Home Theater Forum and Systems banner

21 - 33 of 33 Posts

·
Registered
Joined
·
2,604 Posts
Re: My Behrenger UB802 mixer won't power on

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
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...
 

·
Registered
Joined
·
16 Posts
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.
 

·
Registered
Joined
·
2,604 Posts
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.
 

·
Registered
Joined
·
28 Posts
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.
 

·
Registered
Joined
·
16 Posts
"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.
 

·
Registered
Joined
·
28 Posts
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.
 

·
Registered
Joined
·
2,604 Posts
"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.
 

·
Registered
Joined
·
2,604 Posts
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...
 

·
Registered
Joined
·
2,604 Posts
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...
 

·
Registered
Joined
·
16 Posts
I really enjoy the level of thoughtful discourse around here. :D

I need to brush up on the subtleties of grounding- mainly I have to worry about multi-kilowatt radio transmitters with big lightning rods (aka antennas) attached...

Polyphaser has some good grounding information, mainly with an eye toward protection from direct and very near lightning strikes. You guys (and gals) may enjoy looking at their site.
 

·
Registered
Joined
·
28 Posts
Decades I was a engineer or C.E. at several stations. Lightning and tall towers was always interesting. One of my buddies first job was summer replacement at WLW, that monster and it's monster antenna was even more interesting.
 

·
Registered
Joined
·
1 Posts
Hi azuaro, Would you mind tell me more about using the transformer from Hosfelt on UB802. Like how to connect.
I am a newbie here.:wave:
 
21 - 33 of 33 Posts
Top