Tube-Tech LCA2B Dual VCA Compressor / Limiter
The TUBE-TECH LCA2B is a two-channel unit with an independent compressor and limiter per channel. The unit is all tube based (except for the power supply and side chain circuit). The VCA (1 dual triode) is placed between the input transformer and the output stage (2 dual triodes).
The audio signal is picked up after the VCA and fed to the side chain circuit. The control signal from the compressor and from the limiter is combined and sent to the link switch and to the control amp, which feeds the VCA. The two bi-directional link busses are accessible at two 1/4″ stereo jack sockets on the rear panel. The channels can be linked together for stereo applications and also linked to other LCA 2B’s via the two link busses and a standard stereo jack/jack cord.
The compressor has six att/rel presets as well as manual control. The limiter att/rel is fixed. The LED display is fed from the control amp. The limiter LED is fed from the limiter buffer. The audio path is fully symmetrical from input to output. Input and output have fully floating transformers. All DC voltages are stabilized, except the anode voltage for the output stage.
A note about calibration:
As with all tube VCA compressors, the LCA2B tube will “drift.” How much and how long it takes is impossible to tell, but Tube-Tech selects the tubes with the least drift for the VCA circuit. This drift is why the manual recommends calibration every six months. Even though the unit may stay calibrated within specs for years, it is important that the calibration is within specs if the unit is used with a high Ratio and low Threshold (this is the maximum strain on the VCA balance). If at extreme settings (threshold and limit at full CCW) the sound starts to “pump,” the unit may need calibration.The following test will tell you if your unit needs to be calibrated:
- Turn the THRESHOLD for both compressor and limiter fully counter clock-wise.
- Connect a 1kHz tone at a level of 0 dBu (-4VU)* to the input and turn the LIMITER THRESHOLD slowly clock-wise until the LIM led (RED) turns slightly on.
- If the LIM led (RED) turns on abruptly or even starts oscillating, the balance should be calibrated.
This test can also be done with audio:
- If the LCA2B needs calibration, the compressor starts pumping with this extreme setting.
How to generate a 0dBU signal with Pro Tools:
- Create blank session in Pro Tools. Add a single track and on that track assign the “Signal Generator” plugin and assign that track to an output.
- Change the settings on the Signal Generator to Frequency: 1000 Hz, Level: -18 dB, and Signal: Sine
- Grab an XLR cable and connect it to the corresponding output previously selected in your DAW. With a DMM (digital multi-meter) capable of measuring a 1000Hz AC signal measure the voltage of pins 2 and 3 on the XLR.
- Adjust the level in the signal generator plugin up or down until your DMM reads 0.775VAC.
- Once adjusted connect the XLR to your LCA-2B and proceed with your test.
- Variable ratio from 1.6:1 to 20:1
- Variable threshold: off to 10 dBu
- Selection between six presets and manual controlled att/rel
- Variable threshold: off to 0 dBu
- On/off switch
- LED indication
- Separate compressor and limiter
- Frequency response @ -3 dB: 5 Hz to 50 Khz
- Low noise: < -85 dBU @ 10 dB gain
- Output gain control
- Clickless bypass relay
- LED display for gain reduction
|Impedance Input:||> 5 kohm|
|Output:||< 60 ohm|
|Frequency response @ -3 dB:||5 Hz to 50 kHz|
|Distortion THD+N @ 40 Hz||0 dBu: < 0.15 %|
|+10 dBu: < 0.15 %|
|Noise Rg||200 ohm|
|Output Gain @||0dB +10 dB|
|22 Hz-22 kHz:||< -85 dBu < -80 dBu|
|CCIR-468-4:||< -78 dBu < -73 dBu|
|Cross talk @ 10 kHz:||< -70 dB|
|CMRR @ 10 kHz:||< -60 dB|
|Max. output @ 1 % THD+N:||> +26 dBu|
|Gain:||-6 dB to +10 dB|
|Ratio:||1.6:1 to 20:1|
|Threshold:||off to -10 dBu|
|Attack:||0.3 ms to 70 ms|
|Release:||0.07 ms to 2 s|
|Threshold:||off to 0 dBu|
|Tubes||ECC 81 2 pc|
|ECC 82 2 pc|
|ECC 83 2 pc|
|Height:||2 units 88 mm 3.5"|
|Width:||480 mm 19.0"|
|Depth:||205 mm 8.1"|
|Net:||6.5 kg 14.3 lbs.|
|Shipping:||7.5 kg 16.5 lbs.|
|Power requirements||@ 115 V/230 V, 50-60 Hz: 30-45 W|