Do It
Yourself Car Amplifier

This article is for those who
wants to make their own car amplifier. The basics of
calculation will be discussed below. If you have understand it you will be able to make car amplifier
yourself.

THE
DESIGN OF CAR POWER AMPLIFIER

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.

As
we knows, the output power of any audio power amplifier is approached by formula :

P
= Vpp2/(8*Rl)

where Vpp= peak to peak supply
voltage, Rl is the speaker impedance load. For car
voltage of 12Vdc, if we connect it to 4 Ohm speakers we will only have power of
144/32 = 4,5 Watt. Bridging the amplifier will
double the power, but will never be more than 40 W.

If
we want to make more powerful amplifier, lets say
170 watt at 4 ohm speaker load, we will need supply voltage of 74Vpp, or +/- 37
Vdc. The way to have this voltage from car supply of
12VDC is to make DC-DC converter.

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

1.
The design of audio power amplifier

2.
The design of DC-DC converter

3.     Miscellenous  tips for
making car power amplifier.

1.
THE DESIGN OF AUDIO POWER AMPLIFIER

In fig1
we can see that audio power amplifier can be splitted 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 stabilized the audio
amplifier, like the gain factor. For first stage built by discrete transistors,
both signals is fed to basis of the transistor, like
in fig1.
Both basis of the transistors is the Non- Inverting
input and Inverting Input, like those in the op-amp.

Second
stage is the stage that responsibles 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 milliampere current. To produce power
of 170 Watt at 4 ohm speaker load, than the signal has to have maginitude
of 28Vpp and current of 6.5A (from the equation of P=I2*R =
V2/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, lets say 28 x. The
output signal from the first stage has not reach 28Vpp, it tends to have the magnitude similiar 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.

Offcourse the explenation 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. This principles must be applied in each
transistor in those 3 amplifier stages.

FIRST
STAGE

First stage
designs have main component, that is Constant Current
Source (CCS) which can be seen in fig2
.
One of the basic of electronic law that works on every circuit is that the
voltage drop of Basis and Emitor (Vbe)
equals the drop voltage of one dioda = 0.67V. It
can be seen in  fig2
that the voltage drop of 2 dioda IN4148 = 2 x 0.67V
= 1,34V. We can see in RE and Q1, then V=0,67 is
substracted by  Vbe
of Q1 and the other 0,67V  will be the drop of RE. So we will have a
Constant Current Source of 0,67/RE. In fig2
the Ic is = 4,4mA. CCS
first stage varies between 1-4mA.

In fig1
first stage, each component will be explained like this:

-
R1 is the impedance of
the audio amplifier, the range is 10 Kohm
– 47Kohm

-
C1 is the highpass
filter from the equation : Fhp = 1/(2 x pi x R1 x C1)

-
RED1 and RED2 is
between 50-150 ohm

-
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 will be
discussed later.

Before we
discuss Second Stage and Third stage, first we will discuss the amplifying
effect of a transistor. In fig3a
we will see a circuit of Common Emitor Mode
(CEM). This circuit will amplifies the voltage. In fig3b we
see a Common Colector Mode (CCM). This circuit is
the current amplifier without voltage amplifier. So if we want to amplifies
voltage we use CEM circuit and to amplifies current we use CCM circuit.

SECOND
STAGE

The Second
stage responsibles 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 consist of a voltage amplifier/CEM transistor(Q5 in fig1)
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 , that is  Miller Capacitor (CC in fig1).
CC defines the pole of the frequency response for an audio amplifier and the
magnitude usually in small order (severalpF).

Bias control
circuit consist of a transistor, resistor and a VR like in fig5.
This circuit uses a transistor that is placed in the heatsink, because the
transistor have good heat compensation factor (for bipolar transistors).
For the amplifier that uses mosfet transistor for
the final device, the bias circuit only needs potentio or dioda only because
mosfets have different heat characteristic than
bipolar transistors. The bias voltage magnitude depends on the type of the third
stage used, which will be discussed later.

THIRD
STAGE

Third stage /
Output Stage is the current amplifier. Third stage
and the bias circuit will defines 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
conected to efficiency and the heat
generated. Heat generated is a very important factor, because if not considered
carefully, it will lead to amplifier breakdown.

Many
configurations of the output stage can be seen in fig4.
Each configuration has different optimum bias voltage. It depends on how many
Vbe's that have to be
passed.  Example : In fig4(a)
the signal has to pass 4 Vbe's, which is Vbe Q1, Q3, Q4 and Q2. So the optimum bias = 4 x 0.67V =
2.8V.

