I don't recommend alligator clipping speakers of any kind to the speaker of a pin. What you are doing is changing the impedance that the pins amplifier "sees". The effect will greatly vary depending on games amplifier circuit, speaker impedance you are clipping to, and the subwoofers input impedance. This can also damage your game. The best way to avoid this is to install a speaker to line level converter and run the line level out into the subwoofers line level input. This will effectively isolate the games amp from the subwoofer and should clean up your noise. I'm using one on my Stern Harley Davidson I bought on Amazon for less than $20. Its a Pyle PLVHL60. Its a stereo converter but I am only using one channel (mono) for my subwoofer.

Yes. I just taped off the unused input and output channel wires to prevent any accidental shorting.

So the other channel can be used for a second pinball machine? Would this then be mounted behind the sub?

I would respectfully disagree with the need for a line level converter. It's not that it's a bad idea it's just not necessary. The high level in on your Polk is a fine way to connect your sub to your machine. I would however solder the terminals or at the very least use female spade connections to the machines speaker terminals and delete the aligator clips. In my 20 plus years as an A/V contractor what your most likely experiencing is a dirty amplifier or volume control in your subwoofer. I've seen this many times where the sub starts to make odd random noises even when there is no signal active until almost inevitably it starts to screech and thump constantly at 3am waking you up and scaring the hell out of you. Try this...disconnect your Polk from the machine but leave it powered up and see if it acts up. It may take a while and it may be random but this is more common than it might seem.

Yes, this is possible but if both machines are being played at the same time the sub will see both signals and producing bass from both machines.

https://pinnovators.myshopify.com/products/pinsub-spike-spike2-stern-kit
The best way to hook a sub up

manadams I had this same issue with a couple of PSW10's that I had purchased from Amazon a few months ago... Amazon randomly had a huge price cut so I jumped on it and purchased 2. Got the subs hooked up using the pinnovators kit on my Spike 2 games and was getting feedback just like you. It has nothing to do with the alligator clips. Un -clip them from the sub but leave the sub powered on and you'll notice that the feedback is still there. Its the actual sub thats defective.

I ended up returning both of the subs to amazon and claimed them to be defective. I purchased 2 more a couple weeks ago (same model/brand but higher price) and I have no issues or feedback now.

My best guess is that Amazon got some refurbished/returned units and price slashed them to get rid of them but this lot of subs was obviously defective and they didnt say anything or test them out... super shady. I would return them if you can.

I had one go bad as well, putting out a wind noise with nothing powered up but the sub. Polk warrantied it and send a new one out.

When you directly connect to the high-level input, you're essentially putting the two woofers in parallel. The cabinet woofer, at least in the Spike 2 Stern's, is 4 ohm. So whatever woofer you connect to it, the impedance ends up being lower.

The Spike 2 uses a TI TPA3123D2PWP audio amp chip, which is rated for 4 ohms. And they run it at 30V, which is pretty much at the upper limit. The backbox LED's are right next to it too, so it's already very toasty. Adding another woofer might be asking a little too much.

The low level adapter is really the proper way to connect an active sub, better safe than sorry. Plus many low-level adapters act as a BALUN. This is important since Stern's woofer chip is configured as a balanced output, not single-ended and they can also break the potential ground loop.

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I've used a bunch of the SNI-35 adapters, then run those into a mixer which goes to 2 active subs. Don't mean to steer people away from Pinnovators products. I'm just an electrical engineer, so I tend to 'roll my own'.

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Audio chip link: https://www.ti.com/lit/ds/symlink/tpa3123d2.pdf?ts=1643909062840&ref_url=https%253A%252F%252Fwww.ti.com%252Fproduct%252FTPA3123D2

A few things come mind:

You just might have the gain (i.e. volume control) turned up too high and it's just picking up some noise. Try turning the pin's volume up a bit, and lowering the subs volume.

If the low level input aren't terminated, they could be picking some noise. Try an RCA with maybe 1K resistor to terminate the inputs. That keeps them from 'floating'.

Ground loop issues

I would check the speaker wire to make sure it's not near any other wires that might be running high voltage. Make sure it's as isolated as possible.

