I think you'll be fine if you run a ground wire the same size as your amp output wire (16AWG?) from one of the speaker connector grounds back to your relay board ground, and then for extra protection run that ground to both open relay contacts to help eliminate arcing. I think that will work fine.
Note that there are skeptics that don't think a relay can disconnect an amp fast enough to avoid destroying a speaker if the DC voltage goes to one of the rails due to amp failure. My vintage amp destroyed one of my speakers years ago when it failed, but there was only a fuse to "protect" the speaker (and feeling slightly guilty, I got the speaker replaced under warranty 😱). I'm hoping the relay will save my speakers if my amp fails, but if it doesn't, it will be a good excuse to replace them (they are vintage also).
If you look at the official schematic here https://cdn.shopify.com/s/files/1/1006/5046/files/Speaker_Protection_Board_Schematic.jpg where the transformer connects (U1.1 & U1.2) one of them (U1.1) shows it connected to GND. Run an additional wire from that terminal to your main GND and you're all set. No modifications needed. They work very well. It does seem a slight oversight in the documentation and it took a little starring at to figure out how get it working.
TJ
TJ
That design appears to be from 2013. More recent relay protection boards I've seen have implemented the improvement of grounding the amp output when the relay switches, including my $8 Ebay board. You'd have a better chance to save your speaker if you make the mod.
Thanks for the advice, i have decided to live dangerously and connected both new speakers after much testing with a junk speaker. It all worked fine, but there's no dc protection without a ground ref. I'll hook the test speaker up and play with grounding tomorrow. I haven't grounded to the chassis yet with a thermister. Might not need to as i don't hear any hum when a signal is connected.
mikedrz wrote he was using the DIY Audio store board and wanted to know how it references ground without modifications. The link is to the official schematic of that circuit.
TJ
TJ
Getting closer!
Here are a few pics of what I've got done so far. Mounting the transformer and building the softstart and speaker protection board actually took me longer than assembling the 2 HB amp modules. The non newbie friendly documentation of the simple soft start and speaker protection left me scratching my head for days as well as multiple orders of alternate parts.
Finally I am almost done, but now if I want volume control I'll have to build a preamp. I don't know if I have the patience to do that at this point, I might just buy a ready made one commercially or off ebay etc.
Here are a few pics of what I've got done so far. Mounting the transformer and building the softstart and speaker protection board actually took me longer than assembling the 2 HB amp modules. The non newbie friendly documentation of the simple soft start and speaker protection left me scratching my head for days as well as multiple orders of alternate parts.
Finally I am almost done, but now if I want volume control I'll have to build a preamp. I don't know if I have the patience to do that at this point, I might just buy a ready made one commercially or off ebay etc.
Attachments
Thought I was done, I put in the CL-60 and it really quieted everything done at idle, but now the left channel is hissing where as the right is dead silent. I tried moving the wires around didn't seem to make much difference. However when I touch the dc offset pot with my fingers the hiss goes away. Not sure what changed aside from adding the cl-60. Taking the speaker protection ground off makes no difference.