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Old 15th May 2002, 05:23 AM   #11
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Quote:
Originally posted by swede
I would go for a decent DC protection circuit. You'll find one on Rod's web-pages. It's a cheap insurance when one fuse fail and burn up the transistor on the other rail.
Two thumbs up for Rod's relay-based protection circuit. Built it and works great.
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Old 15th May 2002, 06:07 AM   #12
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Has anybody tried a procection cricuit that upon sensing DC: the circuit disconects the power supply rails instead of the amplifier outputs?

In other words: put the relay conatcs between the power supply filter caps and the amplifier. Certainly removes the relay contacts from the direct signal path.

The only catch I can see is that you would need to make sure once the circuit has been tripped, it must be reset. Otherwise you could get a on-trip-off-on-trip-off-on-trip-off-on cycle. Getting up to push a reset button is better than replacing drivers or outpout devices.

Aud_Mot
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Old 15th May 2002, 06:27 AM   #13
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Hrmm, it should do the trick, but why would you like to do that? To remove the relay from the signal path?
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Old 15th May 2002, 01:46 PM   #14
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Default fuses

From another thread that I read and came across Kristijan and Panos post a soft start circuit last week. I made the PCB and bought all the parts then I saw the K4700 and K4701. The PCB that I made does not have the DC protection that is why I thought about using the passive kit in combination. But I think Trwh has a good point about using two K4700, I can save the PCB and parts for other projects in the future.

Actually would someone please enlighten me that why when the fuse blown in the supply rail it causes harm to the speaker. I thought that is what the fuses are for to break the circuit and stop doing any more harm to the rest of the circuit(ie speaker as well).
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Old 15th May 2002, 03:05 PM   #15
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Well Chris ma

The fuses are there to protect the circuit in case of transistor malfunction...

BuT, if only one of the two rail fuse break and the other stay intact then there will be a rude voltage swing at the output. Normally the speakers see AC voltage coming at them now they see almost the full voltage of your intact fuse rail being thrown at them. In no time your crossover and speaker elements will cook (quite a particular smell believe me!).

Aud_Mot: I really like your idea to cut the rail of the amplifier instead of the amplifier output to the speaker. This could be an even better way to protect the output transistor. Do you think rod protection circuit could be used for that? (ESP project 33)

-Simon
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Old 15th May 2002, 03:58 PM   #16
paulb is offline paulb  Canada
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Quote:
Originally posted by Aud_Mot
In other words: put the relay conatcs between the power supply filter caps and the amplifier. Certainly removes the relay contacts from the direct signal path.
The power supply IS in the signal path, so this will probably make no difference. This is often misunderstood.
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Old 15th May 2002, 04:08 PM   #17
Geoff is offline Geoff  United Kingdom
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A solution which avoids any relay contacts in the signal path or supply rails is to use a regulated supply and to shut down the regulator by earthing the adjust pin (or equivalent for discrete circuitry) when the dc offset is excessive.

Geoff
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Old 15th May 2002, 08:51 PM   #18
paulb is offline paulb  Canada
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That's a good idea. As it happens, I'm in the middle of designing a power supply and I can easily incorporate this. Thanks, Geoff.
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Old 6th June 2002, 05:07 PM   #19
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Default DC protection kit and fuse?

Hi,
I have built the Velleman K4700 soft start and DC protect kits for my current project JLH amp mono blocks. My question now is that do I still have to have in line fuses (slow blow 5A) for the mains and in particular the (fast blow 3A) fuse in-line with the speaker output?

If the fuses are still required than any suggestion of the hi quality fuses that I can use and purchase. I would love to not have the fuse in the output there since it is in the signal path and the dc protect circuit may already degrade the sound some what.

Thanks,
Chris
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Old 6th June 2002, 07:15 PM   #20
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NU_NRG,

My idea is to shut down both rails. This would prevent the output from swinging to the other rail, and you hoping that other fuse blows before your speakers and/or cross overs.

The relay conatcts would be after the PS filter caps, so there would be zero volts into tamp very quickly. I agree with paulb, the PS is in the signal path. I don't know which is better to have in the signel path, relay contacts or a fuse. So I like the design that seems to be the most fail safe.

Chris,

You are talking about 2 protection devices. 1. DC sensor (the Velleman) and 2. Current limiter (the fuse) One does not replace the other.

Typically current protecting the input and voltage gain stages is not needed. You can concentrate on protecting the output stage.

You are using a very sensible (in my book) approach. Use a soft start to limit turn on current. Then you can use a fast blow fuse on the mains. (power transformer primary side) Use the lowest rated fuse that does not blow under normal listening situations.

IF and this is a big IF..... You are making mono blocks and the output stage is designed modestly, 4:1, 6:1 safety margin, the main fuse will go before the output stages.

Be carefull about that last paragraph. You must look at your design carefully and menatlly short out different devices in a game of "what if...."

Regards,

Aud_Mot
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