Protecting An Amplifier Input

[timtheenchanteruk]

In a social club I work in I need to find a way of protecting an amplifier input from being connected to an am output, The setup is this

 

There is a permanently installed DJ system for the resident DJ, this comprises of 2 amps, one for the main room/speakers and one for the "quiet lounge" so the music etc can be heard albeit at a lower volume,

however when a visiting act is in, they only set-up in the main hall, I have been requested to find a way of patching into the second amp to run the speakers in the lounge without having to re-patch anything as the committee have a hard time wiring a plug let alone set up an amp.

This seems easy enough to do, but I need to account for the not so astute artist, so beyond labelling the stage patch point very obviously as line level only, I need a way to protect the amps input from an amp output (if that makes sense)

 

My first thought would be a fuse, but what kind of voltage would I need and amp capability to accept the line, but to blow very quickly if connected wrongly.

[Lamplighter]

Sounds to be a job for a key operated switch rather than a patch.

 

Brian

[DaveMack]

Hi Brain there is a number of solutions at diffrent costs. There are processors that are a little over spec for the job but this would give you multi amp inputs with limiters and would protect your system fine. This is how ever a fairly advanced thing to do and would if for the first time take a little messing with. You can buy a cheap mixer which you can plug your dj decks into and then into the amps having spare channels for visiting bands on the desk but this does not protect your system as such.

Maybe a compressor with a limiter on the LR send from the band would be a cheap solution but not best as the limiter cant protect 100% but which ones can?! This still means buying a desk or putting a split on the amp inputs. With the compressor output going straight to the amp. This last solution would be the cheapest and easiest to set up. Am sure there maybe other device out there with limiters in but I think this is the area you may wish to look at.

Hope this helps.

[Brian]

My first thought would be a fuse, but what kind of voltage would I need and amp capability to accept the line, but to blow very quickly if connected wrongly.

 

A fuse and a few diodes will do it...

 

http://www.aux99.co.uk/blue-room/ampfuse.jpg

 

...the diodes will limit the total voltage swing at the amp input, the fuse will protect the diodes. With the number of diodes shown you'll be limited to about 2v rms before the diodes start to clip, if this causes distortion under normal use then add more diodes.

[errol]

The idea is good, but in real life the diodes will blow to protect the fuse if a loudspeaker is connected to the input :D .

 

A better solution would be to replace the fuse with a resistor that would limit the current to the diodes. A rough guide would be around 1/10th of the input impedance of the amplifier. Then choose suitable diodes. Another replacement for the fuse could be a fusistor (like a resistor and a very very fast fuse in one). This would limit the current to the diodes and blow if the current gets too large.

[Brian]

The idea is good, but in real life the diodes will blow to protect the fuse if a loudspeaker is connected to the input

A cheapo 1p diode like a 1N4004 will stand 30A for one-half cycle of a 60Hz sinewave (8.3msecs). A 100mA F fuse will blow in that time at 700mA giving you a safety factor of 40+. If you wanted a further safety factor you could move down to a 32mA fuse which would blow at 120mA for a safety factor of 250.

 

Moving to a 10p diode like a 1N5404 you get a single cycle surge current of 200A or 60A for a full 1 second.

 

Transistors might suffer from all sorts of nasty breakdown effects but most GP diodes are as tough as old boots.

[Bobbsy]

I think Lamplighter has the right idea. If the users are as technophobe as you say, spend a bit of time building a switch rather than letting them do any patching.

 

Bob