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   CAR POWER AMPLIFIER FOR SUBWOOFER

design by Risto Panovski from Maceodonia e-mail: elkomak@yahoo.com

      This article is for those who want to make their own car amplifier. The basics of calculation will be discussed below. If you have understood it you will be able to make car amplifier yourself. The design of this car amplifier is based on Kenwood car amplifier KAC-718 for subwoofer with maximum power output of 240W (4oma, at 100Hz).

      Be warned - there is considerable risk. Because of the extremely high current available from a car battery, a tiny mistake may easily lead to catastrophic failure. All electronic components are said to contain smoke (wire contains an enormous amount), and a slip of the soldering iron can liberate an unbelievable quantity. Seriously though, the risk of severe burns and the possibility of causing a fire in your car are very real, and should not be underestimated. 100A from a car battery can do a vast amount of damage in a few milliseconds.

The Specifications for this Amplifier are:

Max Power Output (4oma, at 100Hz) …………………………….…. 240Wx1

Rated Power Output (4oma) (0,08% THD at 100Hz, Supply 14,4 volts) …………………….…. 120Wx1

Rated Power Output (2oma) (0,8% THD at 100Hz, Supply 14,4 volts) ………………………… 160Wx1

Signal to Noise Ratio ……………………………………………...… 100dB

Sensitivity (MAX) (rated output) ……………………………………. 0,15V

Sensitivity (MIN) (rated output) …………………………………...... 4,0V

Input Impedance …………………………………………………..… 10Koma

Damping Factor ……………………………………………. More than 100

Low Pass Filter Frequency (12dB/oct.) ……….… 30Hz - 200Hz (Variable)

Operating Voltage ………………………………….. (11V - 15V allowable)

Current Consumption (1 KHz, 10% THD, 4oma) …………………..……. 15A

INTRODUCTION

There are many designs of good amplifier published, solid state (SS) or tube designs. But few have written the design of car power amplifier. Actually the difficulty of designing the car power amplifier does not lies with the audio power amplifier, but it is more to providing the switching power supply (SMPS). The difficulties of installing an HI-FI system in a car are many, although there is no doubt that the most important is the limitation of the vehicle supply voltage. As most readers already know, the nominal voltage of a car battery is 12V, reaching about 13.8V when charging. The maximum RMS. audio power from a given voltage V is somewhat less than see Picture 0

… where RL is the speaker nominal impedance.

Thus, for a 13.8V system, this power is limited to about 6W on a 4 Ohm load. Note that the lower the resistance of the speaker, the higher the maximum power (this is the reason most audio speakers have a 4 Ohm nominal impedance instead of the more common 8 Ohm in home systems).

Power output can be increased by a factor of nearly 4 by using bridging techniques, so we can obtain up to about 24W on a 4 Ohm speaker. This can be enough for the midrange and high frequencies, but is obviously very limited for a subwoofer application, for example.

So, what can be done to increase available audio power? The answer is a simple derivation of the above formula - either decrease load impedance or increase supply voltage. The lower the impedance, the more current is needed, making the construction of low impedance output stages more difficult (there are some other practical limits), so if we want to make more powerful amplifier, let’s say 200 watt at 4 ohm speaker load, we will need supply voltage of 80Vpp, or +/- 40 Vdc. The way to have this voltage from car supply of 12VDC is to make DC-DC converter.

In this article, I will discuss the car power amplifier in 3 steps:

1. The design of audio power amplifier and preamplifier ;

2. The design of DC-DC converter (Switch Mode Power Supply - SMPS) ;

3. Miscellaneous tips for making car power amplifier.

1. THE DESIGN OF AUDIO POWER AMPLIFIER

In Picture 1 we can see that audio power amplifier can be spited into 3 main functions, that is:

- First stage / input stage

- Second stage / voltage amplifier stage

- Third stage / output stage

- First stage is the stage that receives the input audio signal and Negative Feedback (NFB) signal from the output of the amp. Feedback is the back signal used to stabilize the audio amplifier, like the gain factor. For first stage built by discrete transistors, both signals are fed to basis of the transistor, like in Picture 1. Both bases of the transistors are the Non- Inverting input and Inverting Input, like those in the op-amp.

- Second stage is the stage that responsible for the Voltage Gain in the power amplifier.

- Third stage is the Current Gain.

