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Construction notes for the symmetrical 400 watt amplifier
Introduction The symmetrical amplifier is an update of one of my designs, which appeared in the Australian electronics magazine Silicon Chip in June 1994. The main changes made in this design was, the addition of a clipping detector circuit and bias circuit modifications for the use of International Rectifier HEXFET MOSFET's. Later modifications where made to the error amp and VAS stages which improved the overall sound of the amplifier. Tools you will need to complete the construction of this module A good temperature controlled soldering Iron Resin core 40/60 solder A mini drill or a drill press, which can handle drill, bit sizes down to 0.6mm A small flat blade screwdriver and 1 point Philip's screwdriver. An electric hand drill Pre-cut and drilled Aluminium right angle bracket, 196mm long, 3mm to 5mm thick. To mount the Mosfets onto. The Error Amp Stage The first stage is what I call an asymmetrical input error amplifier. It has the ability to accept an unbalanced I/P source only. Now I will explain how each device in this stage works together. Q2, Q3, R35- R36, form the main differential error amplifier, which then has its collectors connected to a cascode load. Q4, Q5, R4 and ZD2 form the cascode stage, which provides a constant 14.4 volts on the collectors of Q4, Q5. Q1, R8, R7, ZD1 and C1 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. Q6, Q7, Q8, Q9, R15, R14, R12, R13, C3, C7, C8 form the second differential voltage amplification stage. Q7 and Q9 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 R15, which is about 8milliamps. The remaining components, namely the caps provide local frequency compensation for this stage. The Bias Stage As the name suggests Q10, R34, 37, 38, C12form the Bias stage. Its main purpose is to provide the MOSFET Gates with a stable and compensated supply voltage. 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 not possible unless more o/p devices are added. Power supply requirements for the 400 Amplifier
The power supply components for this amplifier are as follows and are expressed for Two Channels. 1 x Toroidal Transformer with a Core rating of 625VA. 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 50 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. A suggested power supply schematic is shown below with the schematic of The amplifier.
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Revised Schematic (C)Copyright 1994-99 Anthony.E.Holton LAST REV.01/06/1998
D
+70 VOLTS D1 1N4004 R15 120 R32 100 Ohm F1 5 AMP
D
C8 100uf
R3 2k2
R4 10k
R5 2k2
C5 100uF 100v
Q4 BC546 RCA1 C2 2.2UF R2 1K R1 15k C6 1.0nF Q2 BC546 ZD2 15v
Q5 BC546 Q3 BC546 C7 10pf
Q6 MJE350
Q8 MJE350 C4 10pF Q11 2SK1530 R17 470 R19 470 Q13 2SK1530 R21 470 Q15 2SK1530 R23 470 Q17 2SK1530
R36 100 Ohm
R35 100 Ohm
R14 10k
C3 100nf R38 4k7 P1 5k Q10 BD139
ZD3 1N4737
R25 0.22 Ohm
R27 0.22 Ohm
R29 0.22 Ohm
R31 0.22 Ohm
C12 .47
R6 18K
C
ZD4 1N4737
R24 0.22 Ohm
R26 0.22 Ohm
R28 0.22 Ohm
R30 0.22 Ohm
C
C11 18pf
R37 1k
R34 82 ohm
R7 10k C1 100uf ZD1 15v
Q1 MJE340
Q7 MJE340
Q9 MJE340
R16 470 Q12 2SJ201
R18 470 Q14 2SJ201
R20 470 Q16 2SJ201
R22 470 Q18 2SJ201
R8 10K
R12 100
R13 100
D2 1N4004 R33 100 Ohm
F2 5 AMP C10 100uF 100v -70 VOLTS
C9 100uf
C13 330uF C14 470nf MKT
R10 470
R11 15k
B
R9 10 Ohm
R41 1k
R42 1m
B
C16 100nf C15 10nf R44 330k
R43 100k
Q20 BC556
Q19 BC546 R45 5k6 R40 82k
LD1
Suggested Power Supply
T1 SW1a Active 240VAC/115VAC SW1b Neutral E1 10,000uf 100V E3 10,000uf 100V A Chassis Ground BR1 600V/35 AMP
+70 Volts
A
625VA 2x50 VAC secondary wi ndi ngs
E2 10,000uf 100v
E4 10,000uf 100V
-70 Volts
Title Size A2 Date: File: 1 2 3 4 5 6 7 23-Feb-2001 E:\SCH\MOSAMP8.S01 Sheet of Drawn By: 8 Number Revision
How to match MOSFETs
When using this type of MOSFET in the symmetrical 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 the Drain and Source pins 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 +-100mv. Please note that you only have to match the n-channel to the n-channel devices and the pchannel to the p-channel devices, not the N-channel devices to the P-channel devices.
