This section assumes you have read this whole
document and you have completed the assembly of the amplifier
and tested all the circuits. There are a few things to configure before
using the amplifier. There are some
programmable settings of the Protection
Controller and the External Load Detector that need to be set.
Protection Controller
Setup
Refer to the Amplifier Protection Controller section
for reference. Use the DIP Switch labeled S1 on the printed
circuit board to configure the
Controller. See Figure 1 for the
location of the DIP switch.
1. Determine
which protection circuits you want enabled and turn ON the appropriate DIP switches , S102 - S502.
If you
want all the protection circuits enabled, simple turn ON all the switches S102
through S502.
2. Determine if you want the amplifier to operate in
Latch Mode (switch ON) or Loop Mode (Switch OFF) and set
DIP switch S602.
Latch mode requires the user to turn OFF the amplifier to reset faults, Loop
mode doesn’t.
In Loop
Mode, the Protection Controller will restart when the faults clears and return
to normal operation.
External Load Detector
Setup
Refer to the Amplifier External Load Detector section
for reference. Use DIP Switch labeled S2
on the printed circuit board to configure the External Load Detector. See Figure 1 for the location of the DIP
switch.
1. Determine the resistance threshold that will cause
the ELD to trigger a fault condition and set the appropriate
sequence of switches S104 – S504.
2. For non-bridged mode
operation, set the amplifier channel to ‘Master” mode by turning OFF the
Master/Slave
switch S604. You can use the Slave Mode for non-bridged
operation, the only minor drawback is the self check
stops functioning 2 seconds after the amplifier is powered
on.
3. For bridged mode operation, you need to jumper two
wires across both amplifier channels for communications
and set one channel for Master Mode and set the other
channel for Slave mode. The Interface
Connector
is used for this function.
Refer to the Amplifier Protection Controller section for information on
the Interface
Connector
and see Figure 1 for additional reference of pin locations.
Interface Connector
Refer to the Amplifier Protection Controller section
for reference. Determine if you require
any additional connections to be wired to this interface connector such as the
clipping LED, temperature sensor, muting and
bridged-mode wiring. The amplifier will still function if you
choose to make no connections, these are extra
features that you access through this
connector. See Figure 1 for additional reference of pin locations.
Amplifier & Speaker Protection
Overview
Design Methodology
The design methodology chosen for amplifier and
speaker protection is based on using a high current relay between the
amplifier’s output and the speaker using the normally open relay contacts. The relay must be turned on to connect the
speaker to the amplifier. When a fault occurs, the relay will turn off and
disconnect the speaker from the amplifier.
Speaker Relay
The common relay used in the home and car audio
industry is the Potter & Brumfield 30A SPST relay (T9AS5D12 series). The relay chosen for this design is the
Magnecraft 30A DPDT relay (W92S11D12 series).
The DPDT relay contacts are connected in parallel for the benefits of
60A capability, contact redundancy for reliability and provide less relay
contact resistance.
Speaker Relay Pros
Disconnect the speaker from the amplifier when a
fault occurs to protect the speakers.
Speaker Relay Cons
Some audiophiles claim that relays may have a sonic
signature typically due to poor electrical contact between the amplifier and
the speaker. The relay chosen and the
configuration used should alleviate the fear of using relays even for the most
critical audiophiles.
The other concern with using a speaker relay is
voltage may arc across the relay contacts causing contact failure. This occurs
during a high voltage DC offset failure or similar failure of the
amplifier. The design uses two diodes on
the normally closed contacts to shunt any high voltage arcs to ground. It’s not
a 100% solution but it may prolong relay contact life. Worse case, if there is a severe amplifier
failure and the relay is damaged in the process of saving your $10,000
speakers, the cost to replace the relay is $10, a better solution than
replacing a speaker system.
Protection Detectors
There are five detectors, Rail Detector, External
Load Detector, DC Offset Detector, Short Circuit Detector and an external input
for temperature detection using a thermal switch or equivalent. Included is voltage rail fuse holders on the printed circuit board
capable of 30A each.
Clipping Detector
The Rob Elliot clipping detector is used in this
design for visual indication when the amplifier is clipping the audio
signal. It doesn’t interact with the
Protection Controller.
Rob Elliot’s website
http://sound.westhost.com/index.html
User Programmable
Settings
There are two banks of DIP switches, one for the
Protection Controller and one for the External Load Detector to select desired
modes of operation.
Status LED’s
There are nine system status LED’s.
Interface Header
There is a 6 pin interface header used for
input/output functions and ease of wiring.
Amplifier Protection Controller
Description
It’s function is to determine when to
turn on/off the speaker relay using data it receives from the detectors/inputs.
Protection Controller
Functions
1. Provides an
8 second delay before turning on the speaker relay to protect the speaker from
turn-on transients.
2. Receives
pass/fail status from all the protection detectors.
3. Controls
the speaker relay.
4. Two modes of
operation, Latch or Loop mode.
