Build A Simple IFB System
By Gary Palamara
With a little effort, building the Simple IFB system will give your remotes studio-quality performance and convenience on a poor man’s budget.
Return to Articles PDF Download Radio World Magazine - Sept. 16, 2005 - Pgs 48 - 50
In its simplest form, an interruptible fold-back system or IFB, allows ON-Air talent to hear their own audio (folded-back) mixed in with other program sources like incoming telephone calls or remote feeds. The interruptible part happens when a producer, telephone coordinator or broadcast engineer breaks into the talent headset system to deliver needed information.
In the studio, some broadcast consoles have IFB circuitry built in; but for work in the field, or smaller studio setups, IFB cueing often is quite often a luxury. Commercial systems are available, but many engineers are under budget constraints that force them to home-brew their systems, or worse yet, they do without an IFB.
Sometimes, a producer’s IFB microphone can be sent through a spare channel of the broadcast console, but that means you run the risk of crosstalk or IFB audio accidentally entering the on-air signal path. Not good.
What’s
needed is a stand-alone system that is easy to set up and simple to operate. Figure 1 shows the Simple IFB block diagram. The Simple IFB satisfies all of
these requirements and has enough flexibility for producers and technical crew,
while also providing a program interrupt channel, all in a small, stand-alone
package.
The Basics
Three components make up the system: microphone switching, audio mixing of sources and headphone amplification. With my bottom line in mind, I looked around the shop and decided to design everything around a longtime audio classic that happened to be sitting on a storeroom shelf.
Although it is no longer manufactured by the Shure Brothers Company, the venerable old M-67 four-input audio mixer is legendary for its reliability and ease of use. The M-67 was probably one of the first mass-market field audio mixer, when it entered production in 1968. Tens of thousands of M-67s were made over the two decades of its manufacture and many are still in use today. The M-67 is reliable, easy to use, and when repair is needed, its discrete circuitry is simple to fix. It also has a fair amount of extra room inside the chassis, which helps when making modifications to the circuitry. The mixer’s four inputs, tone generator and VU metering make setting up and monitoring the IFB system relatively easy.
Naturally, your particular setup might vary from what I have designed, but the
basic idea is straightforward. If you don’t happen to own an M-67 mixer or even
its newer big brother the Shure M-267, either unit may be purchased for a $100 or
less on the surplus market. But most of the frugal engineers I know will probably have a
long-forgotten spare mixer sitting on a shelf somewhere, just ready to use.
Figs. 1-3 show the complete system setup. Most of the IFB circuitry is built inside the M-67. Relay K-1 and the other small parts shown in figure 3, were pre-assembled onto a small, perforated circuit board, and then installed into the M-67. For ease of field setup, the connectors associated with the first three audio inputs were replaced with five-pin XLRs. This allows both microphone audio and the program mute circuitry to flow through a single connector.
Input
4 of the M-67 accepts IFB program audio and
is muted by relay K-1.
Energizing K-1
puts the wiper of input 4’s control pot at ground potential and effectively
kills the program audio when the tech crew is giving cues to the talent.
Input 4's audio can be pre-mixed and from one of the broadcast console's "Aux"
outputs or even taken from an "Off-Air" tuner. The M-67's output signal is
then sent to all talent headphones and can be controlled
individually by a Shure FP-12 headphone amplifier or other similar device. If
on-air guests require a non-interruptible fold-back, either a separate console
output or splitting input 4’s signal prior to feeding the M-67 mixer will do the
trick.
Nice extra
Along with the modifications to the M-67, three control boxes were built and are shown in figure 2. Each control box has one microphone input and two outputs. The control boxes allow a producer, telephone coordinator and broadcast engineer access into the talent headsets.
Output 1 sends the IFB microphone signal to the system along with the relay closure that controls the program mute relay K-1. Output 2 may be omitted, but in my design is used for feeding a studio announce amplifier or self-powered speaker system. That’s a nice feature when you’re trying to pump up an audience prior to air or need a “Voice of God” mic when you're about to come back from a commercial break. Just remember to kill that feed prior to going on the air! In a studio situation, the SA signal can be shut off whenever the on-air lights are turned on.
Both in the studio and in the field, the technical crew normally works fairly close together; so cable runs between the control boxes and the IFB mainframe are usually not very long. My system was made for fieldwork, and for ease of setup, I chose to build “special” cables that carry both the IFB audio and program mute switching in the same cable.
Five or six cables of varying lengths were built to give me some options during setup. To interconnect the IFB system to the control boxes I used Canare brand, star quad cable and five-pin XLR connectors. The quad cable has a total of five conductors: two twisted white wires, two twisted blue wires plus a shield. I used the two white wires to carry microphone audio to the mixer and the two blue cables switch the mute relay. The shield connects to ground on pin 1 of the XLR.
Cable-ready
One drawback of building “special” cables to work this system is the inevitability of not having the right cable at the right time for making the interconnections in the field. So make sure to carry a few spares of various lengths when you go out on the road. To identify the five-pin XLR cables, all of the wiring for the IFB system is a nice bright orange color.
With short cable runs, crosstalk between the microphone audio and K-1s DC voltage doesn’t seem to be a problem. Referring to Fig/ 3, one reason for this is the reversed biased diode D-4 and capacitor, C-4 that are connected across relay K-1. When relay K-1 de-energizes, its magnetic field collapses suddenly, and the D4, C4 combination help dampen any back voltage.
While the Shure M67 is a great mixer, one of the problem areas of its early design is the rather poor voltage regulation of its power supply. Trying to run relay K-1 off of the main voltage source that also runs the electronic circuitry inevitably will cause the mixer’s DC voltage to sag to a very low, unacceptable level. Luckily, there is another option.
The secondary of the 67’s 120-volt transformer has two independent windings. While the center-tapped, high-voltage winding operates the mixer’s electronics, another low-voltage winding provides approximately 6-volts AC to only run the two # 47 lamps that illuminate the VU meter. Fig. 3 also shows a partial view of the M67 circuitry. You might notice that one side of the green 6-volt winding is already at chassis ground potential. By adding diode D-5 and capacitor C-3 to the non-grounded end of the winding, you’ll end up with plenty of DC power to run K-1 without affecting the audio circuitry. A 5-volt regulator and LED indicator were added to help stabilize the relay voltage and provide a visual indication of IFB switching.
I’ve used this simple IFB system many times with good results. The entire package with mixer, control boxes, headphone amps and cables fit into a single small road case and only takes a few minutes to set up and test out. You can modify and expand this system with more channels and to meet other needs. Just remember the KISS principle.
With a little effort, building the Simple IFB system will give your remotes studio-quality performance and convenience on a poor man’s budget. Drop me a line and let me know how you made out.
About the Author - Gary Palamara, AF1US, is a radio amateur and a freelance audio engineer. He began his career with Armed Forces Radio and has spent three decades working in audio and video production. He owns Morningstar Sound, a professional sound services company.
© 2005
