Audio research d 90 d 90b pwr biasadjust

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2. Turn both input level controls fully counter-clockwise.

3. Connect the amplifier to the preamplifier or electronic crossover,
using only the highest grade audio interconnect cables (Sound
Connections Silver Cable is recommended).

4. Connect the power line cord to the AC power, observing Paragraph 2
under USE CAUTIONS on Page 2 of this manual.

5. Proper adjustment of the input level controls can be accomplished very
simply. Turn your preamplifier level control to l2 to l o'clock while
playing a record. Then, advance (from the previously counter-clockwise
settings) the amplifier level controls to your normal listening level.
This provides optimum "bandspread" of adjustment at the preamplifier,
as well as providing optimum signal-to-noise ratio. (The D90 will
normally perform best with its level controls near maximum.)

CAUTION

Make certain the amplifier is installed according to the instructions
under INSTALLATION on Page 3 of this manual.

D90 ADJUSTMENT PROCEDURE AND DISCUSSION

The 090 utilizes very high quality commercial grade components and this,
together with conservative operation of all components and tubes, should
provide long adjustment-free service life.

After long service, or after vacuum tube failure and replacement, or in a
location with consistently low line voltage, it may be desirable to readjust
the amplifier for optimum performance.

CAUTION: The following procedures should not be attempted by the owner unless
he is technically gualified. There are high voltages and currents within this
unit which can be lethal under certain conditions. Refer all such adjustment
to a qualified individual.

There are four parameters which may be adjusted fin the following sequence)in
the D90 after removing the top cover:

I. OUTPUT TUBE IDLE CURRENT (BIAS")

2. CROSS COUPLER DC BALANCE

3. PHASE INVERTER AC BALANCE

4. OUTPUT AC BALANCE (NOT IN SOME EARLY UNITS)

l. OUTPUT TUBE IDLE CURRENT (BIAS)

The output stages of the D90 are partially cathode coupled push-pull parallel
class ABT utilizing our patented circuit which operates the tubes in true
pentode configuration and efficiency with the low distortion of triodes
and at the same time providing better coupling than either conventional
circuit.

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As shipped from the factory, the output tubes are adjusted for a nominal

75 mA. per tube with a stable AC line voltage of lZO Volts. Under these
conditions the tubes are each dissipating approximately 30 watts of their
50 watt rating (44 watt plate, 6 watt screen). This point of operation
provides "enriched class AB], and will satisfy most critical listeners.

It is possible to operate the tubes as high as 100 mA. each at idle for
full class A operation without exceeding their dissipation ratings. While
this may provide an additional measure of quality with the finest of speaker
systems, it will also cause the 090 to run significantly hotter, and the
need for some forced air cooling becomes desirable. Under these conditions
output tube life will also be shorted from an expected 2000+ hours of
satisfactory use downward depending upon the ventilation provided.

lA. "BIAS ADJUSTMENT PROCEDURE

A digital voltmeter capable of accurate measurement of .05 to .l Volt DC
is required to accomplish this adjustment.

There is a l ohm 5% wirewound resistor in the cathode circuit of each output
tube, and test connections (test points referred to schematically and on
the PWB as TPs) are provided on either end of these resistors so that a
voltage measurement can be conveniently made across each resistor. These
test points are identified and accessible from the top side of the PWB.

Because the resistor is l ohm, you can conveniently direct read the total
cathode current in each tube. A .05 Volt reading equals 50 mA. A .1 Volt
reading equals 100 mA. A .075 Volt reading equals 75 mA., etc.

TP
TUBE COMMON 3
v11 5
v13 13 9
v15 7
v17 5 ll
v12 6
v14 14 10
V16 8
Vl8 16 12

Vll and Vl3 are adjusted together by RV9
Vl5 and Vl7 are adjusted together by RVll
Vl2 and Vl4 are adjusted together by RVlO
Vl6 and V18 are adjusted together by RVlZ

It is important that all 8 output tubes be reasonably matched (within 5%)
for highest performance operation.

Observe the following:

1. These adjustments should be accomplished under no signal conditions
and with line voltage at its normal for your location.