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Pobierz
AU
DIO
Versatile
Final Amplifier
audio power with many features
By U. Böhmke
Internet:
www.ub-elektronik.de
e-mail:
info@ub-elektronik.de
Only a few years ago, many music lovers turned up their noses on seeing a
hybrid circuit or integrated circuit in a final amplifier, but now there is a new
generation of output
stage ICs that have
been specially designed
for use in high-
quality audio
amplifiers.
The Variable Final Amplifier is built
using ST Microelectronics TDA7293
ICs, which have MOS outputs stages.
This integrated power amplifier IC is
a successor to the TDA7294, which
was described a long time ago in an
Elektor Electronics
data sheet (May
1993) based on an SGS Thomson
product announcement. Evidently
SGS Thomson experienced difficulties
in the development of this IC, since it
took three full years until the IC
appeared on the market. However, at
the end of 1996 it was ready, and it
was used in the 50-W A.F. Amplifier
(November 1996 issue), which has
been built by many hobbyists.
If we compare these two power amplifier
ICs, the first thing that strikes us is the simi-
larity of their internal circuitry and external
wiring (see
Figure 1
). Two small but important
differences allow the new IC amplifier to be
2/2002
Elektor Electronics
35
AU
DIO
Full amplifier
If we place a selector switch and a
potentiometer ahead of the inputs of
a stereo version of our final amplifier,
we have a miniature version of a full
amplifier. The gain of the compact
final amplifier is fully adequate for a
high-level signal source, such as a
CD player. If more gain is needed, a
small opamp preamplifier stage can
be added to the configuration.
used much more flexibly: first, the connection
between the input and output stages can be
opened to allow the output stage to be driven
externally, and secondly there is a special
charging output for the bootstrap capacitor.
We will see what these features allow us to do.
In addition to having good specifications
and sounding good, the TDA7293 is distin-
guished by high reliability. Its quiescent-cur-
rent stability is very good, the output is short-
circuit proof and integrated thermal-shut-
down circuitry prevents overheating.
Integrated switch click suppression in the
form of a mute/standby function makes an
output relay unnecessary, which helps the
damping factor. And if in spite of everything
the preamplifier overdrives the final amplifier
into clipping, the CLIP DET output announces
the fact. The complete data sheet of the
TDA7293 is available at the following
address:
one amplifier is connected to the
bass/midrange driver, while the out-
put of the other amplifier is con-
nected to the treble driver. In case of
a three-way loudspeaker, one output
is connected to the bass driver and
other one is connected to the
midrange/tweeter unit (true fanatics
even go for ‘triwiring’ and dedicate
a separate amplifier to each of the
three drivers). Naturally, the passive
crossover network in the loud-
speaker must be designed for
biwiring, which means that the
high-pass and low-pass filters must
be built as independent assemblies.
Biamping
Cognoscenti
have long appreciated
‘biamping’ as the logical extension
of ‘biwiring’, but in Germany this
idea has become known to a rela-
tively large group of listeners only
within the last few years. This is no
doubt due to the fact that biamping
requires two stereo final amplifiers,
which naturally means twice the
expense if they are bought ready-
made. However, with DIY construc-
tion the additional cost is not as
great, since all that is necessary is to
fit an extra final amplifier into the
enclosure. Our compact final ampli-
fier is an excellent choice for such
use, due to its small size. In such
applications, its relatively small out-
put power (compared with large
final amplifiers) is by no means a dis-
advantage. Besides better utilisation
of amplifier capacity, biamping can
produce an audible improvement in
spaciousness and resolution.
In purely practical terms, two
final amplifiers are need for each
stereo channel, with their inputs
connected in parallel. The output of
Active loudspeakers
The principal advantage of an active-
loudspeaker system is that the only
thing between the amplifier output
and the loudspeaker driver is a
length of wire. This eliminates both
the complex load on the amplifier
and the reduction in the damping
factor that result from using a
crossover network. As regards
amplifier power, the same consider-
ations apply as for biamping.
An active speaker solution is
always an option when developing a
new speaker design. Generally speak-
ing, existing well-balanced speakers
cannot easily be converted into active
loudspeakers. Attempts to do so often
result in a ‘negative improvement’ in
the sound quality. Biamping is a more
effective approach for improving a
passive system.
http://eu.st.com/stonline/books/
pdf/docs/6744.pdf
Basic concept
A compact final amplifier is particularly inter-
esting for anyone who wants more than sim-
ple stereo operation — in other words, any-
one who wants to configure a system with
biamplification, active speakers or multiple
channels. Such systems sound best when all
amplifiers have the same construction.