We can explain those stages in a simple way like this: Input signal, like from car radio or CD player have low voltage, about 1Vpp with few mill amperes current. To produce power of 200 Watt at 4 ohm speaker load, than the signal has to have magnitude of 28Vpp and current of 6.5A (from the equation of Pmax = (V / (2 x 2))2 / R)

The first stage receives this signal in the non-inverting input and the inverting input receives NFB signal to make sure the voltage gain that the amplifier produces has a constant number, let’s say 28 x. The output signal from the first stage has not reach 28Vpp; it tends to have the magnitude similar to the input voltage. Second stage amplifies the voltage that the first stage generates. Second stage will amplifies the voltage to produce a signal that is enlarge 28x for the amplifier to have a 28Vpp signal from 1Vpp signal, but this 28Vpp signal still have small current , only a few mA and cannot drive the speaker load. The third stage amplifies the current from few mA to 6.5 A.

Off course the explanation for three stages above is not that simple in the real amplifier. We should take the nature's law for a transistor gain, that is G=RC/RE. These principles must be applied in each transistor in those 3 amplifier stages.

FIRST STAGE

First stage design have main component, which is Constant Current Source (CCS). CCS first stage varies between 1-4mA.

In Picture 1 first stage, each component will be explained like this:

 

- R1 is the impedance of the audio amplifier, the range is 10Kohm – 47Kohm, and in our case is R1 - 47Kohm, see Picture 2;

- C1 is the high pass filters, in our case C1 - 1uF/50volts, see Picture 2;

- RED1 and RED2 are between 50-150 ohm, in our case R2 and R3, see Picture 2;

- RM1 and RM2 is picked up so the voltage drop will be 50mV – 150mV

- Q3 and Q4 is the Current Mirror that ensures the current in RM1 and RM2 will have the same magnitude.

- RF and CF, in our case R7 and C4 (see Picture 2);

SECOND STAGE

The second stage is responsible for all voltage gain, (Maximum Voltage Swing) in an audio power amplifier. This is why the Second stage is generally known as VAS or Voltage Amplifier Stage. This stage consists of a voltage amplifier/CEM transistor (Q5 in Picture 1) in the bottom, Constant Current Source in the top, and a bias control circuit in the middle. Second stage CCS has current magnitude between 4-8mA.

In the second stage there is an important capacitor for an audio power amplifier, witch is Miller Capacitor (CC in Picture 1). CC defines the pole of the frequency response for an audio amplifier and the magnitude usually in small order (several pF).

Bias control circuit consists of a transistor T9, resistor R23 and variable resistor P1 see Picture 2. This circuit uses a transistor that is placed in the heatsink, because the transistor have good heat compensation factor (for bipolar transistors).

THIRD STAGE

Third stage / Output Stage is the current amplifier. Third stage and the bias circuit will define whether an amplifier works in class A, class AB or class B.

It can be said that almost 90 % of car audio power amplifier works in class B. Operation in class B does not mean that the sound produced is not good or corrupted. With good design, we will have good audio results, both from class A or class B. The choice of class B in car audio power amplifier is connected to efficiency and the heat generated. Heat generated is a very important factor, because if not considered carefully, it will lead to amplifier breakdown.

Both 3 stages that we have discussed above, if we connect the together will be a circuit that can be seen in Picture 2. Parts of this circuit can be explained like this:

- The transistor T1 and T2 are input differential amplifier;

- The transistor T3 and T4 are 2ND - stage differential amplifier;

- The transistor T5 and T6 are 3ND - stage differential amplifier;

- The T7 transistor is cascade stage, T8 is current mirror and T9 is for temperature compensation;

- Transistors T10 and T11 are driver stage for power output transistors T12 and T13;

- R19 and C15 are output power stabilization.

Car Power amplifier usually loaded by low impedance speakers, usually 4 ohms and can reach ½ ohm on bridge mode. Here we know the term “High Current Amplifier”. The difference is the number of final transistors, or in Picture 2 it is the number of pairs of T12 and T13. A pair of bipolar transistor can handle 100 Watt. The power is raised by parrarelling several output transistors, so the current flowing will be larger. The finished amplifier is sown on Picture 3.

