1

Construction Notes for the AV 800 watt MOSFET Power Amplifier

Introduction
The 800 Watt AV amplifier is based on My 1kw Amplifier and shares the same topology
and basic PCB layout.
The only real difference is the number of Output devices that the unit uses.
The 1kw design has 20 O/P devices, while the AV amplifier has 14 O/P devices.
This amplifier can be used for practically any application that requires High power, low
noise, distortion and excellent sound. Examples would be Sub-woofer amp, FOH stage
amplifier, one channel of a very high-powered surround sound amplifier etc.
The AV amplifier has four main stages of amplification. We will begin by looking at each
stage in reasonable detail.

The Error Amp Stage

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 I/P source. An unbalanced source can be used if either the inverting or non-
inverting I/P is tied to signal ground.

Now I will explain how each device in this stage works together.
Q6, Q7, R28- R29, and form the main differential error amplifier, which then has its
collectors, connected to a cascode load. Q1, Q2, R13 and ZD1 form the cascode stage,
which provides a constant 14.4 volts on the collectors of Q1, 2.
Q23, R42, R66, ZD2 and C19 form a constant current source, which supplies 1.5milliamps
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 o/p stage to full power.
Q3, Q4, Q24, Q25, R3, R54, R55, R40, C2, C9, C16 form the second differential voltage
amplification stage. Q54 and Q55form 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 8milliamps.
The remaining components, namely the caps provide local frequency compensation for
this stage.

The Bias and Buffer stage

As the name suggests Q5, Q8, Q26, R24, R25, R33, R34, R22, R44, C10 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 an extra dominant pole in to the amplifiers feedback loop.
 2

The Output Stage or Current Amplification Stage

Once again as the name suggests 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.

Power supply requirements for the 800 AV Amplifier

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. eg: for Australia One single primary winding with a 240VAC
rating. For USA, 110VAC, 115VAC 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 10,000uf 100 volt electrolytic.
Ideal capacity would be 40,000uf 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 amp bridge rectifier
4 x 10,000uf 100-volt electrolytic capacitors
2 x 4.7K 5-Watt ceramic resistors
1 2 3 4 5 6

D Suggested power supply for Mono D

T1
BD1
Bridge Rectifier +90 Volts DC
Local Mains Supply
47uf 390 15k
+
C1 +
C3 R2
Toroidal Transformer 10,000uf 100 Volt 10,000uf 100 Volt 10k 5 watt 1uf
AV800 Amp
Power GND 15k
+
C2 +
C4 R1
C 10,000uf 100 Volt 10,000uf 100 Volt 10k 5 watt C

-90 Volts DC
Suggested power supply for Stereo
+90 Volts DC
BD2
Bridge Rectifier
C1 C2
+
10,000uf 100v
+
10,000uf 100v R1
47uf 390 15k
B
10k 5 watt B
T2
1uf
Local Mains Supply AV800 Amp
Power GND 15k

Toroidal Transformer +
C3 +
C4 R2
10,000uf 100v 10,000uf 100v 10k 5 watt
Bridge Rectifier
BD3
A -90 Volts DC A

Title

Size Number Revision

B
Date: 6-Feb-2001 Sheet of
File: C:\WINNT\..\av800psu.Sch Drawn By:
1 2 3 4 5 6
 3

