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Hydrogenaudio Forum => Scientific Discussion => Topic started by: Nichttaub on 2019-07-10 18:16:13

Title: Speaker fuses
Post by: Nichttaub on 2019-07-10 18:16:13
I've done a web search, and other than the usual "manufacturers flogging their wares" I haven't seen any scientific tests on the contribution of speaker fuses to distortion (audible or not).  Clearly the resistance changes as the fuse wire heats up, but the big companies like Littelfuse don't publish that data openly.  I did stumble across a graph which showed, without attribution, measured distortion in the range of 0.01% to 0.1% for a 2A FB fuse operating at rated current.  If true, that would pretty clearly be negligible.

Is anyone aware of proper tests (measurements or listening) being run on speaker fuses, or is it considered not worth the effort?  Has anyone characterized the linearity of a standard fast-blow fuse vs. current and published the data?  I will not run my main system without fuses, but I would like to be able to point others to a good justification that it won't harm their listening experience.
Title: Re: Speaker fuses
Post by: saratoga on 2019-07-11 05:14:46
A fuse is a resistive element.  In terms of circuit representation, a 10 amp thin film fuse is about equal to a couple meters of 12 gauge wire.  The effect will be similar to wire,  negligible. 
Title: Re: Speaker fuses
Post by: Nichttaub on 2019-07-11 13:41:27
No, because a couple of meters of 12ga wire won't heat up when you run 2A through it; its resistance will not change measurably.  Even the fuse manufacturers acknowledge that a fuse will heat noticeably and its resistance will change non-linearly when you approach rated current.  Non-linear resistance = distortion.  The question is not "will it cause distortion", the question is "how much".  The fuse manufacturers don't publish this info, so I was hoping someone might have done the research and could actually attach numbers to the effect.
Title: Re: Speaker fuses
Post by: pdq on 2019-07-11 15:56:17
In this case the resistance is not changing non-linearly with voltage, but with temperature, which is slow enough that it will not introduce distortion in the audio range.
Title: Re: Speaker fuses
Post by: saratoga on 2019-07-11 17:04:34
No, because a couple of meters of 12ga wire won't heat up when you run 2A through it; its resistance will not change measurably. 

If you size the fuse correctly, it's temperature should not be changing much (or at all) during use, and certainly not at audio frequencies.

Non-linear resistance = distortion.

Nonlinear resistance with voltage causes distortion.  Fuses/resistors are linear with voltage. 
Title: Re: Speaker fuses
Post by: Glenn Gundlach on 2019-07-13 00:49:08
No, because a couple of meters of 12ga wire won't heat up when you run 2A through it; its resistance will not change measurably.  Even the fuse manufacturers acknowledge that a fuse will heat noticeably and its resistance will change non-linearly when you approach rated current.  Non-linear resistance = distortion.  The question is not "will it cause distortion", the question is "how much".  The fuse manufacturers don't publish this info, so I was hoping someone might have done the research and could actually attach numbers to the effect.
With program material and not test tones, I sincerely believe you will never get near actual heating of the fuse element unless you
have such a small value fuse. Having played with audio for 50+ years (most of that time for a living) I can tell you fuses are borderline
 useless for protecting speakers. Inside an amplifier they will blow right after the output transistors fail.

Title: Re: Speaker fuses
Post by: Rotareneg on 2019-07-13 16:15:32
Non-linear thermal effects can cause significant distortion on AC current passing through a wire.

Ok, sure... an extremely thin wire: https://en.wikipedia.org/wiki/Hot-wire_barretter
Title: Re: Speaker fuses
Post by: Speedskater on 2019-07-15 23:12:09
Bob Cordell in his book 'Designing Audio Power Amplifiers' measured fuse distortion.
He found at 20 Hz about 0.0033% THD, with less at higher frequencies.
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