5. User can
enable/disable each detector via a DIP switch bank.
Protection Controller
Inputs
1. Rail
Detector Input.
2. External
Load Detector Input.
3. DC Offset
Detector Input.
4. Short
Circuit Detector Input.
5. Temperature
Detector Input.
6. Muting
Input (External).
7. Interrupt
input (External).
Protection Controller
Outputs
1. Controller
Status Output.
2. Clipping
Detector LED Output
3. External
Load Detector Output (used for Bridged Mode Testing)
User Programmable
Settings
The Protection Controller has a DIP switch bank (S1
on the circuit board) to enable/disable individuals Detectors.
S102 ON = enable the External Load Detector.
S202 ON = enable the Rail Detector.
S302 ON = enable the DC Offset Detector.
S402 ON = enable the Temperature Detector.
S502 ON = enable the Short Circuit Detector.
S602 ON = Latch Mode, OFF = Loop
Mode.
Latch mode requires the user to turn OFF the amplifier to reset
faults, Loop mode doesn’t. In Loop
Mode, the
Protection Controller will restart when the faults
clears and return to normal operation.
Status LED’s
There are nine status LED’s
to help the user in determining any system faults. The LED’s are not latched,
therefore if a fault clears the LED will turn off automatically.
1. Six
Detector LED’s (Red), one for each Detector.
When a Detector sends a fault signal, the LED will light up.
2. Status
Output LED (Green). The LED with light up when the speaker
relay is turned on.
3.
Master/Slave LED (Blue). The LED
with light up when placed in the Slave Mode.
Used by ELD.
4. Interrupt
LED (Yellow). The LED
with light up when the amplifier channel is being interrupted. Used by ELD.
Interface Header
Use the interface header to wire any inputs or
outputs as needed.
1. Status
Output (ST). This is a ground output
when “armed”.
2. Clipping
LED Output (CD). This is a positive
output that connects to the LED anode.
Connect the cathode
to power supply ground.
3. External
Load Detector (ED). This is a ground output
when a fault is detected.
4. Temperature
Switch Input (TD). Use a ground signal
to indicate a fault.
5. Muting
Input (MT). Use a ground signal to mute
the speaker relay (turn it off).
6. Interrupt
Input (INT). Use a ground signal to
interrupt the External Load Detector.
Amplifier External Load Detector
Description:
The
External Load Detector (ELD) monitors the speaker terminal for an abnormal
resistive load during the initial amplifier power on sequence to detect wiring
errors, shorts, or incorrect speaker load. The ELD is NOT directly connected to the speaker terminal,
it uses the speaker relay normally closed contacts to sense the load present
only when the relay is turned off. When
the speaker relay turns on, the ELD is disconnected from the speaker
terminals. The user determines the
triggering threshold by programming the DIP switch (S2 on the circuit board).
The ELD is
a comparator that measures the resistive load on the amplifier’s speaker
terminals in reference to ground and compares this value to the threshold
resistance value that the user programmed.
If the load present at the speaker terminal is lower in resistance than
the preset value, an error output will trigger.
For bridge mode operation and additional measurement is made between
both positive terminals of the two channels.
Anytime the speaker relay is off, the ELD is measuring the
load on the speaker terminal.
Speaker Relay COM contact = Amplifier Speaker Terminal
Speaker Relay N.O. contact = Amplifier Output
Speaker Relay N.C. contact = External Load Detector
Programmable Threshold Setting:
DIP switch bank (S2) on the circuit board is used to
program the ELD, specifically switches S104 through S504. S604 is used for
bridge mode operation to set the Master/Slave function of the ELD.
Referring to Table 1, determine the detection threshold
that will create an error output.
Example, if you want to the ELD to trigger when the load resistance is
less than 1.5 ohms, you turn ON switches S204 and S404, the rest are turned
OFF. If there is a resistive load present on the speaker terminal in reference
to ground that is less than 1.5 ohms, the ELD will trigger.
Table 1 – External
Load Detector DIP Switch Settings
Threshold
|
Equivalent Detector Ohms
|
Switch S104
R104 = 900 Ohms
|
Switch S204
R204 = 300 Ohms
|
Switch S304
R304 = 200 Ohms
|
Switch S404
R404 = 100 Ohms
|
Switch S504
R504 = 50 Ohms
|
0.5 Ohms
|
25 Ohms
|
ON
|
ON
|
ON
|
ON
|
ON
|
0.8 Ohms
|
40 Ohms
|
OFF
|
OFF
|
ON
|
OFF
|
ON
|
1.0 Ohms
|
50 Ohms
|
OFF
|
OFF
|
OFF
|
OFF
|
ON
|
1.5 Ohms
|
75 Ohms
|
OFF
|
ON
|
OFF
|
ON
|
OFF
|
2.0 Ohms
|
100 Ohms
|
OFF
|
OFF
|
OFF
|
ON
|
OFF
|
2.4 Ohms
|
120 Ohms
|
OFF
|
ON
|
ON
|
OFF
|
OFF
|
3.26 Ohms
|
163 Ohms
|
ON
|
OFF
|
ON
|
OFF
|
OFF
|
4.0 Ohms
|
200 Ohms
|
OFF
|
OFF
|
ON
|
OFF
|
OFF
|
4.5 Ohms
|
225 Ohms
|
ON
|
ON
|
OFF
|
OFF
|
OFF
|
6.0 Ohms
|
300 Ohms
|
OFF
|
ON
|
OFF
|
OFF
|
OFF
|
1. The comparator Vin resistor is 1K
Ohms (R704) in series with the Rload (load across speaker terminals).
