Analogue Tube AT-1 User manual

AT-1 Mono Limiter
User Manual
Serial No ________

Thank you for purchasing the AT-1. This compressor and its development represent state
of the art passive design using tried and tested tube technology that includes the
development of the 6386LPG dual triode. The re-introduction of these special tubes
represents several years of research and in many ways is at the heart of the operation of
these compressors. The operation and performance of the AT-1 is the same as similar
Fairchild 660 units of the day, but now has a new creative element in the production of
music with some additional features allowing the engineer to develop individual tracks and
titles.
Through careful adjustment the unique features of this compressor allow all types of
instruments and sound - typically in complete mixes or as individual tracks - to sit ‘up
front’sounding ‘fatter’ and ‘bigger’ in the mix. For example, a vocal track that sits back in
the mix can carefully be brought forward adding depth, dimension and clarity, gluing the
image together for all types of programme, making this natural sounding compressor an
indispensible tool.
The AT-1 uses a double triode gain reduction amplifier and a push-pull amplifying stage
that produces a high voltage side-chain for gain reduction control. The result is that the
gain-controlled amplifier never produces any audible or observable thumps or pops.
Contrary to most limiting amplifiers of the day, this unit has extremely low distortion and
noise under all conditions, both as a straight-through amplifier and under maximum
limiting conditions.
The attack time of this compressor is made extremely fast (micro seconds) in order to catch
short transients the release time is adjustable from 0.3 seconds to 25 seconds in six steps.
Two of these have release times which are automatic functions of the program material,
providing fast recovery for short-duration peaks and an automatic reduction with a very
long recovery time of overall gain should the program level remain high.
Owing to the wide choice of attack and release times, as well as the automatic recovery
positions, this unit can be used to limit program material severely without producing the
audible thumps or pumping so often associated with limited program material.
All TC settings are the same as the original Fairchild 660/670 units
Like the original Fairchild 660, the AT-1 incorporates a single independent feedback
limiter which can compress or limit signals. It is designed to be placed in any normal line
level circuit across the mix bus (if 2 are used) or inserted in a channel.
The AT-1 has a few distinguishing features that set it apart from the AT-101 stereo units.
Particularly the attenuator control as this is an original tapered step attenuator remade as it
was back in the 50’s. This control sweeps from ∞to 0 db. Your limiter is a versatile tool
and is capable of handling and manipulating a large variation of signal program and
provides more functionality than the basic original units by having a separate balanced
*key* input splitting up the GR stage and AC threshold stage at the push of a button.

Threshold Adjustments
It is possible (with careful adjustment of the AC and DC threshold controls) that the AT-1
can be used in any compression or limiting situation. The DC threshold control is found at
the rear of the unit marked *Thresh*
1. Set the AC threshold control to zero and adjust the input gain control for
unity. (between 16 and 12)
2. Turn both AC and DC threshold controls to their full clockwise positions.
3. Apply a 1k signal to the input 3db higher than the desired output level, and
adjust the DC threshold control to the desired output.
4. Increase the input signal to 10db higher than the desired output level, and
adjust the AC threshold control to the desired output level.
Normal operation
For normal operation adjust the attenuator controls for unity advance the stepped AC
threshold control CW until the desired output level is achieved. As can be heard the
compression / limiting action is removed completely when the AC threshold control is
rotated fully CCW. Note that milder use of these controls will allow a greater dynamic
range for compression.
The time constant switch positions give a wide choice of attack and release times. Position
3 may be the first point from which to start, this is a general purpose position;
Attack times (micro seconds) Position 3 400 micro seconds
Release time (seconds) Position 3: 2 seconds
With certain types of music or speech a faster time constant is needed and positions 1 or 2
could be better. For classical music a much slower position 4 is needed.
Getting your sound!
Experimentation is he key here as the unit’s performance and operation are dependent on
several factors; Attenuator position, AC/DC Threshold controls and the Attack/Release

