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Old 28th September 2008, 06:50 PM   #11
Did it Himself
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I never fit secondary fuses, but fuses after the PSU before the amp are essential. If the PSU shorts the transformer can take a massive overload for short term, this should blow the primary fuse before anything is damaged. This is why it's so important to fuse the primary correctly and have a soft start, rather than an inappropriately big fuse to stop nuisance blowing.
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Old 29th September 2008, 06:11 PM   #12
infobot is offline infobot  United States
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Default LM3886 fuse

Well, amps on hold, fuse holders are on the way still choosing a soft start.
Always liked th3e idea of soft start wish most things used them.

The PSU to amp board fuse sounds good.

Wondered about between amp and speakers but that's just curious about the "fuse everything" approach. Must be a little tricky to not lose sonics, fusing the speakers.

Safety before sonics though-or figure out how to, or so I keep hearing.


thanks
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Old 29th September 2008, 06:20 PM   #13
Mooly is offline Mooly  United Kingdom
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You can always include speaker fuses within the feedback loop eliminating "their presence"
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Old 29th September 2008, 08:32 PM   #14
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Quote:
Originally posted by Mooly
You can always include speaker fuses within the feedback loop eliminating "their presence"
What are you saying! You are in the chipamp section, where gaincloners live, and where it is assumed that every superfluous micrometer of copper in the feedback path will influence the sound catastrophically. Imagine a fuse holder with several milliohms in that path. Shocking!

But let us use science. Imagine to use an 8 Ohm speaker with the LM3886's 5,6 A of peak Ieff. Ignore the fact that the output voltage will be too high for that IC. It would be 44,8 V, clearly worse than the worst case scenario. Now let us use the most beloved feedback resistor combination of 680R/22k, giving a current of less than 0,002 A for that situation. Where do you buy fuses with that rating?
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Old 29th September 2008, 08:47 PM   #15
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The fuse goes straight from the output pin before the feedback resistor and speaker connection. It is rated an amp or so to pass normal current, but provide last ditch attempt to prevent speaker fires. It's really very simple!
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Old 30th September 2008, 06:37 AM   #16
Mooly is offline Mooly  United Kingdom
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That's it. The fuse and all it's non linear effects are now within the global feedback loop.
It's only a small step from this to a 3 or 4 wire "sensing" set up that includes the speaker cable and terminations as well.
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Old 1st October 2008, 04:41 AM   #17
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Once again a drawing says more than a thousand words.

The Zobel should connect between IC output and fuse then to prevent oscillating, when the fuse is blown, right?
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Old 1st October 2008, 06:39 AM   #18
Mooly is offline Mooly  United Kingdom
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As Richie says, the fuse is the last ditch protection for the speaker. On a DC coupled amp I would always use a DC offset protection scheme. Always ! Fuses are not quick enough and give no protection against smaller but still damaging offsets.
The Zobel should be close to the output pin really.
To more fully answer your question. When the fuse is blown the feedback is removed. Almost certainly this means the IC output will swing to one of the rails and sit there. It can't oscillate anyway then, for that it needs feedback. You can add something like a 1k 1 or 2 watt resistor across the fuse. This maintains some measure of feedback if the fuse is blown. How much though depends on whats connected to the output. Does no harm anyway.
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Old 1st October 2008, 10:41 AM   #19
AndrewT is online now AndrewT  Scotland
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Hi,
use separate T rated fuses on each primary.
Try for <T2.17A for each 250VA transformer. (250/115=2.17Amps).
You may find that the amps will work long term with T1.6A fuses.

Now, consider what is happening with your T5A feeding a combined load of two 250VA transformers.
Let's assume one channel is operating perfectly and consuming just quiescent power of ~10W.
The T5A fuse will pass 10A continuously for many minutes before rupturing.
That is 1150 Watts passing into the amp chassis.
With just 10W going to the good channel that would leave >=1140W going into the faulty channel. How hot will it get before that T5A ruptures?

I don't want to insure your house!

Fit a soft start to avoid the need for massive overfusing on transformer start up.

If you want to avoid fast charging of the smoothing capacitance, with it's enormous charging pulses flowing through rectifiers and capacitors, then also consider fitting slow start after the secondaries.
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