2. The comparator Vref resistor is 10K Ohms (R604) in series
with the resistance values shown on Table 1.
3. The DIP switch resistors were scaled upwards in Ohms by a
factor of 50 times to allow better precision, i.e., Threshold x 50 = Equivalent Detector
Ohms.
Normal Operation
(single amplifier channel):
1. To enable the ELD, S102 must be set to the “ON”
position.
2. Set the Master/Slave switch to 'Master' on the
amplifier channel.
3. Set the triggering threshold that you desire as shown
in Table 1.
During the amplifier power on sequence, the ELD will
immediately measure the resistance between the speaker terminal
in reference to ground. If it measures less resistance than the programmed
threshold, the ELD will trigger an output ground signal.
Bridged Mode Operation
(two amplifier channels connected to form a more powerful single channel
amplifier):
In order for the ELD to function correctly in this
configuration, you need to jumper two wires across both channels for
communications. These two wires do not need to be removed if you are in “Normal
Operation”. Refer to the section below
called “Bridge Mode Wiring” for details.
1. Bridge Mode Wiring for
inter-channel communications must be enabled.
2. To enable the ELD, S102 must be set to the “ON”
position.
3. Set the Master/Slave switch to 'Master' on one
amplifier channel, set the other amplifier channel to “Slave”.
4. Set the triggering threshold that you desire as shown
in Table 1.
Due to the loading effects of both amplifier channels, the
ELD will only trigger when the measured load across both speaker terminals is ½
of the programmed threshold value.
Example, if you program the ELD for 1 ohm, in bridged mode only, if it
measures less than 0.5 ohms between both speaker terminals, it will trigger.
For measurements from speaker terminal to ground, it will function the same as
“Normal Operation”, it will trigger if it measures 1 ohm in reference to
ground.
There are two types of Bridged Mode tests.
1. Self Test – The speaker terminal to ground is measured.
This test is the same as “Normal Operation”.
2. Bridged Test – If Self Test passes, the Bridge Mode
testing will occur ~2 seconds after Self Test.
The load across both speaker terminals (positive terminals of both
channels) is measured. To measure the load across two floating terminals, the
Slave channel will enable the Bridge Mode circuit which forces a low current
ground signal to it’s positive speaker terminal to
allow the Master channel to measure the load in reference to the ground present
on the positive speaker terminal of the Slave channel.
Bridge Mode Sequence of Events;
- Amplifier
is powered up.
- Self
Test.
- If
Self Test passes, it enters Bridge Mode testing ~ 2 seconds later.
- Slave
channel Bridge Mode relay turns on forcing a low current ground on the
positive speaker terminal.
- Master
channel measures the load across both speaker terminals.
- If
Master channel fails, it will force a failure signal on both amplifier
channels to prevent both amplifier channels from arming.
Note: If Self Test fails, the master channel will interrupt
the Slave channel and force the Bridge Mode relay on the Slave channel to turn
on. This is a “don’t
care” situation because Self Test fails.
Bridged Mode Wiring:
1. Jumper a wire from channel #1 “External Load
Detector Output” to channel #2 “Interrupt Input”.
2. Jumper a wire from channel #2 “External Load Detector
Output” to channel #1 “Interrupt Input”.
3. One channel must to be set for 'Master' mode
and the other channel must to be set for 'Slave' mode.
Notes:
1. These two jumper wires allow interrupt communications
between both channels.
2. The Master channel will measure the load across both
speaker terminals and it’s terminal to ground.
External Load Detector
Control Pins:
1. Reset – Turns off the Bridge Mode circuit including the
Bridge Mode relay.
2. Interrupt - Force the Bridged Mode relay to turn
on immediately or force it to stay on and reset the delay circuit.
If the Slave channel Self Test fails, it will not enter the
Bridged Mode test. If Master channel
detects a fault across both speaker terminals, the Master channel will send an
interrupt signal to the Slave channel controller and it will also be connected
to the Interrupt input of the delay/relay
driver circuit to force the relay to stay turned on and to reset the
delay. When the fault across the speaker
terminal is removed (if the amplifier has not been turned off), then these
sequence of events will occur;
1. The relay will turn off immediately.
2. The Slave channel will redo the self test.
3. If self test passed 2 seconds later, the delay circuit
enables the relay and the Master channel repeats the test across both speaker
terminals.