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D
D
+15 VOLTS VCC
C
DRAIN GND R1 150 Ohm 1 watt DRAIN M1 DC VOLTMETER Q2 P-channel MOSFET GATE SOURCE
C
M1 DC VOLTMETER
GATE
Q1 N-channel MOSFET SOURCE
R1 150 Ohm 1 watt
B
B
GND
VCC
+15 VOLTS
A Title Size B Date: File: 1 2 3 4 5 1-May-2001 C:\WINNT\..\FET_MATCH.Sch Sheet of Drawn By: 6 Number Revision
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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 1watt 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-126 pack = 1.0mm Power Output devices such as the IRFP240 require a hole size of 2.5mm Start constructing the PCB by inserting any wire links, which are shown on the component overlays 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 Q10, which is the BD139 device, are to be left till last. Pre-flight test OK at this stage I am assuming you have populated all of the PCB except Q10 and the main output stage devices IRFP240's and IRFP9240's For the time been temporarily wire up Q10 via flying leads. Making sure that you match up the Collector, Base and Emitter pinout's on the PCB, with the Collector, Base and Emitter pinouts on the BD139. Don't insert Q10 directly into the PCB. 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 across ZD3, On the screen-printed side of the PCB. What this does is to connect the feedback resistor R11 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 +-70 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. R3~1.6 volts R5~1.6 volts R15~1.0 volts R12~500mv R13~500mv R8 ~14.6 volts ZD1~15 volts Offset voltage at R11 should be close to 0 volts, but can be as high as 100mv.
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 the pre-drilled alloy bracket. Now first get the N-channel devices and a pair of long nose pliers and bend the pins at right angles. The same needs to be done for the p-channel devices. Once this is done get 8 x TO3-P insulation washers and 8 x M3 x 25mm bolts and nuts and mount the devices onto the alloy bracket and thereby clamping the alloy bracket to the main PCB. After completely mounting and insulating the N and the P channel devices. Solder the devices in on the copper side of the PCB. Now its time to get the 0.22 Ohm 5 watt resistors and bend the pins at rights on each device and using a pair of side cutters, trim the leads back so about 10mm to 12mm of lead is protruding from the body of each resistor. Then solder each resistor on the copper side of the PCB. 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 Q10 and this device is the Vbe multiplier or bias compensation device, which needs to be mounted off board on top of Q11 in the output stage. Q10 will need flying leads soldered from the Base, Collector and Emitter pins of the BD139 to the appropriately marked pads on the PCB shown as Q10. Now having completed the power module and tested the Error, VAS and Buffer stages and you are confident that it is working OK. 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 0.5 degrees/watt or better.