1.2 PREAMPLIFIER

Car power amplifier has specific accessories like preamplifier gain circuit, an inverting channels so that the power is bridgeable, in our case we don’t use inverting channel. These functions usually done with preamplifiers. The circuit can be seen in Picture 4 and the supply circuit can be seen in Picture 2 it is consist of several parts. The circuit is placed before the audio amplifier circuit.

The Preamplifier is divided in 3 stages:

- Isolation Amplifier

- Input Sensitivity

- Low Pass Filter Frequency

The Isolation amplifier is consisting of IC1 and several other parts. The main purpose of the isolation amplifier is summing the left and right signal from car CD player and providing an input sensitivity with variable resistor P1 witch is use for regulation the output volume of a car amplifier.

The Low Pass Filter is variable (12dB/oct) from 30Hz-200Hz made with IC2 and stereo variable resistor P2. The high power car amplifiers for subwoofers use this kind of filter for reproducing low frequency for subwoofer speakers. With this preamplifier you can make an active filter in order to you lead a loudspeaker of very low frequencies.

The supply for this preamplifier is +/- 16 volts witch is providing from main supply and it can be seen on Picture 2. The parts for positive supply are (T14, D1, R32, R34, C13, and C14) and negative supply is (T15, D2, R33, R35, C11, and C12).

The finished preamplifier is sown on Picture 5.

2. THE DESIGN OF DC-DC CONVERTER (SMPS)

      For building car power amplifier, we need symmetrical power supply (+, 0, -) by building DC-DC converter. The converter system discussed below will be the SMPS (Switch Mode Power Supply) type PWM (Pulse Width Modulation). This system will deliver stable output voltage, regardless of the input voltage (usually the car electrical system will range in 9-15Vdc).

To explain the SMPS type PWM, first I will start with basics of SMPS.

Switch Mode Power Supply Basics

The vast majority of high-powered audio amplifiers use SMPS (Switch Mode Power Supplies) to generate higher voltages from the available 12 (13.8) volts. An extensive theoretical explanation on how these things work is beyond the scope of this article, but these are some fundamental ideas you should know about switch mode power supplies (SMPS) for car amplifiers:

• The DC voltage at the battery has to be switched in some form to generate an AC waveform suitable for a transformer. As you already now, a transformer basically converts the AC voltage in its "primary" to a scaled version of it in its "secondary", the scale factor being the turn’s ratio of the primary to the secondary. (Again, take this as an extreme simplification). A transformer doesn't allow DC voltages to pass, and there is electrical (galvanic) isolation between both windings.

• The AC waveform is usually a square wave that is relatively easy and efficient to generate. The frequencies usually fall between 25 kHz and 100 kHz or more, thus allowing smaller transformers than the used in main appliances (its construction is also different; their cores are not laminated, but made from ferrites or "iron powder"). The switching elements have to be capable of high currents and must also be fast and have low switching losses. Usually, power MOSFET’s or high speed bipolar transistors are used.

• Once this waveform is stepped-up by the transformer, it has to be rectified again and filtered back to DC, since that is what we want. For audio applications, we usually need a symmetrical supply, +/- 40V, for example. The rectification is done with a diode bridge, as it would be using a conventional transformer at 50 or 60 Hz. Note that for the frequencies we are talking about, fast or ultra-fast diodes are needed.

• If we need a regulated power supply, some kind of feedback must be provided from the output rails to a controller that can change some parameters of the AC waveform at the primary of the transformer. This is normally accomplished with PWM (pulse width modulation). In our case we will not use regulated power supply, the supply will depend of a secondary turns from ferrite transformer.

• Always keep in mind that no energy is created … given a (total) rails to battery voltages ratio, the current drawn from the output will be (at least) be multiplied at the 12V input by the same ratio, thus the total power stays the same (assuming 100% efficiency, and that is never the case). A generic transformer "transforms" the voltage by a factor of Tr, current by a factor of 1/Tr, and impedance at the secondary by a factor of 1/sqr(Tr), Tr being the turns ratio. Impedance is of little importance in this context.

• A well built SMPS can reach 90% efficiency. So, if you expect to produce +/-40V at 5A (per rail) supply then be prepared to draw more than 20A from the battery! Fortunately, when talking about audio amps reproducing music, power requirements are always much lower than with pure sine waves.

At this point, the reader should realize the magnitude of the currents involved in a high power SMPS for a car amplifier, and that extreme caution should be taken especially when connecting "the creature" to the car electrical system.

