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Old 14th April 2013, 10:27 PM   #1931
gootee is offline gootee  United States
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Why? Are you trying to protect the silicon? Fuses change resistance with changing current amplitude. And they raise the impedance of the rails.

My opinion: Fuse the speakers, to protect them (if you must). Fuse the AC input, to protect people. Done.

The sound is usually better without any fuses anywhere after the reservoir caps, according to the golden-earred opinionistas. For example, defeating (shorting across) speaker fuses is one of the most-recommended tweaks for new Magnepan speaker owners. (I do still use my speaker fuses, so far.) There is even a healthy market for special (and very expensive) "hi fi" fuses. None of that is actual "evidence". But a lot of people swear by it.

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Old 14th April 2013, 10:45 PM   #1932
AJT is offline AJT  Philippines
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no golden eared, but i do it for peace of mind.....better safe than sorry....and the amp sounds fabulous....for speakers i use relays....
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Old 15th April 2013, 12:03 PM   #1933
AndrewT is online now AndrewT  Scotland
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rail fusing is just as effective as speaker fusing at preventing burning out the speaker driver/s
But a speaker fuse is AFTER the NFB and thus is not usually corrected for the distortion a speaker fuse usually adds.
Whereas a rail fuse is before the decoupling and before the NFB and thus the amplifiers PSRR quite effectively prevents rail fuse distortion effects from being audible.

But there is a problem.
If one rail fuse blows due to slight overload, then most amplifiers send the other rail voltage to the speaker, creating a bigger problem than was first detected. You need to then ensure that the second rail fuse ruptures very soon after the first.
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Old 15th April 2013, 01:50 PM   #1934
a.wayne is offline a.wayne  United States
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Originally Posted by gootee View Post
Why? Are you trying to protect the silicon? Fuses change resistance with changing current amplitude. And they raise the impedance of the rails.

My opinion: Fuse the speakers, to protect them (if you must). Fuse the AC input, to protect people. Done.

The sound is usually better without any fuses anywhere after the reservoir caps, according to the golden-earred opinionistas. For example, defeating (shorting across) speaker fuses is one of the most-recommended tweaks for new Magnepan speaker owners. (I do still use my speaker fuses, so far.) There is even a healthy market for special (and very expensive) "hi fi" fuses. None of that is actual "evidence". But a lot of people swear by it.
Inline fuses makes a difference , fact everything makes a difference ..

I usually up the fuse rating and type for better sonics or bypass with silver wire , does make a difference in my setup and others we have tried, Of course in less revealing systems or running at a pretty benign 8 ohm load may not yield the same results and may not show any sensitive to such changes ..


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Old 15th April 2013, 01:59 PM   #1935
a.wayne is offline a.wayne  United States
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Originally Posted by AndrewT View Post
rail fusing is just as effective as speaker fusing at preventing burning out the speaker driver/s
But a speaker fuse is AFTER the NFB and thus is not usually corrected for the distortion a speaker fuse usually adds.
Whereas a rail fuse is before the decoupling and before the NFB and thus the amplifiers PSRR quite effectively prevents rail fuse distortion effects from being audible.

But there is a problem.
If one rail fuse blows due to slight overload, then most amplifiers send the other rail voltage to the speaker, creating a bigger problem than was first detected. You need to then ensure that the second rail fuse ruptures very soon after the first.
They are usually of different sizes , 10 with an 8 for eg. My Thresholds are like that ...
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Old 15th April 2013, 03:04 PM   #1936
AndrewT is online now AndrewT  Scotland
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If I am fusing a 100W into 8ohms amplifier, I have a choice of using two rail fuses @ F2A and/or one speaker fuse @ F4A.
They would definitely not be the same fuse value in those two quite different locations.
The two Rail fuses would be of the same value.

The mains fuse which I will always fit will be T1A, or maybe T800mA
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Old 16th April 2013, 01:53 AM   #1937
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Could someone recommend a definitive treatment of how protection should or can be added to amplifiers? I had a quick look around, and wasn't particularly taken by what I found ...
Smaller transformer, bigger/more output devices. For example, 36va with LM1875, or 300va with Honey Badger. The conservative transformer selection has decreased the likelihood of blowing the outputs/fuses. These BJT's have more capacity for peak power, so it is possible to build the power supply for. . . at some risk of terminology mishap. . . speed. Practically speaking, you'd just build the power board that can get charged on transformer current insufficient/unlikely to blow the output devices. And, it is the concept of: Don't put more in than can come out. This also applies to eating cheese--far too much in could blow the output device or fuses. See? Easy to remember!
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Old 16th April 2013, 04:07 AM   #1938
AJT is offline AJT  Philippines
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Originally Posted by AndrewT View Post

But there is a problem.
If one rail fuse blows due to slight overload, then most amplifiers send the other rail voltage to the speaker, creating a bigger problem than was first detected. You need to then ensure that the second rail fuse ruptures very soon after the first.
this is where the speaker relay comes in handy, the output offset will trip the relay off...
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Old 16th April 2013, 04:16 AM   #1939
AJT is offline AJT  Philippines
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Quote:
Originally Posted by danielwritesbac View Post
Smaller transformer, bigger/more output devices. For example, 36va with LM1875, or 300va with Honey Badger. The conservative transformer selection has decreased the likelihood of blowing the outputs/fuses. These BJT's have more capacity for peak power, so it is possible to build the power supply for. . . at some risk of terminology mishap. . . speed. Practically speaking, you'd just build the power board that can get charged on transformer current insufficient/unlikely to blow the output devices. And, it is the concept of: Don't put more in than can come out. This also applies to eating cheese--far too much in could blow the output device or fuses. See? Easy to remember!
a balancing act.....
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Old 16th April 2013, 09:16 AM   #1940
AndrewT is online now AndrewT  Scotland
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Originally Posted by Tony View Post
this is where the speaker relay comes in handy, the output offset will trip the relay off...
Tony,
I think you have missed the point.
If a rail fuse blows due to an AC overload condition, then the speaker and the amp may be protected from damage due to the short term AC overload, if the fuse was small enough and thus blew fast enough.
The problem then becomes worse.
The amplifier with one live rail sends a very damaging DC to the speaker.
Some or many speaker relays will find it impossible to break the DC fault current flowing to the inductive speaker coil.

The fuse blow due to AC, can become a major DC event.

That is a good reason to NOT fit rail fuses. I have not made up my mind which is better. I have amplifiers of both types. Fused supply rails and unfused supply rails. I have no amplifiers with speaker fuse. I have no amplifiers with secondary fusing before the smoothing capacitors.
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