My Behringer UB802 mixer won't power on - Page 3 - Home Theater Forum and Systems - HomeTheaterShack.com

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post #21 of 33 Old 11-03-09, 09:14 PM
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Re: My Behrenger UB802 mixer won't power on

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bobdole369 wrote: View Post
I just picked up a UB802 for a great bargain - no AC adaptor. Luckily I'm versed enough to make it work. The unit powers up and works just fine with the same 18.5V rail on both inputs. However just as an exercise I'm intending to build an SG2525 based forward boost converter to go from 12V -> 2x 18.5V, just as an exercise in learning.
Should be fun
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I did put an MOV (130V) on the input AC lines, and bypassed both lines to ground with small value ceramic caps to shunt transients. That's all that's really necessary, the mixer makes the bipolar DC and filters it.
Is that 130V RMS on the MOV? Ceramic caps won't do much for transients... anything substantial will run the risk of blowing them... also, MOVs deteriorate with each time they fire...
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post #22 of 33 Old 11-04-09, 09:42 AM
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Re: My Behrenger UB802 mixer won't power on

Yes, 130 RMS- a GE MOV designed for 120VAC surge suppression. Ceramic caps are intended to have a low impedance for signals at very high slew rates (RF or other fast spikes) and it would take a sustained dose of high frequency input to damage one. I've never seen a cap go out in this duty, although I have seen MOV's break down. The cap helps protect the MOV by shunting some energy before the MOV conducts.
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post #23 of 33 Old 11-04-09, 07:18 PM
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Re: My Behringer UB802 mixer won't power on

I was more concerned about overvoltage on the cap... but I think I see now, the MOV helps protect the cap which in turn takes stress off the MOV... I guess that'll work the first time, but as I said, there's reams of data showing that after the first hit the MOV fires at a different voltage the second time... I guess you're mitigating the effect somewhat, but it's absolutely inevitable.
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post #24 of 33 Old 11-05-09, 07:35 AM
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Re: My Behringer UB802 mixer won't power on

It's probably a bad idea to connect either MOV's or capacitors from the AC power lines to the ground. ExactPower has a white paper "The Truth" on page 14 the explain exactly why it's a bad idea.
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post #25 of 33 Old 11-05-09, 10:46 AM
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Re: My Behringer UB802 mixer won't power on

"It's probably a bad idea to connect either MOV's or capacitors from the AC power lines to the ground. ExactPower has a white paper "The Truth" on page 14 the explain exactly why it's a bad idea."

I just downloaded the paper, and it's page 17 in the latest edition... However, I don't see what their objection is, exactly, as every surge protector and power strip with integrated surge suppression uses exactly this technique.

As far as over voltage on the cap, I usually use 600V caps. It's a very common voltage rating for small ceramics. Any signal over that voltage is very likely to be the edge of an inductive kick back somewhere, and won't last for more than a few microseconds.

Nearby lightning strikes call off all bets.
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post #26 of 33 Old 11-05-09, 11:04 AM
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Re: My Behringer UB802 mixer won't power on

The page numbers seem to drift around.
They write:
A disadvantage with using this design on branch circuits is that the MOV’s capacitance couples higher frequency line noise to the safety ground wiring.
Jim Brown of Audio Systems Group also touches on this subject in some of his papers. Maybe it was "Poer and Grounding for Audio/Video Systems". Or maybe it was in one of the Power-Point slide shows.
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post #27 of 33 Old 11-05-09, 08:23 PM
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Re: My Behringer UB802 mixer won't power on

Quote:
jimfrank wrote: View Post
"It's probably a bad idea to connect either MOV's or capacitors from the AC power lines to the ground. ExactPower has a white paper "The Truth" on page 14 the explain exactly why it's a bad idea."

I just downloaded the paper, and it's page 17 in the latest edition... However, I don't see what their objection is, exactly, as every surge protector and power strip with integrated surge suppression uses exactly this technique.

As far as over voltage on the cap, I usually use 600V caps. It's a very common voltage rating for small ceramics. Any signal over that voltage is very likely to be the edge of an inductive kick back somewhere, and won't last for more than a few microseconds.

Nearby lightning strikes call off all bets.
Agreed. Except for the 600V... I see 500V, 1kV etc all the time, but 600? Ceramics ARE great for noise suppression, but in UL approved IT equipment, they're supposed to be able to handle surges to a few kV, and usually are specially rated and approved components specifically for that purpose.

Though I haven't read the paper as I didn't want to give yet another company my contact info, I suspect the objection to these Y-cpas is that they introduce ground currents in to the line, which can cause noise, and can be a safety hazard. UL also has specifications for the maximum amount of this leakage that's allowed.
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post #28 of 33 Old 11-05-09, 08:38 PM
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Re: My Behringer UB802 mixer won't power on

You can download the paper without registering. It's the right hand box.
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post #29 of 33 Old 11-06-09, 08:06 AM
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Re: My Behringer UB802 mixer won't power on

Aha! I had found the paper by following a link from a search... it didn't give me the options. I see them now. I'll download both, but I don't know when I'll have a chance to read them...
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post #30 of 33 Old 11-06-09, 08:18 AM
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Re: My Behringer UB802 mixer won't power on

Well, as soon as I saw the cover I realized I'd read it before and had a copy on my machine. I have to say that my experience with some very noise sensitive equipment using MOVs at the AC input for surge protection, says that they're HF noise objections seem exaggerated.
That being said, is it better to do the protection at the service entrance? Of course. But safety issues aside, and assuming we're not talking about an extreme of putting banks of MOVs into every device you have...
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