Both 3 stages
that we have discussed above, if we connect the together will be a circuit that
can be seen in  fig5.
Parts of this circuit can be explained like this:

-
The value of Negative
Feedback (NFB) resistor is determined by determining the gain factor with the
equation :  Gain = 1+(R10/R8) = 1+10k/500 = 21
x. The value of R10 = value of R1 to balance input. R20 and C7 are
the pole and slope compensator.

-
C2 limits the DC gain
factor, value ranging from 47-220 uF, usually using
a nonpolar capacitor.

-
R21, R22 and C11 will
stabilize CCS. Here we use CCS with 2 transistor system,but the equation used
still the same, that is  Ic = 0,67/RE
.

-
The output of
differential pair tapped from collector of T10 and send to VAS which is built
by T12 and T4. This configuration is called Darlington VAS and the value of R8
is standard.

-
C3 is the Miller
capacitor with value of 100pF.

-
C5 is called Speed
Up Capacitor. Several designs do not use this
capacitor

-
R18, C6,L1
and R19 are output power stabilisator. If there is
any oscilation occur in the audio power amplifier,
the first tobe effected
is R18 besides the final transistors.

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 fig5
it is the number of pairs of T7 and T8. As a rule of thumb, the number of
transistor needed first has tobe calculated by
equations above, and then we determine the number of final transistor needed
with assumption that 1 transistor can handle 50 Watt output. A pair of bipolar
transistor can handle 100 Watt. The power is raised by parrarelling several output transistors, so the currrent flowing will be larger. For large number of final
transistors, we change the predriver stage with
darlington configuration.

Several
designs uses symetrical design, like those used
in  AXL and Crescendo schematic. this
design is developed from the basic principal above, but the signal handling for
+ and - part is handled by complementary circuits.

I have an
example about another kind of power amplifier, that
is a non-feedback amplifier. You can view the principles of the "millenium
power amplifier" in the http://www.lcaudio.com/ . This amplifier has
a certain gain factor in first and second stage, while the third stage is only
current amplifier.

2.THE DESIGN OF DC-DC
CONVERTER

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, it can be analogued
by the next example. Look at  fig6.
There is a voltage pulse V1 on-off with 50% wide. These pulses if passed  through suitable  L and C filter will be transformated into straight voltage of V2 which is V2 = ½
V1. (noticed the marked area below pulsed V1 is the
same total  area of the marked straight V2 ). With the same logic, if the
pulse width of V1 is narrowed, we will have a lower V2 and if we enlarge the
width of V1 pulse, we will have higher V2. Some may ask, how can we get 30VDC from the car's 12VDC? The answer is
simple. If we get the V1 voltage to 60VDC, then in the 50% duty cycle, we will
get 30VDC straight. This is the part where the power switching transformer takes
control, to make the 60VDC from 12VDC, and then chopped by the PWM.  This
is the princip of PWM. (Like the
principal of class D digital power amplifier). 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 raise equally to 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 20-70Khz.
In push pull sytem like in fig7,
Q1 and Q2 gives alternating switched current pulses so the transformator will be objected to maximum flux swing
change without saturating the core.

In this design
we will use PWM IC with SG3524 from SGS Thompson. Specifications can be seen
in  SGS Thompson's website. Fig8
shows the configuration of 16 pins on this IC. To make is simpler, lets
design a SMPS by explaining the function of each pin.

For the stereo
power amplifier in  fig5,
we will need a  SMPS 12Vdc input and summetrical output of  +/- 37Vdc with 8A
rating.

1.
First we make the Remote Turn On circuit , which is
connected from the car radio / CD player. The circuit can be seen in fig9a.
This circuit will turn on the SMPS by giving 12Vdc to pin 12, pin 13 and pin
15.

2.     The
SMPS switching frequency is determined 50Khz. For
this, the clock inside IC SG3524 is adjusted  2
x 50 Khz = 100Khz. This clock is built up by pin
7(Ct) and pin 6(Rt). The
approach can be done with  equation Fclk
= 1 /(Rt x Ct). Here we use Ct = 1nF and Rt = 10Kohm like in fig9b

3.     Pin
2(Non Inv In). In pin 2 we put stable reverence output for the SMPS. Here we use
reference voltage of ½ from reference pin 16.