Amplifier Rail Detector
Description
The Rail Detector monitors the power supply rail
voltages and triggers a fault when there is an imbalance between both
voltages. The voltage differential
threshold is programmable via two zener diodes. The default threshold is 10V
+/- 2V.
The Rail Detector will detect these type of failures
and trigger a fault condition;
1. One of two rail fuses blows. The
Rail Detector may trigger when both fuses blow.
2. If one
transformer fails in a dual transformer power supply design.
3. If the
transformer is unbalanced or not loaded equally.
Turn Off Muting
Operation
Turn off muting is a bonus feature that may function
depending on your amplifier power supply design. When the amplifier is turned off and the
power supply discharges quickly, the Rail Detector may detect the rapid
discharge and trigger a fault condition that turns off the speaker relay which
is a desirable feature to prevent turn-off transients reaching the speakers. Another method to achieve turn off muting is
to create a condition that causes one power supply rail to discharge faster
then the other rail. The Rail Detector will detect the voltage differential
when exceeding the threshold and trigger a fault condition.
Amplifier DC Offset Detector
Description
The DC Offset Detector is a classic design used in
the audio industry. It monitors the output stage and when sufficient DC voltage
is present, it will trigger a fault condition.
The sensitivity is programmable via three resistors with detection as low as 1.5V of DC
offset.
Amplifier Short Circuit Detector
Description
The Short Circuit Detector is a classic design used
in the audio industry, but implemented slightly different than conventional
means. Normally an output series resistor is added between the amplifier’s
output stage and speaker and the voltage across the resistors is senses for
over-current conditions. Adding this
series resistor was rejected for “sonic reasons”. An alternative solution was to tap the sense
line across on of the output transistor’s source resistor. In theory, all
transistors should be sharing current equally and if an overload condition
occurs, excess current with also flow thought the source resistor and trigger a
fault condition. The sensitivity is programmable
via a single resistor.
Amplifier Temperature Detector Input
Description
Temperature Detectors are not integrated into the
design, but an input is provided to connect whatever temperature detection
methods you prefer. A simple thermal
switch (or switches) can be mounted on the heatsink and connect the switches to
the temperature detector input wire on the 6 pin Interface Connector.
Component Reference Designators
The
industry standard for component reference designator “prefix” is typically “R”
for resistor, “C” for capacitor, “D” for diode, “Q” for transistor, “J” for
connectors, etc. followed by a number (suffix) to identify the specific
component. Example, R1, R2, R3, etc.
The
reference designator scheme used is this design is different. The prefix is the same, but the suffix is
different to identify which components below to a particular circuit. Example, the suffix “02” represents the
Protection Controller. Any component
with suffix “02” belongs to this circuit.
The audio amplifier section use the industry standard method except for
the output stage transistors and power supply capacitors See Table 2 for the list.
Table 2 – Model 2800
Reference Designators
Circuit
Name
|
Reference
Designator Suffix
|
Audio Amplifier Section
|
Industry
Standard Suffix, i.e., R1, R2, R3, etc.
The output stage transistors and power supply capacitors use “01”
suffix, i.e., Q101, Q201, Q301, Q401, C101, C201, C301, C401, etc.
|
Protection Controller
|
“02”
suffix, i.e., R102, R202, R302, etc.
|
Rail Detector
|
“03”
suffix, i.e., R103, R203, R303, etc.
|
External Load Detector
|
“04”
suffix, i.e., R104, R204, R304, etc.
|
DC Offset Detector
|
“05”
suffix, i.e., R105, R205, R305, etc.
|
Output Short Circuit Detector
|
“06”
suffix, i.e., R106, R206, R306, etc.
|
Clipping Detector
|
“07”
suffix, i.e., R107, R207, R307, etc.
|
Speaker Relay
|
“08”
suffix, i.e., R108, R208, R308, etc.
|
Protection Power Supply
|
“09”
suffix, i.e., R109, R209, R309, etc.
|
Component Assembly Order
The recommended component assembly order is shown in
Table 3. Install and test each section
one at a time for ease of debugging problems.
Refer to the parts list and schematics at the bottom of this document. Figure 1 below shows the component placement
map for the protection circuits located on the right side of the printed
circuit board, the SMD components. For a
more detailed explanation, see the installation guide below.