switches. Generally a good starting point is to set the Attenuator controls to 16 and the AC
Threshold controls to 5 (midway) The DC threshold controls (At the back of the unit) are
generally set for the 1 o’clock position) this has been set in this way for general equipment
performance. But you may need to readjust for increased or decreased AC threshold
sensitivity.
Remembering that sometimes this may be on an un-balanced insert where signal levels may
be lower than say across a mix bus. Run the program and carefully adjust the Attenuator’s
CW gradually you’ll begin to see the meters move indicating gain reduction. Note also that
the position of the Attack/Rel switches may also gently change the tone and character of
your signal. This may be most noticeable on a position with a short release time like
position 1 for example. Gradually as the Attenuator is turned CW progressively more gain
reduction is observed on the meters. You will find after some experimentation that this may
also increase the flexibility of your controls. Consider also parallel compression.
The AT-1 has several new features
Stepped AC Threshold
External *Key* input
Stereo link
Bypass
The AT-1 provides a repeatable *stepped* 21 position AC threshold control particularly
useful when linking 2 units together and recalling tracks. This control has a 1% accuracy
per step.
The External *Key* input (when activated) splits the GR and AC Threshold circuits up and
provide access to the compressor side chain. The AT-1 provides access to a 2nd balanced
input which is particularly useful for De-essing, Ducking and Recording Stems. In this
mode the AT-1 can also be used as a straight line amp!
The AT-1 has a switchable stereo link where you can link 2 (or more) units together for
linked stereo operation. The link operation is the same as the stereo AT-101 where either
left or right channel has control.
Side chaining opens all sorts of possibilities
For example in De-essing one can heavily EQ the control signal i/p so when only selected
frequencies appear at the ext key i/p, does the compressor kick in. Such a compressor can
be used as a de-esser, reducing the level of annoying vocal sibilance in the range of 6-9
kHz. This arrangement involves for example an AT-1 compressor and an external equalizer
by feeding a 6-9 kHz-boosted copy of the original signal into the side-chain i/p or external
i/p input of the compressor.
Finally the AT-1 has a full bypass function where the unit can be switched fully out of the
signal chain during operation.

Balance and Zero adjustments
The unit normally maintains the balance and zero adjustments over a wide range
temperature, power line voltage and tube aging, small unbalances of <1/4db do not
normally produce any problems.
1. Allow the unit to warm up for at least 1/2hr
2. Adjust the Balance control until the same meter reading are achieved in each position
3. With the meters switched to Zero adjust the Zero control to read 0vu
4. If this adjustment does not produce a reasonable balance replace one or more of the
6386LPG tubes or the original 5 star GE version tubes if you are using them.
Your new AT-1 limiter comes fitted with XLR connectors at the rear of the unit for In and
OUT operation including the Ext Key i/p stage. EUK models are pin 2 hot whilst US
models are pin 3 hot. The AT-1 limiter is fully balanced in and out, as well as working
normally in mix insert applications.
In some cases these limiters are supplied with their own shipping Peli 0370 flight case.
Each case is filled with soft foam protecting the unit to during transit. These flight cases
should be retained for future use if the unit is to be transported or relocated. Please keep the
flight cases somewhere safe. Each unit it shipped with a full set of instructions that includes
signal graph specification and information relevant to each unit. These documents should
be kept in a safe place for future reference.
Adjustments
There are no internal adjustments.
1/ Meter calibration: The *Zero* control adjustment adjusts for the meter position. When
you make a *Zero* adjustment the meter needle moves. Calibration is made by adjusting
the meter needle for 0vu on the meter scale after the unit has warmed up for 20 mins and
measuring the voltage: pin 3 of V9 (6BL7) should read 200vdc
The *Zero* control is located on the front panel marked *Zero* This adjustment has been
set. After the unit has been switched on the meter needle will take at least 15 mins to
stabilise and reach 0vu
2/ The adjustment for the gain reduction stage heaters is fixed to 6.3vac

SPECIFICATIONS
Input impedance 600ohms
Output impedance 600ohms
Source impedance <4ohms
Input level >28db before clipping
Ext Key i/p line in Balanced
Output level Output 27dbm before clipping
Gain 18 db (no limiting)
Frequency response 20hz to 20khz <1 db (Straight line amp)
Thd+N vs Freq <70db @ 10khz
Limiting noises Below audibility
Attack times (micro seconds) 0.2 milliseconds in positions 1, 2, and 6.
0.4 milliseconds in positions 3 and 5
0.8 milliseconds in position 4
Release times
(from 10 db of limiting) Position 1: 0.3 seconds.
Position 2: 0.8 seconds
Position 3: 2.0 seconds
Position 4: 5.0 seconds
Position 5: Automatic function of program
Material:
2 seconds for individual peaks,
10 seconds for multiple peaks.
Position 6: Automatic function of program
Material:
0.3 seconds for individual peaks
10 seconds for multiple peaks
25 seconds for consistently
high program level
Compressing/Limiting Variable from 1:1 to 1:20 depending on DC threshold setting.