In this design, we use regulating PWM IC's, like TL494, TL594, SG3524, SG3525. These IC's will compare the output of DC-DC converter with a reference voltage. If the output of DC-DC converter is smaller than reference voltage, then the IC will enlarge the pulse width so the voltage will rise equally to reach determined voltage. So as if the output of DC-DC converter is higher than the reference voltage, the IC will narrow the pulse width so the output voltage will be lowered to the determined voltage.

Generally SMPS used in car audio amplifier is the push-pull system with switching frequency between 20-120 KHz. In push pull system like in Picture 6, Q1 and Q2 gives alternating switched current pulses so the transformer will be objected to maximum flux swing change without saturating the core.

The complete schematic of the SMPS is shown on Picture 7.

There are three main blocks described for this DC-DC Converter:

A - Switching MOSFETs

B - Rectification and filtering

C - Control circuitry

A-SwitchingMOSFET’s

The selected switching topology is called a "push-pull" converter, because the transformer has a double primary (or a "centre-tapped" one, if your prefer). The centre tap is permanently connected to the car battery (via an LC filter to avoid creating peaks in the battery lines, which could affect other electronic equipment in the car). The two ends of the primary are connected to a pair of paralleled MOSFET’s each that tie them to ground in each conduction cycle (Vgs of the corresponding MOSFET high).

These MOSFET’s should be fast, able to withstand high currents (in excess of 30A each if possible) and have the lowest possible Rds(on). The proposed IRFIZ48 (T7 and T8) can withstand 40Amp and has a Rds(on) below 16 milliohm. This is important, because the lower this resistance is, the less power they are going to dissipate when switching with a square waveform. Other alternatives are MTP60N06, or the more popular BUZ11 and IRF540.

 MOSFETs and Thermal Runaway

It has been claimed that MOSFET’s are immune from thermal runaway, since they have a positive temperature coefficient for their "on" resistance. While this may be partially true for a Class-AB power amplifier, it is completely false for a switching supply.

For example, a push pull SMPS using one IRF540 MOSFET a side draws 30A at full load. If we check the data sheet, we find that Rds(on) is 0.044 Ohm (44 milliohms) at 25 degrees C, then we know that it will generate

P=I2 x R = 302 x 0.044 = 900 x 0.044 = 39 W peak (per transistor).

At 50 degrees (not uncommon in a car that has been in the sun for some time), Rds(on) will be about 1.25 times the value at 25 degrees (this is from the datasheet), or 0.055 ohms. Power dissipation will now be 49W, so the heatsink has to dispose of more heat. We can guarantee that the extra heat will cause the heatsink temperature to rise further, which will increase Rds(on), and that will make the heatsink hotter, and … BANG

Ensuring that you use parallel devices and a good heatsink will reduce the likelihood of this dramatically. Two MOSFET’s sharing the load will dissipate 1/4 the power (each) of a single device, and have a lower thermal resistance to the heatsink as well.

P=I2 x R = 152 x 0.044 = 225 x 0.044 = 9.9 W peak (per transistor) - 19.8 W for both

The power shown per transistor is the peak - actual (RMS) power (per device) is half that calculated. The total power dissipated by both transistors (and sets of transistors in the case of paralleled devices) is the full value shown, since when one device is "on", the other is "off" and vice versa.

Naturally, the maximum dissipation will only occur at maximum (continuous) amplifier power - the real life requirements are usually somewhat less, however, it is essential that the design is capable of continuous "worst case" dissipation to ensure an adequate safety margin.

I strongly recommend that you do the calculations yourself, and make sure that you understand the implications.

 B-Rectification and Filtering

If one looks to the secondary side of the SMPS, it resembles exactly the scheme of a typical mains PSU, with one fundamental difference - the switching diodes have to be FAST or ULTRAFAST (D6 and D7), if you use a standard diode bridge the system will simply blow up (and this can be very impressive, believe me!) Although a diode bridge is represented, it can be made with discrete diodes as well. Use high current (10 A minimum and a suitable voltage rating) diodes.

You may be surprised that the capacitors aren't too big. This is due to the high switching frequency. It is important that they are good quality ones and must be rated for 105 degrees operation. Ripple current rating and low ESR (equivalent series resistance) is very important for any switching supply. In my opinion, 3000uF per rail is enough.

 Also I have added additional LC filter with fuses preventing over current, it can be seen on Picture 8.