Table 3 – Recommended
Assembly Order
Component
|
Comments
|
Fuse 1
|
This is the positive power supply voltage rail
fuse. Insert a fuse on both clips and
solder the clips their location.
|
Fuse 2
|
This is the negative power supply voltage rail
fuse. Insert a fuse on both clips and
solder the clips their location.
|
K1 Speaker Relay
|
This is the large speaker relay in the center of
the printed circuit board.
|
J1 Molex Connector
|
This is the power supply input, +V, -V, and
ground. Solder the 6 pin Molex
connector in this location.
|
J2 Molex connector
|
This is the speaker output connector. Solder the 2 pin Molex connector in this
location.
|
Protection Power Supply
|
The power supply is approximately +18v.
|
Speaker Relay Driver
|
Figure 1 - GRAY area.
|
Protection Controller
|
Figure 1 - RED area.
|
Status LED’s
|
Figure 1 - TEAL area.
|
Short Circuit Detector
|
Figure 1 - CYAN area. Q106 & R106 are the sensing components
located next to Q1401 on the upper right location of the pcb.
|
External Load Detector
|
Figure 1 – Blue area.
|
Rail Detector
|
Figure 1 – GREEN area.
|
DC Offset Detector
|
Figure 1 - Light PINK area.
|
Clipping Detector
|
Figure 1 - YELLOW area.
|
Interface Connector
|
Figure 1 - PINK area.
|
Audio Amplifier Section
|
Left side of the printed circuit
board.
|
Component Orientation
Resistor Installation
Orientation
Resistors have no polarity and can be installed in
any orientation.
Capacitor Installation
Orientation
The SMD capacitors are small in value and have no
polarity and can be installed in any orientation. The larger capacitors do have polarity
indicated on the schematic and labeled with a + sign on the printed circuit board. For example, if you look at figure 1 you will
see capacitor C105 and C106 which has a plus sign (+) next to them indicating
the installation orientation.
Diode Installation
Orientation
All diodes including zener diodes have polarity and
must be installed in the correct orientation.
Zoom in on Figure 1 and you will see the printed circuit board
silkscreen diode orientation. The diode
prefix is labeled “D” and the symbol has two
beveled edges to represent the diode cathode. You must place the diodes in the correct
orientation for proper function.
Transistor
Installation Orientation
All transistors have three leads and must be
installed in the correct orientation.
Bipolar transistor leads are base, emitter, collector. Mosfet transistor leads are gate, drain, source. For SMD
transistors, the installation is easy because there is only one way the
transistor will line up with the SMD component pads. For non-SMD transistors, the silkscreen will
indicate “B”, “C”, “E” for the larger bipolar
transistors. The output stage
transistors are just labeled “G” for gate. The small signal transistors in the
T0-92 package have not markings, but you can look at the silkscreen picture for
correct match.
Component Installation
Reference the parts list, schematics, and Figure 1
for component values and installation locations. It’s easier to install the surface mount
devices (SMD) first then install the larger components. You will need a fine tipped soldering iron,
fine awg solder, liquid flux and tweezers to hold the device or any other
method you prefer. One technique to
solder SMD devices is to place a tiny drop of flux on the SMD pad, apply a
little solder to the pad using the soldering iron to coat the pad with a thin
layer of solder. Place the SMD component
on top of the “pre-tinned” pads and hold the device with the tweezers and heat
up the intersections where the component meets the pad causing the soldering
iron to reflow the solder.
Fuse Clip Installation
(Fuse 1, Fuse 2)
The fuse clips are beryllium copper rated for 30
amperes. To get the best electrical
connection install the fuse clips on the fuse and position
the clips for best alignment. Install
the fuse/clip assembly on the printed circuit board. Use a high wattage solder
iron.
Speaker Relay Installation
(K1)
This is the large speaker relay located in the center
of the board. It’s a DPDT relay with
parallel contacts for high current capability and contact redundancy for
improved reliability. Use
a high wattage solder iron.
Molex Connector Installation
(J1)
This is a 6 pin connector used for the power supply
input, + voltage rail, - voltage rail, and ground. Solder the
6 pin Molex connector in this location. The two center pins are ground, the two top
pins are positive rail, the
bottom two pins are the negative
rail. The “two pin” parallel design
allows a higher current capability up to 100
amperes and also provides contact
redundancy to improve reliability. You
can solder two wires in parallel on the
Molex receptacle, one set for the positive voltage
rail, one set for the negative voltage rail, and one set for ground.
If you chose not to use the Molex connector, you can
insert and solder a large diameter wire in the largest hole provided on the
printed circuit board. Use a high wattage solder iron.
Molex Connector
Installation (J2)
This is a 2 pin connector used for the speaker output
connector. Both pins are shorted
together on the printed circuit board.
The “two pin” parallel design allows a higher current capability up to
100 amperes and also provides contact redundancy to improve reliability. If you chose not to use the Molex connector,
you can insert and solder a large diameter wire in the largest hole provided on
the printed circuit board. Use a high wattage solder iron.
Protection Circuit
Power Supply Installation
Install the protection circuit power supply. Temporarily mount a TO-220 heatsink or
equivalent to the voltage regulator for testing purposes, then
later you can remove the heatsink and mount the voltage regulator to the main
heatsink used by the output stage transistors.
Plug in the Molex connector and turn on the power supply. The voltage regulator should output
approximately +18 volts +/- 1 volt.
