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Amplifier



It is unlikely that there will be a circuit diagram or specifications for the amplifier PCB


Description

The amplifier PCB has a power input of 15-0-15v AC. There is an on-board regulator section which converts this to the different voltages required by the amplifier - see the section at the bottom of the page.


The audio signal is fed to the PCB via the audio in socket and is then split to two amplifier chips - IC101 which drives the left and right speakers, and IC302 which drives the subwoofer speaker. There is no volume control for the left and right amplifier, that amplifier is always at full volume. As a consequence, there is usually a faint hum from the speakers, even when there is no audio input.

There is a bass control which controls the volume level on the subwoofer speaker. This is the only control directly associated with the amplifier.



PCB connections

There are five plugs / sockets on the PCB and one soldered connection:


3-pin 5v to relay

7-pin from 7-pin mini-din plug

3-pin audio in

3-pin bass control

3-pin AC supply

Soldered wires to subwoofer speaker


All the plugs are accessible without having to remove the PCB. The AC supply plug is most easily accessed from the front or top of the box.



There are three main chips on the heatsink:

IC101 - Left and Right amplifier

IC302 - Subwoofer amplifier

BD400 - Bridge rectifier


The heatsink gets fairly hot - around 39 degrees C when running without any audio input.





















Fig. BA1:  Subwoofer audio block diagram



Removing the amplifier PCB


You will need to remove the front of the box


Disconnect the subwoofer from the mains supply

Remove the 7-pin mini-din cable from the wallwasher


Remove all plugs from the PCB

The bass control cable from the front of the box plugs into the PCB and is also held in place by two twist ties which are soldered to the PCB and are wrapped in foam.

Remove the foam and untwist the ties.

The AC In plug / socket on the PCB can be accessed through the round hole at the front of the box. The plug is the same type as used for the mains connections on the relay PCB and has a tab which must be pushed in and held in as you gently remove the plug (see the Subwoofer Relay PCB page for the tab information).


The PCB is held in place by 6 screws

The screws can be accessed from the square hole in the rear of the case and the round hole from the front of the case by using a “stubby” (short) screwdriver. A Philips size 2 should be OK..

You will probably find it hard to replace all of them, though as it will be difficult to get some of the screws back in the holes unless you can also access the PCB through the top of the box (use a longer screwdriver).


There is a rectangular block of wood glued inside the box, above the black heatsink. This keeps the heatsink / PCB in place and needs to be removed - a gentle tap on the end with a screwdriver should do it.


The PCB needs to be rotated 90 degrees, so that the heatsink is above the PSU before the PCB can be removed from the case or put back.


PCB

Fig. BA2:  Removing the amplifier PCB - the PCB is rotated                         Fig. BA3:  Chocolate block connector



The red and black wires from the subwoofer speaker are soldered directly onto the PCB and will restrict how far the PCB can be removed from the case. If you want to remove the PCB completely, then you could cut the wires at a suitable point - say 9mm from the PCB - and join them together afterwards. There is no shielding involved, so a simple “chocolate block” screw connector could be used. A neater way is to fit a a plug on one end and a socket on the other end so that the wires could just be plugged / unplugged as required.


Twist ties
































Fig. BA4:  Amplifier PCB and socket identification



































Fig. BA5:  Amplifier PCB track side - this is a mirror image so that the orientation matches the photo above  




Testing the amplifier

The amplifier PCB also includes power supply regulation components. Details of these and testing the power supply are on the Subwoofer and speakers / Subwoofer PSU page.

If you are testing the amplifier with the PCB out of the box, make sure that the PCB is not resting on anything metal that can cause a short circuit on the PCB track side.


Before you carry out any tests on the amplifier, check the PCB for any obvious signs of problems, such as a “cooked” / overheating component, loose component etc.

Check the track side of the PCB with a magnifying glass, looking for any signs of dry or loose solder joints, or for any hairline cracks on the tracks or solder.

Flex the PCB VERY GENTLY! a tiny bit to see if there are any obvious signs of cracks.

See cracked PCB and joint problems at Faults and Repairs / Repairs / Repairing a PCB



The amplifier can be tested without the bass control cable connected, but the subwoofer speaker will not work as the input for the subwoofer speaker amplifier IC302 goes through the bass control pot. The white wire (OUT) is the input to the amplifier from the pot.















Fig. BA6:  Bass control pot connections


To test the amplifier, you will need to have:

The 7-pin mini-din cable connected to the wallwasher and the wallwasher working (to close the relay)

The subwoofer mains cable connected to the wall socket and switched on

The speakerlights connected to the wallwasher

The audio cable connected between the PC and the subwoofer

An audio output from the PC



Testing the amplifier when on

It’s possible that you may be able to test the amplifier PCB, with the PCB in place, through the top of the box with the speaker removed. This is the simplest way because it means that everything except the speaker can remain connected as normal. However, access is restricted and you may find it easier to test it with the PCB out of the box.


Because the 15-0-15v PSU connector cable is very short, the amplifier PCB will have to be kept near the front of the box. This then gives problems for making the other connections. The subwoofer speaker will have to be disconnected.  The relay PCB will have to be removed from the side of the box as the mains connections aren’t long enough.


Warning

The track side of the relay PCB will be accessible and will have mains voltage present!

Connect the subwoofer mains plug to the wall socket via a RCD (see the Faults and repairs \ Repairs page for more information). Cover the relay PCB the best you can to prevent any accidental touching.


The bass control pot will have to be disconnected because of the short cable. If you want to test the subwoofer speaker or the subwoofer part of the amplifier, you will need to make temporary connections between the pot and the ends of the (unsoldered) wires, or just twist the red and white wires together. This will give you full subwoofer speaker volume.






























Fig. BA7:  Live testing the amplifier


This photo shows one way of connecting the amplifier PCB out of the box (the relay PCB and connections are out of sight on the right). The white and green cables go to the subwoofer speaker