Altec 770a xover service manual

Page 1

770A ELECTRONIC
CROSSOVER BIAMPLIFIER

MIR

3 LANSING:

OPERATING

INSTRUCTIONS

Figure I. The ALTEC 770A Electronic Crossover Biomplitier

Your new ALTEC 770A Electronic Crossover Biamplifier represents a superior design in an ampl r For high fidelity music-sound systems.
Compact - it may be mounted in the rear panel of yourspeoker enclosure. This autstandinghiamplitier was developed after extensive
circuit evaluation and testing by ALTEC engineers to provide you With years of reliable high-quality performance.

FEATURES
0 Separate amplifier For each speaker component For greater dynamic range
0 Electronic crossover selectable at 500 or 800 H: with attenuation of 12 dB octave at frequencies beyond the crossover Frequency
0 High power output - totals 90W RMS - the bass amplifier output is 60W RMS and the treble amplifier output is 30W RMS
0 Full advantage oF amplifier damping is pro/idea by direct coupling of the speakers to the biampliiier
0 Increased dynamic range provides undistorted pOWer that may be compared to asingle amplifier having apowerautputot I75Vi RMS
0 Total harmonic distortion (THDI is less tnan 0.3:: for all frequencies From 20 H: to 20 KHz at Full rated output
0 IM aisrortion is inherently decreased to its lowest point - Vlruully unmeasurable by normal IHF merhacl
0 Signal Sentry? on aft' switching circuit automatically turns on' when program material is being recei led and automatically
turns atF' two minutes after program ceases
I Fail-Safe protection system with active dissipation sensing :ircuit :9, protection a. output transistors
o All plug-in modular solid-state circuitry
0 Separate gain controls For 'aoss and treble ampliiiers
o ieli-cantoined power supply

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1515 5. Manchester Ave., Anaheim, Califi 92803

4 Mi :Jrr'pan'y L r; 4 rec r: New York

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SPECIFICATIONS
Power Output -
Bass Amplifier:
Treble Amplifier:

THD:

lM Distortion:

Frequency Response:

IHF Power Bandwidth:

Damping Factor:

Hum and Noise:

Input Sensitivity:

Crossover Slope
(Dual Section):

Control Range:

Overall Dimensions:

Panel Cu tout D imensions:

Weight:
Maximum Gain at Full
Rated Output --
Bass Amplifier:
Treble Amplifier:
Input impedance:

Load Impedance:

Output Impedance:

'Turn-On' Sensitivity
Threshold:

'Turn-Off' Hold Time:

Bass and Treble Level
Controls:

Accessories:

DESCRIPTION

60 watts RMS - 8 ohms
30 watts RMS - 8 ohms

Less than 0.3% at a I l frequencies
20 Hz to 20,000 H7.

Unmeasurable by normal IHF method

tI/ZdB 20 Hz to 20,000 Hz (normal-
ized composite output)

l5 Hz to 25,000 Hz (normalized com-
posite output)

25

80 dB below rated output (each chan-
nel)

0.5V RMS for rated output

I? dB/octave

26 dB

6-l/2" H x 9-7/8" W x 9" D
S-I/Z H x 9-I/2" W

16 pounds

73 dB
70 dB

80 kilohms
8 ohms, nominal (each channel)

Less than l0°b of rated load imped-
ance (each channel)

lOmV

2 minutes

Separate slide-type controls, con-
tinuously variable from +6 to -20 dB
when referenced to a setting of 5

4 speaker hookup wires and miscel Ia-
neous hardware

The A L TEC 770A Electronic Crossover Biamplifier is a solid-
state amplifier designed toprovide separate power amplification
for speaker components in a two-way system . The common input
for the 770A is provided from an external single-source pre-
amplifier that plugs into the INPUT iack on the front panel of
the 770A. The preamplifier drives the bass and treble ampli-

l\)

fiers of the 770A. The bass and treble amplifiers are separated
byan electronic crossover network with the crossover frequency
selectable for 500 or 800 Hz . The bass amplifier provides 60W
RMS of power to the LF speaker a nd the treble amplifier pro-
vides30W of power to the HF driver. With separate amplifiers
for each speaker component, the full extent of amplifier damp-
ing can be achieved and an extreme power demand in the low
frequencies wi l | not affect the high-frequency reproduction.
Panel-mounted slide controls provide adiustableshelving for the
bass and treble amplifiers. From a nominal setting of '5', the
shelving may be boosted to +6 dB or attenuated to -20 dB.

In a conventional sound system, the crossover network is placed
betWeen the outputamplifier and the speakers - where the in-
sertion loss of the network may consume more than I096 at the
amplifier power output before the remaining power isdistributed
to the speakers. The crossover network in the ALTEC 770A is
placed between the preamplifier a nd the output amplifiers so
the full output from the amplifiers is distributed to the speakers.
Trarfint response is improved as never before. The increased
dynamic range of an ALTEC 770A biamplifier sound system may
therefore be compared to oconventional single amplifier system
having a poweroutput of I75W RMSI The results insound quality
from an ALTEC biamplifiedsaund system are dramatic and ex-
citing .

A special feature of the 770A is ALTEC's exclusive Signal
Sentry. This is a low-level sensing circuit that instantlyener-
gizes the amplifier when program material equal to or greater
than 10 mV is applied to its input. The amplifier remains fully
energized for two minutes after the program material has been
removed; then it shuts off automatically, leaving the Signal
Sentry circuit ready to accept the next program.

ALTEC's Active Dissipation Sensing Circuit is included in the
770A to protect the output transistors. The action of the sensing
circuit is immediate and effective atall frequencies within the
passband of thearnplifier, limiting only that portion of the pro-
gram material that would damage the transistors.

Completely self-contained, including its own dc power supply,
the 770A plugs into any 120V, 50/60 Hz outlet and requires
only simple connections to the LF speaker and HF driver.
ALTEC's space-savingdesign for the 770A permits easy installa-
tion in the rear of the speaker enclosure, if desired. Figures
2 and 3 show front and right side viewsand Figure 4 illustrates
wiring instructions .

INSTALLATION

The compact design of the ALTEC 770A allows it to be installed
in the rear of aspeaker enclosure. If installation is made in an
existing speaker system, the passive crossover network already
present in thesystem must be removed because the 770A contains
the only crossover circuitry required. Only six steps are then
necessary to complete the installation:

l. Cut a rectangular hole 5-1/2" H x 9-l/2" W in the back
panel of the speaker enclosure.

2. Insert the 770A in the cutout and mark the location of the
six holes for the securing hardware, then remove the 770A.

3. Drill each of the six holes to accept on 8-32 'T nut.

4. Insert the six 'T' nuts in their respective holes from the
inner side of the back panel of the speaker enclosure.