Wednesday, August 17, 2022

Stereo Brick Amplifier

By Raj K. Gorkhali

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Fig.3: Pin details of LM1875
Pin details of LM1875


Construction steps involve PCB design, mounting of components on the PCB, cabinet design, connecting the power supply to PCB and extending wires from the PCB to the front and rear panels.

Having conventional circuitry, the amplifier has to be squeezed into a low-cost brick shaped box. 21V-0-21V/2A power transformer occupies almost half of the internal space. To make room for the PCB of amplifier circuit in the brick cabinet, part of power supply section is mounted on a separate PCB so that it can be fixed on the rear side of the cabinet. Pin details of IC LM1875 are shown in above. Note that proper finned heatsinks should be used for power amplifier ICs (LM1875). While designing PCB layout, there is need to determine a proper location on the board to mount the heat sinks for IC2 and IC3.

A single-side PCB and its component layout can be downloaded from the link below. The PCB for the power supply section can be separated from the amplifier section by cutting along the straight line shown in the attachments.

Download PCB and component layout PDFs: click here

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The construction steps should be followed in the following sequence: Mount the PCB terminals and connectors, followed by low-profile components (resistors and zeners), then smaller capacitors such as ceramics, and finally larger electrolytic capacitors. Ensure correct placement of zener diodes in the PCB as per schematic diagrams. In fact, you may like to fit a good-quality 8-pin DIL socket to the board to allow the amplifier IC1 to be conveniently plugged in later on.

The PCB can fit inside the box by proper drilling, reaming and cutting holes on the front and rear panels and the base of the cabinet.

Note that besides mounting holes in the rear panel for the speaker terminal, there should be clearance holes for other terminals also. On the front panel, the holes for volume control, mono/stereo switch and RCA input sockets are all circular, so they can be drilled and reamed. The holes for RCA sockets should be large enough to accept plastic insulating sleeves. Two holes have to be drilled for volume controls on the front panel.

Larger holes for the mains fuse holder (F1) and power switch (S2) are irregular. So, if you prepare them by hand, careful drilling is required besides a nibbling tool and/or small jeweller‘s files.

Once the wires are connected, carefully place the PCB inside the cabinet. Use the insulation sleeves, flat washers, lock washers and nuts, so that everything is in place before screws are tightened. Mount volume control knobs into their respective holes on the front panel. (You may need to slip a couple of thin washers into the spindles of the knobs, to prevent them from protruding inside the cabinet.) With the PCB fitted at the bottom of the cabinet, bring speaker
wires from connecting points, marked as LS1 and LS2, respectively, on the PCB, to the rear panel.

Fit four screws along with suitable spacers into the holes located in four corners of the cabinet bottom to tighten up the PCB firmly in its place. You can place the power transformer inside the cabinet and tighten it to the cabinet bottom with nuts and bolts.

Now take a pair of tweezers or needle nose pliers and carefully push the tip of each wire through holes for left and right RCA input sockets and stereo/mono switch, respectively. Fix the sockets and switch firmly to the cabinet by tightening the respective nuts. Make sure that the wires are not twisted together or unduly strained. Also, make sure that sufficient length of wires from the PCB to the cabinet are provided for speakers, switches, volume controls and power supply. Following these steps, solder each wire to the PCB pad and clip off any excess parts. This essentially completes the wiring.

Testing subwoofer amplifier circuit

As there are no adjustments to be made, connect the amplifier to a pair of speakers, source of power and provide mono or stereo signals, before listening to the sound.

It is a good idea to perform a functional check of the power supply before IC TL072 (IC1) is inserted into its socket on the PCB. So, turn on the power, then measure the voltages across the supply, which should give +21V and –21V outputs, respectively. Now try checking the voltages at pins 8 and 4 of the socket for IC1, which should be +12V and –12V, respectively. If all the four voltages fulfill the measurement criteria, then turn off the power, unplug the mains cord, insert IC1 in the socket and fit the top of the cabinet.

If you are concerned about the proximity of the sleeved transformer’s primary connections to the side of the cabinet, you may attach a protective inverted-L cover of ‘elephant hide’ or a similar fireproof insulating sheet over the side facing the transformer.

The amplifier is now ready for use.

The project was published in December 2009 and has recently been updated.

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  1. Comment: the 21-0-21 v transformer used in the circuit will give 21×1.41 = 29.61 volts which is equal to 59.22 volts in total to be fed to lm1875 ic which is the maximum supply volt of the ic. But not all ic available in market can sustain such high voltage as some ics may be chinese ones. So please suggest safe working voltage


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