Fairchild 670 User manual

INSTRUCTION MANUAL
MODEL 6 7 0
STEREO LIMITER
Serial No,
Fairchild Recording Equipment Corporation
10-b0 U5th Avenue
Long Island City 1, New York
December 1959
(Supersedes all previous issues)
1

MODEL 670 STEREO LIMITER
GENERAL DESCRIPTION
The FAIRCHILD MODEL 670 incorporates on one chassis two independent
limiters which can limit either two independent signals, such as the
left and right channels of a stereo signal, or the vertical and lateral
components of same. The latter is accomplished by first bringing the
two stereo channels through a matrixing network, dividing them into
their vertical and lateral components, limiting them independently,
and recombining them through a second matrixing network into left and
right channels.
Each half of the MODEL 670 uses only a single push-pull stage of audio
amplification and an extremely high control voltage, with the result
that the Automatic Gain-Controlled Amplifier never produces any audible
or observable thumps. Contrary to most limiting amplifiers heretofore
available, 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 the unit is made extremely fast in order to catch
short transients, and the release time is made 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 very
long recovery time of overall gain should the program level remain
high.
In the circuitry used, each half of the amplifier consists of two
separate amplifiers: one is the Automatic Gain Amplifier, and the
other provides the necessary power for producing the DC control voltage.
Because of this, the controls are somewhat different from those of most
other limiters. Essentially there is one control common to both channels
which will switch between independent limiting and vertical-lateral
component limiting, besides four controls for each channel: a Switch
for Metering; an Input Level Control; a Limiting Threshold Control;
and an Attack and Release Time Switch.
Owing to the wide choice of attack and release time, as well as the
automatic recovery feature, this unit can be used to limit program
material severely without producing the audible thumps or pumping so
often associated with limited program material. A limiting meter is
provided with provision for connecting a remote meter to the terminals
exposed at the rear of the amplifiers.
The MODEL 670 is designed to be placed into any normal line level
circuit and can be set to have a unity gain at no limiting. Since all
critical components are of highest quality, working well below their
maximum ratings, a dependable operation can be expected.
2

LIMITING IN STEREO DISK RECORDING
The cutting of STEREO DISKS has uncovered a number of new problems
heretofore unknown. The normally modulated STEREO groove requires
approximately twice the space of lateral grooves for similar modula¬
tion, yet the available space is no greater. In addition, the two
STEREO channels, depending on their phase relationship, may result
in either lateral or vertical modulation, or a combination of both.
It stands to reason that some peaks may result in purely vertical
modulation on the disk, others purely lateral. If this were allowed
to happen continually, the result would be only 15 minutes on the
STEREO LP side or else it would be necessary to reduce the recorded
level radically. Otherwise, judicious use of variable pitch and
variable depth would have to be exercised. The fact is, because of
the smaller playback tip radii and the response of the playback
pickups to vertical as well as lateral motions, the signal-to-noise
ratio on STEREO DISKS is already reduced over the lateral counterpart.
Besides, radical use of automatic variable depth will further increase
processing problems already severe in STEREO DISKS.
Another problem exists because of the difficulty of tracking large
vertical amplitudes. Most commercially available STEREO pickups have
considerably less vertical compliance and consequently are less capable
of tracking large vertical modulations. Also, we should not forget
that the tip radius on the STEREO playback stylus is still 0.7 mil,
which necessitates a minimum of 1 mil groove width, or poor tracking
might result.
The one apparent solution to these problems is to break the STEREO
signal down to its respective vertical and lateral components. Limit
the vertical and lateral canponents independently corresponding to
the available groove space and depth, then recontoine these components
to regain the original STEREO signal. This can be done with the
FAIRCHILD MODEL 670 LIMITER and many thousands of STEREO masters
have been cut successfully with the help of this FAIRCHILD unit.
The limiting of the vertical and lateral components instead of the
left and right channels has additional merits. Such limiting will
retain the spatial distribution of instruments and soloists as orig¬
inally recorded without producing any annoying image drift. Of course
some program material of the ping-pong type requires independent
limiting of each channel, and this is also available in the MODEL
670 LIMITER.
3

SPECIFICATIONS (Each channel, unless otherwise specified)
INPUT IMPEDANCE
OUTPUT IMPEDANCE
OUTPUT LEVEL
GAIN
FREQUENCY RESPONSE
NOISE LEVEL
LIMITING NOISES
INTERMODULATION OR
HARMONIC DISTORTION
ATTACK TIME
(adjustable)
600 a
600 Q
+U or +8 VU (+27 dbm clipping point)
7 db (no limiting)
UO cycles to 15 kc ± 1 db
70 db below +h dbm
Below audibility.
Less than 1% at any level up to +18 dbm
output (no limiting).
Less than 1% at 10 db limiting and +12 dbm
output. v
.2 milliseconds in positions 1, 2, and 6.
• U milliseconds in positions 3, U, and 5.
RELEASE TIME
(from 10 db of limiting)
*3 seconds.
.8 seconds.
2 seconds.
5 seconds.
Automatic function of program
2 seconds for individual peaks,
for multiple peaks.
Automatic function of program
.3 seconds for individual peaks,
10 seconds for multiple peaks, 25 seconds
for consistently high program level.
Position Is
Position 2:
Position 3:
Position U:
Position 5J
material:
10 seconds
Position 6:
material:
COMPRESSION RATIO
SEPARATION
POWER REQUIREMENTS
Variable from 1:1 to 1:20 above a predeter¬
mined level. Predetermined level factory-
adjusted to +2 dbm.
Left-Right position: 60 db
Vertical-Lateral position: hO db
117 volts, 50-60 cycles, 3 amps
Unit maintains stability of gain, gain
reduction and balance over a range of line
voltage fluctuations frcm 100 to 127 volts.
STABILITY