Assembling the Printed Circuit Board

One of the first things to do is to look at the PCB and see if all of the holes on the
board are of the correct size for the components you wish to insert. The holes that
have been drilled into the PCB should be OK. However it does pay to check before you
start. If you find that some of the holes are not big enough then you will need to drill
them out to the correct size. The standard holes sizes used for most electronic
components are as follows.
¼ watt ½ watt resistors = 0.7mm to 0.8mm
1–watt resistors = 1.0mm
¼ watt to 1-watt zener and normal power diodes = 0.8mm
Small signal transistors such as BC546 of the TO-92 pack = 0.6mm
Medium signal transistors such as MJE340 of the TO-220 pack = 1.0mm
Power Output devices such as the IRFP240 require a hole size of 2.5mm
However on this PCB these devices are mounted on the copper side of the PCB facing
down flat.
The next thing that needs to be done is the cutting out of the PCB section, which has
the output stage devices screen-printed on top of the PCB. This needs to be done so the
mounting of the o/p devices can be properly clamped to the main heat sink. The best way
of cutting this section out is to either use an electric router or drill 5mm holes in each
corner and use a coping saw to remove the unwanted fibreglass and file the inside edges
clean.
Start constructing the PCB by inserting any wire links, which are shown on the
component overlays as R5, R23, R41, and R45. The wire links are made from spare
component leads such as from 5-watt ceramic resistors or ¼ watt resistor leads.
Once the links have been taken care of the insertion of all the resistors is next,
followed by the capacitors and then the small signal semiconductors. You will need to
cross-reference the parts list with the white screen component overlay on the PCB to
see where to insert the required component. Be careful to always insert the polarised
components in the right way as shown on the screen-printed overlay. Failure to do this
will most likely result in the module not functioning properly or damaging one or more of
the components in the module.
The output stage transistors and Q8, which is the IRF610 device, are to be left till last.
The buffer stage transistors are to be mounted on 10 degrees/watt heats sinks with a
one-inch pitch mounting.

The buffer stage devices do need to be insulating from the heat sinks, As they would be
180 volts across both of these heat sinks.
 4

Pre-flight test
OK at this stage I am assuming you have populated all of the PCB except Q8 and the
main output stage devices IRFP240’s and IRFP9240’s

Pre-flight test continued

It is important to test the function of the amplifier at this point in time so as to make
sure it is working properly. This is achieved by soldering a 10-Ohm ¼ watt resistor from
the output of the amplifier PCB to one side of the 330-Ohm 1W resistor found at R38
On the screen-printed side of the PCB. What this does is to connect the feedback
resistor R37 to the output of the buffer stage. In doing so it bypasses the output stage
and turns it into a very low powered amplifier, which can be tested without damaging the
expensive output stage. Assuming you have connected the resistor from o/p to the
buffer stage. It is now time too connect the +-90 volt supply to it and power it up.
Be sure to have 4k7 Ohm 5-watt bleeder resistors across the power supply capacitors.
Now assuming that there was no smoke, with a multimeter on volts. Measure the
following voltage drops across these resistors locations marked in blue and if they
match to within +-10% then you can be sure that the amplifier is OK.
When you have done the checks, be sure to power down and remove the
10 Ohm resistor.

R1~1.6 volts
R2~1.6 volts
R3~1.0 volts
R55~500mv
R56~500mv
Offset voltage at R37 should be close to 0 volts, but can be as high as 100mv.

Completing the Module

Now we have come to the soldering in of the output devices. It is assumed at this point
that you have all ready matched the output stage devices as outlined in the
accompanying document on How to match output devices.
If this is already done then you can proceed by getting the PCB and flipping it over so
the copper side of the board is facing you.
Now identify which is the positive supply side of the PCB and start soldering the
IRFP240 devices face down on the appropriate pads so the metal tabs on the back of
the power MOSFET’s are facing you.
Once all of the N-channel devices are done proceed with the IRFP9240
P-channel devices, in the same way
After completing this task the module for the most part is completed.
The only other thing you will need to decide is wether to use PCB stakes to solder the
external wiring too or solder the cable directly into the PCB pads.
Now there is one device that requires some special attention. This is Q8 and this device
is the VGS multiplier or bias compensation device, which needs to be mounted off board
on a strip of 10cm x 2cm x 4mm thick piece of aluminium and it is this piece of aluminium
that will clamp down the output stage. Q8 will need to be insulated with a TO-220 mica-
washer kit from this piece of metal and flying leads need to be soldered from the Gate,
 5

Source and Drain pins of the IRF610 to the appropriately marked pads on the PCB shown
as Q8.
One other thing that needs to be done is to mount some 3mm high rubber feet on copper
side of the front and back of the PCB. This is to hold the PCB off the heat sink, so as
not to allow the PCB to touch the heat sink in any way.

