The Gates Sta-Level Amplifier is an automatic gain control device designed to maintain a consistent average level within a programme circuit. While it is not a peak limiter, it pairs perfectly with one, helping the peak limiter operate more effectively by smoothing out large signal variations before they reach the limiter.
Here are four key applications of the Gates STA-Level Amplifier:
- Automatic Program Level Control: It can manage the output level from the speech input console automatically, reducing the need for the operator to manually adjust gain. This ensures the right level is sent to the telephone line and peak limiter.
- Input Level Adjustment: It adjusts automatically for differences between the levels of various input sources like microphones, turntables, remote feeds, and projectors, ensuring consistent output even when the input levels vary.
- Automatic Fader: The Sta-Level can be used as an automatic fader, lowering turntable output by 20 to 30 dB when the microphone is active and then automatically restoring the turntable to its original level once the microphone is off.
- Remote Line Control: During extended remote broadcasts, it can regulate individual incoming remote line levels, making it possible to adjust a remote amplifier’s output and control it through the Gates STA-Level for up to a ±15 dB variation, freeing the operator to focus on other tasks.
The amplifier comes with built-in input and output attenuators, allowing it to function in most scenarios without the need for external pads. These attenuators, adjustable from the front panel, enable precise control of input and output levels. Additionally, engineers can modify the attenuator values and impedance settings using standard resistors if non-standard values are required.
The Gates STA-Level Amplifier is designed to be mostly self-regulating once set up and requires minimal ongoing attention. After allowing the unit to warm up for a few minutes, it can be calibrated for optimal performance. The system, as outlined in drawing A-11361, will be covered unless another specific application is noted.
To calibrate, begin by adjusting the compression meter to zero with no input signal. This meter is located behind the panel, mounted on a bracket. Reduce the console amplifier output by 10 dB from its normal level, then increase the input control on the Sta-Level until the meter reads 15, which corresponds to the heavy line on the compression meter. It’s best to use program material to set this level for peak readings.
Once the compression meter peaks at 15, adjust the output control on the Sta-Level until the console VU meter peaks at 0 VU or 100% modulation. This allows the Gates STA-Level to function within its mid-range, adjusting for signal levels that vary by ±15 dB from normal. You may observe that the VU meter will read about 4 dB low with input levels 15 dB below normal and 4 dB high with inputs 15 dB above normal. This slight variation allows the console to function normally without needing constant monitoring of the compression meter.
The Gates STA-Level can compress up to 40 dB with low distortion, providing an additional 10 dB of safety margin beyond the usual 30 dB range. Amplifiers preceding the Gates STA-Level in the signal chain may not operate over this wide range, so their levels may need to be adjusted to stay within the Sta-Level’s limits.
Program material with different characteristics will require varying recovery times. The Gates STA-Level offers a front panel switch labelled “Double” and “Single” to adjust the recovery time. In the “Single” setting, recovery is slower, making it ideal for classical music or content with heavy peaks requiring a broad dynamic range. In this setting, compression attack time is also slower, at a minimum of 75 milliseconds. For speech and most popular music, the “Double” setting provides faster recovery times, better suited for handling frequent short peaks and maintaining a higher average modulation level. In this mode, the attack time drops to 25 milliseconds, making it more responsive to rapid signal changes.
The Gates STA-Level’s operation principles rely on a gated automatic gain control. A rectifier signal is used to adjust bias on the variable mu tube, reducing the gain. Push-pull stages are implemented throughout to cancel bias thumps and prevent motor-boating. Negative feedback in the output stages (V2, V3, and V4) helps reduce noise and distortion, ensuring smoother audio output. V1 is a high-quality tube that does not require feedback for stability.
The input pad of the Gates STA-Level is rated at 20 dB and helps prevent input overloading, allowing signal levels as high as +24 dBm without distortion. If this pad is removed, the maximum input level drops to +4 dBm, but the amplifier gain increases by approximately 20 dB.
The output pad offers between 10 dB and 16 dB of attenuation. One side of the shunt is variable to achieve a precise 6 dB adjustment. If this pad’s impedance is modified, care must be taken to maintain proper attenuation characteristics and ensure that any changes do not negatively impact frequency response. The variable resistance across the shunt may need to be adjusted to fine-tune the attenuation.
If both the input and output pads are removed, the amplifier’s maximum gain increases to approximately 62 dB. Under these conditions, the minimum input compression threshold is -44 dBm, with typical compression occurring at -30 dBm for 15 dB of gain reduction. The output level would be around +24 dBm under these conditions.
The amplifier’s circuit uses conventional push-pull amplification. Rectification in the 6V6 stage is triggered when the peak signal exceeds the positive bias of +25 volts. At this point, the RMS signal voltage must be greater than 17.5 volts to initiate rectification.
Compression starts when the bias reaches around -2.4 volts for 5 dB of compression, scaling to -76 volts for 40 dB of compression. The compression ratio can be altered from 3.3:1 up to 6:1 by adjusting the bias. Reducing the bias below +25 volts lowers the compression ratio, with practical ratios as low as 1.6:1 under certain conditions.
The Gates STA-Level’s attack and recovery times are governed by the RC circuit components, specifically C10, C11, and resistors. Recovery times vary between the “Single” and “Double” switch positions, with the latter allowing a more complex recovery profile that can handle sustained signal peaks.
