Altec Lansing M-20 Operating Instructions Manual

SPECIFICATIONS

Frequency Response: See curve.
Output Level with various loads:

Unbalanced—High impedance (10,000 ohms or higher)—

30 db re 1 V. 10 dynes/cm-600 ohms—49 dbm re 10
dynes/cm-150 ohms—53 dbm re 10 dynes/cm-30 ohms—
59 dbm re 10 dynes/cm-Balanced—(Using 4665 plug-in
transformer)

600, 150 or 30 ohms—48 dbm re 10 dynes/cm-Power
Requirements: 117 V., 60 cycles, 15 watts Physical: P-525A
Power Supply—width 85/8", height 23/8", depth 71/2".

Finish—21 microphone, stainless steel; 165A base—

gray anodize; P-525A Power Supply—silver gray
cover with gray anodized panel.

Mounting—166A holder attachment threaded 5/8"—27
to mate with 22C floor or 24C desk stand or other
standard microphone stand. Weight—M-20 System
(unpacked)—6 lbs., 1 1 oz.

Accessories: 4665 plug-in transformer for balanced output line.

167A extension cable 25 ft. long equipped with

Cannon RWK 6-22C and RWK 6-21C cable con-

nectors.
22C floor stand, adjustable 35" to 64".
24C desk stand.
11853 rack mounting assembly for two P-525

power supplies, 31/2 inches high x 19 inches

wide x 71/2 inches deep.
168A or 170B wind screen
169A shock mount

OPERATION

The "Lipstik" microphone system is designed to work with any existing microphone preamplifier or facility regardless of the input
impedance. Aside from the outstanding advantages of this microphone, it differs from the conventional only in that the P-525A
power supply must be connected between microphone and amplifier. The 165A base, containing the 5840 vacuum tube and 21
microphone, is terminated at the end of a 15' cable in a Cannon connector which in turn mates with its companion fitting on the
panel of the P-525A power supply. Extension cables may be used as described below. The output appears at the end of the 2­conductor shielded cable extending from the power supply. The power supply must be energized from the 117 VAC 50-60 cps sup­ply. The 166A holder will snugly grip the 165A base providing a light streamlined swivel attachment for floor or desk stand. Its
modern line is achieved with ruggedness by combining a molded, high impact Styrene part with a satin chrome die cast swivel,
containing the standard RTMA 5/8—27 thread. A jumbo size fountain pen clip provided, will allow the microphone to be attached
to the lapel affording great freedom of action to the user.

OUTPUT CONNECTIONS

Unbalanced: As supplied, the P-525A power supply is equipped with an internal shorting plug which connects the output cable to the

cathode follower, through isolating resistor and condenser. This condition is preferred when the following amplifier has a high imped­ance or unbalanced (transformerless) input. It may also be operated into amplifiers having low impedance input connections where
line balance is not of importance and a small reduction of sensitivity can be tolerated. An instance of this operation would be a
portable P.A. application in which microphone lines of limited length are used. When the plug is inserted, the black wire of the
output cable is ground, thus is common with the shield.

Balanced: Broadcast and recording applications normally require balanced or at least ungrounded signal lines in the interest of
suppression of unwanted longitudinal signal currents and in maintaining the highest possible signal to noise ratio. This feature is
provided by means of a plug-in transformer (4665) which may be inserted in the P-525A power supply (installation instructions are
given under P-525A power supply). The socket into which the transformer is inserted is strapped to provide a load impedance of
150 ohms. Other load impedances of 30 and 600 ohms may be obtained by re-strapping the socket as indicated on the transformer
schematic. Because of the great latitude in loading permitted by the cathode follower, it is suggested that all load impedances be­tween 30 and 250 ohms be fed from the P-525A without modification of the transformer socket strapping.

Termination: When the 4665 balancing transformer is used, no termination other than that provided by the input transformer of the
following amplifier is required. The unbalanced output (no 4665 transformer) requires termination when working into transformer
loads of 600 ohms or less. A 1/2 watt resistor matching the transformer impedance within ± 20% should be connected across the
power supply output. This is most easily accomplished by installing the resistor in the power supply connector which will plug into
the amplifier.

Sensitivity: The voltage developed at the grid of the first tube following the P-525A power supply, for a given microphone signal
input, will be identical for all balanced line impedances where the amplifier input is matched, using the 4665 transformer. It will
have the same value when the unbalanced (direct cathode follower) output is fed into the first tube of an amplifier not having an
input transformer. However, when the unbalanced output is fed into low impedance transformers and loaded with a resistor as de­scribed under Termination, the sensitivity will vary depending upon the load impedance. With a 600 ohm resistor termination work­ing into a 600 ohm transformer, the voltage will be comparable to the above; with a 150 ohm resistor and 150 ohm transformer,
the voltage will be 4 db lower and with a 30 ohm resistor and 30 ohm transformer, the voltage will be 10 db lower. These sensi­tivities are listed in the specifications relative to microphone inputs of 10 dynes/cm-.

Extension Cable: The cable length supplied between microphone base and Cannon connector is 15 feet. Heavy duty, 25' extension
cables are available as listed, or single lengths of Suprenant 5426 or equivalent six-conductor cable may be utilized to separate
microphone base and power supply up to 400 feet. Belond 400 feet, large conductors should be used for the heater leads to avoid
excessive voltage drop.

21 Microphone (Fig. 1): The condenser microphone depends for its operation upon variations in its electrical capacitance caused
by microscopic deflections of the diaphragm to a fixed electrode, when exposed to the pressure varations of a sound wave. The
capacitance variation is converted to electrical signal by supplying a polarizing potential to the microphone through an extremely
high resistance. In this case, the potential is 200 volts and the charging resistance is the insulation resistance of the associated vacuum
tube and socket. Since the voltage e across the condenser is proportional to the charge Q divided by the capacitance c, it will de­crease for a positive pressure applied to the diaphragm and increase for a negative applied pressure. Thus, the output is propor­tional to the diaphragm displacement for all frequencies above those at which the microphone reactance, 1/wc, is commensurate
with the leakage resistance. The capacitance of the condenser is approximately 6 mmf. Because of this high internal impedance, it is
necessary to place the cathode follower tube, which serves as an impedance transformer, in close proximity to the microphone.