Completing the Module Continued

Now having completed the power module and tested the Error, VAS and Buffer stages
and you are confident that it is working OK. Now its time to bolt it down to a suitable
heat sink. Remember that all of the o/p devices must be insulated with either silicon
rubber washers or mica –washers and heat sink compound. The type, size and shape of
heat sink are left up to you and the local availability of heat sinks. But be sure to have a
heat sink rated at 0.2 degrees/watt or 0.5 degrees/watt with fan cooling.

Testing the module

So we have come to the point where we need to do a full test on the amplifier module.
There are a few checks that need to be done first.
• The Drain pins on all the o/p devices need to be checked for S/C to the heat sink.
• The power supply wiring has been checked for correct polarity to the PCB.
• The Multi-turn pot P1 has been turned back to 0 Ohms, so that a measurement of
approximately 4.7k is measured across the Gate and Drain pins of Q8 IRF610.
• When wiring up the power supply, be sure to have 8 amp fuses inserted on each of
the supply lines.
• Connect a multimeter on DC volt range to the o/p of the amplifier.

Ok now that you are happy that the module is setup correctly apply power via a VARIAC
if you have access to one, otherwise just power the amplifier up.
Looking at the voltmeter you should get from 1mv to 50mv offset voltage.
If this is not the case then power the amplifier down and check your work.
Assuming all is well then power the amplifier down and find a small flat blade
screwdriver so you can be ready to adjust P1 for the biasing of the o/p stage.
But first connect the voltmeter across one of the o/p stage Source resistors using
Alligator leads.
Now reapply power to the amplifier and slowly adjust P1 while watching the voltmeter,
for a reading of 18mv.
Now check across the rest of the Source resistors and find the one, which has the
highest reading, and adjust P1 till 18mv is read.
Now connect a load and signal source to the amplifier and with a CRO if you have access
to one observe that the waveform is clean and free from noise and distortion.
If you don’t have a CRO and Signal generator, connect a pre-amp and loudspeaker and
have a good listen. The sound should be very clean and dynamic.

Congratulations, the amplifier is complete.

Best Regards

Anthony Eric Holton

8th February 2001
www.aussieamplifiers.com
 6

Specifications for the AV800 MOSFET Amplifier

All measurements were taken at an AC Mains input of 240 volts.

And with a 2kva Toroidal Transformer powering the Amplifier module.
Filtering with only 10,000uf per voltage rail
One channel only was been driven.

Frequency response 10hz to 100khz

THD measured at 100 watts into 8 Ohms 0.01% @1khz
Power Output into 8 Ohms = 450 Watts RMS
Power Output into 4 Ohms = 820 Watts RMS
Damping Factor = 400

Notes and Errata

It has been shown on the first batch of PCBs sold that Q3 and Q4 silkscreen
overlay has an error.
The B C E markings are in the incorrect place. They should have the same
orientation as Q25 and Q25.

This error has now been corrected on the latest PCB version 1.0.5
 7

Component List for the AV800 MOSFET Amplifier

All of the resistors are ¼ Watt metal film 1% tolerance unless otherwise stated.

Designators Component Value

C1 100uf 160v RB
C2 10PF Ceramic
C3 47UF/100V RB
C4 47UF/100V RB
C5 47UF/100V RB
C6 47UF/100V RB
C7 47UF/100V RB
C8 47UF/100V RB
C9 10pf Ceramic
C10 1uf MKT
C11 2n2 MKT
C12 68pf Ceramic
C13 2n2 MKT
C14 220uf 25v RB
C15 47uf Bipolar
C16 100nf MKT
C17 100nf MKT
C18 47uf Bipolar
C19 47uf 25v RB
C20 47UF 100V RB
C21 47UF 100V RB
C22 47UF 100V RB
C23 47UF 100V RB
C24 47UF 100V RB
C25 47UF 100V RB
C26 100uf 160v RB
C27 100nf-x2/250vac Mains Rated
D1 1N4007 1 amp diode
D2 1N4007 1 amp diode
HS1 1 inch pitch, Min 10 Degrees/Watt
HS2 1 inch pitch, Min 10 Degrees/Watt
LD1 LED any colour
LD2 LED any colour
P1 5K POT Multi turn or 10 turns
Q1 2SC2240 TO-220
Q2 2SC2240 TO-92A
Q3 2SA1306 TO-220
Q4 2SA1306 TO-220
Q5 IRF610 TO-220
Q6 BC546 TO-92
Q7 BC546 TO-92
Q8 IRF610 TO-220
Q9 IRFP240 TO-3P
Q10 IRFP240 TO-3P
Q11 IRFP240 TO-3P
Q12 IRFP240 TO-3P
Q13 IRFP240 TO-3P
Q14 IRFP240 TO-3P
Q15 IRFP240 TO-3P
Q16 IRFP9240 TO-3P
Q17 IRFP9240 TO-3P
Q18 IRFP9240 TO-3P
Q19 IRFP9240 TO-3P
Q20 IRFP9240 TO-3P
 8