Completing 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 Base and Collect pins of Q10 BD139. · 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. This sets the bias current in the output stage to just under 100ma per device 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
Testing the module
Specifications for the symmetrical 400 watt amplifier Power rating of 200 Watts RMS into 8 Ohms Per Channel 400 Watts RMS into 4 Ohms Per Channel. Total Harmonic Distortion is typically 0.005%, Signal to Noise Ratio of 122dB unweighted (20Hz to 20Khz) A-weighted -126dB, Damping Factor Greater than 200 at 8 Ohms. 1.2 volts RMS for full power O/P
Part Used PartType Designators 2 1 .47 C12 3 8 0.22 Ohm R24 R25 R26 R27 R28 R29 R30 R31 4 1 1.0nF C6 5 1 1K R2 6 2 1N4004 D1 D2 7 2 1N4737 ZD3 ZD4 8 2 1k R37 R41 9 1 1m R42 10 1 2.2UF C2 11 4 2SJ201 Q12 Q14 Q16 Q18 12 4 2SK1530 Q11 Q13 Q15 Q17 13 2 2k2 R3 R5 14 1 4k7 R38 15 2 5 AMP F1 F2 16 1 5k P1 17 1 5k6 R45 18 1 6k8 R14 19 3 10,000uf 100V E1 E3 E4 20 1 10,000uf 100v E2 21 1 10K R8 22 1 10 Ohm R9 23 2 10k R4 R7 24 2 10nf C3 C15 25 2 10pF C4 C7 26 2 15k R1 R11 27 2 15v ZD1 ZD2 28 1 18K R6 29 1 18pf C11 30 2 47 Ohm R35 R36 31 1 82k R40 32 1 82 ohm R34 33 2 100 R12 R13 34 2 100 Ohm R32 R33 35 1 100k R43 36 1 100nf C16 37 2 100uF 100v C5 C10 38 3 100uf C1 C8 C9 39 1 120 R15 40 1 330k R44 41 1 330uF C13 42 9 470 R10 R16 R17 R18 R19 R20 R21 R22 R23 43 1 470nf MKT C14 44 1 600V/35 AMP BR1 45 1 625VA 2x50 VAC secondary windings T1 46 5 BC546 Q2 Q3 Q4 Q5 Q19 47 1 BC556 Q20 48 1 BD139 Q10 49 3 MJE340 Q1 Q7 Q9 50 2 MJE350 Q6 Q8
Part Cross Reference Report For : C:\WINNT\Profiles\aholton\Desktop\My Documents\MOSAMP8_BOM.XRF 2-Mar-2001 11:45:59 C1 100uf E:\SCH\MOSAMP8.S01 C2 2.2UF E:\SCH\MOSAMP8.S01 C3 10nf E:\SCH\MOSAMP8.S01 C4 10pF E:\SCH\MOSAMP8.S01 C5 100uF 100v E:\SCH\MOSAMP8.S01 C6 1.0nF E:\SCH\MOSAMP8.S01 C7 10pf E:\SCH\MOSAMP8.S01 C8 100uf E:\SCH\MOSAMP8.S01 C9 100uf E:\SCH\MOSAMP8.S01 C10 100uF 100v E:\SCH\MOSAMP8.S01 C11 18pf E:\SCH\MOSAMP8.S01 C12 .47 E:\SCH\MOSAMP8.S01 C13 330uF E:\SCH\MOSAMP8.S01 C14 470nf MKT E:\SCH\MOSAMP8.S01 C15 10nf E:\SCH\MOSAMP8.S01 C16 100nf E:\SCH\MOSAMP8.S01 D1 1N4004 E:\SCH\MOSAMP8.S01 D2 1N4004 E:\SCH\MOSAMP8.S01 E1 10,000uf 100V E:\SCH\MOSAMP8.S01 E2 10,000uf 100v