C - Control Circuitry

In this design we will use PWM IC with TL494. Picture 9 shows the configuration of 16 pins on this IC. To make is simpler, let’s design a SMPS by explaining the function of each pin.

1. First we make the Remote Turn On circuit, which is connected from the car radio / CD player. The circuit can be seen in Picture 7. This circuit will turn on the SMPS by giving 12Vdc to pin 12, pin 11 and pin 8. Transistor T2 is used as switch for powering the TL494.

2. The SMPS switching frequency is determined 62,5Khz. For this, the clock inside IC TL494 is adjusted 2 x 62.5 KHz = 125 KHz. This clock is built up by pin 5(Ct) and pin 6(Rt). The approach can be done with equation Fclk = 1 /(Rt x Ct). Here we use Ct = 1nF (C2) and Rt = 16Kohm (R7) like in Picture 7.

3. For output filter capacitor of 1500uF, we will need approximately 4x 1500uF or 6000uF in the SMPS's input 12Vdc . The larger the value of this capacitor, more energy stored for the SMPS.

4. Output filter inductor Lo is determine by: Lo = 0,5 x Vout/ (I x F).

Example: with Vout = 2 x 37V = 74V, I = 8A and F = 50Khz, we will have Lo = 0,092mH or Lo = 0,046mH on each supply rail + and – 37Vdc.

5.Pin 9 and pin 10 are output pins that will drive the primary winding switching mosfets. Inside IC TL494 both pins have already operated in mode push-pull. The circuit for driving power mosfets can be seen in Picture 7.

7. Transformator (trafo) for SMPS is self would from ferrite toroidal core (like donuts) like in Picture 10 or we can use ETD-type cores Picture 11.It is very important that for SMPS frequency above 20Khz, we cannot use iron core transformator like we use in homes. The ferrite core transformator will have black color like in the speaker magnets, but do not have magnetizing force. The basic of equation for switching power supply with 12Vdc input is:

 (1) Np = 1,37 x 105 / (F x Ae), where Np= primary number of turns, F = switching frequency, Ae = X x Y = window area of ferrite in cm2. Look at Picture 12. To make it easy to wound the transformator, we will have to choose the toroid core with minimal diameter of 2,5 cm and window area minimal of 0.75cm2.This is necessary for the easiness of self handwound. Remember that in push-pull system there is 2 primary windings.

(2) Ns/Np = Vo/8,8, where Ns = secondary number of turns, Vo = secondary output voltage

(3) Ap = 0,004 x Vo x Io, where Ap = window area of primary wire in mm2, Vo = output voltage, Io = output current.

(4) As = 0,13 x Io, where As = window area of secondary wire in mm2.

Example (+/- 37volts, 8A): If we use toroidal ferrite core with window area of Ae = 1 cm2 , in my case I also use ferrite core with window area of Ae = 1 cm2 , then from equation (1) we will have number of primary turn Np = 1,37 x 105 / (50Khz x 1 cm2) = 2,74 turns. In practice, number of minimal primary turns is 4 so the primary will cover the whole toroidal core. So we use 4 turns for T7 and 4 turns for T8.

From equation (2) we have that Ns/Np = 37/8.8 = 4,2. From here we can calculate that the number of secondary windings is = Np x Np/Ns = 4 x 4,2 = 16,8 or 17 windings. Like the primary, in secondary we use 2 x 17 turns, that is 17 turns for +37V –> 0 and 17 turns for 0-> -37V, better calculate +/- 37volts than +/-40volts because the SMPS is not regulated and the output voltage always is higher then calculation +/- 3%.

Equation (3) is used tp determine the number of primary winding wires. We have Ap = 0,004 x 74 x 8 = 2,36mm2. If we use a 1mm diameter magnet wire, we will have window area of 0,785mm2 so we will need 3 wire magnets for each primary winding.

Equation (4) is used to determine the number of wire needed for secondary windings. We have As = 0,13 x 8 = 1mm2 So if we use wire magnet with diameter of 0,8mm(window area = 0, 5mm2), then we will need 2 wires with diameter 0,8mm for each secondary windings.

