Margules U280sc 25th User Manual

Margules Audio

Amplifier U280SC 25 anivesary

Congratulations!

The Margules U280-SC 25th Anniversary Stereo Tube Amplifier
combines the superior sound of vacuum tubes with new and
innovative technology. The results are clearly apparent in its quick
detailed dynamics and ultra-smooth transient response. Unusual
versatility and unprecedented reliability provide an exceptional
performance upgrade to an already remarkable amplifier.

Class-A Output Stage & Active-Servo Bias

Vacuum tube power amps with class-A outputs are rare. Class-A
operation causes the tubes to run quite hot, and increases the
chance of saturating the output transformer.
Margules has resolved this problem by incorporating an Active Servo
Bias (patent pending). The bias actively tracks the input signal,
permitting full class-A output, without overdriving the tubes.
Active Servo Bias also sets DC bias and AC drive levels, assuring
optimum electrical and sonic performance. Throughout the useful life
of the output tubes, no drive or bias adjustments are ever needed.
New tubes can be installed without test equipment or time­consuming setup. Active Servo Bias also makes it possible to use a
variety of output tubes. The 6550, KT88, KT99, KT100, KT 120,
KT150 can be interchanged without modification.
Active Servo Bias is implemented with semiconductors, which are
designed for the aerospace industry, assuring long, trouble-free
operation.

The New U280-SC 25th Anniversary Stereo Tube Amplifier includes
a new and improved differential output, as well, further reducing the
THD and transient distortion.

Ultralinear / Triode & Strapped Operation

Audiophiles have argued the merits of Ultralinear v Triode operation
for 50 years. Triode operation provides lower distortion, at the price
of reduced power output.
The U280-SC resolves this predicament by providing switchable
Triode / Ultralinear operation. For those listening sessions where you
prefer a more soft sound, you simply switch to Triode Mode. And
when more power, dynamics, detail and punch is needed, you simply
switch to Ultralinear Mode. .
Unlike most vacuum tube amplifiers, the U280-SC's outputs can be
strapped, for twice the output. Start with a single amp, then add a
second U280-SC for increased power—or to convert to triode
operation at the same power level.
Separate gain controls for each channel make precise balancing
easy. This simplifies the setup of bi-amped systems, as well.

Regulated Shunt Power Supply

The input stages of the new U280-SC 25th anniversary include a
special SHUNT power supply for these critical circuits. Shunt
technology provides a dramatic improvement in Q damping, the key
factor in regulating voltage for those stages.
Good regulation reduces the loss of signal integrity that occurs when
the signal itself shifts the amplifier’s operating point. Each channel
uses separate regulators that further improve the U280-SC’s already
excellent imaging.
Damping is critical to overall performance including smoother
transient response and overall staging. Further improvements in
quickness, dynamics and the overall life-like listening experience
result.
Our new Power Supply is virtually indestructible due to specially
designed regulation technology that protects the amp from major
tube failures.

Zero Feedback

The U280-SC does not use negative feedback as do most tube
amplifiers. Negative feedback results in a loss of detail and sound
richness. Margules has developed the BEFS (Back Electromotive
Force Sensor), which changes the amplifier's output impedance per

the load requirement. BEFS permits the amplifier to drive difficult
speaker loads with exceptional efficiency, dynamics and extended
controlled bass.
The U280-SC's innovative design is complemented with a fine new
finish, new silver coated epoxy pc boards, military grade
potentiometers, precision metal-film resistors and audio grade
interstage capacitors.

SETUP

The amplifier should be unplugged from the wall outlet.
Choose the proper output impedance accordingly to your speaker;

both channels should be the same.

2 ohms from 1 to 2 ohms load
4 ohms from 4 to 8 ohms load
8 ohms from 8 up ohms load

In Mono mode always use both channels of the amplifiers in
ultralinear or triode mode in the same output impedance range.

The recommended settings are the following:
2 Ohms should

4 Ohms from 2 to 4 ohms load
8 Ohms from 4 to up ohms load

If used as Stereo or Mono the Triode/Ultralinear mode should be the
same in both channels.

Connect the speakers to the output binding posts, red to red black to
black the input signal of the amplifier to the preamplifier.

If you are not using a preamplifier set the level controls to minimum
otherwise full clockwise.

Check the power switch in the back of the amp to be off.

Connect the amplifier to the AC outlet of the wall, check
that all connections are OK and flip the switch to on.

Wait until the meter reads 43 ma

The panel meter is the actual current passing through
the right front output tube. When overpasses the 100ma,
you are overdriving the tubes; around 120ma, coincides
with the maximum power.

It is not important if you overdrive the tube in short
periods, if you do it continuously, it will shorten the life
expectancy of the tubes. To change the tubes just
unplug the A.C. power line cord, pull out the old ones,
and insert the new tubes; the active bias will balance
them.

We recommend matched pair output tubes, but this is
not indispensable.

Do not substitute with other type the two 12AU7 tubes in
the back of the amplifier. You will unbalance the circuit
and will raise the distortion.

Break-in

The break in period is about 150 hours of music
reproduction.

Tube life expectancy

For optimal performance we recommend to change the
set off tubes every two years, although they should last
a much longer period.

Precaution and care

This product is intended to be plugged directly to the
power supply line of your house.