SPECIFICATIONS contd
CONTROLS
MECHANICAL DIMENSIONS
WEIGHT
TUBE COMPLEMENT
a ) 2 Input Gain Controls
Step attenuator: 1 db per step.
b) 2 Threshold Controls
Continuously variable.
o) 2 Time Constant Switches
6 positions each, so as to provide fixed
and variable time constants for any type
of program material.
d) 2 Metering Switches
3 positions each, which permits the
measurement of plate current of each
set of control tubes.
e) Mode Switch
Left-Right position: 2 independent
limiters.
Vertical-Lateral position: matrixing
input and output, left and right in and
out, limiting action vertical-lateral.
f) QN-QFF Switch
Standard 19" rack, 1U M panel space, depth
behind panel 11".
Approximately 65 lbs.
8-6386; 1-608U; 1-5651; 2-12AX7; 2-12BH7;
1-EL3U; U-6973; 1-GZ3U (5vU).
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LIMITING

INPUT AND OUTPUT CONNECTIONS
LEFT CHANNEL - INPUT
(Z = 600 8)
RIGHT CHANNEL - INPUT
(Z = 600 S2)
Terminal 1 Tie point (no internal
connection)
Terminal 2 High
Terminal 3 Low
Terminal h Ground (chassis)
Terminal 5 Tie point (no internal
connection)
Terminal 6 High
Terminal 7 Low
NOTE: If a floating circuit is
used, it is necessary that Terminals
3 and 7 be strapped to Terminal k
(chassis).
LEFT CHANNEL - REMOTE METER Terminals 8 and 9
RIGHT CHANNEL - REMOTE METER Terminals 10 and 11
CAUTION: Terminals 8, 9, 10, and
11 are 2lj0 V above ground.
LEFT CHANNEL - OUTPUT
(Z = 600 8)
Terminal 12
Terminal 13
High
Low
Terminal 11; Ground (chassis)
RIGHT CHANNEL - OUTPUT
(Z = 600 a)
Terminal 15
Terminal 16
High
Low
Terminal 17 Ground (chassis)
Any standard W-Meter can be used as a Remote Meter if the internal
rectifier is removed. A variable resistor of approximately 10K should
be used in connection with the Remote Meter. This variable resistor
(Ohmite CLU-1031) enables the zero of the Remote Meter to coincide
with zero on the meter in the unit (see "REMOTE METER CONTROL”
illustration).
If desired, Fairchild Recording Equipment Corporation can supply such
Remote Meter (order Fairchild Part No. A-2329il).
6

REMOTE meter control
R-ohmite.ab or similar quality
POTENTIOMETER
M-ANY STYLE VU METER WITH
RECTIFIER REMOVED OR
FAIRCHILD PART A'23294
CAUTION - REMOTE METER LEADS ARE
APPROX 250V ABOVE
GROUND
REMOTE THRESHOLD CONTROL CIRCUIT
THE FADERS MARKED "f"aRE GANGED 20STEP'20DB
BRIDGED "T" FADERS

BALANCE AND ZERO ADJUSTMENTS
The unit normally maintains the balance and zero adjustments over
a wide range of power line voltage changes as well as tube aging.
Small unbalances in the order of l/h db do not normally produce any
ill effects in operation. If it is necessary to re-adjust, the
procedure is as follows:
1. Allow the unit to warm up for at least a half hour.
2. Adjust the BALANCE control until same meter reading is
obtained in both BALANCE positions.
3. With the METERING switch in zero position, adjust the ZERO
control for a meter reading of zero.
U. Repeat step 2 if necessary.
5. If above procedure does not produce reasonable balance,
exchange one or more of the 6386 tubes.
6. Repeat the above procedure for the other channel if necessary.
NOTE: Do not adjust R313 (B+ ADJ) inside the amplifier unless the
voltage on Pin 8 of V302 (EL3U) deviates from 2U0 V.
7
k

