Kit 106. 50 Watt Audio Amplifier
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his kit is based on an amazing IC amplifier
module from ST Electronics, the TDA7294. It
is intended for use as a high quality audio
class AB amplifier in hi-fi applications. It has very
low noise and distortion, wide bandwidth and good
output current capability, enabling it to supply high
power into both 4
Ω
and 8
Ω
loads. It has both short
circuit and thermal protection, so is quite robust.
With the addition of a handful of parts and a suitable
power supply, this module will deliver over 50W
RMS into 4 or 8 ohms with < 0.1% Total Harmonic
Distortion (THD) and <
0.1% Intermodulation
Distortion (IMD). A similar circuit was published in
Elektor
magazine, 11/96.
It is also suitable as a replacement power amp stage,
or upgrade for many existing amplifiers of between
30W-50W, provided they have a suitable dual
supply, and most do.
Specifications (
±
35V D.C. power supply)
Output power :
> 50W RMS, 4-8
Ω
load
~ 80W into 4
Ω
max.
~ 60W into 8
Ω
max.
Gain 24
dB
(30dB modification)
Input sensitivity :
1.3V for 50W, 8 ohm
0.9V for 50W, 4 ohm
(0.6V, 50W, 8
Ω,
G=30dB)
Input impedance :
10k
Ω
Signal-to-Noise ratio : > 95 dB, (>105 dBA)
Frequency response : ~ 20Hz - 200kHz, –3 dB
Slew rate :
> 10V/uS
THD :
< 0.01%, 1W-40W,1kHz
< 0.1%, at 50W, 20-20kHz
IMD :
< 0.01%, 1W
Construction :
* Please note C6 on the PCB overlay is shown the
wrong way around. The positive should go to pin 6
of the IC, as shown on the circuit diagram.
Start with the lowest height components first,
resistors and capacitors. Keep one of the longer lead
off cuts to use as the link. Be careful to get the
electrolytic capacitors in the correct way around. The
positive lead is marked on the overlay. The negative
lead is marked on the body of each capacitor. Leave
the IC and the big electrolytic capacitors to last.
Make sure that the IC is at right-angles to the PCB
and the heatsink chosen will fit, before soldering.
Important : You must supply a heavy duty heatsink
rated at least 1.4
o
C/W or better.
This is not supplied
with the kit, it must
be obtained separately then
drilled to suit. If building two kits for a stereo amp,
you may prefer to use one larger heatsink, and mount
a board at either end. This is often cheaper than two
smaller ones. A silicon impregnated insulating
washer and insulation bush are provided in the kit.
How it Works :
Most of the circuitry is contained within the IC
module. The input signal is applied to pin 3 via
capacitor C1 and low-pass filter R1/C2. The filter
improves the pulse response and helps stop RF
signals. The lower -3dB point is determined by
R2/C1 and R4/C3. This is approximately 20Hz for
the values used. The upper -3dB point is over
200kHz. C7/C8 and C9/C10 provide extra power
supply filtering or decoupling.
R3/R4 are the feedback resistors. The gain is
1+R3/R4 which is approx 16 times, or 24dB. If you
wish to increase the input sensitivity you may
change the resistors to suit. Changing R3 to 22k
would increase the gain to 30dB and lower the input
required for 50W into 8
Ω,
to 0.6V, without affecting
performance too much. If you reduce the value of R4
you will also need to increase C3 to maintain bass
response, as this sets the feedback low frequency roll
off.
Pin 10 is a mute input and pin 9 provides a standby
mode. Muting should always take place before
standby mode is selected. Connecting these pins
permanently to the supply rail (insert links provided)
ensures that the amplifier comes on immediately on
power up. Any switch-on clicks may be eliminated
by increasing the time constants of R5/C4 and R6/C5
if necessary. See the ST data sheet for more details.
Power Supply :
The maximum supply voltage of the IC is
±
40V.
However the maximum dissipation of the IC can be
exceeded even at a lower voltage. Therefore the
supply voltage used need not be more than
±
35V
.
This can be constructed using a 50V center tapped
transformer, a diode bridge rated at 5A (min.) and a
pair of electrolytic capacitors, as shown below.
A lower secondary voltage transformer could also be
used but the reduced DC voltage will result in less
power output into 8 ohms. You can still obtain 50W
into 4 ohms with only 24V supply rails. A 36V C.T.
transformer will give you approx
±
25V rails. The
mains transformer used should be rated at a
minimum of 80VA. If you want to run two modules
in a stereo amplifier you can use a common power
supply. In this case the transformer should be rated
at 150VA or greater.
T
