ALM672
AUDIO LEVEL MASTER
OPERATION MANUAL
IB612901
TABLE OF CONTENTS
GENERAL DESCRIPTION1.0
INSTALLATION2.0
CONNECTION AND SETUP3.0
FUNCTIONAL DESCRIPTION4.0
MAINTENANCE5.0
APPENDIX B MAINFRAME
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1.0 GENERAL DESCRIPTION
The ALM672 Audio Level Master is a full stereo two-band audio level controller designed to maintain consistent audio program output levels over a wide range of input levels. Db-linear control characteristics allow smooth gain control over a 30 dB range. Program input levels can vary plus or minus 15 dB while the output level remains relatively constant. Gain is controlled without the usual compressor artifacts common to less sophisticated designs, such as "ducking", a sudden drop in average overall level after a loud transient like the sound of gunfire in a movie.
Due to the split-band design, high-level audio at low frequencies, such as thunder or heavy bass in music, will not modulate or reduce the level of mid-range audio, where the ear is most sensitive. Heavy low frequency program material will cause gain to be reduced in the Bass band only, allowing a much more natural sound. However, with material that lacks low frequency content, the unit will not make a mistake and excessively increase the low frequency gain, due to inter-band referencing circuitry that "watches" the Bass band versus Main (high) band gain.
Noise rush-up during pauses in program audio is eliminated by a gate circuit. The gate prevents gain increase when audio drops below a preset threshold level.
The ALM672 is a high performance audio controller that is virtually inaudible in its operation. It automatically produces an output that sounds as if the level had been correctly set, even with varying input levels.
2.0 INSTALLATION
One to three ALM672s reside in a PMS610 mainframe/power supply, which mounts in a standard 19 inch rack unit (1 3/4 inches) high. The unit is grounded via the AC power cord and the rack mount.
See Appendix B for instructions on installation of additional modules in empty spaces in a PMS610 mainframe.
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Before mounting in rack, check the input level requirements of the equipment to be fed by the Level Master. Left and right output levels are factory set for 0 dBm average level, unless otherwise specified. If higher level is required, remove top cover and refer to the drawing for programming of output level select jumper plugs. These plugs are on the ALM672 PC board. Average output levels of 0 dBm, +4 dBm, and +8 dBm can be selected. Mono (sum of left and right channels) output is provided to conveniently feed an additional, monaural modulator. Mono output is unbalanced, low impedance, at a fixed average level of 0 dBm into a high impedance or -6 dBm into a 600 Ohm load.
3.0 CONNECTION AND SETUP
Audio connections to the unit are made to a removable connector block at the rear of the unit, For ease of connection, unplug the connector block, connect wiring, and plug block back into unit. Connect wiring by inserting stripped wires into the appropriated holes in block, and secure by tightening the screws on top. PUSH FIRMLY ON THE CONNECTOR BLOCK TO "SNAP" IT INTO PLACE.
STEREO CONNECTION:
Refer to the connector block drawing and connect the program source to the Left and Right inputs. Make sure that HI or (+) connects to TIP, and LO or (-) connects to RING at both inputs. A reversal of polarity will cause cancellation of any center or monaural signal. Connect shields to G (GROUND).
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Connect outputs of Level Master to equipment to be fed, in same manner as above. Observe correct polarity. The mono output is available at the rear panel RCA jack. It is a sum of both left and right channels, and is convenient for applications such as feeding a monaural TV channel modulator, while the main stereo outputs feed an FM stereo modulator.
STEREO SETUP
Turn both Left and Right INPUT LEVEL controls fully counterclockwise. These are adjusted through holes in front panel, using the adjustment tool provided, or a small flat-blade screwdriver. With program material at normal level feeding the unit, turn up Left INPUT LEVEL control until GAIN reduction LED display is indicating 15 dB, (green LED) most of the time. This sets the input level to the center of the gain control range. This display will move to the left during lower level passages, and to the right with higher levels, which will cause more gain reduction.
Turn up Right INPUT LEVEL control until the STEREO BALANCE display is centered, (green LED). This indicates equal limiting in both channels. With normal stereo program material, this adjustment is best done during a mono portion, such as center stage speech in a movie, or a single announcer speaking. In music, the lead vocal is usually centered. The balance display will respond to normal programs channel imbalances, so if you get a stable centered display, the unit is probably being fed a mono signal. If a mono portion of program is not available, try to get the best average balance. If a mono portion of program is not available, try to get the best average balance. If the opportunity exists to set up your entire audio chain with test tones, balance can easily be set with equal level tones in both channels of you system.
