Paul Thurst – Page 177 – Engineering Radio

Proper termination of long audio wire runs

This is standard telephone company stuff, however, it would seem that several radio engineers have forgotten this. I was reading on one forum where an AM station was using 1000 feet of 12 gauge Romex to send audio from the studio to the transmitter out back. The owner was complaining that the audio sounded bad.

Longer wire runs need to be terminated with the characteristic impedance of the cable being used, normally 110 ohms or so for typical audio wire. This is because impedance mismatches can cause return loss just like in an RF circuit. Exactly what the effect of the mismatched impedance depends on the length and frequencies involved. On shorter cable runs of less than 100 feet or so, this usually is not an issue.

The result of return loss is part of the audio energy gets reflected back to its origin (a standing wave), where it mixes with newer audio. This can cause out-of-phase issues and usually, the result is high tinny sounding audio with distortion in the mid-range frequencies. In other words, it ain’t pretty. This can really become an issue with digital audio because of the higher bandwidth requirements for high sample rates. It has always struck me as odd that AES/EBU audio uses XLR-type connectors. An XLR connector does not maintain the characteristic 110-ohm impedance of most digital cables and itself can cause pretty significant return loss. But anyway…

There are a number of options for proper termination:

1. Transformers are often used to match the impedances of circuits. A transformer converts alternating current at one voltage to the same waveform at another voltage. The power input to the transformer and output from the transformer is the same (except for losses). The side with the lower voltage is at low impedance, because this has the lower number of turns, and the side with the higher voltage is at a higher impedance as it has more turns in its coil. Western Electric 111C audio transformers were often used in equalized TELCO circuits sending audio over long distances on copper pairs.

WE 111 repeat coil, one of the best such transformers ever made

2. Resistive network impedance matches such as H or T or L pads are the simplest to implement. They limit the power deliberately and are used to transfer low-power signals, such as unamplified audio or radio frequency signals. Almost all digital circuits use resistive impedance matching which is usually built into the structure of the switching element.

3. Active balanced converters using opamps with high input impedances (10 Kohm bridging resistance) that first greatly reduce the voltage, then amplify it are often used an audio circuits. They have the advantage of active gain control and are often used in conjunction with gain reduction and limiting circuits.

The above diagram shows an active unbalanced to balanced audio converter. The advantages of such a circuit are active gain controls can be added to set levels. With additional feedback circuit elements, it can also be used for automatic gain control, gain reduction, limiting, and so forth.

For most inter and intra-studio wiring, professional audio equipment is designed for 0 dBm 600 ohm balanced audio (AKA line level audio). Audio cables such as Belden 8451 or multi-pair cables terminated on punch blocks or connectors works well. Cable impedances and matching are generally not design considerations. Long cable runs, longer than 150 feet or so, do need to be terminated in a high-quality audio installation.

Studio Builds, the never ending cycle

The lease is up, it’s time to move! Yay, we get to rip apart the old place and redo it! Again! It seems to be a matter of course that every few years a radio station will move. Such is the case with WKZE in Red Hook (the town, not the area in Brooklyn). Their lease is up on the “Grotto” location, so the owner has decided to move to a new location, closer to the center of town.

The new location was the former thrift shop. I know this because while I am working there, a constant stream of older people stop by and tell so. Once, while working alone doing some pre-move work punching down wires and computer network cables, I had to use the facilities. There I sit, on my porcelain throne, when I hear, “Hello?” in an old shakey voice. A quick glance at the door reveals it is not locked. Oh, NOs! Okay, don’t say anything, she’ll go away.

“Hello?”

“Hello, is anybody here?”

“Hello? Very strange, the doors are open but nobody is here. Hello?”

Oh for the love of Pete, “I’m in the bathroom,” I finally said.

“Where is the bathroom?” said the interloper.

I refused to say anything else and she finally left. She could have taken all my tools if she wanted to.

Anyway, the studios themselves are pretty simple, one production studio and one air studio. A T-1 line to the transmitter site, turned out the be the hardest thing about the entire operation. We moved the old Radio System consoles rather than purchasing new equipment. Radio Systems has a program called a Millennium upgrade, where you buy a new control surface, which replaces all moving parts, for something like $2,300.00 or so. For that, basically, a new console is had.

Radio Systems Former RS-12 now Millennium 12 console

The new production room is long and narrow.

The air studio is large and spacious. They often have live music from this studio, which is really cool. The station uses Prophet Systems automation equipment, although it is live most of the time.

WKZE air studio before furniture is installed — WKZE air studio before the furniture is installed

The main office area is one large room where desks will be located.

We are moving in stages:

Prep work, installing all the computer network cable, phone system cable, pulling all the audio and control wiring. Then the contractor finished up the drywalling and painting. Nice Colors!
Ordering phone lines and T-1 line. Ahhh, the phone company, such a pleasure to deal with, we had to pull a new cable through the underground conduit from the street to the building because the old cable did not have enough pairs. The conduit length is about 75 feet or so.
Removed the old production room console and took it to the shop to rebuild. It was not that difficult really, although a little cumbersome. I throughly cleaned out all the dust dirt and other detreious materials from the console frame and install the new control surface. I also checked all the power supply voltages with an oscilliscope to make sure there was no ripple. The original consoles were made in 1992, not bad for an 18 year old board.
Built a new production room with the rebuilt board.
Tested all computer jacks, audio wiring, etc prior to move.
Move T-1 circuit and all office and studio telco lines to the new location. Fortunately, the phone company is a local company not the big V we have in other cities. They were able to work with us and get things paralleled to the new location, something a large company might not have understood.
On the air from the production room at the new location
Remove the main rack, intact and move it to new location
Remove office phone system and install at new location
Remove and rebuild old air studio console
Install rebuilt air studio console in new studio, wire
Transfer operation to new studio

Right now, we are on step #6. That is going to be done next Tuesday (the day after memorial day) morning I believe. We should have the move completed by the end of the week. I’ll post updates as they become available.

