Start-up Surge

[chance]

Just as a light bulb (most times) will blow when you turn it on due to the surge of power even tho it is not an over-surge of power.
My console is getting up in years, it's an Allen & Heath sigma 32 X 24 buss with automation. I have grown to love this board but due to the high electric rates here in California I really can't leave it on 24/7. Most of my work is on the weekends at this time which leads me to my question. Does anyone know of a device that will allow the power to pass thru in a gradual manner?
I have blown my power supply on this board a few times on power-up and perhaps a slower power-up might make it more reliable
Thanks in Him

 

[skippy]

Most power supply designs won't like that any better than the abrupt turnon transient- if you ramp the AC input over a period of time, it is not exactly clear what will happen internally. The worst case would be to get mistracking of the + and - rails, and end up lunching some of the opamps.

Arguably the correct thing to do would be to get the schematics for that supply, and spec out some nice closed-frame Lambda or equivalent industrial supplies to simply replace it- I'm doing that now for my Soundcraft Ghost. If the supply really is a weak sister OEM design, then going with a set of industrial-duty supplies would probably be a significant improvement in reliability.

Usually, though, startup inrush effects are easily handled by most pro audio supplies. Are you certain that that's what is doing in the supplies, or are there other long-term environmental issues (low/high input line voltage, poor airflow to the unit) that might be the _actual_ cause of the problem?

Example: Power amp blows up at turnon (literally: the PS electrolytics _explode_, bulging up the top of the case a couple inches...). Bad caps, right? Yes, but the reason they went bad was not that particular inrush surge, it was the 500 hours of operation at 175degF with a wad of dust-bunnies blocking the air intake that _actually_ caused it. The inrush that day just finished them off.

What can you tell us about the circumstances of the previous supplies' demise?

 

[chance]

Thank you for your reply. The last time it blew it blew the LM-273 IC and I think it's a voltage regulator. The tech said that he could build me a power supply from army surplus parts that would be rock steady (used for missle guidence systems.) for about $500.00. I just get worried every time I go to turn it on right before a session and I need reliability

 

[skippy]

Ahhh. LM723, I betcha- that's a very common IC voltage regulator used in a lot of supplies. And it has an Achilles heel: if you reverse bias it, it *will* blow up. Right away.

Circuit designs based around the 723 need to ratio the filter cap values very carefully, with the bulk of the capacitance on the input side if the regulator rather than the output side, and bypass the device with a *big* low-Vt diode to clamp the voltage and prevent reverse-biasing the regulator. When the power is turned off, it is very important that the output (load) side voltage decays faster than the input side, so that the 723 never sees a reversed bias (output voltage higher than the input)... It's best to select the bypass diode for a very low voltage drop- it has to be less than the breakdown voltage of the 723, which if I recall correctly is only about 600mV.

Betcha that's what blew it up: a turnoff transient condition, rather than a turnon problem! This used to happen all the time with techs who would add lots of additional bypass capacitance all over the place in mixers, increasing the load-side capacitance to well beyond the line-side capacitance- and then wondering why the supplies blew up... I'd also heatsink the thing all to hell and gone, since it doesn't have any thermal protection. But if your tech is good, he can come up with a supply design using one of the more current IC regulators, with built-in reverse bias and thermal protection. It sounds like you'd do well to go with a better design anyway: the 723s are downright ancient history these days...

But for serious bombproof supplies, I'd just shell out for the closed-frame industrial units. A pair of those for the + and - rails, and one more for the +48v phantom, and you'd have truly high-reliability supplies- and no more headaches or worries... I'd also add a balance clamp (a device that would shut down both supplies if the voltages exceeded some mismatch, like 250mV or so, just for good measure. That'll keep the opamps happy!