3. MISSCELLANEUS TIPS FOR MAKING CAR AMPLIFIER

Handwound the transformator core can be done as follow (Picture 14):

- First we wound the secondary winding of 4 wires of 0.8mm magnet wires at once with 17 numbers of turn. The turn can be made in any direction as long as we consistent with the direction of the wound. If we have finished wounding it, the toroidal core will look like Picture 14a. We named the wires with wire A, B, C, and D. If we start the wound on top of the core, the end will be at the bottom of the core. Make sure each wire edges with AVO meter. Connect start edge of wire A and B to point S1 and the end edge of wire A and B to point G. The start edge of wire C and D is connected to point G and the end edge of wire C and D is connected to point S2. Point G will be the secondary ground of the power amplifier and point S1 and S2 will be connected to bridging ultra fast diode like Picture 14b.

- After we finished with secondary winding, we start to wound primary winding. Edges of primary wires are placed diagonally to the edges of the secondary wires like in Picture 14c. Like winding the secondary wires, we wound 6 wires of 1mm diameter at once. Name them wire A, B, C, D, E and F. Connect the start edge of wire A, B, C to point P1 and the end edge of wire A, B, C to point P+. Connect the start point of wire D,E,F to point P+ and the end edge of wire D, E, F to point P2 (Picture 14d)

If you have finished winding the primary and the secondary, the whole transformator will have the same wire directions like in Picture 13. Connect point P+ to the +12VDC of the car battery, point P1 to the drain of power mosfets T7 and point P2 to the drain of the power mosfets T8.

It is important to remember that all tracks in PCB layer that is connected to the power transformer has to have sufficient width due to large current will be involved. Also it is better if we soldered those tracks to have more current transfer see Picture 15.

After finishing winding the transformator, place all the rest of the component and finish assembly of the SMPS. You can test it by connect it with 12VDC input from the battery. Don't forget to connect the remote turn on with 12VDC. There should be output voltage of +37V, 0 and –37V without any large current draw in the 12VDC line. Check for any mistakes, if the output voltage does not present or if the SMPS draws large current from 12VDC input. Finished SMPS and audio amplifier are shown on Picture 16 and finished whole system is shown on Picture 17. Also I have provided bloc diagram of a full car amplifier for better understanding the principle of working for this amplifier, Picture 18. From Picture 19 you can see the front and back of the car amplifier and how it can be made.

In the assembly process of car audio power amplifier, we have to pay attention in mounting all transistors to the heatsink. We must use sufficient heatsink surface so the heat won't damage the amplifier. Use mica isolator and white silicon pasta to make sure the heat transfer. Firmly tighten all the bolts to press all the transistors. Car amplifier works in vigorous environment like in the trunk of a car. Placing an extra fan always a good idea in making car power amplifier. In my design I have put fan from an old PC supply see Picture 20, and it turn ON when the amplifier is on. The schematic is shown on Picture 21 and the system work as a remote control system from the car amplifier.

After we connect the SMPS to the audio amplifier, we are ready to test the car power amplifier. First trim the bias potentiometer fully left side to have minimum bias. Turn on the SMPS and look for the current draw in 12VDC line with amp meter. The amp meter indicator will raise for a moment to fill all the capacitors. After a few moments, the amp meter indicator must turn back to minimum indication of ampere. If not, there is some problem. Then we trim the bias to optimal point.

PROTECTION CIRCUIT

The Protection circuit protect amplifier in various ways. The main part of protection is uPC1237 witch is monolithic integrated circuit. I will design protection system by explaining the function of each pin.

For overload I have put fusses each 2,5A by rail, so Pin 1 we don’t use in this design. Pin 2 is for speaker detection it has functioned for output offset detection. If too much DC current flows through a speaker voice coil due to large output offset DC level, the voice coil might be overheated and the speaker might be broken. To prevent the damage, it is necessary to detect the Output offset Dc level and to disconnect speaker from the power amplifier by breaking off a relay if the detected DC level is shifted beyond a threshold level. Pin 3 and Pin 5 are contented to GND. Pin 4 is use to prevent causes a shock-off noise, because when the amplifier is turn off, sometimes causes a shock-off noise, therefore it is necessary to break off the relay and then to keep the power amplifier apart from loud speaker at the moment that power switch is turned off from Car CD player. Also TH1 witch is mounting on heatsink prevent over hitting the amplifier. Pin 6 has uses for driving the relay and mute transistor, R (set) has value witch will determinate 80mA maximum flow to pin 6, the value of R (set) depends of internal relay resistance. Pin 7 is for time delay. To suppress shock-on noise generated by power on, a time delay is provided by connecting a circuit with a time constant. This time delay is set to make relay ON to connect speaker after enough time for the power amplifier and the preamplifier to reach a stable operating condition. And also the last Pin 8 is for supplying the whole protection system. Also as you can see I have use two resistors (1W) in serial for supplying the relay with nominal voltage. See Picture 22.

