T L Audio User Manual C1 VALVE COMPRESSOR Tony Larking Professional Sales Limited, Letchworth, England. Tel: 01462 490600. International +44 1462 490600. Fax: 01462 490700. International +44 1462 490700. INTRODUCTION The T L Audio Valve Compressor combines classic valve techniques with low noise solid state circuitry to produce a unit offering a fully controllable compressor on each of its two channels. Both channels feature balanced and unbalanced line level inputs, plus a mic input with switchable 48V phantom powering. Further flexibility is provided from a front panel auxiliary input which accepts mono or stereo keyboard and guitar inputs. Typical applications include direct recording to tape, instrument preamplifier, use as outboard equipment during mixdown, and as a main stereo compressor for monitoring and PA systems. The block diagram of one of the channels is shown in fig.1. A solid state, electronically balanced input amplifier is used, to achieve state of the art performance with very low noise, low distortion and wide bandwidth. The input includes a switchable second order high pass filter, -3dB at 90Hz, to remove unwanted LF noise. A triode valve is used as a second stage voltage amplifier, to obtain the classic valve sound and gradual overdrive characteristics. Two XLR input sockets are provided per channel, for mic and line inputs. The mic input is suitable for low impedance (150-600 ohm) microphones, with a gain control range of +16 to +60dB. 48V phantom power is available on each channel. The line input has an effective overall gain range of -10 to +35dB, allowing the valve stages to be fully driven from line level signals in the range -20 to +20dBu. A front panel mounted auxiliary jack socket for each channel allows mic or line level inputs to be easily connected to the compressor. The aux input may be used for a stereo keyboard or mono guitar, etc. An insertion point is provided in the sidechain circuit to allow an equaliser or other processor to be used for frequency conscious compression, etc. The insertion point is unbalanced, at a nominal operating level of -2dBu. The sidechain processing is solid state, whilst a second triode valve stage is employed in the compressor circuit. The compressors feature continuously variable controls for threshold, attack, release, ratio and gain make-up. A certain degree of programme dependant variation is incorporated, particularly of the attack and release times. Full stereo operation is supported, with ganged controls and linked control voltages. The compressors may be by-passed for A-B comparison. Twin illuminated VU meters are provided, which may be switched to monitor the output signal level or compression applied. A solid state, electronically balanced output stage with XLR connector completes the channel. Dedicated unbalanced outputs are also provided on jack sockets. The outputs are switchable for +4dBu or 10dBu nominal output level. Please read this manual fully before installing or operating the Compressor. PRECAUTIONS The T L Audio Valve Compressor requires very little installation, but like all electrical equipment, care must be taken to ensure reliable, safe operation. The following points should always be observed: - All mains wiring should be installed and checked by a qualified electrician, - Ensure the correct operating voltage is selected on the rear panel before connecting to the mains supply, - Never operate the unit with any cover removed, - Do not expose to rain or moisture, as this may present an electric shock hazard, - Replace the fuse with the correct type and rating only. Warning: This equipment must be earthed. INSTALLATION AC Mains Supply. The equaliser is fitted with an internationally approved 3 pin IEC connector. A mating socket with power cord is provided with the unit, wired as follows: Brown: Live. Blue: Neutral. Green/Yellow: Earth (Ground). All mains wiring should be performed by a qualified electrician with all power switched off, and the earth connection must be used. Before connecting the unit to the supply, check that the voltage selector switch on the rear panel is correctly set. The unit may be set for 115V (accepting voltages in the range 110V to 120V, 60Hz AC), or to 230V (for voltages in the range 220V to 240V, 50Hz AC). The fuse required is 20mm anti-surge, 1AT. Warning: attempted operation on the wrong voltage setting, or with an incorrect fuse, will invalidate the warranty. Audio Inputs. Each channel has two female, 3 pin XLR connectors, for mic and line sources. Both are compatible with either balanced or unbalanced signals, when the mating connectors are appropriately wired: Balanced inputs: - Pin 1 = Ground (screen). - Pin 2 = Signal Phase ("+" or "hot"). - Pin 3 = Signal Non-Phase ("-" or "cold"). Unbalanced inputs: - Pin 1 = Ground (screen) - Pin 2 = Signal Phase ("+" or "hot"). - Pin 3 = Signal Ground When using unbalanced signals, the signal ground may be obtained by linking pins 1 and 3 in the mating XLR connector. Good quality screened cable should be used, particularly for microphone or low level sources, to prevent hum or noise pickup. Unbalanced Line Inputs. An unbalanced line level input is provided for each channel, on a 0.25" mono jack socket. The mating plugs should be wired as follows: - Tip = Signal Phase ("+" or "hot"). - Screen = Ground. Auxiliary Inputs. A 2 pin (mono) jack plug is required, which should be wired as follows: - Tip = Signal Phase ("+" or "hot"). - Screen = Ground. Insertion Points. The insertion points are interfaced via a 3 pin, 0.25" switched jack socket on the rear of the unit. The pin connections are: - Sleeve = Ground, - Tip = Send, - Ring = Return. The insertion point is unbalanced, and operates at a nominal level of -2dBu . If used as an additional send only (e.g. as a send to a tape machine or monitor mixing desk), the Tip and Ring should be wired together, to preserve the signal path through the insertion point. Outputs. The outputs are via balanced, 3 pin male XLR connectors. The mating connectors should be wired as follows: - Pin 1 = Ground (screen), - Pin 2 = Signal Phase ("+" or "hot"), - Pin 3 = Signal Non-Phase ("-" or "cold"). If an unbalanced output is required, pins 1 and 3 should both be connected to ground. Nominal Output Level. A switch on the rear panel allows the output to be matched to equipment at a nominal operating level of +4dBu or -10dBu. Most professional equipment requires +4dBu (approximately 1.2V