Power 230 volts 50hz 1 amps (EUK)
120 volts 60hz 2 amps (US 110w)
Peak I 2 amp RMS (US) 120v
Stability Unit maintains gain stability, gain
reduction stability and balance over a wide
range line voltage
CONTROLS
1 Input gain controls Original tapered step attenuator: ∞to 0 db
1 AC Threshold controls Stepped 21 position
1 DC Threshold controls Continuously variable
1 By-Pass switch Switches AT-1 completely out of circuit
1 Ext i/p switch Provides a 2nd balanced i/p and splits up the
GR and AC threshold circuits within the AT-1
when active.
1 Time Constant Switches
6 positions each, so as to provide fixed and variable time constants for any type of
Programme material.
1 Metering Switches
3 positions which allows measurement of plate current for each set of 6386 gain
reduction tubes.
Stereo Link Switch 2 or more independent AT-1 limiters
XLR left and right in and out Pin 2 Hot (EUK) Pin 3 (US)
Dimensions
Standard 19” rack. 6U panel space and 11” depth behind panel
Weight Approximately 20kg.
Tube complement 4-6386LPG; 1-6084; 1-5651; 1-12AX7;
1-12BH7;1-6BL7; 2-6L6GTA; 1-GZ34.
All i/p’s & o/p’s fully transformer balanced

Safety first!
Like all tube equipment extremely high voltages and potentials are present inside this
equipment, no attempt should be made to adjust the unit internally without the help of a
properly qualified service engineer. If in the unlikely event adjustments need to be made
please contact [email protected] first.
Looking after your AT-1 Limiter
Your AT-1 limiter should give you many years of uninterrupted service when observing the
following simple guidelines:
Operational Guidance.
Whilst you’re limiter is capable a great deal of limiting it also performs as a very clean and
dynamic line amplifier giving around a 20db increase.
However when heavy limiting is used a very high –Ve voltage is fed back to the grids of
the 6386 gain reduction tubes. As the input sensitivity becomes quite high at this level it is
generally not desirable to operate the Attenuator controls close to ‘open’ (fully CW) whilst
in this mode. This can allow a significant amount of signal through the Attenenuator’s. The
lf and hf performance and general operation will be greatly enhanced giving you more
flexibility with greater Attenuation if these controls and operated conservatively. For
example; try to increase your signal level into the unit, the input headroom has an excess of
20db!
Do not move the unit whilst ON
Do not move the unit whilst hot
Do not operate in small un-ventilated spaces
Allow the unit to cool down before moving
Fit soft rubber wheels to your free standing trolley
Allow free cool air to flow over unit when ON
Operate at the correct voltage!
Always fit correct tubes to your unit
Always switch unit OFF when not in use
Always use the Peli 0370 flight case for transportation
The AT-1 is very heavy around 20kgs, use 2 people when lifting or when fitting
unit to a rack
Temperature
When installing the AT-1 Limiter, please ensure that adequate ventilation is available.
These units consume around 110w (120vac 60Hz) of power. This equipment should not be
operated in an unventilated area like a flight case.

A 1U low noise 3 fan unit can be provided, These 19” rack units are operated at a ¼ of
their fan speed and sit approximately ½ u above the unit. If the limiter is to be fitted into a
free standing rack, It is recommended the limiter occupy it’s own space and not share it
with other equipment it should have a vented 3U space directly below the unit and a vented
3U space above to include a fan rack.
If the Limiter is operated in a stand alone trolley, it can be operated at a 60 degree angle
with fan assistance, as described above.
There are 12 tubes and a regulating power transformer for the 6386LPG tubes. The chassis
will become warm during operation –this is normal.
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attention to detail and an open ended commitment to customer satisfaction we believe that
once you own Analoguetube products you own them for life.
Simon Saywood - CEO Analoguetube Ltd
Rev SS04072014