4.     Pin
1(Inv In) is the output voltage detector . Pin 1 is
connected to the optoisolator type 4N35 like in  fig9b.
Optoisolator is an important component in making
this SMPS so we can have  Floating Secondary
Ground which will prevent noises (especially whine/storing) if the power
amplifier is placed in car. The value of  zener
diode is 2 x 37V = 74V. If it is difficult to have zener voltage of 74 V, then
we can series several zener values until we have total of 74
V.

5.     Pin (4) and  pin(5) are not used and connected to
ground, pin(8) and pin(10) connected directly with ground.

6.      Pin
no 9(Comp) determines slope and pole of feedback from the whole SMPS system. In
this design we use only 1 capacitor of 100nF.

7.     Pin
no 16(Vref) gives reverence voltage of 5,1
Vdc . This pin is placed with 10nF as a voltage
stabilisator.

8.     The
output ripple (Vr) of the SMPS is determined by
equation :

Vr = 8 x 10-6 x I / Co. With I = 8A and Vr =
0,029V we will have Co of 2.200uF    in +37Vdc ->-37Vdc
rail or 4400uF each in +37Vdc_0  and
4.400uF in 0_-37Vdc.

9.     For
output filter capacitor of 2.200uF, we will need approximately
4x 2.200uF or 8.800uF in the SMPS's input
12Vdc . The larger the value of this capacitor, more energy stored for the
SMPS.

10. Output
filter inductor Lo is determine by : Lo = 0,5 x Vout/ (I x F). With Vout = 2 x
37V = 74V, I = 8A dan F = 50Khz, we will have Lo = 0,092mH or Lo = 0,046mH on each
supply rail + and – 37Vdc.

11. Pin
11 and pin 14 are output pins that will drive the primary winding switching
mosfets. Inside IC SG3524 both pins have already
opereated in mode push-pull. The circuit for driving
power mosfets can be seen in  fig9b.
The number of power mosfet used is 3 in each transformator primary. So total there is 6 power mosfets type BUZ11.

12. Transformator(trafo)
for SMPS is selfwould from ferrite toroidal
core (like donuts) like in fig10.
It is very important that for SMPS frequency above 20Khz, we cannot use iron core transformator like we use in homes. The ferite
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 fig10.
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 easyness 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 :
If we use toroidal ferrite core with window area
of  Ae = 1 cm2. then from equation no. 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 Q1 and 4 turns for Q2.

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

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
windings

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.

13. The
secondary output voltage is rectified by full bridge configuration like in fig11.
Bridging diode must be the type of fast rectifier, usually looks like
transistor TO220 with plate heatsink. For SMPS we cannot use ordinary 50/60Hz
rectifier diode. For this design we use diode type
BYW29-150, which have rating  of 8A, 150V. We can also use other
diodes like with prefixes FE…,MUR..., as long as it is a fast rectifier diode with
minimal specification like above.

3.
MISCELLENEUS TIPS FOR MAKING CAR POWER AMPLIFIER

Car power
amplifier has specific accesories like preamp gain
circuit, an inverting channel so that the power is bridgeable. These
functions usually done with opamps. The circuit can be seen in fig12a
and the supply circuit can be seen in fig12b.
The circuit is placed before the audio amplifier circuit.

The transformator is handwound on
toroidal ferrite core. The output filter inductor
can be made with ferrite core material or MPP core material. It can be made with
1.2mm wire magnet, handwound and measured until we
have 0,046mH

Handwound the transformator core can be done as follow (fig13b):

-
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 fig13a.
We named the wires with wireA,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 AVOmeter. 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 diode of
BYW29.

-
After we finished with
secondary winding, we start to wound primary winding. Edges
of primary wires is placed diagonally to the edges of the secondary
wires like in fig13c.
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 (fig13d)

If you have finished winding the primary
and the secondary, the whole transformator will have
the same wire directions like in fig12e.
Connect point P+ to the +12VDC of the car battery, point P1 to the drain of
power mosfets Q1 and point P2 to the drain of the
power mosfets Q2.

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.

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 do not
present or if the SMPS draws large current from 12VDC input.

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.

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 ampmeter. The ampmeter
indicator will raise for a moment to fill all the
capacitors. After a few moment, the ampmeter
indicator must turn back to minimum indication of ampere. If not, there is some
problem. Then we trim the bias to optimal point. Usually for car stereo power
amplifier total quiscent ampere will not exceed 2A
of 12VDC line.