Mercury 66 MKIII Tube Limiting Amplifier

The new Mercury 66 Studio Limiting Amplifier Mk. III is an ‘All Tube - All Transformer’, fully balanced, variable-bias limiter based on the most sought after product in this genre, the legendary Fairchild 660. This limiting amplifier is recognized as an essential tool for making great recordings. 

The Mercury 66 Studio Limiting Amplifier uses a single push-pull stage of amplification which is transformer coupled input and output (XLRs) with no capacitors in the signal path) with a high control voltage. The result: the unit never produces any audible thumps, and has low distortion and noise under all conditions (either as a limiter or straight through amplifier). The Control amplifier is also fully balanced, transformer coupled in/out. The unit makes use of a very stable high current vacuum tube voltage regulator. It has ample and accessible input, threshold, attack, release time and metering controls allowing for convenience in the control room when making adjustments. The attack time is very fast and catches short transients. The release time of .3 to 25 seconds provides real flexibility: Three positions of the six available make the release time a function of program material, with fast recovery for short duration peaks and overall reduction of program level should the program level remain high. It can be used as a limiter or compressor. For example, use it as a 2 to 1 ratio compressor with a threshold 5 dB below average level, or as a peak limiter with a 20 to 1 ratio with threshold 10 dB above normal program level, or anywhere between these extremes.

One notable change on the Mercury 66 is the addition of the DC Adjust Control. On an original Fairchild 660 this was called DC Threshold and was on the rear of the unit (not accessible from front panel). Mercury put that control on the front panel and labeled it DC Adjust to not confuse the Threshold function. The DC Adjust lets you control DC (Direct Current) in part of the circuit to give the user more control of the functionality and level of compression. 

Front Panel Controls:
Input Gain, AC Threshold, Attack/Release, DC Adjust, Meter Zero (Trim), Stereo Link and Power on/off Switch. M66 Attack and Release Times:

• Position 1 - Attack [ Fast - 0.2 ] Release, in Seconds [ 0.3 ]
• Position 2 - Attack [ Fast - 0.2 ] Release, in Seconds [ 0.8 ]
• Position 3 - Attack [ Med. - 0.4 ] Release, in Seconds [ 2.0 ]
• Position 4 - Attack [ Slow - 0.8 ] Release, in Seconds [ 5.0 ]
• Position 5 - Attack [ Med. - 0.4 ] Release, in Seconds [ Auto: 2.0 peaks, 10.0 program ]

Specifications:

• Transformers: 600 ohm Input and Output
• Gain: 7 db (no limiting)
• Frequency response: 40 Hz- 15 kHz +/- 1 db
• Noise: = -74 dB below OdBm
• THD: = < 1%
• Compression Ratio: 1:1 to 1:20
• Attack Time: 1] 0.2, 2] 0.2, 3] 0.4, 4] 0.8 5] 0.4 6] 0.2 ms
• Release Time: 1] 0.3, 2] 0.8, 3] 2 sec, 4] 5 sec, 5] auto 1, 6] auto 2
• Tubes: (1) 6BC8, (2) 6V6, 12BH7, 12AX7, EL34, 5AR4 or GZ34, 5751
• Rack Size: 3u