Crown OC-150A - Circuit Description and Operation; Headphone Attenuator Switch Functionality

CIRCUIT DESCRlPTlON
The AG power supplied to the OCe150A powers the
meter boards and the display lights. Dual 8 volt
secondaries ern the transformer power the meter.
amps separately, with each amplifier having its own
rectifier and capacitive filter. She resulting un-
regulated supplies are k lO volts.
The uniqueness of the OC-150A metering system is
that the information fed to the mebevs is elecfrcani~al-
ly derived. Peaks are held at the meter terminals so
that the readout accuracy dws nQI depend an metea
response time.
To accmplish this, the audio signal (from thepwrer
amplifier output), goes through a rectificaticrro
process to produce a series sf negative pulses. These
are pmeessad by a uaity-gain invefiing op-amp
which in turn drives a transistor stage. The output of
the transistor stage charges the peak storag~ and
timing RC networks which permit the unique opera-
tion of the circuitry. $a3 reduce leakage into the
storage network, the circuit is isolated through a pair
of transistors biased as diodes. The voltage im-
pressed on the storage network drives another unity
gain IC stage, which in turn drives the meter move-
ment. This final-stage IC incarporates a$-FET input
to provide the extremely high impedance neded to
maintain a stable voltage in the storage network. An
auxiliaw IC stage provides compensation for thg in-
put sHseteurrenl (in the picoamp range) and the out-
put offset voltage of the finlaf stage, "fhr; precise ad-
justment required is dane during the factory sel~up
procedure, Peaks stared in the network are thus held
until a discharge transistar is turned ogl (by the tim-
ing circuit). Using this overall mnfiguration, higher
peaks ar@ always allowed to daminate the meter
movement, and peaks can be held with re35 than Q.5
db of meter drin For longer than 20 minutes (ar
roughly the time needed to play one side of an LP).
At the time the peak storage netvvork is charged, the
timing network is also charged by a current sauree
tumed on by the initial transistor stage sf the
amp! ifier. The positive voltage thus impressd on the.
timing network is applied to the inverling input sf an
op-amp camplarator circu if a The comparator main-
tains a negative output a5 long as the timing network
charge
is positive. In the VU mode this charge is
removed very quickly by discl-rarging the netwsrrh
tkmugh a resisBor to the -16 woft supply, Re hold
time is extremely shsrt and the meter simply
averages %he siigna t level. I rz the peak mode, however,
the nebork may kdischsrgd tograund (maximum
hord time) or to the -10 volt supply (minimum hold
time) OP any wint between these Wo sxtremes, The
hold time csntaol wsrks by varying the effedive
reference voltage discharging the timing network.
When the
timing network is su8icien"tly discharged
for its voilage to become negative, the camparator
op-amp switches to a positive output which turns on
the
discharge transktsr and removes the drive to the
meter, if the discharge circuit is referened to
ground (meter hold control at the potential on
the nework will tllever became negative and the hold
time will consequently ba infinite* (Hawever, the ac-
curacy will vary with the drift, as described above.)
With the hold time control referencing thc eixuit lo
the -10 volt supply $mine hold), the neWork wilil dis-
charge in about 600 milfisecands.
Meter calibration is provided by a pd in series with
the meter movement,
HEADPHONE A"$lfEQslUQ\"O"OR SWITCHES
Bwause of the relative efficiency sf most
headphones, it. Is easy ta produce large sound
pressure levels with low or medium pwer amplifiers.
A 200 was amplifier would represent a "lethal dose"
of power to most headphones. at is for this reason that
Btkerauator~ are inserted in series with the
headphone jacks an the front pame! of the 8C-150As
Two levels sf attenuation are available, 17db and
2463b. The attenuators are activated by three-
position slide switches on the reas panel,
Tgs illustrate the usefugness sf the attenuators, let's
set up a typicaa headphone monitoring system, We?/
assume an impedance of 8 ohm$ with an eMiciency
such tlaat 3 volts acradjs the 8 ohm impedance
produces a casmfa&abie listening !eve/. Using Ohm's
law the power is % .I3 watts. Now if we WOW td switch t~
the %7db aRenuation position, we could increase the
voltage ts 21.4 volts and still keep 3 volts acrsss our
headphones, The new pswr is 57.2 waHs. Similarly,
by using the 24db a"ctenuatiora position, the
voltage/poweb- could be increased to 49,8 waits 08"
318 waEs.