Monoblock
In the simplest case, a complete final ampli-
fier and power supply are built into a single
enclosure, resulting in a small, ready-to-use
monoblock.
Multichannel systems
Due to its small size, the compact
final amplifier is naturally also espe-
cially suitable for use in multichan-
nel systems. Separate amplifiers are
preferable for the front, centre and
rear channels. For the subwoofer, a
parallel or bridge circuit is an ideal
solution.
Amplifier circuit board
with options
The schematic diagram of the com-
pact final amplifier (
Figure 2
) repre-
sents a standard application circuit
for the TDA7293, although it has a
few unique features. The input con-
nections are duplicated to allow the
music signal to be ‘daisy-chained’ to
a following final amplifier. After the
input we find the usual filters. C1
Figure 1. Internal circuitry of the TDA7293V.
36
Elektor Electronics
2/2002
AU
DIO
+UB
K5
+ U
B
R4
560
R5
24k
C11
C9
Ω
150n
1000
µ
63V
*
C3
2x
100
µ
25V
C4
C6
voir texte
*
*
siehe Text
22p
zie tekst
C5
R6
100k
*
see text
100n
*
R3
7
13
+VS
+PWVS
K3
2
11
IN–
BD
(20k)
PARALLEL
C1
R1
390
IC1
AMP
3
14
Ω
IN+
OUT
C8
1
µ
LSP
*
TDA7293
47
µ
R2
4
6
50V
SGND
BOOTSTRAP
C2
C7
*
10
5
NORMAL
MUTE
STBY
CLIP
47
µ
470p
9
12
50V
BL
JP3
R7
JP2
STBYGND
–VS
–PWVS
1
PAR/
SLAVE
8
15
+UB
D1
1N4148
C15
100n
K2
K4
R8
24k
R9
47k
R10
100k
C13
S1
on
off
2x
9
10
10
µ
63V
C14
7
8
5
6
3
4
1
2
K6
K1
C10
C12
9
10
7
8
150n
1000
µ
63V
5
6
– U
B
3
4
1
2
JP1
010049 - 11
SM
Figure 2. The circuit diagram of the amplifier corresponds to the standard application but has a number of special configuration options.
and R3 form an input high-pass filter
that isolates the input from any dc
voltage present at the output of the
previous stage.
The TDA7293 can be seen as a
non-inverting operational amplifier.
The gain is set to around 35 by the
negative feedback network. This
amount of gain results in the best
balance of speed, bandwidth and
stability. In order to avoid amplifying
the input offset voltage, the amplifier
is ac coupled. Capacitor C6 improves
the square-wave response.
Good-quality components must
be used for the Boucherot network
(R7 / C15). R7 must be a low-induc-
tance type, while a foil capacitor
must unconditionally be used for
C15.
Although it is possible to obtain
more output power by operating two
modules in parallel (for a 2-
load) or in a bridge configuration (for
an 8-Ω load), the TDA7293 gives the
best results (in terms of both mea-
surements and listening tests) when
used alone. Consequently, parallel
and bridge configurations should be
used only for subwoofers.
If only a single amplifier module
is used, the load impedance should
not be less than 4 Ω . Since the pro-
tective circuitry cannot cope with
extremely low-impedance or com-
plex loads (such as the Infinity
Kappa), the amplifier will be
destroyed!
However, two amplifiers can eas-
ily be connected in parallel. The
power dissipation is then divided
over two packages and can thus be
greater than with a single module.
The internal resistance drops in pro-
portion to the number of modules
used. This yields certain advan-
tages, particularly with load impedances less
than 8 Ω , and is the only manner in which a
2-
load, the
supply voltage can be raised to the 8-Ω level,
with the result that the output power can be
increased to more than 100 W.
In bridge operation, one amplifier works
against the output of a second, inverting
amplifier instead of against ground. Theoret-
ically, this doubling of the output voltage
swing results in quadrupling the output
power into a 4-Ω load. However, the loud-
speaker must have an impedance of at least
8 Ω , due to thermal considerations, so only
half of this theoretical increase can actually
be realised, but that is still good for up to
150 W (depending on the quality of the power
supply). The damping factor is reduced by a
factor of 2 relative to a single amplifier driving
an 8-Ω
load can be driven. With a 4-
load.
