Compression Obsession: Why does the 1176 sound so cool?…Continued
In my previous installment, we talked about the 1176’s famous ‘all-button mode”. Let’s backtrack a little, and explore the 1176 as a whole, and look again at the controls as well as some of the other reasons the 1176 sounds the way it does.
Dialing in the 1176 properly
The basic controls for the 1176 are Input, Output, Attack and Release, plus Ratio Selection.
Selecting ratio is typically your first step. The 4:1 and 8:1 ratios are commonly used for compression, 12:1 and 20:1 are used for peak limiting.
As I said in the last installment, there is no threshold control on the 1176LN. I suggested that the input knob doubles as the threshold control, but there’s a deeper wrinkle: the threshold is also determined by the ratio selection (the higher the ratio, the higher the threshold). The Output knob controls make up gain and final output level. Remember, cranking up the Input knob also affects post-compression output levels.
Note: The top button on the right side of the meter switches the meter to show the amount of gain reduction, while the central two buttons display the output level referenced to either +8dBm or +4dBm levels. Be aware that the two output metering positions can introduce a little more distortion. Depending on your view, this may or may not be a bad thing.
Now set attack and release to taste!
Remember that the 1176 is a program dependent compressor: The attack and release are program dependent, and so is the ratio. This to me is the heart of the 1176 sound. But there are many other contributing factors…
Factors of Cool:
- During the time of the 1176’s incarnation, the majority of audio equipment employed 600ohm input impedances and therefore demanded a lot of signal current. As a result, output stages had to be robust. Bill Putnam Sr. addressed this with a Class A amplifier based around a special custom-wound output transformer, the high output performance of which became another major element to the sound of the 1176. This transformer performed two functions. It converted between the unbalanced internal circuitry of the limiting amplifier and the balanced external connections, and it also provided the correct impedance matching for the 600ohm line.
- However, Putnam knew that transformers were notorious for introducing distortion, so he used additional sets of secondary and tertiary windings to provide feedback signals, a practice already employed in tube amplifiers. In this way the transformer was enclosed within the negative feedback of the output amplifier and its non-linearities were compensated and corrected for automatically, producing very low output distortion.
Since the 1176 uses a mic-level input transformer and a FET for gain reduction followed by this custom high output transformer, by its nature it has a hot sound. The 1176 has a 0.5% THD specification with 45 dB of gain. This is more gain than most modern compressors. - The original 1176 emerged in 1966 after Putnam had successfully designed and built a remotely controlled amplifier around the newly invented Field Effect Transistor (FET): The transistorized 1108. For minimal distortion, FETs require careful circuit design so that they operate within their narrow linear range. In the case of the 1176, the FET is used as a voltage variable resistor to control compression. Here’s how it works: Signal goes through the input stage, including going through the FET itself. The signal then goes to the output stage, but before the signal hits the output stage, the signal is tapped, sent to a sidechain circuit where the voltage is fed back into the FET, and controls compression. Depending on how hard you crank the input knob, the FET will be affected by the voltage and compress to a greater or lesser degree, by shunting the voltage to ground. So the FET works as a voltage variable resistor and determines how much of the signal is shunted to ground. Hence, the 1176 works as a “feedback” compressor, another important factor in the sound of the 1176.
- In 1970 the 1176 received a significant update largely designed by an engineer named Brad Plunkett. His modifications improved the noise performance tremendously and resulted in the 1176LN ('Low Noise'). The D and E 'black-face' LN revisions are widely considered to be the best-sounding models, therefore Universal Audio modeled our reissue after these two models.
Enjoy!
-Will Shanks
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