What could the possible use of the fuse on the XLR output?

Hi guys,
Got a Pioneer dj mixer suffering from buzzing on the XLR output. Turns out its a common problem and the culprit is the fuse on the 'hot' signal out. The issue and the fix is shown in the following video.
Pioneer DJM S9 XLR buzzing or no output issues repair fix - YouTube
Same as the owner of the video I also thought what would be the purpose of this fuse. At first I also thought it could a guard against accidental plugin into phantom powered input - but since phantom power is limited to mA range and though its not ideal it could not cause much harm in this situation and its not common to see a fuse on this end in other similar mixers.
Also the exact fuse is hard to source and its sold as 'pioneer fuse' :eek: . Which leads me to think its kind of a manufacturing decision to make the life of the repair tech harder.
What are your thoughts on the possible use for this fuse?
Thanks in advance.
P.S. any suggestion on an equivalent replacement. Its rated as a 1A fuse.
 

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Phantom power is limited to 10mA, its not why fuses are added - I'm pretty sure they are there to pass safety tests (like not catching fire in any circumstances short of BBQing it).


I do wonder how a DJ manages to put 1A into a mixer - is there some other use of an XLR cable that carries, I don't know, like mains or something?
 
Well its clearly needed if people manage to blow these fuses frequently enough to be a known thing. The alternative would be burnt pcb traces or worse damage I suspect.

The extra resistance is probably lost in the noise of all the trace and wiring resistances, being in the milliohms ballpark. Standard pcb signal traces (15mil) are 30 milliohms per inch, contact resistance in a connector can be 50 to 100 milliohms too, its all utterly tiny compared to input impedances of audio equipment (5k - 50k usually).


And furthermore many line-driver audio circuits have 100R resistor or so in the signal path for stability - adding a fuse to this is immaterial to performance!
 
Hello Mark,

While a fuses resistance is small, it is there. And a balanced interconnect is supposed to have common mode noise rejection - which is pretty much negated completely if the two signal lines are not balanced quite closely.

So, adding a fuse to only one of them pretty much makes no sense at all if you expect a balanced connection to do any good. If you want to see a little more on this, find the article below on the internet.

The G Word, or How to Get Your Audio off the Ground
Bruno Putzeys

Regards,
Greg
 
Thanks everyone for the input, quite a mysterious fuse this is. I think I'll just jumper the pads.
But then again I think to myself what might have caused the fuse to blow. Could it be just a dud brand of fuse maybe? as nothing else in that path has failed.
 
Phantom power is limited to 10mA, its not why fuses are added - I'm pretty sure they are there to pass safety tests (like not catching fire in any circumstances short of BBQing it).


I do wonder how a DJ manages to put 1A into a mixer - is there some other use of an XLR cable that carries, I don't know, like mains or something?
Not "Mains", that would be highly illegal, but for a long time and before speakon became widespread, large/rack power amps sometimes used XLR as speaker *out* connectors.

The ones on the left are speaker connectors (8-16 ohm), the one on the right is for Mains, but uses incompatible 4 pin XLR:

3490_04.jpg


70s-80s Soundcraft Power amps (and a few others) used 3 pin XR for speakers, so much so that there are instructional YT videos showing how to replace them by Speakon, go figure.
 
While a fuses resistance is small, it is there. And a balanced interconnect is supposed to have common mode noise rejection - which is pretty much negated completely if the two signal lines are not balanced quite closely.

Let's get real here. From the schematic given, there are 330R resistors in serie with each output. The fuse's resistance is lost in those resistors' tolerance.

Furthermore, it's a DJ mixer. Good enough is good enough. As Bruno Putzeys says in his article, expect a few ohms of imbalance from real world sources.
 
Guys another (maybe) interesting curve ball - basically when my customer brought in the faulty mixer in to my shop his complain was that there was a buzzing (the sort of buzzing you get when you have the jack connected in half way into an amplifier) sound from the PA when he had the mixer connected to it. Nothing connected to the inputs just XLR outputs to the speakers. This has been his setup for years and no issues. So he changed the mixer with an exact working mixer - same cables,PA,power cord/outlets etc. and no buzzing.
When I got the faulty mixer before opening up as usual I checked it and I didn't get the same issue as him. I didn't hear any buzzing and I got good signal out of my speaker. So after trying to recreate the problem that the customer had. I only got the buzzing when I had one end of the RCA connected to the Line input and the other end of the end of the cable not connected to anything. The buzzing goes if I connect the loose end to my signal source. I noticed that when I had the buzzing, when I touched the outer jacket of the RCA which was connected the buzzing noise does change.
When I opened the unit the fuses were open. Bypassed them put the unit back and it works. I don't get the buzzing when I have one end of the RCA connected and the other end loose as before.
I was hoping someone could explain why this happened, and why it was different scenario for the customer. Also I wonder why I got any sound in my speaker with the open fuses, because it's my understanding that with a differential input (which I believe my speaker would have) needs the 'hot' and 'cold' signal to amplify the signal or one of them needs to be grounded which wouldn't have been the case with an open fuse at the 'hot' output of the mixer.
Thanks in advance
 

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> why it was different scenario for the customer.

Some "balanced" inputs float. If one side falls off, no sound (actually usually sizzle).

Many "balanced" inputs are really ground referenced. They will take half a signal just fine.

We don't know what is in yours and the customer's amplifiers.
 
Not all xlr connectors were wired the same in days gone by. Sony used a different pin out than almost everyone else. Sony flipped the plus and the ground pins. So if you plugged into a piece of Sony gear you were driving your plus signal directly into ground, a short. Negative signal was not moved so it wasn’t subject to accidental short. The fuse popped to save the plus drive circuitry. Other brands also had flipped + and ground but Sony was the big culprit. How old is the unit?