Key features of the Western Electric 110A included automatic peak compression to safeguard against sudden, excessive audio peaks, thus maintaining consistent signal quality and reducing distortion. Its design incorporated a control network with varistors, adjusting resistance to moderate the audio output during loud peaks and preventing harsh clipping or “monkey chatter” interference. This innovation enabled radio operators to maintain a strong audio presence within safe modulation limits.
The Western Electric 110A Program Amplifier also allowed operators visual confirmation of levels through an indicator lamp that flashed when peaks surpassed a set threshold. Its durable construction was ideal for rack mounting, featuring intuitive front-panel controls and a self-contained power supply for easy integration. With a transmission range of 30 to 10,000 Hz and less than 1% distortion, the Western Electric 110A was an engineering milestone in early broadcast compression technology.
Features
- 3 dB increase in average audio signal level or an increase in primary service area equivalent to that which would be obtained by doubling the carrier power.
- Continuous visual indication of the correctness of operating levels.
- Automatic graduated compression of excessive program peaks.
- Protection against over-modulation in the event of accidental changes in program level.
- Freedom from distortion and extra-band radiation due to over-modulation, thus eliminating “monkey chatter.”
- Program amplification of line output for feeding the transmitter at proper level.
- Increase in the effective area of coverage without increasing the power consumption of the transmitter.
- Safety from overloading of amplifiers and loudspeakers in public address and program distribution systems.
The Western Electric 110A Program Amplifier is built on a recessed metal panel, measuring 19¼ inches high and 7½ inches deep, designed specifically for rack or cabinet mounting.
The main components, including vacuum tubes, capacitors, and pads, are located at the rear of the panel, while the terminals and wiring sit in the recessed front portion, covered by a face mat. The front panel hosts all controls, which include input and output gain adjustments, a peak indicator lamp with an adjustable level control, a current meter, an on-off power switch with an indicator light, and a transfer switch to toggle an external volume indicator between the input and output circuits. This setup allows for accurate reading of output levels in relation to various input levels, making it easier to fine-tune the amplifier’s operation.
In radio receivers, the audio signal is directly related to how much the carrier is modulated. By increasing the average modulation level, the effective audio signal and the transmitter’s coverage can be extended. For programmes that don’t utilise the transmitter’s full dynamic range, it’s advantageous to maximise the upper range, providing stronger audio signals to receivers. However, as the modulation level nears the transmitter’s maximum capacity, there’s a greater risk of over-modulation from sudden audio peaks.
Manual adjustments to control gain are too slow to prevent over-modulation accurately, as these adjustments require precise timing, magnitude, and duration. Thus, it’s standard practice to reduce the overall programme level to prevent overloads, though this sacrifices potential audio strength.
Recognising this limitation, Western Electric developed the Western Electric 110A Program Amplifier. This amplifier includes a network that automatically increases resistance when the audio level surpasses a set threshold. For input levels within the set limit, the amplifier maintains a low, fixed resistance. However, as the programme level rises above this threshold, the resistance increases significantly, preventing excessive output. This feature allows the amplifier to be set so that peak levels do not push the transmitter’s modulation above 100%, ensuring consistent audio quality and transmission stability.
The core component of the Western Electric 110A Program Amplifier is a specialised network designed to compress a specific portion of the audio signal’s volume range. This network utilises varistors (identified as X1 in the schematic) whose resistance is determined by the voltage applied to them. Each side of the balanced line includes a varistor arranged in series and one in parallel, ensuring the impedance remains constant while adjusting the loss through the network. A DC potential is applied to control this loss, as described below.
Transformers T2 and T3 play a key role by introducing the DC control potential and ensuring impedance matching between the varistor units and the surrounding circuitry. Positioned after transformer T3, an amplifier (V1) and a full-wave rectifier (V2) generate a DC potential in the rectifier’s load circuit (comprising R4, C1, and C2) that corresponds to the program level at T3’s output. The return circuit of V2 includes a biasing potential (R9—R8), ensuring that no rectifying action occurs for programme levels below a certain threshold, thus keeping R4’s potential at zero for these levels. This network (R4—C1—C2) connects to the grid of control tube V3, whose plate load is the varistor network X1.
When the programme level is below the rectifier’s bias level, the grid bias on tube V3 is zero, and the voltage divides evenly between X1 and V3’s internal impedance. In this state, the loss through X1 is around 4 dB. As the output from T3 rises above the bias threshold of V2, V3’s grid receives a bias proportional to the excess signal level, which increases the plate impedance of V3, reducing the voltage across X1 and enhancing the loss in the audio path. This relationship between input at T2 and output at T3 is mapped on the schematic, with point “A” marking the input level where the rectified component matches the V2 bias.
For consistent performance across a wide frequency range (30 to 10,000 Hz), it’s crucial to prevent the network from responding to low-frequency cycles. This is managed by shunting resistor R4 with a condenser, allowing the amplifier to operate more smoothly, almost in a syllabic manner. The selected values of R4 and its shunt condenser balance insertion and release times, with about 20 milliseconds for insertion and 250 milliseconds for release in the standard position (position 1) on switch D1. Position 2 allows for faster operation, reducing these times by half.
To monitor the amplifier’s operation, a relay tube (V4) runs parallel to the varistors (X1), with an adjustable bias setting via P3 and R8. This bias allows V4 to be adjusted to trigger at a specified level, corresponding to “A.” A lamp (E1) lights up whenever V4’s bias threshold is surpassed, allowing operators to observe the frequency and duration of loss insertion at various levels. The varistors’ characteristics are tightly controlled during manufacturing, selected to work with a control tube (V3) of standard size.
Given that the network requires stable levels, it’s optimally inserted at the transmitter in the program chain. Input and output amplifiers are built-in to accommodate varying program levels, ensuring proper impedance matching, gain, and frequency response. These two-stage amplifiers incorporate feedback, with gain controls (P1 and P2) offering 19 steps of 1 dB each. Input padding (R5—R6—R7) and output padding (R1—R2—R3) align with these gain controls. Despite the high gain and loss insertion method, the amplifier’s distortion is effectively negligible.
