Hacking a faulty “Panasonic DVD Home Theatre Sound System” player to function as a powered sub woofer for my “Bram Stroker’s Dracula” pinball machine.

The objective of this hack was to modify the powered sub woofer to enable a speaker level input (from the pinball), a volume control and an automatic power switch.

Having come across a faulty home theatre system I found that it had multiple faults.  The circuitry however on the powered woofer enclusure was looking to be in good order and importantly – all analogue!

The enclosure houses a sub woofer speaker, power supply and an integrated amplifier for all 5 channels.  I set to tracing out the connection so that I might hack into the unit and make use of the sub woofer channel to server as a powered driver for my BSD pinball.

 

Working with the powered speaker was particularly dangerous due to the “live” AC inlet PCB.  This PCB provides the standby 6v power supply and a relay to switch the main transformer on and off. I recommend this type of work be left to people with a lot of experience in servicing live electronics.

Circuit

I found a PDF online with the full schematic for the active speaker. This was invaluable for the project.  The document covered the subwoofer / amplifier unit only.    Whilst I was certain the DVD player originally attached to the unit was faulty,  I also found that two of the bridge rectifiers in the active sub woofer had cracked joints.  These needed re-flowing.

Power Switch

The power switch was achieved by tapping into the SB-WA840EE power on relay circuit. The standby 6v supply can be used to trigger the mains relay.  Connecting CN502 pin 2 to (PCONT) to pin 5 (6v) will power the unit up.

A relay was installed in the pinball to complete this circuit when the pinball was powered on.  Using a piece of vero board and a small relay, I used a 12v supply from the coin door interface board to activate the relay.  The N/O contacts were wired to the 4 wire plug hanging out of the cabinet bottom.

 

Low Pass Filter

The filter circuit employed is a 2nd order active low pass filter.  I found this resource from TI to be very useful in the design of this circuit.

Note that the first opamp in the circuit is configured as a 12x inverting buffer.  This takes the low level input signal from the isolation transformer and amplifies it to approximately 100mv P-P (with a low to moderate input speaker volume).

The 10K potentiometer provides an adjustment of the level accessible from the back of the speaker enclosure.

Balanced Input

I used a salvaged audio transformer to decouple the incoming audio from the active speaker. Using this means the polarity of the speaker connection is not important. It also keeps the pinball fully isolated from the speaker.   I felt this was important as the sub woofer was a double insulated device (no earth) and the pinball utilises a fully earthed design.  I thought this prudent for safety as well as for minimising mains hum.

Op Amp Power Supply

I was able to tap the +/- 30v supply from the main amplifier.  Feeding to two 1K 1W resistors and 16v zener diodes gave me a suitable +/- 16v supply for the filter.  I used an online calculator [here] to size the resistors. I allowed for 5 mA of current for this simple regulated supply.

Pinball Tap

The pinball tap was a 4 wire connection.  The two speaker wires from the cabinet speaker, and two wires from the relay contacts make up the 4 wire plug. I user a 4 circuit Molex plug to enable the powered speaker to be disconnected as required.