8.11 ATTENUATOR AND 10
dB
RANGE EXTENDER
The attenuator
(U301)
is a voltage controlled amplifier.
Although it does have amplification gain, the majority of its
d
dynamic range is used as an attenuator.
The total range of the
attenuator is 112
dB
which is adjustable using a variable dc
voltage applied to Pin 3 (EC) of the device.
In the normal
operating mode,the attenuator is set to 5
dB
resolution.
In the
calibration mode,
however,
0.5
dB
resolution is available and
made possible by an 8 bit digital-to-analog converter (DAC).
Earphone calibration is accomplished by storing an HL to SPL
value per frequency in a look-up table in EEPROM.
For example,
at 1 KHz and 80
dB
HL the CPU will set the attenuator to a
position which will establish 87
dB
SPL in either earphone.
Therefore,it can be stated,
that the attenuator for a given
frequency is offset from the nominal indicated HL by a decibel
value equal to the Reference Threshold Value
(RTV) to a
resolution of
dB.
The
amplifier
stage
(U300)
following the attenuator has a
switchable
feedback resistor. This stage serves as the
+10
dB
range extender circuit used during normal operation.
The gain of
the stage is changed by switching Resistor R304 in parallel
with
R303
or grounding R304 at one end
(+10
dB
position).
8.12 POTENTIOMETER ADJUSTMENT PROCEDURES
L
LINEARITY ADJUST
()
I
_*
a.
Set front panel controls to: 1 KHz, 80 HL, Tone Steady,
Channel ON.
b.
Set a reference on Sound Level Meter (SLM) with either left
or right earphone coupled to it.
C.
Reduce HL dial to 60 HL and note reading on SLM.
d.
Adjust R404 for a
-20
dB
change from the reference level
obtained in step
"b".
e.
Re-check 80 HL for 20
dB
change,
readjust R404 as necessary.
II. DISTORTION ADJUST (R403)
NOTE:
Tone may be locked
"ON" in the Normal Mode by connecting
TP100
to the cathode of CR3.
a.
Set front panel controls to:
1 KHz,90 HL
(+10
dB
activated),
Tone Steady, Channel ON.
b.
Connect a distortion analyzer to either left or right phone
output,
load with 60 ohms and select the appropriate output
using the front panel switch.
68
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