Should I renew my free subscription to Radio World?

When I left my former place of employment and set out as a contractor, I also left my Radio World subscription behind. After I was let go, or quit or whatever it was, I was thinking that my days in radio were over or at least winding down and I had bigger things to worry about than trade magazine subscriptions.

I have been checking the Radio World website and find that it gets updated from time to time, usually several weeks behind the magazine. So the question is, what am I missing? As one commenter on the radio info board put it, the magazine keeps getting smaller and smaller, soon they’ll have to rename it Radio Pamphlet.

I’d like to stay abreast of things in the technical end of the radio business, but is Radio World the way? There was a time when it was a technical magazine full of good timely articles well written by fellow engineers. Occasionally I still find something interesting to read, and that is true. Interspersed with that are lots of advertisements, useless information, and many pro-HD radio articles. Is it worth the price? I don’t know, I am still pondering that one.

Wire terminations

Radio studios involve quite a bit of wiring. Runs between the console and equipment are pretty straightforward, from whatever the connector required for the equipment to whatever the connector required for the console. When it comes to trunk runs between the rack room and the studio, however, some type of terminating block is required.

alt= — 66 block or M block insulation displacement wire termination

This particular cabling installation is for low-level signaling, contact closures, and the like. It uses a Belden cable with 37 un-twisted wires which do not follow the standard Western Electric color code. The color code can be found here. If it were audio or data, the wires would be terminated differently. That color code can be found here. For more information on color codes and pinouts, see this post.

Many engineers use the venerable 66 block or M block insulation displacement termination. These terminal blocks were designed by ATT to terminate 25 pair 22 through 26 gauge solid wire. The original design was rated for category 3 (16 MHz or 10 mb/s) communications standards. Newer designs are category 5 or 5e compliant (350 MHz or 100 mb/s). Notice the part about the solid wire. Most audio wire is stranded and as such, the metal fingers on a 66 block will cause stranded wire to spread out losing contact with the terminating finger. This causes intermittent connections and audio dropouts, which I have experienced often (before I knew better, I used 66 blocks when building studios). The way to cure audio dropouts on a 66 block is to heat the termination fingers with a soldering iron. This melts the wire insulation and gets it out of the way. In the long run, it is better to use more suitable terminations.

Krone LSA-PLUS 110 type wire termination block

The 110 block is an updated version of the punch block for high-speed networks. it is also designed for 22 through 26 gauge solid wire. This is the termination used on category 5, 5e, 6 patch panels and RJ-45 jacks. They are also formed into block-type terminations the size of small 66 blocks. The 110 block is designed for 500 MHz (1 gb/s) or greater bandwidth. Krone makes a version of a 110 block called LSA-PLUS which is an acronym that stands for: Lötfrei, Schraubfrei, Abisolierfrei, Preiswert, Leicht zu handhaben, Universell anwendbar, Sicher und schnell. This translates to: no solder, no use of screws, no insulation removal, cost-effective, easy to use, universal application, secure and fast. Unlike a standard 110 block, the Krone block is designed for solid or stranded wire. 110 blocks are acceptable for use with AES/EBU digital audio at sample rates greater than 268 KHz as well as gigabit networks and analog audio.

In very old installations, I have seen Christmas trees. This is a wire wrap system where wires are wrapped around metal fingers that form the shape of a pine tree, hence the name. They were very popular in the fifties and sixties and only work with solid wire. It is also time-consuming work and requires special tools and skills. Wire wrapping is a bit of a lost art.

Christmas Tree wire wrap termination block

Screw barrier strips have been used to terminate audio cables from time to time. I wouldn’t consider this method because it is too time-consuming, takes up too much space, and is difficult to label.

ADC ICON wire termination block

ADC makes a good termination block called ICON (Integrated Cable Organization Network) which uses QCP (Quick Connect Panel) connectors. the connectors are small square devices that are insulation displacement termination (like 66 and 110 blocks) but require a special tool to “punch down.” This particular type of connector is well suited for stranded wire from 22 through 26 AWG. QCP connectors are also used on some of ADC’s patch panels and other audio products. Like any other termination technology, they are only as good as the person punching down the wires. QCP connections are small high-density devices, I have seen them get mangled by someone in a hurry who got his punch-down tool across two of the terminals by accident. ICON blocks can be used for digital audio, however, they do not maintain the 110 ohms impedance of most digital-type audio cables (neither do XLR connectors, by the way). This can lead to some return loss, which on longer cable runs can cause problems.

Radio systems prefer RJ-45 connectors with Category 5 cable, something they call Studio Hub. These are 110 blocks as noted above, but designed primarily for computer networks. Radio Systems discovered that the impedance of most audio cables is very close to that of computer network cables, audio cable is designed for 110-ohm impedance vs. computer network cable which is designed for 100-ohm impedance. Therefore, RJ-45 connectors and shielded or unshielded twisted pair work well with balanced professional audio, either analog or digital.

For analog audio wires, ICON blocks seem to be the best, most secure high-density termination system. In all my years of using them, I have never had a connection go bad. 110 block and other category 5 or 5e systems also work well. For digital audio, Krone blocks or 110 blocks need to be used in order to maintain the full bandwidth characteristics of the cable being used. Using appropriate cable and or terminations in digital audio circuits often leads to impedance mismatches and high return losses in the system.