Chris Reitz dot Com

OK, so I promised pictures. here they are. Yup, got my computer fixed, so now I should be able to post a little more regularly.

Fig 1 - The layout I'll be using. The ciircuit board will go where the ribbon cables are.

This next one made me mad, because it means I’ll probably have to buy about $12 more worth of switches, and spend another hour soldering them all up.

Fig 2 - The broken switch

Fig. 3 - Most of the TRS jacks in place. Still need to paint the back plate.

Well, sorry, I know that this is a pretty lame post, but I didn’t really have time to do more. Hopefully the pictures make up for the lack of new info. Part of it is that I’m waiting for a nice day, so I could paint my back plate black, and part of it (ok…ok…I admit it) is laziness. I’ll have more next time.

Pictures coming soon…

I got set back on this a little, because I needed to shell out for a new suspension and a 60,000-mile checkup on my car. In the meantime, some good things and some bad things have happened to the project since.

The Good

I got some more TRS connectors! I’m almost ready to put the whole thing together! I only need one more connector.

The Bad

One of my switches broke. Yes, that’s right, broke. The ones I got were made of plastic, and apparently I bumped a switch and the threads broke, so I can’t bolt it down anymore. I’ll have to decide whether to buy another switch, and just hope it won’t happen again, or whether I should replace all the switches with better quality ones. That’d take a lot of desoldering, and resoldering though…we’ll see.

I’m probably also going to need to get some pretty long, pretty skinny bolts from somewhere. Again, we’ll see

The Obligatory Rant

Some pretty smart people have worked at Radio Shack, but unfortunately, usually the ones who make a career of working there seem to be complete morons. I went into the Shack today to buy my TRS jacks, and I had to tell the guy five times that I didn’t want any batteries, cell phone accessories, batteries, batteries for my smoke alarm because Daylight Savings Time is coming up, batteries, or anything else (including batteries). Maybe next time I’ll just tell him, “if you keep going with your litany, I’m going to put my connectors back, and get them from Full Compass.” That ought to shut him up. I guess at least he’s not like the guy who used to work there. You’d ask that one for a resistor and he’d take you to the remote control cars.

Anyway, I’ve been having computer problems, so as soon as those are done being fixed, I’ll post pictures.

Fig. 1 - Circuit Diagram for the Relay Rack

So I’m building a relay rack for my home studio.

First, for those who don’t know, a relay rack is just that; a rack of relays. The relays are used to switch between audio stuff. Most often, you’ll see relay systems built into higher-end mixer boards in radio stations or production studios. Usually, the relays are set up so that, if you turn on a microphone, it kills the speakers. That way, it is impossible to get any feedback while you’re recording. You can also wire up a relay so that if you turn the microphone on, an ‘on-air’ light would light up.

So anyway, I’m building a 4-channel relay rack. Four mono microphone channels, which switch a set of stereo speakers on and off.

Q: Why don’t you just put switches between the amp and the speakers?

A: Two reasons.

1) (and this doesn’t answer the question), I have active monitors, so I’d need to tear my monitors apart to do that.

2) (and this does answer the question) assuming that I had an external amp running to a couple of speakers, I still wouldn’t run switches between the speakers and the amp. I wouldn’t do it for the same reason why you never ever run an 6Ω speaker to an amp that’s designed for a 8Ω load.  Electronics nerds will be able to tell you that the smaller the load you’re running, the larger the current you’ll be running. So if you attach a 6Ω speaker to an 8Ω amp, you’ll be making the amp run higher currents than it is designed to do.

Q: OK, so what do speakers have to do with switches?

A: Well, a switch is the same as attaching a 0Ω speaker to your amp. The amp will end up supplying WAAAAY higher currents than the amp is designed for. So if you want to be replacing your amp all the time, you can run switches between the amp and the speakers, but a relay rack saves the expense of buying a new amp every couple of months.

I first got the idea to build the relay rack when a friend of mine gave me his old homemade relay rack as payment for driving a mixer board from Madison to Milwaukee for him. He had just a single channel, running to two RCA cables that go between your mixer and your speakers. The switch was a DPDT (middle-off) toggle on a 1RU plate, and the relays were screw-terminalled and zip tied to a 4RU plate. It worked, but I thought I might be able to make it slicker.

I took my friend’s relay rack, and drew out the circuit diagram, and then expanded it to a two-channel version (See Fig. 1). Four channels is cooler than two (and most radio stations would need four channels anyway), so I’m building a 4-channel, 1-RU version of the relay rack.

Fig. 2 - A front view of the relay rack

So here’s the front view. There isn’t a split in the rack plate; that was my cheesy attempt at panoramic photography with my cell phone, and the two halves of the plate ended up different colors. The way the rack works is if the main power (big switch on the right) is off, all microphones are off and audio passes straight through from the board to the amp. If you turn on power, same thing happens, but if you turn any of the mikes on (the little switches on the left), the speakers turn off, and the mic turns on.

Fig. 3 - Wiring the switches and status LEDs

OK, so first off, I drilled holes in the rack plate and mounted the switches and status LEDs. Easy enough. I’m a little bummed that I got Channel 3 a little off where it should be, so it doesn’t look completely professional, but I’ve seen (and done) worse. I just used a center punch and a drill press.

Now the fun begins. Wiring. Between the LEDs and the switches, there were a total of 24 connections, so I decided to use some ribbon cable I had lying around. Two cables, with 12 conductors each. I could have done a lot of the wiring at the switches, but somehow, I wasn’t that smart. If I ever build another one of these, I’ll do more wiring at the switches, so I don’t have to run so much cable.  That having been said, the ribbon cable does give it a kind of a hot-roddish look!

Fig. 4 - The switches wired up

I also used a couple cable clips from Radio Shack to route the cables. It makes them lie more nicely, and fit better in the small 1RU space. Anyway, neatness is good.

Next, I soldered the relays to a little circuit board (Fig. 6). The blue ones are for the microphone channels (Normally Open), and the black ones are for left and right audio (Normally Closed). The switches will be connected to the board via a Parallel-style connector. I bought a 25-pin one, thinking I’d need all of the inputs, but all I needed was 6 (See Figs. 5 and 7). That’s ok. I’ll probably still use the 25-pin one.

Fig. 6 - The brains of the relay rack.

Fig. 5 - Wiring the ribbon cables

Fig. 7 - Wiring the ribbon cables

If you’re curious about what’s under all the blue tape in Figs. 6-7, Fig. 8 is a circuit diagram of all of the connections. All that ribbon cable for six connections. No use crying over spilled milk, but next time I’m going to wire up 90% of this right at the switches.

Fig. 8 - Circuit Diagram of the LEDs and switches

Well, that’s how far I’ve gotten for now. A quick look at what I’ve still got to do (and therefore, a sneak preview of future posts), I need to figure out how to mount the 25-pin connector to my little circuit board, wire it (and the corresponding one on the ribbon cable) up, figure out how to mount the circuit board in my rack, make a back plate for the rack, where audio I/O will happen, and wire up the audio I/O jacks. Wish me luck; I’ll need it!