The component connected to pins 1, 9 and
10 provide switch click suppression. The
or 4-
2/2002
Elektor Electronics
37
AU
DIO
COMPONENTS LIST
Amplifier Circuit Board
JP3
IC1
C7
JP2
H1
LSP
C11
R1
R2
Resistors:
R1 = 390
Ω
R2,R5,R8 = 24k
R4
C1
C8
S
R3
C12
C6
K3
C5
R6
M
C15
R5
K2
R3 = 19k
6 (20k
)
C10
C9
R4 = 560
Ω
R6,R10 = 100k
C4
C3
R7 = 2
2 2W
R9 = 47k
Ω
R9
R10
010049-1
K5
R8
K6
+UB
D1
Capacitors:
C1 = 1µF MKT (lead pitch 5 or 7.5mm)
C2 = 470pF
C3,C4 = 100µF 25V radial
C5 = 100nF
C15 = 100nF (lead pitch 7.5mm)
C6 = 22pF
C7 = 47µF 50V radial
C9,C10 = 1000µF 63V radial
(max. dia. 17 mm)
C11,C12 = 150nF (lead pitch 7.5mm)
C13,C14 = 10µF 63V radial
-UB
K1
K4
H3
S1
off
on
Semiconductors:
D1 = 1N4148
IC1 = TDA7293V (STMicroelectronics)
Miscellaneous:
JP1 = 3-way pinheader *
JP2 = 2-way pinheader with jumper *
JP3 = 2-way pinheader *
K1,K2 = 10-way boxheader *
K3,K5 = 2-way PCB terminal block (lead
pitch 5mm)
K4 = 3- way pinheader
K6 = 3- way PCB terminal block (lead pitch
5mm)
Heatsink *
Enclosure *
PCB, order code
010049-1
(see Readers Services section)
* see text
Figure 3. Small and double-sided — the printed circuit board for the Versatile Final
Amplifier.
TDA 7294
The TDA 7294, which is the predecessor to
the TDA 7293 and well known to readers of
Elektor Electronics, can also be used with
the circuit board for the Versatile Final
Amplifier if the following considerations are
taken into account:
– The maximum supply voltage must not
exceed ±40 Volts.
–Parallel operation is not allowed.
– Bootstrap capacitor C8 must be fitted,
with C7 being omitted.
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Elektor Electronics
2/2002
AU
DIO
Bridge mode
Parallel mode
only 0.5 mA.
Naturally, if several amplifiers are con-
nected to a common power supply (for
biamping, active loudspeakers, parallel oper-
ation or bridge operation), this function
should be controlled using a single common
switch. This can be achieved by using 10-
way flat cable to interconnect the K1 connec-
tors of the individual amplifiers, which is
already the case for parallel and bridge con-
figurations.
Normal
Master
Slave
Master
Slave
JP1
JP2
JP3
PC1,2
open
installed
open
input
to M
installed
open
input
to S
installed
open
bridge
open
open
open
installed
open
input
installed
open
Table 1: Jumper settings
selector switch (S1) is connected to
connector K4. When S1 is switched
to the supply voltage, the TDA7293
awakens from the standby mode
after a brief delay, and shortly there-
after the mute circuit activates the
output. If S1 is switched to ground,
the output is first muted and then
the IC goes into the standby mode,
in which its current consumption is
Caution:
This must only be done with final amplifiers
that are powered from a single
common
power supply!
COMPONENTS LIST
Power Supply
(depending on number of final amplifiers)
C5,C6,C11,C12 = 3
µ
F3 250VDC /
160VAC MKT (size 11x21x31.5mm)
(e.g., Epcos B32524-Q3335-K, Farnell
# 331-3311)
C7-C10 = 10,000
µ
F 63V radial, lead
pitch 10mm, max. dia. 45mm), PCB
mount
Miscellaneous:
K1-K10 = 2-way PCB terminal block
(lead pitch 5mm)
Mains transformer, 2 x 22V at 225VA
PCB, order code
010049-2
Resistors:
R1-R4 = 0
15 5W
R5,R6 = 4k
Ω
7
(see Readers Services section)
R7 = 12k
Semiconductors:
D1-D4 = BYV29-200
D5 = high efficiency LED
Capacitors:
C1-C4 = 47nF ceramic
K3
C1
47n
R1
0
Ω
15
5W
+ U
B
K4
D1
D2
R2
0
Ω
15
5W
R7
K1
K5
K7
C2
47n
R5
C7
C9
C5
C11
D5
3
µ
3
3
µ
3
POWER
Tr1
K2
R6
C8
C10
C6
C12
3
µ
3
3
µ
3
K6
K8
C3
47n
R3
0
Ω
15
5W
D3
D4
R4
0
Ω
15
5W
K9
– U
B
D1 ... D4 = BYV29-200
C7 ... C10 = 10 000
µ
F / 63V
C4
47n
K10
010049 - 12
Figure 4. A classic design, but with fast rectifier diodes instead of a bridge module — the power supply for the Versatile Final Amplifier.
2/2002
Elektor Electronics
39
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