Manley MONO VARIABLE MU LIMITER / COMPRESSOR 5670 MODELS - Manual (Page 9)

TECHNICAL NOTES
SWITCHING ON
The power switch is located on the right hand corner of the front panel. Flip the switch up to turn on the
Limiter Compressor and down to turn off. Do not do this up and down rapidly - could cause damage.
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. One
symtom of failing tubes is increased noise floor, another is microphonics.
NOTE
Allow this unit to sufficiently 'warm up' before operating. This unit employs solid state rectification and
reaches optimum operation within 30 minutes. Furthermore, any adjustments such as meter calibration
should be performed when the unit has sufficiently 'warmed up'.
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 benefits 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 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.
HUM
This unit is meant to use the third pin of the mains as the ground reference. The GROUND TERMINALS are one
method to reduce hum. Verify that the two terminals are connected with the ground strap or that the hum is less with the
strap off one terminal. If all else fails try a three pin to two pin AC adapter rather than break off the AC ground pin.
More than likely it is the input/output cabling at fault. Transformer isolated balanced inputs and outputs are most
immune to ground problems. These balanced inputs and outputs will interface to unbalanced inputs and outputs
automatically. This is a prime benefit with transformers.
Alternative ground schemes are common in studios. The two most used are the "textbook style" where cutting shields at
either inputs or outputs to prevent ground loops and the "star style" of not using any AC grounds but supplying a
separate ground wire from a central ground to each piece of gear. The textbook style could either rely on CHASSIS
GROUND being connected to CIRCUIT GROUND with the supplied strap or get its ground from the rack rails. The
supplied strap is safer. The star ground method would likely require not using the strap and the CIRCUIT GROUND is
connected to the ground wire with a "U" or "#10 fork" crimp. CHASSIS GROUND may be connected to the rack rails
deliberately or not. There are other variations that sort of work too. It is likely that this unit will be the least headache
because of: A) low current draw, B) transformer isolated & balanced ins and outs, C) the grounding options available
with the terminals, D) low input and output impedance. Note however that the low input impedance may be a difficult
load for cheap IC based gear. Less "headroom" or more distortion would be a symtom. The "cure" - use "pro" gear
(designed to drive 600 ohms) to drive this unit.
Another source of hum can be equipment stacked on top of one another. This is not a good plan from the ventilation
standpoint generally and it is likely to introduce hum, buzz or noise into the system. Certain gear radiates magnetic
fields or high frequency noise around its chassis and other gear may be prone to receiving these fields. Distance helps
greatly. This is often worth a try.