Just to note, I run almost exclusively PSW10s on my modern games (except one Sony 8" that I got super cheap). One sub per game, (I think 7 games total) most of them, if not all, are clipped to the cabinet speaker with alligator clips. Needless to say, I have no issues with noise. B/W, Spooky, SAM, Spike 2, all are good. If you have 2 games on one sub, disconnect one game to see if that helps.

I've had a few games where the subwoofer would feedback loop with the cabinet speaker when the game was off. Because the gain is turned up so high on the external subwoofer the cabinet speaker acts like a microphone and you get a feed back loop. Slide the sub away from the game and see if the feedback goes away. When this is the case I'd just wire the subwoofer on a power strip to cut power to both.

I don't know anyone who would want to run a subwoofer with the game off. Sure you can manually power both the sub and game on and off every time you use them, but that gets old quick. Use a power strip if your sub is external. Mine is internally mounted, wired to mains power to turn on and off with the games switch.

I've been a professional audio technician and now supervisor for over 30 years.
We only alligator clip circuits when running tests. Permanent connections soldered or connectors are the way to go. Alligator clips for permanent connections can be noisy, fall off, etc. Just not a very professional way to connect electronics, to me thats kind of hillbilly, just a shade better than taping a wire to your speaker terminals,
but that's just my opinion shared with many in this field. To do it right for an external sub, I mount panel RCAs on the bottom of the cab. Easy to unplug and move if you need to.

+1, i buy these for all of my machines. so inexpensive and completely reversible.

When working on games I sometimes flip the power off and get a fun feedback hum. This is with using the line level converters or the built in subwoofer line level inputs. Doesn't matter. Depends on the subwoofer though.

Of 32 games with subs on them it mostly only happens on gottlieb games.

My guess is it’s the subwoofer itself. These polks are good for the price when they work but a quick google search will get into how the amps go bad. I have the 12” version of this subwoofer and out of nowhere it had a heartbeat like sound that was there no matter what. Disconnect from everything and power up sub.

I wonder if the sub would do this all by itself if you just turn up the volume? Nothing attached to the low level inputs, just the sub, volume all the way up. If that's the case, terminating any unused inputs with a resistor might help.

I sort of think the subs aren't bad (the amp part, anyways), the amp is just picking up something (i.e. 60Hz) and just amplifying it. I guess it could be some 60Hz from a so-so power supply filtering tho.

Just an educated WAG on my end.

Any chance the pins and subs are on a different breaker? Just wondering if they share a common ground or not.

Same WAG here really. Again they aren’t bad I just know the amps can go bad. I am actually going to try and shotgun the 12” I have in hopes of using it on my next game. If it is consistent from pin to pin my guess is the amp even if it works OK on some of the pins.

Well...if you connected a passive speaker directly to the pinball machines cabinet speaker in a parallel configuration then yes, you would reduce the impedance by half and damage the pinball machines amplifier. In this case the amplified subwoofer's high level input is being connected to the pinball machines speaker as designed and is not affecting the impedance of the pinball machines amplifier. Think of it this way...if what your saying is absolute then there would be tons of blown pinball machine sound boards. Paper vs. real world.

I don't disagree with your comment, JB. I just prefer to not find the out the limit on the audio chip, that's all. Like I said - better safe than sorry (i.e. $10 adapter vs. $350 Spike 2 board). I know heat can be an issue. ​

My Munsters woofer audio is fine 100% of the time when I first turn it on. After it's been on for a while, if I turn it off (letting the power supplies discharge), then back on...the audio chip has a problem (it's been replaced too). It's only a +85C chip and the LED's sink a lot heat into the board at 100% brightness and there must be residual heat in the board - enough to cause a problem. After doing some tests, it's definitely heat related. Could be as simple as using cheap caps (Y5V vs. X7R, X7R's being temperature stable) around the amp. People don't usually turn the pin off, then back on - so they typically don't see the problem. But I'm aware of others that have the same problem.

So chances are, with a low impendence load, that probably makes the chip run warmer than normal. Even in TI's data sheet, they only run the chip at 27V and Stern is running it at 30V, so that aggravates it more. And if the sub's woofer is at ground, that's definitely not good since the pin's woofer '-' output is not ground. Some of the older pins, the woofer '-' is at ground, so less of an issue there.

