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 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.

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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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.