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The original Project 72 design is for audio - this one is for the workbench. It can be used as an amplifier, but can also be used as a variable source/ sink power supply. The output voltage and current are obviously limited, as it's not a powerhouse IC, but it's one of those things that you never knew you needed until you had one. Of course, you may not need it at all, but it's still worth a look.
As an amplifier, it's only low power (about 8W into 8Ω), but small amps are always handy on the workbench. As a power supply, you can get from ~2V to 22V output, with a maximum suggested current of around 500mA. Unlike almost all standard power supplies, it can both source (supply) and sink (absorb) current. The tab on the IC is conveniently connected to the negative supply (pin 3), so it can be connected to a thick chassis or a heatsink with no insulation - thermal compound will provide the best possible thermal transfer.
You can also use the TDA2050 (from SGS-Thompson), which has almost identical performance and (remarkably) the same pinouts! The amp is easy to build on Veroboard. These ICs are normally a cow to wire on Veroboard, but this simplified version is simple enough.
Note that the TO-220 SGS-Thomson (now STMicroelectronics or 'ST') TDA-series IC power amps are discontinued, leaving only the LM1875 as an 'official' option. There are many on-line sellers offering the TDA series of IC power amps, but they are not official distributors and the devices offered are probably not genuine. This doesn't necessarily mean they won't perform as expected, but it does mean that you can't be certain of their provenance.
Figure 1 shows the schematic - this is a very different circuit from those described elsewhere. The speaker/ Gnd terminal must return to the central 'star' earth (ground) point at the power supply filter cap. If connected to the amplifier's earth bus, you will get oscillation and/or poor distortion performance. R2 is shown as 100Ω and is there to help suppress RF interference. VR1 and VR2 are linear 10k pots, with one used as a volume control and the other to set the DC output voltage. The separate pots allow you to set the DC level independently of the audio volume.
When used as an amplifier, VR2 should be set to 50% (centred) for maximum output level. For other experiments, set the DC level as required, and audio (AC) can be added as needed. For minimum noise in DC mode, turn VR1 fully anti-clockwise (off). C1 then adds good input noise bypassing.
The AC voltage gain is 27dB as shown, but this can be changed by using a different value resistor for the feedback path (R4, currently 1k). Increasing the value of R4 reduces gain and vice versa. The amplifier must not be operated at any gain less than 10 (20dB) as set by R3 and R4, as it will oscillate. Gain above 33dB (R4 = 470Ω) is not recommended as the distortion will increase, but that's probably not an issue here (you'll need to increase the value of C3 to maintain low-frequency response). The low frequency -3dB frequency is about 5Hz. The inductor in series with the output is essential to prevent instability with capacitive loads (10 turns of 0.5mm wire wound around a 2.7Ω 1W resistor). The most common capacitive load for an audio amp is the speaker cable, but when the amp is used as a 'power supply', capacitive loads are almost always a fact of life.
The DC gain is unity - the amplifier simply buffers the DC output from VR2. The amp will remain stable, because the AC gain is not changed. If the Audio input is used, the gain is the AC gain set by R3 and R4, but with a variable DC offset thanks to VR2.
The 2.7Ω resistor for R6 should be 1W, as the larger size makes winding the inductor easier. All others should be 1/4W 1% metal film (as I always recommend). All electrolytic capacitors should be rated for at least 35V, and the 100nF (0.1µF) cap (C7) for the supply should be as close as possible to the IC to prevent oscillation.
The supply voltage should be +24 Volts at full load, which will let the amp provide a maximum of about 8W. 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, and the heatsink will be at ground (zero volts) and does not need to be insulated from the chassis unless you will connect the circuit to another power supply or other earthed equipment.
Note that the supply voltage should not exceed +24V - this is the maximum allowable voltage for VR2, which will dissipate about 60mW (carbon pots are not designed for high dissipation). Ideally, the supply will be regulated, either using a small switchmode supply (24V at 2A or so) or a discrete linear supply. Use of an unregulated supply is not recommended, because the DC voltage will not be stable or predictable.
| The drawing shows the pinouts for the LM1875, viewed from the front of the IC, 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 tab is connected to -VEE (used as ground in this circuit) so it can be bolted directly to the chassis. The pins can be spread easily to fit standard 0.1" Veroboard, but be careful not to bend the pins too close to the plastic package. The same pinouts are used for the ST devices (TDA2050 and lower power versions, such as the TDA2030 and TDA2040). Most of the TDA series are considered obsolete - they are no longer available from the major suppliers, so you have to resort to other suppliers who may or may not be offering the genuine article. |
Note that if you can get the TDA20xx ICs, you should consult the datasheet to verify the recommended component values. Those shown will work with any variant from any manufacturer. Unfortunately, the supply requires an odd transformer voltage for the power supply, but you have some leeway because of the regulator.