THRESHOLD ADJUSTMENTS
Through the use of the AC and the DC THRESHOLD controls, it is possible
to adjust this unit to any operating point between a limiting amplifier
and a compressing amplifier (see "INPUT VS. OUTPUT CURVES" where (3)
indicates the curve the unit is adjusted to from the factory).
The slope and position of the curve are functions of the desired output
level and the position of the AC and the DC THRESHOLD controls. Before
adjustirg these controls, the desired output level at 3 db and 10 db of
limiting respectively, must be chosen. The 3 db limiting point is
controlled by the DC THRESHOLD, the 10 db limiting point by the AC
THRESHOLD.
The AC THRESHOLD control is located on the front panel as a knob adjust¬
ment, whereas the DC THRESHOLD control is located inside the chassis as
a screwdriver adjustment. The adjustment procedure is as follows:
1. Sat the AC THRESHOLD control to zero and adjust the INPUT GAIN
control for unity gain.
2. Turn both the AC and the DC THRESHOLD controls to their full
clockwise rotation.
3. Apply a signal to the input 3 db higher than the desired output
level and adjust the DC THRESHOLD control to desired output level.
U. Increase the input signal to 10 db higher than the desired output
level and adjust the AC THRESHOLD control to desired output.
5>. Repeat Steps 3 and h until the desired slope and position are
obtained.
6. Repeat the above procedure far the other channel if necessary.
REMOTE THRESHOLD control is possible in different ways. One way which
does not involve changes in the 670 circuit is the external method,
where ganged T-faders are used at in- and output (see "REMOTE THRESHOLD
CONTROL CIRCUIT" illustration). This will involve a loss in level of
approximately 10 to 16 db which, for example, can be compensated for
by using a line amplifier, such as FAIRCHILD MODEL 722.
3

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NORMAL OPERATION
To simplify the initial setting up, here is the standard procedures
1. Observe if any shipping damage has occurred and make certain that
all tubes are firmly seated in their sockets.
2. Mount the unit into a relay rack.
3. Connect input and output leads as specified under "INPUT AND
OUTPUT CONNECTIONS."
u. Check the LEFT-LAT and RIGHT-VERT metering as outlined in
"BALANCE AND ZERO ADJUSTMENTS."
5. Set the INPUT GAIN controls to unity gain (approx. 10 db attenua¬
tion).
6. Set the LEFT-LAT and RIGHT-VERT TIME CONSTANT switches to position 3.
7. Turn the LEFT-LAT and RIGHT-VERT AC THRESHOLD controls on the front
panel to zero.
8. Turn the MODE SELECTOR switch to Left-Right position.
9. The unit is now set up as TWO independent limiters. It can be
used for TWO separate program sources or for the "ping-pong" type
of stereo.
10. To secure the desired degree of limiting, advance the AC THRESHOLD
control until the desired amount of limiting or desired output
level is achieved.
11. Turning the AC THRESHOLD controls completely counterclockwise
removes the limiting action completely. The unit is now a simple
Unity Gain Line Amplifier.
12. The unit is adjusted in the factory to conform to curve 3 (see
"INPUT VS. OUTPUT CURVES"). This curve is achieved by advancing
the AC THRESHOLD controls fully clockwise.
13. Should a different Input vs. Output Curve be desired, see
"THRESHOLD ADJUSTMENTS." Generally speaking, a pure peak-limiting
action is required for operations having a definite overload point
(such as 100$ modulation in transmitters), whereas a lesser sloped
curve, somewhere in between compressor and peak-limiting, is
useful in recording applications. The pure peak-limiting opera¬
tion generally produces maximum volume within given overload
limits; but if a great deal of limiting is used, it tends to
become somewhat obvious since all peaks, irrespective of their
initial value, will get reduced to the same level. Milder sloped
curves, on the other hand, will tend to give a greater apparent
dynamic range.
9

NORMAL OPERATION contd.
111. It should be remembered that the standard VU-Meter is only an
average reading instrument. A VU indication of ”0” in complex
program material may result from actual peak values as much
as 10 db highefr. In setting up limiting curves, this should
be kept in mind •
15. Individual beliefs and previous habits often influence the form
of limiting desired. It is with this in mind that we have
provided a tool flexible enough to be set up to fulfill almost
any requirement.
16. The TIME CONSTANT switches give a wide choice of constants.
Position 3 is merely a first suggestion for a general purpose
timing circuit. With certain popular music or speech a much
faster time constant, such as positions 1 or 2, might be more
desirable. For classical music a much slower position, such
as U, is useable. Positions 5 and 6 combine fast release with
slow release and can be useful if a great amount of automatic
level correction is required. These positions should also
make the limiter action least audible since they will reduce
overall program level if continual limiting persists.
17. So far the unit has been described as TWO separate limiters.
If it is desirous to use the unit for correcting the respective
lateral and vertical components in transferring tapes to stereo
disks or dubbing original tapes to the master tape, the Vertical-
Lateral position is advantageous. In this position, each stereo
channel is divided into its respective lateral and vertical
components. The upper channel now acts as a limiter for the
lateral component, the lower channel as a limiter for the
vertical component. As long as the amount of lateral and
vertical component reduction in each channel is identical, no
deterioration of separation will occur. However, often it is
advantageous to reduce the vertical component more than the
lateral, resulting in some loss of separation. Limiting the
lateral and vertical components can be of great help in producing
good stereo masters with only negligible loss of separation.
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