Output fuse

[Anvil]

Hi. I have a little problem. I cant decide whether to put output fuse to my amp or not. My amp is AV800. I have read about fuse distortion, can it be heard? Please help me.

[Jan Dupont]

I don't think you will be able to hear a fuse on the output.
Anyway I prefer to put the fuses on the rail supply to the output devices (just in case ).

[lucpes]

Fuse the rails and solder the fuses in place

[slowhands]

The fuse on the output is only there to possibly protect your speakers in the event of too much output, or shorted output devices. If you consider your speakers valuable, you use it. Personally I always use output fuses, with relays too in larger amps.

I say "possibly" because it is difficult to size so that it passes enough current for low frequency transients without nuisance tripping, but opens quickly on an output transistor short, the most common failure mode for bipolar outputs. So you have to know what your speakers can handle, and fuse accordingly. You have to make a judgement call whether avoiding nuisance trips is as important as a higher degree of protection for your speakers. Unfortunately, it won't protect against a fault in the amp which puts only partial power out which is still enough to burn out your speakers. For that case, more complex protection circuits using cutoff relays are needed.

I don't know for sure if "fuse distortion" is a real effect or mere speculation. Regardless, if you are concerned about it, you can include the fuse in the feedback loop (sense the output after the fuse). If you bypass the fuse with a large resistor and cap, your fuse can blow and the amp remain stable. This was actually done in some of the early Hafler amps, but when I remove it, I really can't tell a difference. So I question if this is a real or imagined effect. This needs objective testing to be verified.

[sam9]

Other than just hopping to be lucky, the alternative to a fuse is a relay that opens when DC or near DC is sensed. A fuse has definately saved me from some expensive repairs on two occassions. Relays are suppossed to be faster but in the case I cited the relay would most likely not have opened because the over-current situation was not related to DC.

I guesss this means there is no perfect answer to this.

[Anvil]

Hi. I have fused my rails with 10,3mm * 38,1mm special fuses. I would use that kind of fuse in the output too. I have a DC protection circuit and turn on delay circuit that controls a big relay. It is very hard to shortcircuit a Speakon-connector . Thanks for your replies.

[metal]

To place the fuse in series with the loudspeaker wiring "fig 1"
as it stands would cuase cosiderable problems. This is because a fuse has a relatively high series resistance. Its not too good for the muting factor of the amplifier, nor for the bass reproduction quality. But there is more to it. When the current passes through the fuse it gets hot causing non-linear thermal behaviour and the quality of bass will show a negative temperature coefficient.

Some thing can be done about this.Include the fuse in the negative feedback loop, in other words, tap off the negative feedback voltage from a point behind the fuse. The fuse is bypased by means of R3 which is small compared to R2 (slight influence on the DC set-up of the amplifier) , but larde compared to 4 or 8 ohm load impedance, a value of 100-220 ohm / 2 watt is fine.

[sam9]

And what happens to the amplifier when the fuse blows and removes most of the NFB from the amplifier?

[ChocoHolic]

Hi!

I had some less convinient issues with fuses in the
speakers wires.
But in fact the reason was not the fuse itself, but the fuse connector!

The amp was a wild design with OP + BJT VAS + BJT current output stage.
Designed for 100W at 8 Ohms, or 150W at 4 Ohms.
8 amplifiers clubbed together, you could parallel or bridge them or combine all that to get 1.2kW at 2 Ohms......
Rails were +/-54V DC, without signal load.
Ten years ago I felt this was not so poor.
The fuse was a 5 A slow type, european style (20mm).

We measured at 10kHz/8 Ohms/100W.
..yes we measured 10kHz, because at 1kHz I had no
doubts about the performance...
7 chanels showed less than 0.02 % THD.
But one chanel was at 0.07 % !!!
We found the reason at the fuse connector, so be carefull about that.

I fact I am not sure if I would have heard the difference.
During the years I did several changes, especially in order
to reduce the crossover distorsions.... this was audioable....
Unlucky thing, it was just good luck that I got connections to
an audio lab were I could measure the THD. Since I moved to
Munich, this opportunity is lost.... So I have no idea about the
THD of the current modification...
But probably I still underestimated the cross over distorsions,
since I upgraded my NAD receiver two weaks ago.....

For cross over distorsions I would now say: VERY CRITICAL, you may hear even very low levels of distorsion.
For distorsions of fuses I am not sure if they are that critical,
but I never made a listening test to this.

Bye Markus

[fab]

Originally posted by sam9
And what happens to the amplifier when the fuse blows and removes most of the NFB from the amplifier?

You will only hear LOW level VERY distorted sound that will advise the listener that the fuse has blown. Just change the fuse since the opening of the fuse itself will not damage the amplifier. Of course, investigate why the fuse has blown... Sometimes, it is only a matter that the fuse is "dead" because of age (fatigue).