Ok, there are so many ways to improve to the WPC era sound for pinball I almost hesitate to add another, but why not too much info is better than not enough.
So first my goal for my particular situation is that I was underwhelmed by my Party Zone sound. (like many others)
This pin has some good tracks, like Hendricks an Who, so why not make that sound better? (stock sound is pitiful)
First of all I have to manage my expectations since this pinball machine is NEVER going to be a quality stereo system.
Also I am no audiophile, so not looking for too much with this modification.
That being said, I have played new Stern and JJP machines and they sound WAY better.
But, I'm a serious cheap skate and a pre-fab aftermarket solution like PinWoofer or Pinball Pro are outside my price range.
(I will say after going through the process those products look very attractive after the amount of work required for DIY)
OK, the first thing to do was replace the original speakers, obvious move.
I felt lucky to find speakers that matched the bolt patterns of the originals from Parts Express.
Cabinet 6 1/2 IN Woofer $26.98
https://www.parts-express.com/Dayton-Audio-DC160-8-6-1-2-Classic-Woofer-295-305
(no modification bolt pattern aligns perfect)
Mid Range $26.98
https://www.parts-express.com/Dayton-Audio-PA165-8-6-PA-Driver-Speaker-295-015
(a little modification to the head to make room for the really big magnet, cutting a notch in the wood cross member)
Tweeter $3.00
https://www.parts-express.com/T-25-2-1-2-Paper-Cone-Tweeter-8-Ohm-279-452
(Very slight modification to ream out one of the holes to fit the bolt pattern)
After changing the speakers there was no noticeable improvement
The sound from the cabinet speaker was almost non-existent.
OK, searching Pinside I found a link for improving WPC sound by attenuating the top speakers so you can hear the woofer.
Guide to add L-Pad - http://www.dziedzic.us/wpc_speaker_sound_info.html
Did that and there was some improvement, but nothing that made all the changes to this point worth it.
OK, time to get serious, I'm adding a Sub Amp - I am determined to hear some bass.
This is when the choice of a external sub woofer or internal has to be made. (you can do either with about the same cost)
I'm sure adding an external is easier, but I have a personal preference to keep the pinball machine looking stock if at all possible.
Maybe I'm stupid for having that in my head. I decided to go internal.
Well there are many options to make that happen, but being a cheap-skate I go to Amazon for an Amp that would drive my 50W cabinet speaker.
Anyway here is the China Sub Amp $16.98
https://www.amazon.com/dp/B01N5DGK37
Well not quite that simple because the input of that amp it designed for a high impedance and the WPC amp is a low impedance output. So, I ordered the cheapo China impedance converter from Amazon.
Impedance Converter $8.99
https://www.amazon.com/dp/B009T5S3HE
I had a 19V laptop power brick that could deliver >6 amps, so that should more than cover the 50W I am looking for.
That brick power supply mounted easily into the cabinet beside the transformer. (wired to curtesy plug)
Alright I've got some power NOW !
I wired the output of the early WPC amp to the impedance converter (I just soldered the wires to the PCB).
Then I soldered the wire harness included with the amp to the RCA jack of the impedance converter. (mono only one RCA needed)
The impedance converter has a potentiometer to adjust level, so I did that.
Now I went into the 'test' mode for audio and put this new configuration through its paces.
Freakin Aye, now that woofer is thumping !
Oh, damn now the whole pin is shaking like there an Earthquake ! (mostly the playfield glass, it is bulging like it is going to burst)
OK, how am I going to fix that?
Well all that air pressure from the woofer needs to be re-directed outside the cabinet.
Maybe a pre-fab woofer box would keep that pressure from the inside of the cabinet and shaking the playfield glass.
Do I have room inside for a box? Looked around and found a small box designed for 6 1/2 speakers.
IMG_3340 (resized).JPG
Rhino Base Box 6 1/2 IN $70 for a pair
https://www.rhinobassbox.com/productdisplay/65-single-hole-speaker-box-poly
Got a pair of those from eBay for $50
Unfortunately, when I place my 6 1/2 IN woofer in there it did not quite fit. (5.5 IN diameter hole speaker fit diameter 5.7 IN)
So, I broke out my hole cutter and drill press and opened up the hole by 0.2 IN
IMG_3365 (resized).JPG
Now the speaker fits perfect.
OK, I did some re-engineering of the bottom of the cabinet to mount the new box. (pried up the speaker ring and screwed that to the box)
The speaker and box are now a removable and serviceable module.
IMG_3343 (resized).JPG
Installed that and discovered a ton of blow back air escaping around the speaker terminal connections on the back of the box. UGH
Alright I need to get the power of all that air behind the speaker into my ears, ported woofer !
How to add a port without making the pin look like a kludged Frankenstein?
Rummaging through my junk box I found a drain elbow from my new garbage disposal I never used.
Garbage disposal elbow $8.88
https://www.amazon.com/gp/product/B07BF2R9LZ/ref=ox_sc_act_title_2
I just drilled a 1 1/4 IN hole in the side of the box and screwed the elbow pipe into the box.
IMG_3346 (resized).JPG
Now I have a ported speaker box.
IMG_3348 (resized).JPG
Then drilled a 1 1/5 in hole in the bottom of the cabinet.
Mounting required some T-nuts and #8 1in long screws with 1/4 IN hex heads. (hard to get a screwdriver to the corners, used 1/4 IN wrench)
IMG_3356 (resized).JPG
IMG_3371 (resized).JPG
IMG_3354 (resized).JPG
With all that mounted and the volume turned up the glass is not vibrating and the bass thump is satisfying.
Now that extra air is blissfully blowing out that port with enough pressure to blow women's clothes off
The L-pad also allows me to balance the head speakers volume to the woofer.
Still this setup is in no way a high fidelity setup, but it is as far as I think I need to go for a pinball machine.
My overall look back at this mod is that it was pretty difficult and I might have been better off going external on the Sub.
Still, the idea of hauling around an external Sub with the pin does not appeal to me.
So, maybe this is OK.