The Technical Stuff
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25
Output (Makeup Gain)
Finally, an output control is employed to make up for the gain reduction applied by the gain reduction
circuitry; on the 6176, this is the function of the Output knob. Makeup gain is generally set so that the
compressed signal is raised to the point at which it matches the level of the unprocessed input
signal (for example, if a signal is being reduced in level by approximately -6 dB, the output makeup
gain should be set to +6 dB).
As you are adjusting a limiter or compressor, a switchable meter such as the one provided by the
6176 can be helpful in order to view the strength of the incoming signal (displayed when the meter is
set to PRE), the strength of the outgoing signal (displayed when the meter is set to COMP), or the
difference in levels between the original input signal and the gain-reduced output signal (displayed
when the meter is set to GR). When in GR mode, the 6176 meter will read 0 dB when there is no
incoming signal or when no compression is being applied.
About ALL (“All-Button”) Mode
One of the most unique features of the 6176 limiter / compressor is the ability to select a ratio value
of ALL. In this mode (sometimes also known as “British Mode”), the 6176 duplicates the so-called
“4 button trick” that occurs when all four ratio buttons on an original 1176 are pushed in
simultaneously.
In ALL mode, distortion increases radically due to a lag time on the attack of initial transients (a
phenomenon which might be described as a "reverse look-ahead"). The ratio goes to somewhere
between 12:1 and 20:1, and the bias points change all over the circuit, thus changing the attack and
release times as well. The unique and constantly shifting compression curve that results yields a
trademark overdriven tone that can only be found in this family of limiter/compressors.
About “Class A”
Most electronic devices can be designed in such a way as to minimize a particularly unpleasant form
of distortion called
crossover distortion.
However,
the active components in “Class A” electronic
devices such as the 6176 draw current and work throughout the full signal cycle, thus eliminating
crossover distortion altogether.
About Negative Feedback
Negative feedback is a design technique whereby a portion of the preamplifier’s output signal is
reversed in phase and then mixed with the input signal. This serves to partially cancel the input
signal, thus reducing gain. A benefit of negative feedback is that it both flattens and extends
frequency response, as well as reducing overall distortion. Turning the 6176 front panel Gain switch
clockwise (i.e., increasing it) reduces negative feedback, which has the effect of also increasing the
amount of the input tube’s harmonic distortion, a major contribution to the “warm” sound
characteristic of tube equipment. (
see #1
on page 3
)
