PA Questions

PolarityMan

We have an existing QSC poweramp that uses jack inputs. We're thinking of buying a behringer SR16 digital mixer but this has speaker outputs on XLR. I am assuming that I can use a cable something like this:

https://www.thomann.de/gb/cordial_cfm_3_mv_klinkexlr_kabel.htm?ref=search_rslt_XLR+to+jack_199094_11

Do I need balanced or unbalanced in this situation?

Similar question but to run IEMs we're going to intially go with wired from the aux sends into a headphone amp for each person. Again the output is a XLR but the input to the headphone is a jack. Will the same kind of cable do?

We're only going to have 4 aux sends for 4 members so these will obvs be mono mixes but I do want the mono mix coming out of both earphones for the IEM.

ESBlonde

Firstly they are not speaker outputs on the mixer, just regular line feeds.
Secondly the model of the QSC power amp will be reguired to determine if it has balanced or unbalanced input jacks. QSC is generally high quality so I would expect them to be balanced anyway, I am surprised that a QSC amp only had jack inputs unles it is very old or low powered.
Again without knowing the make/model of the headphone amp it's hard to give pertinant advice on it's suitability, some headphone amps might use a TRS jack as a stereo unbalanced input rather than a balanced mono feed.
Balanced becomes more important over distances and in 'noisy' environments, inside a small rack you will likely get away with unbalanced but if the option is balanced go with it.

Danny1969

That's the wrong gender, you need female XRL to jack to connect the mixer to the amp, balanced preferred but signal is so strong at this point it's unlikely to make a difference, 

To connect a single XRL output to headphones and get a mono signal in both ears you need to 

Get an XRL lead and short pin 3 to pin 1 ............... now you have an unbalanced signal between pins 1 and 2   ..... Then 

Connect pin 1 of your XRL to the sleave earth of the jack that's going to feed the headphone amp and connect pin 2 to both tip and ring of the jack that feeds the headphone amp ......... that will send the same unbalanced aux send from the desk to both ears 

My other little company Dr Watson makes all kinds of clever cables for stuff like this, pre made cable is £15

PolarityMan

yeah the QSC is pretty old but its a total tank been running it with unbalanced leads so far with no issues. headphone amp we have a variety of cheap ones that we already have. 

@Danny1969 ;

You got a website for that?

BigMonka

Rather than getting a 3m cable like you linked to I would recommend using adaptors - that way you can use standard XLRs for the bulk of the distance which are way more useful to have in your PA kit bag anyway (if you haven't got them already).
You can either get short (10cm) adaptor cables made up or a solid one like this: https://www.thomann.de/gb/thomann_sk083_adapter.htm

PolarityMan

the adapters are a good thought, i have quite a few XLRs already. Not sure how they are wired though, are they going to give stereo output from a mono input on the aux sends? They look perfect for the mixer -> power amp job though.

Danny1969

Adapters tend to be for converting one type of connector to another, not so much moving a signal from one pin to another. I'm not a fan of adapters to be honest, they tend to place strain on sockets with the extra weight if used directly. Used in line though they are not so bad but it is another point for potential failure 

The cable I described earlier use's a standard mic cable with the adjustment one end. If you need to make it longer you just stitch on another normal mic lead ... or 2 or 3 or send it down a stage snake etc ........ that's the beauty of it

@PolarityMan I don't have a website for cable building, I just tend to build what people ask for, generally PA based rather than guitar. I'm happy to do try before you buy though if you want something

PolarityMan

I'd be looking for 4 of the cables for the XLR to jack with stereo in both ears thingy. what length would you get for 15quid a piece? If you want to pm me a quote that would be cool.

BigMonka

Danny1969 said:

Adapters tend to be for converting one type of connector to another, not so much moving a signal from one pin to another. I'm not a fan of adapters to be honest, they tend to place strain on sockets with the extra weight if used directly. Used in line though they are not so bad but it is another point for potential failure 

The cable I described earlier use's a standard mic cable with the adjustment one end. If you need to make it longer you just stitch on another normal mic lead ... or 2 or 3 or send it down a stage snake etc ........ that's the beauty of it

@PolarityMan I don't have a website for cable building, I just tend to build what people ask for, generally PA based rather than guitar. I'm happy to do try before you buy though if you want something

That's a fair point about the extra weight of adopters. My personal preference for any converters would still be short (30cm) cables though as I find it easier to do the main runs with cables and then open up the little bag of adaptors/converters for the end connections - saves me getting confused and pulling out the wrong cables. If not that then I'd definitely have any funky type cables like these in a different colour so that you can quickly see what's what.
(I appreciate that everyone likes to do it differently and @Danny1969 you've got a ridiculous amount more experience than I have in these things)

PolarityMan

Ok so about to pull the trigger on our PA order but had another stupid question. Do I also need to buy any fittings for current gear? Im thinking particularly about the power amp which is one of these:

http://medias.audiofanzine.com/images/normal/qsc-usa-850-954591.jpg

It seems pretty heavy to support only via the front panel screw fittings, even if i put it on the bottom I dont think the bototm of the amp will be in contact with the bottom of a rack case.

Im pretty sure for everything else just screwing in via the fron rack mounts is fine but do I need to get some kind of rails or supports for the amp?

EDIT:

More stupid questions, am I right in thinking that the poweramp is 2U and do you need to leave space above the power amp for heat dissipation?

Danny1969

Well you can get rear fixings for some very heavy power amps, it's a kind of inverse fitting to the front but in general an amp like that is fine just bolted to the front  ..... on the bottom of a rack with a spacer  (made of carpet tile, thin wood or whatever) taking up the space between the bottom of the rack and the bottom of the amp to help the load. 
That amp vents through like most but I still like to leave at least one U between the top of an amp and something else above it. How hot the amp gets depends on how loud your driving it and the impedence it's driving into ..... to be honest once you put more than one power amp in a rack it gets pretty heavy .... so I wouldn't put much else in the rack with it

PolarityMan

Plan is to have the power amp, a behringer sr 16, a focusrite saffire pro 40, and a behringer ada 8000. By my count that will take 7u which leaving one space above the power amp should nicely fit an 8u. Might be a little heavy but the qsc is easily the chunkiest thing in there.

I'll prob get back to you on Monday about those cables btw.