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The balance display allows accurate balance adjustment, even with program audio, typically with less than 1 dB difference between channels.
NOTE: Be sure to set balance using a properly balanced source. If balance is set with a source that has a fixed imbalance between channels, input gains of the unit will be mis-calibrated. Then, when switching to a source that is properly balanced, (which may, for example, be a local ad insert), this source will now appear to have a channel imbalance.
ALTERNATIVE SETUP:
A more accurate balance setup could be done with KNOWN equal levels applied to both input channels of the unit. This can be done by connecting both left and right inputs in parallel with jumper wires, feeding one program channel at normal level into both inputs. Then set gain reduction and balance as in previously described stereo setup. This calibrates the ALM672 to precisely equal gain in both input channels.
Remove jumpers and connect left and right program lines to their respective inputs.
Do not re-adjust INPUT LEVEL controls.
The ALM672 can now be used as an instrument to help set up the channel balance of your audio system. With its input gains matched, it will indicate any imbalance present in the system feeding it, and you can use the STEREO BALANCE display to align the channel balance of the audio sources in your system.
MONO CONNECTION:
Connect program audio line to both left and right inputs of unit, in parallel, using short jumper wires. Connect left TIP to right TIP, and left RING to right RING. This parallel connection is necessary for the Gate circuit to receive the proper level. If input polarity is reversed between channels, the gate will not work properly due to cancellation of its input signal. This reversal will also cause cancellation at the mono output. Any or all outputs of the ALM672 can be used.
MONO SETUP:
Mono setup is exactly the same as the stereo setup.
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4.0 FUNCTIONAL DESCRIPTION
The ALM672 is made up of the following subsections:
4.1LEFT AND RIGHT INPUT AMPLIFIER / LOW-PASS FILTER
4.2INPUT CROSSOVER, VCA
4.3OUTPUT CROSSOVER / LOW-PASS FILTER
4.4OUTPUT AMPLIFIER
4.5CONTROL
4.6GAIN REDUCTION DISPLAY
4.7BALANCE DISPLAY
4.8POWER SUPPLY
4.1 LEFT AND RIGHT INPUT AMPLIFIER / LOW-PASS FILTER
The input amplifier is a differential amplifier. It responds to signals applied between the tip and ring inputs, while rejecting common mode hum and noise. The INPUT LEVEL control attenuates the input signal before it is applied to any active circuitry, allowing maximum headroom before overload occurs.
A low-pass filter follows the input amplifier. It is flat to 15 KHz, designed to prevent out of band noise from influencing the gain control action.
4.2 INPUT CROSSOVER / VCA
Left and right Main band input crossover filters are high-pass filters contained within U3 and its associated circuitry. The crossover frequency is 340 Hz. Bass band input crossovers are low-pass filters contained within U5 and its associated circuitry. Output from these filters feeds four essentially identical voltage controlled amplifiers, the Bass left and right channel VCA's and Main band left and right channel VCA's.
The VCA is realized using a gain cell in the feedback loop of an op-amp. Maximum control voltage forces maximum gain through the gain cell which produces more negative feedback through the op-amp, producing minimum overall gain. Main band gain cells are U4, Bass band gain cells are contained in U6. U7 contains DC servo biasing networks for U4 and U6.
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4.3 OUTPUT CROSSOVER / LOW-PASS FILTER
Left and right Main band output crossover filters are high-pass filters contained within U16 and its associated circuitry. Left and right Bass band output filters are low-pass filters also contained within U16 and its associated circuitry. Main and Bass band filter outputs are summed, and the combined signals are applied to left and right 15 KHz low-pass filters.
4.4 OUTPUT AMPLIFIER
Left channel output amplifier is U17. Right channel output amplifier is U18. Each output driver is an active balanced, floating configuration. these are low output impedance amplifiers, designed to feed loads of 600 Ohms or higher.
4.5 CONTROL
Output from each VCA is applied to one of four identical rectifier circuits. Each circuit is a full-wave precision rectifier with a preset threshold. When the peak value of the signal exceeds the threshold in either polarity, the rectifier produces an output pulse. Left and right Main band rectifiers are comprised of U8 and its associated circuitry. Bass band rectifiers are U9 and associated circuitry.
The rectifier output pulses are applied to timing and exponential converter circuits. The timing circuits can be thought of as RC filters that smooth the rectifier pulses to produce a DC control voltage. These filters have a fast attack and slow decay characteristic. An exponential converter produces an anti-log of the DC from the filters, which becomes the VCA control voltage.
