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Speaker protection
Speaker protection
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Old Yesterday, 12:57 AM   #1
3robert8 is offline 3robert8
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Join Date: Feb 2016
Default Speaker protection

Hi All
I am finishing up the final sub assemblies for a new DIY power amp (LM3886). Two of these being separate Thiele networks which will mount on the chassis. But I am beginning to worry about speaker protection if I upgrade to more expensive speakers. First thing that comes to mind is a clever relay based circuit, but I don't think I have the room for them. I could easily mount a PCB fuse holder on the Thiele network board at no additional real estate cost. I would like to know your thoughts on this type of protection. This would be a 2 amp fast blow that probably would pop at about 2.3 amp or so in a reasonable amount of time. That RMS current level would put an 8 ohm speaker at the 40 watt level (which it would rarely see in normal operation). I have done some tests on quality Little Fuses and found the DC drop to be 750 mw at a 2 amp current and less than 100 mw at a lower listening level of about 4 Watts. This represents 2% power loss at 40 watts and <100 mw loss at 4 watts listening level." Quick and Dirty" - cheap and simple, but am I missing anything here?
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Old Yesterday, 03:25 AM   #2
tomchr is offline tomchr  Canada
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Location: Calgary
Fuses tend to be rather nonlinear devices, thus cause distortion.

A better option is a MOSFET transmission gate driven by a photovoltaic driver. That's what I did for my Guardian-86, which I developed for use with LM3886-based amplifiers. It works great and I can't measure any impact on the distortion of the amp before/after the protection circuit.

Relays are not fantastic speaker protection devices as they tend to arc over and self-destruct during a fault, sometimes taking the speakers with them. You can find some rather spectacular videos of that here: Speaker DC protection with relays

That said, I don't personally use speaker protection on my LM3886-based amplifiers. The LM3886 is pretty rugged and not likely to blow up. While the chances of a catastrophic failure are small, they're not zero. So whether you use a speaker protection circuit with the LM3886 really depends on your risk tolerance.

MOD686: Up to 240W/8Ω, <0.00025% THD. MOD86: 40W/8Ω, 0.000054% THD. HP-1: 3W/20Ω, -130dBc THD, 128dB DNR.
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Old Yesterday, 04:07 PM   #3
3robert8 is offline 3robert8
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Join Date: Feb 2016
Default speaker protection

Thanx for the info Tom. As far as your opinion of using no protection, I hear this from other people like yourself who are knowledgeable on this subject. I guess I will go with the flow on this one and omit speaker protection. I have used fuse protection on a 50 watt amp used for general testing over the years. Loads were speakers (cheap) and other various devices. In most cases the load was saved by the fuse, but occasionally the fuse was saved by the load (smoke).
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