Designators Component Value

Q21 IRFP9240 TO-3P
Q22 IRFP9240 TO-3P
Q23 MJE340 TO-126
Q24 2SC3298 TO-220
Q25 2SC3298 TO-220
Q26 IRF9610 TO-220

All of the resistors are ¼ Watt metal film 1% tolerance unless otherwise stated.
R1 2k2
R2 2k2
R3 120
R4 100
R5 Wire LINK2
R6 0.22 3 or 5 WATT
R7 0.22 3 or 5 WATT
R8 0.22 3 or 5 WATT
R9 0.22 3 or 5 WATT
R10 0.22 3 or 5 WATT
R11 0.22 3 or 5 WATT
R12 0.22 3 or 5 WATT
R13 10k 1watt
R14 470
R15 470
R16 470
R17 470
R18 470
R19 470
R20 470
R21 47K
R22 100
R23 Wire LINK4
R24 100
R25 4k7
R26 4k7
R27 4k7
R28 47
R29 47
R30 4k7
R31 4k3
R32 10 Ohm 1watt
R33 1k
R34 100
R35 470
R36 390
R37 15k
R38 330
R39 470
R40 10k 1 Watt
R41 Wire LINK3
R42 10k 1watt
R43 100
R44 100
R45 Wire LINK5
R46 47K
R47 470
R48 470
Designators Component Value
 9

Designators Component Value

R49 470
R50 470
R51 470
R52 470
R53 470
R54 100
R55 100
R56 10k
R57 100
R58 0.22 3 or 5 WATT
R59 0.22 3 or 5 WATT
R60 0.22 3 or 5 WATT
R61 0.22 3 or 5 WATT
R62 0.22 3 or 5 WATT
R63 0.22 3 or 5 WATT
R64 0.22 3 or 5 WATT
R65 10 Ohm 5 WATT
ZD1 15v 1watt Zener Diode
ZD2 15v 1watt Zener Diode
 10

Component Value Quantity

Misc
1 Inch pitch 10 Degree/watt heat sinks 2
Wire Links 4
Resistors
0.22 Ohm 3 Watt Resistor 14
100 Ohm Resistor 8
100 Ohm Resistor 1
10k Ohm Resistor 1
10k Ohm 1watt Resistor 3
10 Ohm 1watt Resistor 1
120 Ohm Resistor 1
15k Ohm Resistor 1
1k Ohm Resistor 1
2k2 Ohm Resistor 2
330 Ohm Resistor 1
390 Ohm Resistor 1
47 Ohm Resistor 2
470 Ohm Resistor 17
47K Ohm Resistor 2
4k3 Ohm Resistor 1
4k7 Ohm Resistor 4
10 Ohm 5 Watt Resistor 1
5K multi turn POT 1
Capacitors
10PF Capacitor 1
10pf Capacitor 1
100nf MKT Capacitor 1
100nf MKT Capacitor 1
100nfx2 250vac Capacitor 1
100uf 160v Capacitor 2
1uf MKT Capacitor 1
220uf 25v Capacitor 1
2n2 MKT Capacitor 2
47uf 100V Capacitor 12
47uf 25v Capacitor 1
47uf BP Capacitor 2
68pf Capacitor 1
Semiconductors
15v 1watt Zener Diode 1
15v 1watt Zener Diode 1
1N4007 1 amp Diode 2
2SA1306 Transistor 2
2SC2240 or 2SC2546 Transistor 2
2SC3298 Transistor 2
BC546 Transistor 2
IRF610 Transistor 2
IRF9610 Transistor 1
IRFP240 Transistor 7
IRFP9240 Transistor 7
Light Emitting Diodes 2
 11

How to match Hexfet MOSFETs

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 if 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 way force the
devices to current share.