Timing and exponential converters are U10 for Main band, and U12 for the bass band, along with their associated circuitry.
4.6 GAIN REDUCTION DISPLAY
Gain control voltage from U11 is fed to the input of U14, which is an LED display driver. The degree of gain reduction taking place is indicated by LED's D1401-D1405. Gain reduction is indicated in 6 dB steps, with LED's for 3,9,15,21, and 27 dB.
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4.7 STEREO BALANCE DISPLAY
Left and right rectifier pulses from U8 are applied to a differential amplifier within U11, which produces a filtered voltage corresponding to the difference in amount of limiting between left and right channels. This voltage is applied to U15, an Led driver. With no limiting or equal limiting between channels, the input voltage to the driver causes the Center LED, D1502, to illuminate.
More limiting in the left channel will cause the voltage to go more negative, causing the Left LED, D1501, to light. Heavier right channel limiting will produce a more positive voltage to be applied to the driver. This will cause the Right LED, D1503, to illuminate.
4.8 POWER SUPPLY
The power supply is part of the PMS610 mainframe. It is a conventional series regulated supply, output voltage is -24 volts.
5.0 MAINTENANCE
No routine maintenance or adjustment is required. If a problem develops, contact the factory. It is strongly recommended that if servicing is necessary, the unit be sent to the factory for any adjustment or repair.
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APPENDIX B
PMS600 SERIES MAINFRAME AND POWER SUPPLY
The PMS610 is a mainframe power supply for mounting FM SYSTEMS 600 series equipment cards. Up to three circuit board modules may be accommodated. These modules may be readily installed in the field with common hand tools; no soldering is required.
PMS600 SERIES SPECIFICATIONS
MECHANICAL:1-3/4" High x 19" Wide x 10-1/4" Deep.
CAPACITY:Holds up to three 600 series modules in any of three card positions.
POWER:105-125 VAC 50-60 Hz. 0.4 Amps. USA grounded three prong cord set.
POWER: (610-24)-24 VDC Battery supply.
POWER: (610-48)-48 VDC Battery supply.
CONNECTORS:Accessible on rear panel, provided as integral part of circuit board modules.
CIRCUIT BOARD MODULE INSTALLATION
Select which of the three positions will be occupied by the new circuit board module. Disconnect the power and remove the mainframe from the rack. Remove the bottom cover and the 4-40 x 1/4" mounting studs where you wish to mount the circuit card. Next remove the rear metal panel in preparation to mounting the new card. Then remove the old Nameplate by carefully lifting one corner and slowly peel the nameplate from the aluminum panel.
Install the new circuit board module with the components toward the top cover. Take care to avoid moving any of the pre-set controls. Slip the connectors through the holes in the rear panel and drop the front edge of the circuit board onto the brackets attached to the front panel. Next install two 3/8" -32 nuts and lockwashers on the outermost "F" connector barrels. Gently tighten the nuts while holding the circuit board against the front mounting brackets. Install tow 4-40 x 1/4" studs (without the lockwashers) to secure the front of the board to the mounting brackets.
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CIRCUIT BOARD MODULE INSTALLATION (continued).
Next remove the top cover from the mainframe and connect the power supply and the circuit card with the power wire harness that is supplied. The power to multiple cards is wired as a "daisy chain". Push any L.E.D. (Light Emitting Diodes) straight into the appropriate mounting holes. Mount switches according to the panel graphics.
Temporarily set the new nameplate in place and check that all necessary cutouts are clear. Remove the paper peel coat from the adhesive backing of the nameplate, then slide the name plate down around the L.E.D.'s onto the panel. Press gently to set the adhesive. Mount any additional panes components (switches, meters, potentiometer, etc.) with the hardware supplied. Gently tighten the fasteners to secure the components: DO NOT OVER-TIGHTEN.
Mount the new rear panel connector identification stripes as illustrated in the instruction book for the particular module being installed. Connect input, output, and power cables. The green "power-on" indicator should be illuminated. Set any internal controls as required.
CAUTION
Most circuit board modules have several adjustments which are carefully factory set with precision instruments for optimum performance. Change only those which must be adjusted. Some controls when mis-adjusted produce little change under "normal" operating conditions, but can seriously reduce the ability of the unit to function correctly under other conditions which may be encountered. Therefore, if you must adjust a control, place a mark on it before moving it, so that it may be returned to its original setting with reasonable accuracy.
Disconnect power. Replace top and bottom covers and mount mainframe to rack. Reconnect power and check for normal operation of each module.
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