rcbrown316- I have extensive experience with audio electronics so let me see if I can help you out. It's a long but informative post.
1) Yes it is possible to share your powered-sub between multiple pins however you need to make some adjustments in your set-up. If you are combining 2 machines on 1 powered-sub by tapping the stock pinball speaker of each machine
Quoted from rcbrown316:
I am sharing each sub between two games. i had read that was ok but now i am wondering about the powered subs ability to isolate the two amps from each other.
then yes you are potentially creating a problem. Without yet discussing the powered subwoofer you have essentially connected the audio output from both of your machines together. Music signals are basically electricity converted to sound by speakers. So even if 1 of the 2 machines connected to the powered subwoofer is "off".. its audio electronic section is still possibly receiving electric signal inputs through its output section from the other machine that is "on" and producing sound. There are surely diodes in all audio equipment circuitry to keep certain electrical signals going in one direction but how far up the chain they are in various pinball amplifier sections is a guess without schematics and it's just not good practice in audio electronics to electrically combine outputs. Powered-subs were not built to isolate multiple inputs....It may work but it could be a cancer to all connections.
2) Because you have basically combined each machines speaker system you have either dropped or increased the ohm load between each machine to possible limit status. This load most likely includes the back box speakers & amplifier board as well due to most pinball audio amplifiers being mono and all speakers tied into a single output circuit on the amp board. Being that the machines speakers have been tied together we have to treat them as a single unit! You may not hear music from #2 machine when it is off and you are playing #1 machine but that does not mean #2s electrical load is not present or completely isolated. Electricity takes the path of least resistance and that could very well be through a circuit getting tired of power trying to come in its "out" door.
A) Purchase a "speaker selector switch" such as this (http://www.mcmelectronics.com/product/50-15618) and use it in reverse. These types of switches are used to connect multiple speakers to a single output but in your case we would use it to connect multiple outputs (pinball audio) to a single input (powered subwoofer). This switch would allow you to isolate the audio signals between pinball machines being used. You would connect +/- wires from the powered-sub speaker level "input" to the switch "input" section and each separate machine +/- speaker "output" to the switch's separate speaker channel "output" section. Mind you we are using this switch in reverse so the instructions I just gave are backwards and that is why "input" is going to "input" instead of "input" to "output". All the switch is doing is opening a gate and allowing that specific audio signal to pass through.
B) I would consider disconnecting the machine subwoofer or replace it with a better quality unit. Although many people on the forum use the machine woofer and a powered sub-woofer together, your machines woofer will surely be under powered compared to the powered-sub and you may or may not hear distortion (clipped electrical audio signals) in the pinball woofer. Distortion is a sign of amplifier strain and or speaker damage and is bad for all involved. If you wish to keep the pinball woofer connected, turn the volume of your powered-sub up moderately above the machines woofer or the machines volume down so it does not have to work as hard to keep up with a much better piece of audio equipment. Also if you desire to keep the pinball woofer connected with the powered-sub it is imperative that you test reverse the -/+ wires on the pinball woofer while connected to the powered woofer. This will place the two woofers 'in" & "out" of phase respectively and the bass will sound better one way or the other for your system. When performing this test I would suggest you close your machine up completely including the glass and listen a bit because air pressure is very important for woofers and makes a hell of a difference in sound on an open -vs- closed machine. You should try the reverse -/+ test for any home or other decent sound system you have sa well. You only have to do it on one speaker.
C) Consider a separate powered-sub for each machine to simplify and reduce the chance of multiple connection problems. This would be my best choice in your situation to keep things simple .
D) The adapter that lllvjr shows above seems nice also...I am not familiar with it but seems a great alternative. I would still follow the volume matching considerations described in section (B) above with any speaker additions.
I replace and up-size my woofers as well as replace my back box speakers with car audio multiaxial type speakers. I install a home theater sub-woofer amp and incorporate it into the machine itself. Most don't go that far but I love and like to feel my music. I even installed an amp on my back box speakers for my Dr. Who machine because it could not keep up with the bass (lol). Hope this helps and feel free to ask any other questions if needed!