TUBE LIFE
As with all tubes, their quality degrades with age. This is due to cathode emission, a natural process found in all tubes.
We recommend that you have your unit checked every 4-5 years, depending on usage, usually the limiter will require re-
tubing after this time has elapsed. Furthermore, any adjustments such as meter calibration should be performed when the
unit has sufciently ‘warmed up’.
Almost all compressors have only two “gain” controls of a possible three: Input, Threshold and Output. With all three
theoretically one is redundant. Utilizing a Dual Input has confused some “engineers” when it was needed for two different
sounds. Simply nd a good compromise position on the INPUT LEVEL and use the Threshold and Output as usual. The
Input control is very useful with stereo material - try it - but it is not meant to be used for stereo “fades”. Use it to set up a
general level and nal tweaking.
OPERATION
The principle of operation of this unit is based around a 5670 variable-mu (variable gain) double triode operating in a
fully balanced or symmetrical circuit.
Because of the need to preserve symmetry, a balance-adjustment pre-set potentiometer is placed on the vertical circuit
board adjacent to the 5670 variable-mu double triode. This control is set so that the two individual triodes amplify
equally. (See Adjustments Section).
The benets of utilizing a variable-mu vacuum tube as the ‘heart’ of the limiting (or compression) function is that a
widely varying range of input signals can be handled in a rapid-acting fashion without introducing harmonic distortion -
unless the operator / engineer wishes to do so for creative reasons.
As with all compressor limiters there will be little change of gain reduction with various settings of ATTACK and
RELEASE controls using a sine wave source. With music there will be changes in the amount of gain reduction with
changes of these controls.
A complete re-calibration is typically only required when the unit has new tubes installed; most of the adjustments (with
the exception of Steps 1 and 6) affect nothing but calibration of the meters. However, the front-panel DC Balance adjust-
ment in step #1 should be checked periodically (every 6 months or so) as this can affect the distortion level of the unit.
EQUIPMENT REQUIRED:
Digital multimeter
Steady-state signal source (audio signal generator, console test tone) capable of generating +4dBm into 1kOhm load
Meter or other means of measuring audio signal level in decibels (multimeter with a decibel scale, console meter, Pro-
Tools meter, etc.)
Trimpot adjustment tool, Spectrol/Vishay #008T00 , or equivalent (a tiny athead screwdriver will sufce)
INITIAL SETTINGS:
Refer to labeled front panel drawing on page 6 for control locations.
In/Bypass switch: IN
Comp/Limit: LIMIT
Recovery: FAST
Threshold: FULL CW
Output Attenuate: FULL CW
Attack: FULL CW
Input Level: FULL CCW (initial setting only)
Link-Sep: SEP
HP side-chain: OUT
(continued on next page)
TECHNICAL NOTES
15
SERVICE ADJUSTMENTS
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