Global Media Education - SAE Institute
Audio Wiring
The main thing about audio wiring is understanding how the earthing works. Lets take the connection of a 24 track analogue recorder as and example. You send a balanced lead from the balanced output of the console to a balanced input on the recorder. You then return to the console with another balanced lead. Now remember, in the balanced system the audio runs through the +ve and -ve leads. The earth is just a shield established to drain unwanted interference off to ground. But if you connect the shield at both ends of each lead you are establishing the potential for an earth loop. It is in fact joined to itself in a loop. In fact it's two loops because each machine is connected to an earth as well. If on the other hand you disconnect the shield going to the recorder at the recorder end any interference generated in the shield must go to the console earth. Now if you disconnect the shield in the return lead but disconnect it at the console end any interference will be drained to the recorder's earth. No loop! Because the recorder and the console both have mains power in their circuits there must be a link to earth for safety so don't de-earth to get rid of the hum when there is a safe way such as this.
Balanced
Now the same problem but with unbalanced leads.
Remember that the earth is now the negative as well as the ground
Unbalanced
As you can see the recorder has a connection with the positive but also a connection to the negative but via the earth (ground) and once again there is no loop and both machines are earthed safely.
In the modern studio there are lots of simple external power supplies that just feed a single unit like a reverb unit (wall warts we call them). Have you ever noticed that they are not earthed to ground. Their mains connection has only two pins. These units allow the circuit to float above ground so the shield must be connected for the unit to receive the negative feed.
Some may say that by not earthing the wall warts it is dangerous but as they don't feed high voltage (typically 9 - 12 volts) to the units it's not necessary. If you did disconnect the shield in this circuit, because there is no negative, the sound would become what they call one legged and the sound would be thin and low in level.
So when you start wiring up your studio think of what is earthed and what is not and then you can establish when it is safe to de-earth a unit to minimise ground loops.
Additional things to consider
- Maintaining Phase: It is essential that your wiring maintains constant phase. With unbalanced leads it's pretty obvious - the centre wire is positive and the shield is negative/earth. But with balanced leads you can run into problems. Unfortunately the world has two standards. On your standard microphone plug (often called by the brand name Cannon) Europe uses pin 3 as the positive and pin two as the negative and pin 1 is earth. On the other hand the US has pin 2 as the positive and pin 3 the negative. The same applies to inputs and outputs on equipment. It is essential that you check each piece of gear and work out which pin is positive and which is negative and wire accordingly. Pin 1 is always earth and is usually a little longer so it connects first. In ring, tip and sleeve plugs the tip is always positive, the ring negative and the sleeve earth. A handy little piece of gear is a microphone line phase checker that has three lights that check the three lines in the lead for continuity and phase.
- Guitar Lines: It is advisable to incorporate guitar leads between rooms. This allows you to plug a guitar into a jack in the control room and pick it up in the studio and plug it into an amplifier. Guitarists often like to play in the control room, especially if you are using effects, so a cable between rooms saves having to run leads through doorways. I have seen ads for a product that has a battery powered amplifier in the cable that compensates for the high frequency loss experienced when running long unbalanced guitar leads.
- Speaker Leads: There is a lot of discussion on this topic and proprietary speaker cable can be purchased, but it is expensive. If you can't afford it use the standard power cable as used by your electrician. When running your speaker leads run at least two sets per side. This allows for a replacement if one cable gets damaged also it allows you to go BI-amp later should you want to without having to climb behind the speakers to add the extra cable.
- Transfer Lines: It's a good idea to have some standard line level lines between the control room patchbay and the other rooms. You can feed line level instruments like keyboards down them and plug them straight into the line inputs on your console.
- Telephone Lines: It is a good idea to incorporate a telephone line into your control room, especially now that computers are common and you may need to hook up to the web for software update downloads. We are not far off having the ability to record in real-time down these lines and can now transfer Wave and MP3 files.
- Direct Boxes: Direct boxes are designed to match impedance between your guitar and the microphone input. Without going into the full electronic detail here basically what happens is that a guitar is designed to plug into a high impedance input whereas a microphone input is a low impedance input. Plugging a high impedance magnetic pickup into low impedance results in a loss of highs. Direct boxes can match the impedance by either using a transformer (passive) or using a circuit (active). Active direct boxes are identified by the fact that they have power - either as a battery or powered by the phantom power system. DI boxes also have a pad switch to reduce the level of a line level instrument down to the lower microphone input level.
Patchbays
Patchbays can save a hell of a lot of trouble when interfacing your recording equipment. Even if you only have a bedroom studio it is a lot easier if all your gear appears on a patchbay and you can easily patch one thing into another. Patchbays can be cheap or expensive depending on the style and construction. They can also come in balanced (Ring tip and sleeve) or unbalanced (tip and sleeve). It really depends on the gear you have and your requirements but don't overlook the advantage of having a patchbay.
Lets look
at the standard layout of a patchbay:
The inputs and outputs usually go like this:
The main
idea here is that each row is normaled to the next. i.e. microphone line
1 is directly connected into preamp in 1. Insert send 1 is directly connected
to Insert return 1 - group 1 is directly connected to recorder in 1 and
recorder out 1 is directly connected to line input (tape return) 1.
In other words, with no patch leads the circuit is complete and you only
use a patch lead if you wish to change from the normal - that's why it's
called normalling
Here we have a standard stereo plug and socket. When the plug isn't inserted the +ve and -ve pins are shorted to the two normalling pins. The normalling pins then connect to the through connections.
As you can see the insertion of the plug breaks the normalling and allows the new connection. Prebuilt patchbays often have the normalling as an option. Tascam have some excellent unbalanced ones but fully balanced normalling patch rows are expensive. A simple check is to count the pins - 3 pins are standard and a normalling patch bay has 5 pins. (Earth is common)
Other normalling areas to consider are your console outputs being normalled to your master compressor input and its output is normalled to the input of your DAT recorder. Then the output of your DAT recorder is normalled to your External Monitor input. That allows you to start mixing without having to setup a huge patch . If you need to access your master compressor you just patch into it and break the normalling.
Another area is your Aux Sends. It is advisable to normal your regular setup - such as 1 &2 to your headphone amp, Aux 3 & 4 to your stereo reverb, Aux 5 to your effect unit 1 and Aux 6 to your effect unit 2. You can go further by normalling the returns of your three effects units into tape/line returns 23 - 28. With such a setup you can start a mix without having to patch a thing!!
If you are the only user of your studio it is probably not really necessary to label the patch bay fully but if you have outside clients it must be labelled clearly.
Bi and Tri - Amping
In a standard speaker the various components,(woofer, midrange and tweeter) and divided from each other with what is referred to as a crossover unit. What actually happens is the crossover divides the frequency response into 2 or 3 bands. The lows drive the woofer, the mids drive the midrange speaker (often a horn) and the highs drive the tweeter.
Here the signal from the console goes to the amplifier and then to the speaker. Within the speaker the crossover circuit splits the frequencies into to three and feed to each speaker.
Alternatively here the output of the console goes to the electronic crossover unit that then feeds to each amplifier that drives a speaker independently. Although you need three amplifiers the amps don't need to be as big. Big PA systems run on this system and are described as being 2 way, 3 way and 4 way - bi - amped, tri - amped, and quad- amped. The additional crossover in the 4 way system feed the low mids.
You can now buy small near field monitors that have the electronic crossover and the amps built in - all you need to do is connect the output of the console into the rear and you are away. The multi crossover multi amping system is extremely efficient and you don't need huge 500 watt amplifiers etc.