Subwoofer Power Supply Unit


The subwoofer connects directly to the mains supply via the fitted mains cable and plug. There is no external mains on / off switch on the subwoofer. Instead, the mains supply is switched on and off via a relay, the relay acting as a switch. See the Subwoofer and speakers / Subwoofer relay PCB page for full details of the relay and its operation.

The power supply is divided into two parts - a transformer and the regulator circuit components. The transformer is housed in a black plastic box in the subwoofer case - for convenience, I refer to this as the subwoofer PSU. The components are located on the right hand side of the amplifier PCB.

The regulator circuit is straight forward.

The transformer supplies a nominal voltage of 15-0-15v AC and this connects to the amplifier PCB via connector CN400. Two fuses - F400 and F401 protect each of the 15v inputs. These fuses are the Wickman type PCB mounted fuses (see the Components page).

The AC is rectified by bridge rectifier BD400 to give a split DC output of -19v6 (-Vs) and +19v6 (+Vs) for use by the two amplifier ICs - IC101 and IC302. Thus the ICs receive a voltage of +/-19v6.

+19v6 also goes to regulator IC400 which gives a +9v DC supply.

D450, D451 and D452, provide a local DC feed.

Fig. BP1: Subwoofer power supply circuit

Testing the PSU output

The transformer rating as shown on the transformer label is 13v-0-13v at 3.6A. The actual reading may vary depending on how high the mains voltage is, if the measurements are taken under load, etc. The readings quoted here and on the above diagram are actual readings taken with a 240v AC mains supply. Your readings may vary slightly, so expect a slight difference to the readings above. The +9v from IC400 should always be +9v DC as it is supplied from a 9v voltage regulator.

Ensure that the wallwasher is on and the relay working.

Make sure the subwoofer is connected to a live mains supply

Warning! - the mains relay PCB will now be live. Do not touch any part of the relay PCB or the components on it - see the Subwoofer and speakers / Subwoofer relay PCB page for more information

The output from the PSU goes to the three pin plug (CN400) on the amplifier PCB. It may be possible to take the voltage reading for the transformer without disconnecting the plug from the PCB by putting the ends of the probes into the top of the plug (where the wires go in) as long as you don’t short out via the probe tips. If not, remove the plug and test by touching the meter probes to the bare contacts on the plug.

Set the meter to AC volts

Check the voltage across the three terminals - black to yellow, then black to the other yellow. You should have a voltage reading of approximately 15v AC between the yellow and black connections and 30v AC between the two yellow connections. This means that you have a voltage supply of 15-0-15 AC to the amplifier.

This reading should be approximately the same whether the PSU is connected to the amplifier PCB or not.

If the readings are correct, it can be assumed that the the PSU is working correctly, although there is always a slight chance that the PSU may be failing under load.

Testing the on-board power supply

All DC measurements are with respect to ground - i.e. one probe of the meter at ground, the other at the point being measured.

Check that the fuses are OK as shown below.

Check for the correct voltage on the pins of bridge rectifier BD400 pins 1 and 4

Check for 9v DC output at regulator IC400 Vout pin.

Fig. BP2: Amplifier PCB fuses

Accessing the PSU

The PSU is secured to the bottom of the box by four screws which are accessed from holes in the top of the PSU.

You will need to remover the speaker grille and speaker (see the Subwoofer opening page).

Whilst one of the screws is easy to get out, you will have problems with the others, especially the back ones as the wooden top of the box blocks their access.

The screwdriver needs to go into the hole by about 10 - 10.5cm to reach the top of the screw. Because of the restricted access, the overall length of the screwdriver, including handle needs to be short enough to be able to fit vertically in the screw hole - no more than about 15.5cm long.

I didn’t bother to attempt to remove the PSU. The only real way to gain decent access to the screws is to drill holes in the wooden top to allow a long-bladed screwdriver to reach through. Personally, I would only bother to attempt to access the PSU if it was proved that the PSU was faulty and this was the last chance to be able to repair the wallwasher. There is only the transformer inside and that can be tested it without needing to remove it.

It may be possible to find a suitable replacement transformer, but it is likely to be expensive. One option is to see if you can pick up a working secondhand amBX subwoofer, or the premium system that is being sold for parts (as long as the subwoofer is working OK)

In the end, it may be better just to scrap the subwoofer and buy a new system, just keeping the speaker lights to use as lights only.

If you want to still use the amBX speakers using another amplifier, see “Connecting a sound source to the sidelight speakers” at the bottom of the Subwoofers and speakers / Subwoofer an speakers - general page.

Fig. BP4:  Screwdriver details
Fig. BP3:  Looking at the PSU from the speaker hole at the top of the box
At least 10 - 10.5cm
Maximum 15.5cm
Philips size 2
Fig. BP5:  The transformer - all that is in the PSU case