Speaker Relay Driver
Installation
Install resistors – R108, R208, R308, R408
Install transistor - Q108
Note: R308 and R408 are 1W resistors.
Protection Controller
Installation
Install integrated circuit – U1
Install resistors - R102, R202, R302, R402, R502,
R602, R702, R802, R902, R1002, R1102, R1202
Install capacitors - C102, C202, C302, C402, C502
Install diodes - D102, D202, D302, D402, D502, D602,
D702, D802, D902
Install transistor - Q102, Q103
Install 6 pin DIP Switch – S1
Install Red LED’s - L102, L202, L302, L402, L502
Install Yellow LED - L602
Install Green LED - L702
Short Circuit Detector
Installation
Install resistors - R106, R206, R306, R406, R506,
R606
Install capacitors - C106
Install diodes - D106, D206
Install transistor - Q106
External Load Detector
Installation
Install integrated circuit – U2
Install resistors - R104, R204, R304, R404, R504,
R604, R704, R804, R904, R1004, R1104, R1204, R1304, R1404, R1504
Install capacitors - C104, C204, C304, C404, C504
Install diodes - D104, D204, D304, D404, D504, D604,
D704, D804, D904
Install 6 pin DIP Switch – S2
Install relay – K2
Install Blue LED – L104
Install Yellow LED – L204
Note: R104 – R704 are 1% tolerance.
Rail Detector
Install resistors - R103, R203, R303, R403, R503,
R603, R703, R803, R903
Install capacitors - C103, C203, C303, C403, C503
Install diodes - D103, D203, D303, D403, D503
Install transistor - Q103, Q203, Q303, Q403
DC Offset Detector
Install resistors - R105, R205, R305, R405, R505,
R605
Install diodes - D105, D205
Install transistor - Q105, Q205, Q305, Q405
Install capacitors - C105, C205
Clipping Detector
Install integrated circuit – U3
Install resistors - R107, R207, R307, R407, R507,
R607, R707, R807, R907, R1007, R1107, R1207, R1307
Install capacitors - C107
Install diodes - D107, D207, D307
Install transistor - Q107, Q207
Note: Clipping LED L107 is mounted on the chassis and
use the interface connector for wiring.
Interface Connector
Install a standard 6 pin Molex header, 100 mil
centers.
Figure 1 – Protection Circuit
Component Map

Audio Amplifier
Section
Install all the components except for the output
stage transistors which will be install last after testing.
Component
|
Reference Designator
|
Comments
|
¼ Watt Resistors
|
R1, R2, R3, R4, R5, R6, R7, R8, R9, R12, R13, R14,
R15, R16, R17, R19, R20, R21, R22, R23, R24, R25, R26, R28, R29, R101 through
R2801
|
|
1 Watt Resistors
|
R10, R11, R18, R27, R30, R31, R32, R33, R34
|
|
Potentiometer
|
P1
|
|
Diodes
|
D1, D2, D3, D4, D5, D6, ZD1, ZD2, ZD3, ZD4, ZD5, ZD6
|
Note orientation on pcb.
|
Transistors
|
Q1, Q2, Q3, Q4, Q5, Q6, Q7, Q8, Q9, Q10, Q11, Q12
|
Note orientation on pcb.
|
3 Watt resistors
|
RS01 through RS28
|
Minimum 3 watt.
Radial mount.
|
Capacitors
|
C1, C2 (optional) - Some people prefer no
input signal capacitors if the preamp has low DC offset. Install two jumper wires on the C1 & C2
locations if the capacitors are not used so the input signal passes through.
|
Electrolytic capacitor is only viable solution due
to space restrictions on the pcb.
|
Capacitors
|
C5, C7, C11, C12, C13
|
Silver mica capacitor preferred.
|
Capacitors
|
C3, C4, C8, C10, C14
|
Polypropylene capacitor preferred.
|
Capacitors
|
C6
|
Electrolytic capacitor, not
polarity.
|
Capacitors
|
C15, C16
|
Polypropylene or any film type
capacitor preferred.
|
Capacitors
|
C17
|
250 Vac X2 rated capacitor.
|
Feedback Capacitor
|
Pick one installation option.
A. Install a 220uF non-polarized in location C9.
B. Install two 470uF polarized in location C9A and
C9B
|
If C9A & C9B is installed,
jumper a wire across the extra C9A & C9B pads.
|
Power Supply
Capacitors
|
C101 through C3401
|
Note polarity on pcb.
|
Input Signal
Terminal Block
|
J3
|
|
Output Stage
Transistors
|
Q101 through Q2801
|
Install these last
after testing.
|
Note:
Transistor Q10 is the Vbe multiplier or bias compensation device. It needs to be mounted on the same
heatsink that the output stage transistors
are mounted to for thermal tracking.
Preliminary Test
The preliminary test will determine if the amplifier
is functioning properly. The amplifier
section of the printed circuit board should have all the components installed
except the output stage transistors Q101 through Q2801.
