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Blow Fuse! [message #50561] Sun, 21 January 2007 08:33 Go to next message
Tuffer is currently offline  Tuffer
Messages: 6
Registered: May 2009
Esquire
Hi Guys,

I have been borrowing some of your crossover designs to try and improve my speakers which are used for mainly DJ use. I am using a Beyma 15G40 and PSD2002 in a vented box. I am using an Eminence PXB2:1k6 crossover.

I have created an lpad using two 15 ohm resistors and a 0.47uf cap for compensation. The resistors are wirewound and so may be inductive. See a pic below.

http://www.tuffer.co.uk/cross1.jpg

I have also disconnected one end of the 10uf cap on the crossover to give me a pseudo first order filter on the woofer.

http://www.tuffer.co.uk/cross2.jpg

Was quite happy with the results. Attenuation is perhaps a little strong. 8db might have been better.

On their first proper outing however both the fuses on the crossover board blew. They were being powered by a Behringer EP2500 so about 450w max to each cab. I wasn't present when they blew but was told that they weren't being driven particularly hard. I had been using these crossovers without the lpad or mod for some time without any problems.

Can anyone tell me if what I have done looks OK ? Could any of these changes increased the likelihood of overload and the fuses blowing or is it just one of those things that happen ? Should I just replace them with stock Eminence fuse or something else ? Obviously while the fuses have done there job and protected the HF drivers I wan't these cabs to be as reliable as possible.

Thanks for your help.


Re: Blow Fuse! [message #50563 is a reply to message #50561] Sun, 21 January 2007 12:47 Go to previous messageGo to next message
Wayne Parham is currently offline  Wayne Parham
Messages: 18791
Registered: January 2001
Illuminati (33rd Degree)

Are you saying the light bulbs blew? You may have presented a dead short or an open, both of which cause excessive current flow through the lamp. An open causes high current near the crossover frequency because of undamped shunt resonance. A short causes high current because it's a short. If you have am ohmmeter, you can check and see if either of these things is the case for you.


Re: Blow Fuse! [message #50564 is a reply to message #50563] Sun, 21 January 2007 13:00 Go to previous messageGo to next message
Tuffer is currently offline  Tuffer
Messages: 6
Registered: May 2009
Esquire
Yes the lamps blew. They worked OK for about 3 hours before they went, which makes me thinks perhaps they were just doing there job. If I replaced them with 3 12v 10w lamps in parallel would this be OK or is there a better way to protect the HF driver ?

Excuse my ignorance but where would I check for a dead short or an open ? I have an ohmeter.

Re: Blow Fuse! [message #50565 is a reply to message #50564] Sun, 21 January 2007 21:15 Go to previous messageGo to next message
Wayne Parham is currently offline  Wayne Parham
Messages: 18791
Registered: January 2001
Illuminati (33rd Degree)

Check across the tweeter output to see if there is a short or open condition there. You can replace the bulbs using a pair of 211-2 automotive lamps. The 211-2 bulb is designed for 1A at 12V (12 ohms) but cold resistance is only about 2 or 3 ohms, so the pair is about an ohm and a half. When used on the input of the crossover, there is no noticable impact until high current makes the filament glow, increasing resistance and limiting current. It acts like a compressor and protects the tweeter, but the compression effect is very natural sounding, you can hardly even tell. And it doesn't happen until the amplifier is generating about 300 watts. If you want to allow more power, you can change the configuration. But watch the voltage limit of the caps if you jumper around the lamps or use a different type of bulb or number of bulbs.


Re: Blow Fuse! [message #50566 is a reply to message #50565] Mon, 22 January 2007 02:10 Go to previous messageGo to next message
Tuffer is currently offline  Tuffer
Messages: 6
Registered: May 2009
Esquire
Thanks for that. Any idea what power would be required for the lamps to pop ? Would adding a polyswitch in parallel with the lamps be a good idea as this seems to be a solution used a lot ? Any idea what rating ?

Thanks for your help.

Re: Blow Fuse! [message #50568 is a reply to message #50566] Mon, 22 January 2007 12:34 Go to previous messageGo to next message
Wayne Parham is currently offline  Wayne Parham
Messages: 18791
Registered: January 2001
Illuminati (33rd Degree)

The current level that causes the lamps to glow brightly is 2 amps. They begin to compress the signal around half that level and fuse somewhere around twice that level.


Re: Blow Fuse! [message #50569 is a reply to message #50568] Mon, 22 January 2007 12:50 Go to previous messageGo to next message
Tuffer is currently offline  Tuffer
Messages: 6
Registered: May 2009
Esquire
So a polyswitch with a trip current of around 2.5a would be a good bet to go in parallel with the stock lamps ?

Re: Blow Fuse! [message #50570 is a reply to message #50566] Mon, 22 January 2007 13:08 Go to previous messageGo to next message
dB is currently offline  dB
Messages: 234
Registered: May 2009
Master
Hi Tuffer,
You said, adding a polyswitch in parallel with the lamps. You mean in series?
PolySwitchâ„¢ is a theoretic idea, but to design (and some factories do it) a xover you need to test the finished xover over the speakers. So, after you chose one from the tables with the right triger current you have to go to the lab and blow a few PSD2002 to know the right Threshold Current. It comes out very expensive and it might not work if they are not fast enough. You can see the lights from the lamps (xover light bulbs) working and it's a much more gentle process, and your clients don't lose the sound in the process because the speaker doesn't turn off.
Best Regards



Re: Blow Fuse! [message #50571 is a reply to message #50570] Mon, 22 January 2007 13:21 Go to previous messageGo to next message
Tuffer is currently offline  Tuffer
Messages: 6
Registered: May 2009
Esquire
Thing is the top end was lost as the lamps blew so I'm looking for a solution that is not quite so all or nothing.

Re: Blow Fuse! [message #50572 is a reply to message #50569] Mon, 22 January 2007 15:47 Go to previous messageGo to previous message
Wayne Parham is currently offline  Wayne Parham
Messages: 18791
Registered: January 2001
Illuminati (33rd Degree)

If you're considering replacing the bulbs with a fuse or breaker, you should know that they're going to have to be de-rated. They aren't very effective for tweeter protection. The reason is that currents in the tweeter circuit are high frequency and the fuse is designed for DC. It isn't designed to respond very quickly. I found that fuses had to be pretty heavily de-rated to be effective.

Honestly, bulbs work better for tweeter protection than any fuse or breaker I've seen, as they compress the signal rather than fusing. The bulbs only blow if the signal is much higher than expected or if the load is open or shorted. I suggest that you get out a meter and start quantifying things. Check the voltage level of the amplifier when the bulbs blow. Those bulbs don't cost much, so it isn't any trouble to make a few tests that push them to the point of destruction. Also, check the impedance across the crossover's tweeter output. Check the voltage across the tweeter output. Once you know more about what's happening, you can make a more informed decision how to correct it.


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