There are many instances where a simple and reliable power amplifier
is needed - rear and centre channel speakers for surround-sound,
beefing up the PC speakers, low powered tweeter amplifier, etc. For
those who want to build their own 'Gainclone' amplifier, this will
certainly do the job :-)
This project (unlike most of the others, but in a similar vein
to Project 19) is based almost directly on the typical application
circuit in the National Semiconductor specification sheet. You can also
use the TDA2050 (from SGS-Thompson), which has almost identical
performance and (remarkably) the same pinouts! As it turns out, the amp
in the NS application circuit is pretty good, as is the (very similar)
one from SGS. The amp is also remarkably simple to build - if you have
a PCB! These ICs are a cow to wire on Veroboard - it is possible, but
results are unpredictable.
Figure 1 shows the schematic - this is almost the same as in the
application note (redrawn), and with added RF protection at the input.
Note that the speaker must return to the central 'star' earth (ground)
point. If connected to the amplifier's earth bus, you will get oscillation and/or poor distortion performance.
Figure 1 - LM1875 / TDA2050 Power Amplifier Circuit Diagram (One Channel)
Voltage gain is 27dB as shown, but this can be changed by using a
different value resistor for the feedback path (R4, currently 22k,
between pins 2 and 4). The amplifier must not be operated at any gain
less than 10 (20dB) as set by R4 and R5, as it will oscillate. In some
cases, an inductor may be needed in series with the output to prevent
instability with capacitive loads (10 turns of 0.5mm wire wound around
a 10 Ohm 1W resistor). The most common capacitive load is the speaker
cable itself, and 'audiophile' leads are often worse than standard
grade cables in this respect.
The 1 Ohm resistor (R6) should be a 1W or 0.5W type, and all others
should be 1/4W 1% metal film (as I always recommend). All electrolytic
capacitors should be rated at 50V if at all possible, and the 100nF
(0.1uF) caps for the supplies should be as close as possible to the IC
to prevent oscillation. C1 should be a bipolar (non-polarised)
electrolytic, or may be plastic film if you prefer.
The supply voltage should be about ±25 Volts at full load, which
will let this little guy provide a maximum of 25 Watts (rated minimum
output at 25°C). To enable maximum power, it is important to get the
lowest possible case to heatsink thermal resistance. This will be
achieved by mounting with no insulating mica washer, but be warned that
the heatsink will be at the -ve supply voltage and will have to be
insulated from the chassis. For more info on reducing thermal
resistance, read the article on the design of heatsinks - the same
principles can be applied to ICs - even running in parallel. I haven't
tried it with this unit, but it is possible by using a low resistance
in series with the outputs to balance the load.
Note that the supply voltage must not exceed ±30V at any time - this
is the absolute maximum voltage rating for the LM1875. Note that the
TDA2050 is rated for a maximum of ±25V.
Figure 2 - IC Pinouts
Figure 2 shows the pinouts for the LM1875, and it should be noted
that the pins on this device are staggered to allow adequate sized PCB
tracks to be run to the IC pins.
The PCB for this amp is for a stereo amplifier, is single sided, and
supply fuses are located on the PCB. The entire stereo board including
four fuses is 115mm x 40mm (i.e. really small). The heatsink needs to
be bigger than you might expect, largely because of the relatively high
thermal resistance of the TO-220 case. National recommend that the
heatsink should be no smaller than 1.2°C / Watt (it is actually
suggested that the heatsink be 0.6°C / W, but this is a very large
heatsink indeed, and is not necessary for normal audio into reasonably
well behaved loads.
Never operate these ICs with no heatsink, even without any load
connected. The quiescent dissipation will cause them to overheat very
quickly, and may damage the internal circuitry.
Output power is rated at 20W per channel, but with music signals you
will probably be able to get a peak power of about 25W into an 8 ohm
load. Refer to the data sheet (see link below) for the full
specification on the IC. Note that the TDA spec sheet claims 50W, but
this is overly optimistic and cannot be achieved in practice.
Photo of Completed Amp (On Heatsink)
How Does It Sound?
The sound quality is very good - as I said at the beginning, I
would not call it audiophile hi-fi (but then again - I might, with
caveats), and provided the amp is never allowed to clip it sounds
excellent. Because of the overload protection (which I have never liked
in any form) this amp provides somewhat nastier artefacts as it clips
than most discrete amplifiers.
For those who think an incredibly short feedback path length is
actually important, a surface mount resistor can be used for R5, either
soldered directly to the leads (pins 2 and 4) or the pads on the copper
side of the board. This will provide a feedback path of less than 20mm
in total, and could be made less than 10mm (at the risk of damaging the
IC with excess heat).
This amp is ideal for Hi-Fi PC speakers, and could also be used as a
midrange and/or tweeter amp in a tri-amped system - there are a lot of
possibilities, so I will leave it to you to come up with more.
A suitable power supply diagram is shown below. This is adequate
for as many amplifiers as needed, simply by increasing the size of the
transformer. 18-0-18 volt transformers are available (they are commonly
used for 12V lead-acid battery chargers), and this provides the
wiring must be performed by a qualified electrician - Do not attempt
the power supply unless suitably qualified. Faulty or inadequate mains
wiring may result in death or serious injury.|
Figure 3 - Power Supply
Although 10,000uF capacitors are shown, the amplifier will operate
quite happily with less - I do not recommend anything less than 4,700uF
for a pair of amps. The transformer rating is up to you. It should not
be less than 150VA, and more than 300VA is unwarranted - the regulation
improves with greater VA ratings, but the law of diminishing returns
comes into play quite quickly.
Signal earth (the triangle) and mains earth should be tied together
at a common point, which will become the 'star' earthing point for the
whole amplifier. This should be as close as possible to the common of
the filter capacitors. The main earth must connect to the chassis to
prevent electric shock in case of a transformer "meltdown".