darTZeel NHB-108 model one
User Manual, version B 1.0
Page
35
of 35
T12. Special adjustments
T12.1. Output DC voltage drift
When using the
NHB-108
with the DC compensa-
tion offset
off
, Eyes could eventually blink, mean-
ing that an output DC voltage greater than ±0.6 volt
is present at the terminal speaker outputs. An ad-
justment is then necessary.
Caution! The procedure described below can pre-
sent some risk, since the
darTZeel NHB-108
model one
must remain powered ON.
WE DECLINE ANY RESPONSIBILITY IN THE
EVENT OF ELECTRIC SHOCK DUE TO IN-
CORRECT MANIPULATION.
WARNING! ONLY A QUALIFIED PERSON
OR A TECHNICIAN CAN SAFELY PER-
FORM THIS ADJUSTMENT.
The DC trimming potentiometers are located on
the audio printed circuit boards attached to the
rear of each heat sink, as shown in the figure
below (left channel shown):
Fig.9 – DC Offset Trimmer
Before doing any task, it is important to ensure that:
-
The "DC Offset Comp" jumper is set to
off
.
Please refer to chapter
6.4
"
Jumper settings
".
-
Ambient room temperature is b15°C
and +35°C.
-
The
NHB-108 model one
is properly vented,
as described in chapter
5.2.
"
Location
".
-
The loudspeakers are connected to the speaker
terminals!
-
The
darTZeel NHB-108 model one
has been
powered OFF for a minimum of 2 hours.
To make this adjustment, you need the following
tools:
-
A No. 5 flat blade screwdriver for removing the
glass cover; this tool is supplied as standard
with the NHB
-108 model one
.
-
The 2 suction cups supplied with the
NHB-108
model one
. Please do not forget to slightly
wet them, to get firm vacuum on the glass.
-
A No. 0 flat blade screwdriver for adjusting the
12-turn trimmer; this tool is supplied as stan-
dard with the
NHB-108 model one
.
-
A universal DMM (Digital Multi Meter), not
supplied.
Procedure:
DO NOT FORGET THAT THE AMPLIFIER
WILL BE POWERED ON!
NEVER TOUCH THE COPPER BUS BARS,
WHICH ARE AT A POTENTIAL DIFFER-
ENCE OF 115 VOLTS DC.
A SHORT CIRCUIT INDUCED BY THE
SCREWDRIVER WILL PARTIALLY
EVAPORATE
THE BLADE BY INSTAN-
TANEOUS MELTING OF THE METAL OF
THE LATTER!
1)
Power the amplifier ON. Connect the DMM to
the speaker terminals. SLOWLY ADJUST THE
DC TO ZERO VOLT, plus or minus 20 mV.
2)
Let the amp gently warm for a couple of hours,
then check the DC again. Adjust the trimmers
in order to HALVE the value you just read.
That's all!
Voltage values can be either positive or negative.
When halving them, and if you got +300mV, set at
+150mV. If you got -200mV, set at -100mV.
The measured output DC drift
does
fluctuate very
slowly around the set value. This is a normal behav-
iour and cannot be interpreted as a failure.
If you intend to use the amplifier in areas where
ambient temperature is subject to great changes, we
recommend you put the "DC Offset Comp" jumper
to
ON
(see chapter
6.4
. "
Jumper settings
").
T12.2. Monitoring circuit
The monitoring circuit, also called supervision sys-
tem, has 3 trimmers intended for factory settings.
Any alteration of these trimmers will void the war-
ranty.
