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Old 22nd March 2009, 04:21 AM   #1
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Default Converting Active monitors to passive. Help!

Hi everyone,
I'm new to the forum and I signed up because I could use some help.

My problem is that I bought some Behringer Truth 2031A Active monitors. (I know... serves me right for buying Behringer. They're just so cheap I had to roll the dice.) The problem is that they pick up the radio. Even without any cables attached they play the radio so I know it's not a ground issue or loose cables or anything like that. I've even tried opening up the amp on the back and installing RF chokes everywhere there was enough wire to get a few good wraps in. While I had the cabinet and amp open I noticed that the amp construction seems sub par and all of the cables are unshielded. The cabinet and the drivers seem to be decent and they actually sounds pretty good when you're playing music loud enough to cover up Star98.9 FM.

I'm thinking about removing the amps and replacing them with a decent 2 channel amp. I think I need something that can push a 4ohm load with 150watts peak. I guess less watts would be fine and I should probably double check the ohm load. The passive models take 4ohms so I'm assuming the active models use the same parts.

I think I have all the skills I need to pull this off. I'm a decent woodworker and I can solder. But I don't know what I need. I assume I need an amp with a crossover, but I've never used one before so I don't know what to look for. Adding a sub later would be nice so I guess I should get an amp that can drive a sub too. I'm running the speakers through a mixer so XLR inputs would be ideal. I'm assuming that I can build whatever cables I need to attach the amp to the speakers. Probably 1/4" (?). I dunno... that's why I'm here!

Or maybe I should just sell the Behringers and get something else. But the budget's tight. $300 max without having to save up. And that will only get me... another pair of behringers.

Thanks in advance!
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Old 22nd March 2009, 11:50 AM   #2
C37 is offline C37  Belgium
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Hi there.

A friend of mine lives close to a radio broadcast station and gets a lot of fm through his actives exactely like you do. Do you live near an FM brodcasting station?In many actives the electronics are not fully enclosed in a metal box but only placed on a metal sheet that acts as an heatsink.
you might want first to decouple the bridge rectifier in your speaker so that it acts less as an FM antenna with small values capacitors ( 10 nF ceramics are fine if i remember well, but please check this) and if this does not solve it , to shield the electronics ( think of a faraday cage).
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Old 22nd March 2009, 03:09 PM   #3
gz76 is offline gz76  Australia
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Adding a faraday cage is the first thing that comes to mind.

Or, line your walls with tinfoil!
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Old 22nd March 2009, 05:20 PM   #4
Pano is offline Pano  United States
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What happens if you play Star 98.9 on the speakers? Is there an echo?

Re-doing these is going to be a challenge. Not only do you need an amp, you need to build a crossover. Probably a passive crossover if you want it in the speaker box.

If you know, or can find the crossover points and slopes already being used, you'll be ahead of the game. It's not going to be a small task to do this "right." Can be done, but it will be real work.
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Old 22nd March 2009, 05:48 PM   #5
gedlee is offline gedlee  United States
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You certainly should be able to measure the crossover and EQ in the existing amps and then try to build that in passively or go with an active crossover with external amps, but that will be beyond your budget. Measuring the amp and then doing a passive design is not too hard if you have the tools, but if you don't then passive is probably beyond what you will be able to do.
Earl Geddes Gedlee Website
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Old 22nd March 2009, 09:55 PM   #6
C37 is offline C37  Belgium
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Possibly the only benefit by going passive is that you will put the amp ( assuming you want to salvage those from the behringers) into a metal box, outside the speaker, thus shielding them.

I have Genelecs 1031A and tannoys AMS8a. none of these are shielded, see my previous post. I do not have FM on any of these speakers, probably because i live far away enough from a broadcast station, but one day I was shoked to hear loud noises form my genelecs seconds before receiving an sms ( i was very close to the speakers). Indeed my mobile makes it trip if i am close enough. Since then i lined the interior of the recess where the electronics are with thick house aluminium foil that is grounded and linked to the amp plate. I can now make a mobile phone call hands free on top of one of my speakers without hearing a thing through them. Should work with your fm problem and at least that's cents of your money if you want to try...

PS: of course make sure you don't create any short circuit doing this!!
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Old 22nd March 2009, 10:20 PM   #7
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open the box remove all the dampening material, then glue 2 layers of tinfoil inside the whole box then redo the dampening. =)
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Old 24th March 2009, 03:55 PM   #8
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Thanks for all the suggestions!
I'll try and hit all the points here in one message.

I do live near a radio tower. I can see it from my front yard. I have a lot of other audio gear in the space (it's a recording studio) and the only time anything else picks up the radio is if there's a loose cable. According to the internets the speakers I own are notorious for picking up the radio so I think I'm dealing with a design flaw or just poor build quality... which Behringer is kind of known for.

I have tried a faraday cage. I wrapped a cardboard box in tinfoil and placed a speaker in the box. It was a good fit but it didn't really help. I guess because the face has to be open to use the speaker there's still an a place for the radio waves to get in. But the good news is that if I wear the box like a hat the aliens can't read my thoughts anymore. Which is comforting...

If I coat the interior of the cabinet with tinfoil that still leaves the amp unshielded. It's attached to the back and if I'm going to use it I'll need to access the power switch so boxing it in is awkward at best. I'd also be a little concerned about ventilation. But I might be over-worrying about that.

I could remove the amps and place them in a faraday cage that I can enclose and use shielded cables to run to the cabinet. Right now that's looking like a good move.

But I guess I'll start by lining the interior of the cabinet with a think layer of tinfoil and just see what happens. But I got a hunch that the problem is in the amp on the back.

Quick question about grounding the tinfoil to the amp plate. If I did that wouldn't the tinfoil just act as an antenna and leak a signal back to the amp through the ground? Because I think the electronics are also grounded to the plate.

If the tinfoil doesn't work I'll do some research and try to understand C37's suggestion of decoupling the bridge rectifier. You lost me there but I'm going to assume that it was a great suggestion since you obviously know more about electronics than I do. Unless you just made that up. In which case you totally got me!

Thanks again!
I'll post the results soon.
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Old 24th March 2009, 04:46 PM   #9
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My understanding of shielding is that the shield must be grounded. Did you ground your faraday cage?

Another possibility is to pull the amps off the back of speakers, install them in an appropriately shielded enclosure, and install normal speaker terminals in the speaker.
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Old 25th March 2009, 06:39 PM   #10
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Thumbs up Theoretical Sucess!

I did not ground my faraday cage until about 10 minutes ago.
And it turns out you do need to ground it.

I put the loudest radio... um I mean speaker... in the tinfoil covered cardboard box and ran a wire to the amp plate and when I touched the other end to the tinfoil >boom< no more radio! That's promising!

Now I just need to decide if I should remove the amp and build a new home for it or just shield the back of the speaker cabinet. I'll report back when I get something working.

Thanks everybody! I know there's a lot of other people out there with the same problem to this particular model of speaker so hopefully this thread will be the gift that keeps on giving. I'll try to document the steps I take to fix it for those guys.

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