Dartzeel-audio NHB-108 B version User Manual download pdf (Page 35)

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 between +15°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.

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Dartzeel-audio NHB-108 B version User Manual Page 35

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