E:\SCH\MOSAMP8.S01 E3 10,000uf 100V E:\SCH\MOSAMP8.S01 E4 10,000uf 100V E:\SCH\MOSAMP8.S01 BR1 600V/35 AMP E:\SCH\MOSAMP8.S01 F1 5 AMP E:\SCH\MOSAMP8.S01 F2 5 AMP E:\SCH\MOSAMP8.S01 LD1 E:\SCH\MOSAMP8.S01 P1 5k E:\SCH\MOSAMP8.S01 Q1 MJE340 E:\SCH\MOSAMP8.S01 Q2 BC546 E:\SCH\MOSAMP8.S01 Q3 BC546 E:\SCH\MOSAMP8.S01 Q4 BC546 E:\SCH\MOSAMP8.S01 Q5 BC546 E:\SCH\MOSAMP8.S01 Q6 MJE350 E:\SCH\MOSAMP8.S01 Q7 MJE340 E:\SCH\MOSAMP8.S01 Q8 MJE350 E:\SCH\MOSAMP8.S01 Q9 MJE340 E:\SCH\MOSAMP8.S01 Q10 BD139 E:\SCH\MOSAMP8.S01 Q11 2SK1530 E:\SCH\MOSAMP8.S01 Q12 2SJ201 E:\SCH\MOSAMP8.S01 Q13 2SK1530 E:\SCH\MOSAMP8.S01 Q14 2SJ201 E:\SCH\MOSAMP8.S01 Q15 2SK1530 E:\SCH\MOSAMP8.S01 Q16 2SJ201 E:\SCH\MOSAMP8.S01 Q17 2SK1530 E:\SCH\MOSAMP8.S01 Q18 2SJ201 E:\SCH\MOSAMP8.S01 Q19 BC546 E:\SCH\MOSAMP8.S01 Q20 BC556 E:\SCH\MOSAMP8.S01 R1 15k E:\SCH\MOSAMP8.S01 R2 1K E:\SCH\MOSAMP8.S01 R3 2k2 E:\SCH\MOSAMP8.S01 R4 10k E:\SCH\MOSAMP8.S01 R5 2k2 E:\SCH\MOSAMP8.S01 R6 18K E:\SCH\MOSAMP8.S01 R7 10k E:\SCH\MOSAMP8.S01 R8 10K E:\SCH\MOSAMP8.S01 R9 10 Ohm E:\SCH\MOSAMP8.S01 R10 470 E:\SCH\MOSAMP8.S01 R11 15k E:\SCH\MOSAMP8.S01 R12 100 E:\SCH\MOSAMP8.S01 R13 100 E:\SCH\MOSAMP8.S01
R14 R15 R16 R17 R18 R19 R20 R21 R22 R23 R24 R25 R26 R27 R28 R29 R30 R31 R32 R33 R34 R35 R36 R37 R38 R40 R41 R42 R43 R44 R45 RCA1 SW1a SW1b T1 ZD1 ZD2 ZD3 ZD4
10k 120 470 470 470 470 470 470 470 470 0.22 Ohm 0.22 Ohm 0.22 Ohm 0.22 Ohm 0.22 Ohm 0.22 Ohm 0.22 Ohm 0.22 Ohm 100 Ohm 100 Ohm 82 ohm 100 Ohm 100 Ohm 1k 4k7 82k 1k 1m 100k 330k 5k6
625VA 2x50 VAC sec 15v 15v 1N4737 1N4737
E:\SCH\MOSAMP8.S01 E:\SCH\MOSAMP8.S01 E:\SCH\MOSAMP8.S01 E:\SCH\MOSAMP8.S01 E:\SCH\MOSAMP8.S01 E:\SCH\MOSAMP8.S01 E:\SCH\MOSAMP8.S01 E:\SCH\MOSAMP8.S01 E:\SCH\MOSAMP8.S01 E:\SCH\MOSAMP8.S01 E:\SCH\MOSAMP8.S01 E:\SCH\MOSAMP8.S01 E:\SCH\MOSAMP8.S01 E:\SCH\MOSAMP8.S01 E:\SCH\MOSAMP8.S01 E:\SCH\MOSAMP8.S01 E:\SCH\MOSAMP8.S01 E:\SCH\MOSAMP8.S01 E:\SCH\MOSAMP8.S01 E:\SCH\MOSAMP8.S01 E:\SCH\MOSAMP8.S01 E:\SCH\MOSAMP8.S01 E:\SCH\MOSAMP8.S01 E:\SCH\MOSAMP8.S01 E:\SCH\MOSAMP8.S01 E:\SCH\MOSAMP8.S01 E:\SCH\MOSAMP8.S01 E:\SCH\MOSAMP8.S01 E:\SCH\MOSAMP8.S01 E:\SCH\MOSAMP8.S01 E:\SCH\MOSAMP8.S01 E:\SCH\MOSAMP8.S01 E:\SCH\MOSAMP8.S01 E:\SCH\MOSAMP8.S01 E:\SCH\MOSAMP8.S01 E:\SCH\MOSAMP8.S01 E:\SCH\MOSAMP8.S01 E:\SCH\MOSAMP8.S01 E:\SCH\MOSAMP8.S01