Tests

This project handles quite large powers, so it is well worth the pain of step-by-step testing before you regret blowing all your work up in a microsecond.

For the tests, use a big 12V to 13.8V power supply, with current limiting if possible and capable of delivering at least 10 to 20 amperes (see project 77). If you don't have that, a PC computer PSU will work (although you won't get more than 80-90W, but it is enough for testing purposes and almost indestructible). Don't connect the SMPS to a car battery the first time you test it (it can be really dangerous!). A 10A fuse in series with the 12V supply is also a good idea. (You don't know to what extent! ;-)

The cables from the supply to the amp should be as short as possible and heavy gauged, to minimise losses. First time I tested the amp I had a 1 volt of difference from one side to the cable to the other in only 1.5 metres: the cable itself was dissipating more than 15W!!!. So, when calculating efficiency, always measure input voltage just at the input of the SMPS to account for this.

• First of all, with only the TL494 chip and its associated components (no MOSFETs), check that you have a very clean 12V square wave in each output (180º out of phase and they do not overlap EVER). Check also that when you turn-on the power, it starts from 0% to 50% duty cycle in about a second or two.

• Once you have this, you can mount the MOSFETs. Do it on a heatsink, but be aware that the tabs are connected to the Drain, so provide insulation (mica + plastic washers, the usual stuff). Then solder the transformer and watch the primary waveform with an oscilloscope (use a 10:1 probe just in case you have large spikes in order to avoid damaging the instrument). You should have a square wave of about 25-26V peak to peak and the smallest peaks (overshoot) as possible. It they are higher than 30V (from ground), you may try to re-wind the transformer to improve coupling. You can also reduce the overshoots using the snubber network shown in the schematic, although they will dissipate a bit of power (use 2W resistors and 100V capacitors), so mount them only if necessary.

• Once you have a clean waveform, you can solder the rectifier and output capacitors and see what you have in the positive and negative rails. You should have the same voltage in both, and it should be similar to what you calculated.

• Now try to load it with power resistors. Start with low power consumption (about 20W) and observe the mosfets, rectifiers and transformer carefully to see that they don't heat up. Also watch the current drawn from the 12V supply. The power (V x I) should be only a bit higher than that at the output load. (Expect a 80% efficiency or so).

 • If everything goes well, increase the load (decrease its resistance value). The mosfets should get warm after a while with heavy loads (about 100W), and the efficiency should maintain high (always above 75-80%). When you are completely sure that everything works as expected, you can proceed to connect it to the car electrical wiring (see "installation procedures" paragraph). First time you will notice an spark due to the sudden charge of the big input capacitor, unless you connect a resistor in series first (very good practice) to allow it charging slowly and then remove it for normal operation. Installation procedures For your car and own safety, it is VERY IMPORTANT that you pay special attention when installing the power supply (and amplifier) in your car. These are some recommendations that everyone should follow carefully:

• The supply MUST be taken directly from the battery, not to the radio or other +12V cables, as you will just blow or burn them, with the risk of a fire in the car. The supply wire must be of adequate section, about 5 mm diameter (excluding the plastic cover) minimum.

• A fuse MUST be connected in series with the supply wire, as near the battery as possible, because otherwise, in case of a collision, the wire can be shorted to ground, which WILL produce a fire. This is not a joke! The battery can produce in excess of 100 A that can burn virtually anything in a fraction of a second. • Another fuse should be put at the +12V input of the amplifier, in order to protect it from over current. My recommendation is to put a smaller value than the definitive and test the amp for a few days to see if it overheats, etc. For example, a 10-15A fuse can be suitable. The FIRST connection you have to make to the amp is Ground, and that must be firmly screwed to the car chassis as near the amp as possible with thick wire. Notice that, if you connected, for example, the signal RCA cables first and then the +12V wire, the input capacitors would try to charge returning to ground via the audio cables, possibly ruining the preamplifier of the head unit.