AMB Laboratories DIY Audio

[amb]

I would replace the regulator with a 7812. You could keep R1 as is -- the LED may be dimmer, but probably not by much.

[oneplustwo]

I replaced the regulator with a 7812. LED is indeed dimmer but no big deal. I verified the relay works fine, but still, I get some sort of funny arcing inside the relay. I'm not sure if this is because the relay was damaged somehow the first time. Or if this problem would have happened before anyway. Guess I should just replace the relay first? Or is there another step I should take first?

[amb]

I think there might be a timing-related issue.

On my build, both the SSR (that turns the amp power on) and the electromechanical relay (that switches the bleeder resistors) are driven together by the ε24 power switch driver circuit. So they mostly switch simultaneously (the electromechanical one just a hair slower due to the physical movement, but then the SSR may also delay very slightly because it would switch only at the AC waveform's zero-crossing point). Since it takes a small amount of time for the transformer to "set up" its magnetic core and for the large amp caps to charge up, the bleeder resistors are switched out first.

On your build, your electromechanical relay is powered up by a σ25 off a different transformer secondary. This means that when the relay coil is activated, the transformer will have already done setting up its magnetic core and the σ25's bulk capacitor is already charged up. This also means that the big amp bulk caps are also already charging up, at this point there is probably a large current flowing through the bleeder resistors. The delay is still very short, but might be long enough to make a difference.

Now that's a total guess. Maybe your relay is wired wrong or otherwise damaged, I don't know. If you have a way to check the relative timing of the events (with a multi-channel digital storage scope running in single-event trigger mode, you could observe the rampup of the main amp rail voltages and the relay coil voltage to see this) it would bear out whether my guess is true. A simpler test is to measure the voltage across the bleeder resistor while powering up. I don't know whether the DMM will respond fast enough to register a reading, but you shouldn't see a big voltage spike.

[oneplustwo]

I'll try to see if I can measure a voltage spike. My guess is you're 100% right. Would a separate trafo for the sigma 25 help this as perhaps it would "saturate" more quickly? I'm pretty sure the relay is wire correctly. There's not a lot that can go wrong. And the system as a whole did work sporadically.

Is there an easy way to manipulate the timing of events to fix the problem? Or is there another relay that would fix the problem assuming timing is indeed the issue? Perhaps one with a higher coil voltage requirement?

[amb]

I'll try to see if I can measure a voltage spike. My guess is you're 100% right. Would a separate trafo for the sigma 25 help this as perhaps it would "saturate" more quickly? I'm pretty sure the relay is wire correctly. There's not a lot that can go wrong. And the system as a whole did work sporadically.

If you could determine that the problem is indeed caused by a timing issue, then you'll need to fix it by making sure that the relay disengages the bleeders before the main bulk caps begin to charge up. I am not sure if a separate small trafo would do the trick. But changing to a AC 120V relay that's triggered at the primary side of the transformer (after the power switch) should work.

Is there an easy way to manipulate the timing of events to fix the problem? Or is there another relay that would fix the problem assuming timing is indeed the issue? Perhaps one with a higher coil voltage requirement?

I don't think so... how would changing the relay in this manner help?

[oneplustwo]

That's a great idea! Why didn't I think of that? Is there a reason a AC 120V relay triggered from the primary side after the power switch is less desireable than my existing solution? Seems like less complexity given there's no need for a sigma 25. I should have thought of it before!

Here's one... should work I think: http://www.mouser.com/ProductDetail/Omr ... 7R3Q%3d%3d

[amb]

Yep, that should work fine.

[oneplustwo]

I got the new relay installed and it works like a charm. Seems like a simpler execution. Now I have a spare sigma 25.

What should I expect the voltage to be for the unregulated sections unloaded? I measured almost 90V! (I'm still waiting for linux-works to return one of my sigma 22's so I can go through the full setup. I'll probably try to get some more casework done in the meantime.)

[amb]

90V = +/-45V which is about right when the amp is mostly unloaded.

[oneplustwo]

Ok, it's time to finally get this thing finished! I'm getting tired of the big trafo mocking me every time I walk by it! Any tips on the next setup steps? Seems pretty straight forward but I thought I would reach out to see if anyone had any "if only I knew..." kinds of moments after they finished their testing.