The best method I have found to work very well utilises just a 150 Ohm 1 watt resistor
and a +15 volt DC power supply.
If you look at the schematic below it shows how to connect and measure the N-channel
devices and the P-channel devices.
With the devices connected, as shown measure across Drain and Source of Q1 and Q2
with a multimeter set to DC volts and measurement of between 3.8 volts and 4.2 volts
will be shown. Simply match the device in-groups to a tolerance of +- 0.1 to 0.2 volt
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, not the N-channel devices to the P-channel devices.

1 2 3 4 5 6

D D

+15 VOLTSVCC DRAIN

GND
C C
Q2
M1 P-channel MOSFET
R1 DC VOLTMETER
150 Ohm 1 watt GATE
SOURCE
DRAIN
M1 R1
Q1 150 Ohm 1 watt
DC VOLTMETER GATE N-channel MOSFET
SOURCE
B B

GND VCC +15 VOLTS

A A

Title

Size Number Revision

B
Date: 3-Feb-2004 Sheet of
File: D:\my docs backup\FET_MATCH.Sch Drawn By:
1 2 3 4 5 6
 12

AV800 Schematic Build 03022004

To see this schematic in more detail use the zoom option in Acrobat Reader.

1 2 3 4 5 6

+92V
C3 C4 C5 C6 C7 C8
C1
220uf D1 47uf 47uf 47uf 47uf 47uf 47uf
R4
100
1N4007
R1 R13 R2 R3 LD1
2k2 10k 1 Watt 2k2 100 Yellow

D D
R21
Q15 47k
R20 IRFP240
470

Q3 Q4
2SA130 2SA130
C9 Q14
Q1 Q2 10pf C2 R19 IRFP240
2SC254 2SC254 10pf 470

Q13
R18 IRFP240
C- 470

68pf
Q12
Q6 Q7 R17 IRFP240
BC546B ZD1 BC546B 470
1N4744
Con? C15
R35 R26
SOCKET 33uf 470 4k7 Q11
R16 IRFP240
470
R27
4k7 R28 R29
100 100
C11 Q5 Q10
C
2n2 R15 IRFP240 C
IFR610
R24 470
100

Q9
R22 R14 IRFP240
100 470
R25
C13 4k7 R6 R7 R8 R9 R10 R11 R12
2n2 0.22 0.22 0.22 0.22 0.22 0.22 0.22 Feedback
Q8
P1
R38
Con? C18 R40 330 1 Watt Con?
R39 R30 C16 10k 1 Watt C10
100nf 1uf MKT
SOCKET 33uf 470 4k7
5k
IRF610
SOCKET
R65
Q23 R58 R59 R60 R61 R62 R63 R64 10 Ohm 5 Watt
R32 NPN R34 0.22 0.22 0.22 0.22 0.22 0.22 0.22
100
10k 1 Watt R33
R31 1K
ZD2 C- 4k3 R44 R47
C19 1N4744 68pf 100 470 Q22 C27
47uf R56 R43 IRFP924 100nf MKP
10k C12 100 Q26

68pf
R48
IRF9610 470 Q21
B
IRFP924 B

R37 R49
Feedback
470 Q20
15k IRFP924

R36 R50
390 470 Q19
IRFP924

C17 C14
100nf
220uf Q24 Q25 R51
2SC329 2SC329 470 Q18
IRFP924

R52
470 Q17
IRFP924 R46
47k
R54 R55
100 100
R53
470 Q16 LD2
IRFP924
Yellow
R57 D2
A 100 1N4007 A

C26 C20 C21 C22 C23 C24 C25

220uf
47uf 47uf 47uf 47uf 47uf 47uf -92V

Ti tle
800 Watt Power MOSFET
Si ze Number Revi si on
C
Date: 3-Feb-2004 Sheet of
Fi le: E:\Protel Projects\..\AVAMP_2.SCH Draw n By:
Anthony E Holton

1 2 3 4 5 6