To test the amplifier connect a 10 ohm ¼ watt
resistor from the output of the amplifier to one side of the 330 ohm
1 watt resistor found at R27. This will connect the feedback resistor R15
to the output of the buffer stage and
bypass the output stage. Turn on the main power supply (+90v/-90v) and
measure the voltage across these
resistors to verify that the amplifier is
ok. If the amplifier passed these
checks, power down the amplifier and remove the 10 ohm resistor.
R8 ~ 1.6 volts
R9 ~ 1.6 volts
R19 ~ 1.0 volts
R16 ~ 500mV
R17 ~ 500mV
Offset voltage at R15 should be close to 0 volts, but
can be as high as 100mV.
Final Assembly
Transistor Matching
The output stage transistors need to be matched to
ensure current sharing of the transistor bank.
The N-channel
IRFP240 transistor bank should have similar
electrical characteristics to each other and the P-channel IRP9240
transistor bank should have similar electrical
characteristics to each other. You don’t
need to match N-channel
characteristics to P-channel characteristics. Refer to the section on “Matching Mosfet
Transistors”.
Transistor Thermal
Pads
The output stage transistors (IRFP240 &
IRFP9240), the buffer stage transistors (IRF610 & IRF9610), and the
protection circuit voltage regulator (TL783) are mounted on the same heatsink
(or metal interface block). Insert a
gray thermal pad (or equivalent) between the transistor and the heatsink. The gray thermal pads are designed for
electrical isolation and heat transfer, better
than mica insulators in most cases.
Refer to the manufacturers recommendation on
whether or not you need thermal compound. If you use mica insulators, you need
to apply
the white thermal compound to both
sides of the insulator to transfer the heat from the transistor to the
heatsink.
Transistor Clamping
There are two common methods to secure the transistor
to the heatsink. You can use a thick
metal bar that
spans the length of the transistor bank
to clamp all the transistors firmly in place or you can individually screw
each TO247 package to the heatsink. If you use screws to secure the TO220 package
make sure the screw doesn’t touch the metal tab on the package otherwise the
tab will short the heatsink. The metal
tab on the
IRFP240, IRFP9240, IRF610, IRF9610 transistors are
connected to the drain pin (middle pin).
The metal tab on
the TL783 voltage regulator is
connected to the output pin (middle pin).
After the transistors and voltage regulator
have been clamped, use an ohm meter to
verify that there is no electrical connection between the transistor
pin (that connects to the tab) and the
heatsink.
Transistor to Heatsink
Mounting Method 1
See Figure 2.
The printed circuit board will mount on top of the heatsink using 1/4”
standoffs minimum to provide ample clearance to avoid the bottom soldered leads
from shorting to the heatsink. The
transistor leads lay flat and are soldered on top of the printed circuit board
pads. Use an aluminum interface block to
couple the transistor to the heatsink.
Transistor to Heatsink
Mounting Method 2
See Figure 3.
The printed circuit board will mount on top of the heatsink using 1/4”
standoffs minimum to provide ample clearance to avoid the bottom soldered leads
from shorting to the heatsink. The
transistor will lay flat on the heatsink underneath the printed circuit
board. The transistor leads are bent 90
degrees and inserted through the holes on the printed circuit board from the
bottom.
Note: There
two sets of holes for the transistor. You can place the transistor further
underneath the board by using the second set of holes.
Transistor to Heatsink
Mounting Method 3
Note: The
printed circuit board will mount on top of a mounting plate or chassis using
1/4” standoffs minimum to
provide ample clearance to avoid the bottom
soldered leads from shorting to the heatsink.
The transistor will mount vertical using the printed circuit board holes
closest to the edge of the board. The
transistor will be attached to the vertical heatsink.
Figure 2 – Transistor to Heatsink
Mounting Method 1


Figure 3 – Transistor to Heatsink
Mounting Method 2


Figure 4 – Transistor to Heatsink
Mounting Method 3

Final Test
Preliminary Checks
1. The drain
pin on each output stage transistors are not shorted to the heatsink or block
used for mounting.
2. The power
supply wiring is the correct polarity.
3. The
multi-turn potentiometer P1 has been turned back to 0 ohms so that a
measurement of approximately
4.7K ohms is measured across the gate and drain pins of Q10.
4. Insert an 8
ampere fuse in each fuse holder on the printed circuit board, labeled FUSE 1
and FUSE 2.
5. Connect the
red probe of a DC voltmeter to the output stage. One method is to connect the meter to
the lead of R15
closest to the relay. Connect the black
probe of the meter to ground.
Turn on the power supply an
the voltmeter should read between 1mV to 50mV of DC offset. If this is not
the case, turn off the amplifier and
debug your assembly.
Output Stage Bias
Adjustment
1. Turn off
the amplifier. With the voltmeter still
connected to R15 (output stage side), connect the black lead of
the voltmeter to any output stage transistor source
lead. You will be measuring the voltage
across the source
resistors.