I realize I'm sort of mixing two potential problems (heat issue, maybe a low impedance load), but in the end - I think the Spike 2 woofer amp is being run a little bit 'on the edge' thermally. Maybe not to the point of failure for most, but for $10, I'm going to eliminate one potential problem (low impedance load) that could make the heat worse. I don't know how the manufacture's handle a high level input (i.e. what impedance it presents to the pin's woofer?).

For what it's worth, here's a FLIR SnipIt of the left side of the Munsters Spike 2 with the LED's at 100%. Woofer chip is circled. I don't mean to imply the temperature readings are 100% accurate, it was just a quick test I ran awhile ago. Rather toasty. Since then, I've turned all my Spike 2's backbox lighting down to around 30% to help keep things cooler.

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Just to expand on this, I've sold hundreds of my subwoofer cables (alligator clip style), and I've never had a single customer complain that the setup damaged one of their machines. There have been a few isolated incidents where a certain combination of games/subs caused an unwanted hum or other type of sound, but that's it. This obviously doesn't prove without a shadow of a doubt that there isn't an element of risk, but I think the anecdotal evidence strongly suggests that it's a safe way to add a sub to your game.

Gweempose,

The Stern Spike 2 woofer amplifier (TI TPA3123D2PWP) is configured in a 'Bridged Tied Load' configuration (sometimes referred to as a 'balanced'), meaning it's a differential output. There's a 'Out +' and a 'Out -' (see the first SniptIt from the Stern schematic), there's no ground return on the speaker. This means the 'Out -' is not the same as ground. When the output is referenced to ground, it commonly called 'Single-Ended' (or 'unbalanced'). There's another SnipIt (2nd one) comparing a balanced and single-ended output. The Spike 2 backbox speaker amps are actually configured Single-Ended. One side of the speaker is grounded. Being balanced (in the case of the TPA3123D2PWP) also means there's a DC voltage on the speaker terminals when measured to ground. There's no DC blocking caps since the same voltage is on both speaker terminals. Not a big deal if the sub has an internal coupling cap.

So a common way to connect a balanced output to a single-ended input (or vise versa) is thru a BALUN (3rd SnipIt), which means BALanced to UNbalanced. A BALUN, in this case, can be as simple as an audio transformer. There's other ways to make one too.

So back to hooking up external speakers. I'm definitely not saying there's an issue. I honestly don't know what's inside an external sub, but that's sort of my concern, at least on some pins. One sub might do it one way, the other does it completely different. Same for the pin's amplifier. The adapter simply just removes that unknown. If the sub's input is balanced, then no problem. Unbalanced - still no problem. Sort of a win-win. But generally speaking, connecting balanced directly to single-ended probably is a little 'iffy'. Another plus to a BALUN is it can help with ground-loop issues, because it can break the the ground path (depends on how it's constructed).

The older Williams (and probably others) used single-ended audio amps. At least the schematics I recall looking at, anyways.

Lastly, I don't want to sound like a 'know it all', truly wasn't my intent. More of just what I see from examining the schematic. And from your experience, you haven't seen any issues. I don't discount that. I'm not even an audio engineer. But I am a RF engineer (retired), and all my high-power amp designs involved designing BALUNS in between stages, so this section of the Stern schematic caught my attention. My lower power stages were single-ended, then switching over to balanced for the high-power amps.

Just my 2 cents!
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Sorta sounds like it was a ground loop issue. I mentioned earlier that I use an inexpensive mixer, then run all the pins into the mixer, the mixer then drives two subs. Nice thing about a mixer is now each pin has a woofer level control.

I think a 4 ch mixer was about $25 on Amazon.

I'm having the same issue now. Only on spike machines. Turn on 1 game (left high level input) and I get noise out of the head speakers from the other machine (right high level input) which is off.

Do your google on inexpensive Polk sub amps.

Spolier: They're junk.

My solution:

Convert sub to a passive sub box by opening it up and wiring the speaker direct to a couple of the binding posts.

But a cheap mono amp designed for use w subs (Amazon $80 and up). I bought Fosi Audio MO-3.

You may need some adapters to hook up to the amp's RCA inputs.

All good for now, let's see how long the little amp runs.

I have 6 or your clips hooked up to Polk subs right now and haven’t had any issues. Thank you for providing these!