monitor selector and headphone amp.

[Chili]

I'd like to build a box that will allow me to select and route from multiple sources to multiple monitors. Plus, have two or three headphone amps with individual source selectors.

The first source is XLR from my D/A, the second is 1/4 unbalanced from my mixer and the last is the 3.5mm from the computer soundcard. I have two sets of monitors, the first uses XLR balanced, the other is 1/4 unbalanced 2.1 bookshelf type.

I expect to only choose one source and one monitor at a time and not have any kind of complicated routing. I'm thinking I can just straight wire these without any active electronics. Am I correct?? Do I need transformers to match up between unbalanced and balanced sources and loads????

To add headphone amps, are there any with Hi impedance inputs so they don't load down the signal going to the speakers??

Does anyone know of a supplier for cheap 1U chassis'???

Thanks much.

 

[dintymoore]

I made a box that routes the signal from two sources (my mixer and stereo playback from my computer) and can send it to the power amp and split it to the sub amp too. I put a big mother volume pot on it too from an old expensive stereo, plus overall and sub kill switches.

I didn't use any active electronics and used bozo Ace Hardware DPDT toggle switches. There's no pops when I use them.

I used an old IU case from a 12-bit Digitech reverb - what else are you going to do with it? I've seen people make rack mount cases out of aluminum channel, but from now on I'm only using wood - I've come to the conclusion that the whole thing of using a metal case as a shield a) isn't needed if you wire things right and b) wood sounds better next to me in the room c) could act like a big antenna. Wood is, to me, superior for cases.

The main thing I'd suggest is to get one of those packs of alligator clips from Radio Shack and build the whole thing with those, and when you get it right then make the real thing.

People will tell you that you need all kinds of stuff that you actually might not in your specific room. For instance, I was surprised to see that the alligator clips with no shielding didn't introduce hum, still I used shielded Mogami wire anyways. The alligator clips will tell you what you really need, and the less the better.

 

[mshilarious]

I expect to only choose one source and one monitor at a time and not have any kind of complicated routing. I'm thinking I can just straight wire these without any active electronics. Am I correct??

Yes.

Do I need transformers to match up between unbalanced and balanced sources and loads????

No, but you might want them, especially if the speakers and other gear do not share a good common ground. This is potentially the classic ground loop situation, which can be solved by isolating the unbalanced signal ground from the balanced chassis ground.

For balanced outs to unbalanced ins, you can just float the pin 3 output unless it's a transformer-balanced output, which I imagine you probably don't have. For unbalanced out to balanced in, you might need to terminate pin 3 input to ground, you might not. Depends on the gear; try it and see.

To add headphone amps, are there any with Hi impedance inputs so they don't load down the signal going to the speakers??

All headphone amps should have a 10K (or higher) line input impedance. Any line output should be able to drive many such loads.

Does anyone know of a supplier for cheap 1U chassis'???

PartsX sells a cheap 1u chassis from Mid-Atlantic.

 

[mshilarious]

I used an old IU case from a 12-bit Digitech reverb - what else are you going to do with it? I've seen people make rack mount cases out of aluminum channel, but from now on I'm only using wood - I've come to the conclusion that the whole thing of using a metal case as a shield a) isn't needed if you wire things right and b) wood sounds better next to me in the room c) could act like a big antenna. Wood is, to me, superior for cases.

That's just preposterous. Sure, if you have fully balanced lines throughout you can often get away without shielding. But few circuits are fully balanced; a simple switching box is hardly a complicated circuit. And internally wiring with shielded cable is a tremendous waste of time. Try wiring up something complicated like a mixer with multiple busses and you will see what I mean. Then try to service it sometime. There's a reason ribbon cable and IDCs are popular . . .

Second, any circuit that radiates EMI needs to have shielding to prevent it from screwing up other gear. Plenty of circuits radiate EMI.

Third, there is no difference between the shielding provided by a metal box and a shielded cable. If a box can be an antenna (which it shouldn't be if wired properly), then so can a cable shield. If you have an incompetent grounding scheme such that your chassis serves as an antenna to inject RF into your audio, then I'd suspect something else is very wrong with your circuit as well.

Fourth, anything that connects directly to wall power really should have safety ground bonded to chassis.

Fifth, wood is flammable, metal is not. No, a switching box probably won't catch on fire, but other circuits, especially improperly designed, can.

 

[Chili]

Thanks for the replies.

Any recommendations on the xfrmrs??

I saw the PartsExpress listing for the chassis. I think I saw them cheaper somewhere else, but still more than I want to pay. I have an old delay unit that no longer works. Gut and Cannabilize.

I'll leave the discussion of wood rackmount cases to others.

Thanks again.