rms), but some small mixing consoles, portable tape recorders or domestic audio equipment require -10dBu (approximately 225mV rms). The switch should be set to the position which results in the best signal to noise ratio, whilst preserving sufficient headroom. Unbalanced Outputs. An unbalanced line output is provided for each channel, on a 0.25" mono jack socket. - Tip = Signal Phase ("+" or "hot"). - Screen = Ground. Ventilation. The unit generates a small amount of heat internally. This heat should be allowed to dissipate by convection through the grill in the front panel, which must not be obstructed. Do not locate the unit where it will be subject to external heating, for example in the hot air flow from a power amplifier, or on a radiator. The compressor may be free standing, or mounted in a standard 19" rack. Rear Panel. The rear panel connectors are identified in fig.3. OPERATION. Front Panel. The front panel controls for one channel are identified in fig.2. Input Stage. Ensure that the correct input connector, mic or line, is being used and that the front panel switch is set to the appropriate position. Note that both mic and line inputs may be connected at the same time, but only one is selected. The input selector switch selects between the line input, mic input, mic with +48V phantom power (signalled by a red LED), and the Aux input. CAUTION: Switching phantom power on, or plugging a microphone in with phantom power applied, may cause a click or thump in your loudspeakers. To prevent this happening, ensure that the system gain is set to minimum (e.g. on your mixing console fader or power amplifier), before operating the switch or plugging in a microphone. If the gain required is not known, set the control to minimum with the EQ by-passed, and gradually increase the gain until the required output level is achieved. Auxiliary Input. The front panel auxiliary input sockets may be switched between high level signals ("Keyboard") and low level pick-ups ("Guitar"), and are provided with an input gain control. Note that the channel gain controls remains operative when the Aux input is selected. The two aux input sockets may be used, for example, for a stereo keyboard. The compressors may be operated independently, or linked for true stereo operation. Aux A input is also normalled to the B input, so both channels may be driven from a mono input (guitar, etc). With different channel gain settings, one channel may be lightly compressed, whilst a higher degree of compression is applied to the high gain channel, resulting in stereo effects from a mono input. Compressor Operation. A compressor functions by reducing the gain of the signal when it exceeds a certain level, or threshold. The amount of gain reduction may be fairly gentle through to limiting, where the signal is clamped at the threshold level. The amount of gain reduction is determined by the ratio control, which is calibrated as a ratio of the output to input signals. The gain of the signal is reduced by a voltage-controlled circuit. Variable time constants are applied to the control voltage to adjust the rate at which the gain is reduced, called the attack time, and the rate at which unity gain is restored after the signal returns to below the threshold, referred to as the release time. The effect of compression is to limit the dynamic range of a signal. It may be used to keep a variable output from a bass guitar, for example, at an even level, or to add punch to vocals, drums, guitar or a complete mix. Whist the subjective sound quality can be improved by compression, the overall signal level may be reduced. A gain make-up control at the output of the compressor stage allows the signal level to be brought back to the same loudness as the uncompressed signal. Frequency selective compression may be obtained by inserting an equaliser into the sidechain signal, from which the control voltage is generated. Threshold. The TL Audio compressor has a variable threshold, set by a rotary control calibrated from +20dBu to -20dBu, resulting in increased compression as the control is rotated clockwise. Attack and Release. The attack time is variable from 0.5msec to 50msec. At 0.5msec attack, the compressor is fast enough to reduce the gain of a 1KHz signal in less than half a cycle, effectively preventing an overload of any following equipment which has limited headroom, such as a digital processor, tape machine or transmitter. The release time is variable from 40msec to approximately 4 seconds. Adjustment of the attack and release times allows unobtrusive compression to be applied to virtually any audio signal, but should very short transients occur the time constants become signal dependant, generally reduced, to prevent a slow release leaving a "hole" in the signal after the transient. Also, a fast release setting will be extended by a slow attack setting. Due to this automatic modification of the time constants, the controls are simply calibrated "fast" to "slow". Ratio. The ratio may be varied from 1:1.5 (very gentle compression) to 1:30 (near limiting). The compressor normally operates with a "soft-knee", i.e. the compression is gradually introduced as the signal passes the threshold, in which case the ratio refers to the compression eventually obtained. Gain Make-Up. Up to 20dB of gain make-up may be applied, to retain the subjective loudness of the signal. Stereo Operation. In normal 2 channel operation, the controls operate independently for each channel. The compressor may also be set for full stereo operation, where the Threshold, Attack, Release and Ratio controls for channel A control both channels. The input gain and gain make-up controls remain separate to allow relative input/output gain balance adjustment. In stereo mode, the control voltages are also linked, ensuring that the same amount of gain reduction is applied to both channels (even if one signal is below the threshold). This ensures that the stereo image is preserved. VU Meters. The compressor is equipped with twin, illuminated VU meters. These meters normally monitor the audio output from the unit, and are calibrated for 0VU = +4dBu. The reference point may be internally adjusted by your dealer if required. The meters may be switched to indicate the amount of compression occurring. If the signal is below the threshold, the meters will indicate 0dB: i.e. no gain reduction. As the signal passes through the threshold, the meter will indicate the gai ...