The PCB for the stereo project version of this amp (Project 72) is not suitable for this application as it uses a dual supply. Instead, use Veroboard, which requires that the IC pins are bent out slightly to fit the tracks. The heatsink needs to be bigger than you might expect, largely because of the relatively high thermal resistance of the TO-220 case. National (now Texas Instruments) recommends that the heatsink should be no smaller than 1.2%deg;C/ Watt (it is actually suggested that the heatsink be 0.6°C/ W, but this is a very large heatsink, and is not necessary for normal audio into reasonably well behaved loads.
Never operate these ICs without a heatsink, even without any load connected. The quiescent dissipation will cause them to overheat very quickly, and may damage the internal circuitry.
The output power of the amp is limited by the power supply, and you can expect around 8W into an 8Ω load with the recommended 24V supply. Refer to the data sheet for the full specification on the IC. When used as a DC source/ sink, the current limit is determined by the IC's internal protection circuits.
A suitable linear power supply diagram is shown below. This is regulated using an LM317, and for a reasonable output current (1.5A typical) the transformer voltage should be around 20-22V at 50VA. The unregulated voltage will be ~28V (nominal), allowing 4V for regulation. This is just enough, and the filter cap (C1) needs to be larger than expected to minimise ripple. There are multi-tapped transformers available that are ideal, and if you can get a 21V secondary (12V + 9V for example) that will be ideal. An example is the Jaycar MM2014.
Remember that if you use any of the TDA series ICs, the power supply voltages are limited, but the supply shown will suit all versions. Make sure that the absolute maximum supply voltage is not exceeded or the IC will be damaged.
WARNING: In some countries it may be required that mains wiring be performed by a qualified electrician - Do not attempt the power supply unless suitably qualified. Faulty or unsuitable mains wiring may result in death or serious injury. All mains wiring must use mains rated cable, segregated from input and low voltage wiring as required by local regulations.
In most cases, the supply will be a switchmode type, as used for laptop computers. 24V versions are readily available, but make sure that it has genuine approvals for your country. If you'd prefer to use a linear supply, the one shown below will fit the bill nicely, but it is bigger and probably more expensive than a suitable SMPS.
Figure 3 - Regulated Power Supply
Although 4,700μF capacitors are shown, the amplifier will operate quite happily with less - I do not recommend anything less than 2,200μF for this circuit, but more than 10,000μF (total) is silly. The transformer rating will ideally be around 50-60VA. The regulator must be fitted to a heatsink. The 1.82k resistor is optimum, but you can use 1.8k to get an output of 23.75V (nominal), or use a 1.5k fixed resistor with a series 1k trimpot so the voltage can be set exactly.
Signal earth and mains earth are separate. The 'star' earthing point for the amplifier is as close as possible to C6 in Fig. 1 - this is the common of the amplifier. The mains earth must connect to the chassis to prevent electric shock in case of a transformer 'meltdown'. While it is usually recommended that the signal and mains earth (ground) connections should be joined, this limits the usefulness of the amp as a 'utility' device.
This is an odd-ball circuit that came about while I was experimenting with something else. It's not a powerful amp or power supply, but it's one of those things that can come in handy. Although the maximum current I suggest is only around 500mA, that's enough for many simple projects. It's not often that most people need a supply that can sink (absorb) current from a load, but having the ability to do so is often very useful.
One place where this ability can be used is for battery/ cell discharge for capacity testing. The battery is simply connected (via a resistor) to the output which has been preset to the minimum allowable voltage. For example, a 3-cell Li-Ion battery has a nominal voltage of 11.1V and a minimum of 10.2V (assuming 3.4V/ cell). You'd set the output voltage of the utility amp to 10.2V, and connect the battery via a 10Ω 1W resistor (90mA nominal current). The current is monitored by measuring the voltage across the resistor. Unlike a resistor discharge circuit, the battery cannot be discharged below the minimum you've set it up for. Higher current is set by using a lower value series resistor.
Warning: Never switch off the supply while a cell or battery is connected. Doing so will discharge the cell/ battery below the point where it can be recovered, and the IC may be damaged. The diodes are included to reduce the chance of damage to the IC, but the connected cell or battery will be fully discharged if left connected. With lithium chemistries, this renders them useless!
You can also use the amp as a load for a power supply (500mA maximum!) to verify that the supply can provide the current required. Be aware that it will mask any ripple breakthrough though, because it has a regulated output voltage. The optional auxiliary input allows you to send an output signal superimposed onto the DC. The 'Aux' input has the same gain as the main input for AC. The input impedance is 10k.
It can also be used as a simple power supply if you happen to need more voltages that you can get from your bench supply. If the entire circuit is floating internally (no mains or chassis earth/ ground), it can be used with reverse polarity to get a negative output voltage.
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