2. Turn on the
amplifier and slowly adjust P1 to achieve an absolute value reading of 18mV.
Matching
Hexfet Mosfets
by Anthony E. Holton
When using this type of Mosfet in the AV800 amplifier
is strongly recommended that the output stage devices be
matched. As it has been found that is
this is not done then there is no guarantee that they will share the current
under load. The source resistors provide
only a bit of local feedback and don’t in any ways force the devices to current
share.
The best method I have found to work very well
utilizes just a 150 ohm 1 watt resistor and a +15 volt DC power supply. If you look at Figure 5 it shows how to connect and measure
the N-channel devices and the
P-channel devices.
With the devices connected as shown, measure across R1 with a multimeter
set to DC volts and measurement of between 3.8 volts and 42 volts will be
shown. Simply match the device in-groups
to a tolerance of +/- 100mV.
Please note that you
only have to match the N-channel to the N-channel devices and the P-Channel to
the
P-channel devices.
Figure 5 – Matching Mosfets

Matching
Devices
by Nelson Pass
http://www.passdiy.com
After you acquire the
devices, you will need to test them. You might consider running lots of tests
on these transistors, but only one is essential: measuring gate-source voltage
versus current. The greatest variations occur here, and it is necessary to do
some matching to get proper performance. This test will also tell you whether
or not the device is broken.
The test is simple and
requires a power supply, a resistor, and a DC voltmeter. Figure 5 shows the
test hookup for N- and P-channel types. The supply source resistance (R1) is
nominal, and is found from I = (V - 4)/R1. Consistency is the most important
thing here. The given voltage is 15 and, adjusting for about a 4V VGS, we will
see about 11V across the resistor.
We are looking for as much
matching of the input Mosfets as possible at a current of 5mA. For this test,
we use an R1 value of 2.2kohm. Measure the voltage between the gate and the
source. Write it down on a piece of masking tape or a sticky label and place it
on the part. Keep in mind the caveats about electrostatic discharge: touch
ground before you touch the parts.
Matching input Mosfets is
critical, because they must share equally the 10mA of bias current from the
current source, and they will not do that unless their VGS is matched. At 5mA
current, they have an equivalent source resistance of about 15ohm. Assuming we
want them to share the current to within 2mA, we calculate the required VGS
match as follows. Using the formula V = IR, we see V = 0.002 x 15, which gives
us 30mV. The VGS of the input devices should be matched to within 30mV at 5mA
current. The matching is only essential within a given pair; you do not have to
match the Ps to the Ns, or match to devices in another channel.
If you are unable to find
input devices matched to within 30mV, you must insert resistance in the source
to make up the difference. The resistance is calculated by the difference of
the two values of VGS divided by 5mA. For example, if the difference in VP1GS
is 100mV, then 0.1/0.005 = 20ohm. You would then place 20ohm in series with the
MOSFET source having the lower VGS.
We use the same test setup
for the Mosfets in the TO-220 packages but at a higher current (20mA), so we
use a 560ohm resistor. No matching is required for these devices; we are just
checking to see that the VGS is between 4-4.6V and that they work.
We will measure the output
device VGS at about 170mA. You can achieve this with either a 56ohm at 2W
resistor, or two 100ohm at 1W resistors in parallel. We are looking to obtain a
reasonable match within a parallel output bank of each polarity of each
channel, so we want two groups of 12 with matched N- channel devices, and two
groups of matched P-channel devices.
The VGS voltages of our
test samples gave the following spread:
N-channel ---- P-channel
Min. VGS ---- 4.00V ------
3.79V
Max. VGS --- 4.57V ------
4.15V
Avg. VGS ----4.42V ------
4.01V
We also measured the
transconductance by taking another reading for each device at a higher current
(0.5A), just to see what kind of variation we got. The transconductances
measured from a low of 1.19 to a high of 1.56, with the average at about 1.35.
Within this amplifier's general operating curve, each output will vary its
current by about 1.3A for every volt of its VGS change. For 12 devices in
parallel, we expect about 15A for each such volt.
By placing 1ohm source
resistors on each transistor, we can assure adequate current sharing for a
fairly wide range of VGS. In Class A bias, we will be operating at about
200mA/device, which will place 0.2V across each source resistor. A variation in
VGS will cause the bias to be unequally distributed between the devices. For
example, for a 4.6V device in parallel with a 4.5V device, the first will run
at about 160mA at 6W and the second at about 240mA at 9W.
Remember that each of these
devices is rated at 75W on a cold heatsink, and maybe 50W on a hot sink. We are
only going to bias them to about 8W each, so they're not going to break from a
little unequal distribution. Nevertheless, we like to see the load shared, and
recommend that you group the outputs by VGS as closely as possible. Matching
within 0.2V will work, and O.1V is even better. Within a population of 150
transistors, you can easily get 12 sets matched to O.1V VGS at 200mA.