Rescue an Orphan Dell Subwoofer

One of the cool things about volunteering at the local non-profit recycling center is getting to see the interesting electronic equipment that gets dropped off.  We get the usual variety of TVs, computers, VCRs, etc. but we’ve also seen broadcast equipment from  the TV station, Point of Sale systems, and things even we old electronics guys can’t recognize.  Some of it is just salvageable for parts but some items are either working or just need a small amount of repair.  Mostly we repair computers and LCD monitors to help raise funds but some items we take on just for fun.  Such is the case for that golden oldie the Dell 5650 5.1 surround sound system.  Unfortunately, we never see these systems with the center speaker/control unit so they have just been sitting around our shop collecting dust.  I suspect that there may be thousands of these incomplete sets sitting in basements, garages, and attics that unsuspecting souls purchased at garage sales only to find that they can’t get them to work.  Well, this is your lucky day because I’m going to show you how simple it is to replace that missing center speaker/control unit.

Dell 5650 5.1 System

As you can see in the pictures, the complete set came with the subwoofer box (which is also the amplifier unit), four side speakers (two front, two rear) and a center speaker/control panel that looks a little like a sound bar.  I don’t know what happens to all of the control panels but what we get at the recycling center are the subwoofer/amplifier boxes and maybe some of the side speakers.  Originally they were made to connect to a computer sound card but they can be easily repurposed to use with a TV.  The one caveat is that when using them with a TV you will not be able to get true 5.1 sound because the TV won’t have the separate audio sources for front, rear, and center that the computer sound card has.  It will work well as a 2.1 system or you can still use all of the side speakers with the understanding that the rear speakers will simply echo what the front speakers are putting out.  You can add a center speaker but you would then effectively lose the stereo effect.  But, hey, 2.1 is a vast improvement over the speakers built into most TVs.

The 5650 provides three stereo plugs for audio inputs.  As shown in the pictures, the original set came with a 3-to-1 adapter that allowed a single audio source to be input into all three of the plugs.  If you had a true 5.1 sound card, the green connector would provide the front side speaker audio, the yellow connector would provide the center/subwoofer audio, and the black connector would provide the rear side speaker audio.  If all you care about are 2.1 outputs, just connect the green plug.  It will provide sound out of the front speaker connections and the subwoofer.  If you want to drive all three inputs, even if you won’t use all of the outputs, then you need an adapter to route the single audio input to all three plugs.  The 3-in-1 adapters are hard to find and can be expensive but all you really need is a cheap 5-in-1 adapter like the one shown above.  Or just make your own out of 1/8 inch stereo audio sockets or you could even hard wire the lines together.

The speaker outputs (except for the center) are clearly labeled on the back panel and will accept a standard RCA plug.  The outputs are keyed with plastic side wings for the original speakers but you can always cut the side wings off of the Dell speakers if you don’t have exactly the matching ones you need.  There is nothing special about the speakers that require them to only plug into the matching color coded slots.

Center Speaker/Control Panel

Adapter Wiring

It should be noted that the knob on the front of the subwoofer only applies to the subwoofer itself.  The main volume control and the on/off switch are all part of the center speaker/control panel.  When you look at the back of the box you can see a non-audio connector that is labeled “CENTER”.  That connector is known as a female DB9 (9 pins) connector and is usually seen as part of the old fashioned RS-232 serial port.  In this case it is simply a convenient way to route all of the connections that need to be made between the control panel and the subwoofer/amplifier box.  In order to even turn on the system we need to understand what those connections are and replace the control panel functions.  Fortunately, the mating male DB9 connectors are readily available and inexpensive.  The list below details the pins of the connector.

1 and 2: Shorted together internally and connect to minus side of center speaker

5 and 9: Shorted together internally and connect to plus side of center speaker

3: To volume control inputs (pins 5 and 7) of M51132FP VCA chips

4: To power control transistor

6: +12 volts

7: +5 volts

8: Ground

The power switch should connect so that pins 4 and 6 are shorted together to turn the system on.  One thing to keep in mind is that the voltages on pins 6 and 7 are current limited internally so they are not able to drive additional circuitry.  They are just provided as control voltages for the internal electronics.

I don’t know exactly what value was used in the original for the volume control but it probably isn’t critical.  The reason why is that the volume control is really just acting as a voltage divider between +5 and ground for the input to the Voltage Controlled Attenuators (VCA) inside of the box.  I tried it with a 5k linear pot I had and it worked fine.  I would suspect that 10k would also work if that is a more convenient value to find.  If you don’t have a pot handy, just jumper pin 3 to pin 7 to get full volume for test purposes.  It really wasn’t unbearably loud when I did that with an MP3 player driving the input.  The volume control pot should have one side connected to pin 7 (+5) and one side connected to pin 8 (ground) with the center connector (wiper) connected to pin 3.  The DB9 adapter wiring diagram is shown above.

Subwoofer

The woofer speaker inside of this box is interesting in that it has two voice coils.  Subwoofers are usually mono but the merging of the left and right channels is normally done at the input to a mono power amplifier.  In this box the left and right audio for the subwoofer are separately amplified and then it is the speaker that combines the channels.  If you wanted to modify the box you could separately drive two normal subwoofer speakers.

Remote Control

I hope that this post is helpful in getting a few more orphan 5650 systems into good homes.  If you want to take it a step further, you can add an IR receiver/controller to this box.  I had already designed one for another subwoofer system so it was easy enough to modify that design in order to build one for this system.  Check out that post if you are interested.