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will also depend upon individual speakers, room
size, type of music and positioning etc.
Do not connect to more than one pair of terminals
for each channel. If two pairs of speakers are
required to be connected, they must both be 8-
ohm and connected 4 ohm terminals. Contact
Icon Audio for more information. Sound quality
may be impaired if the amplifier is not correctly
loaded.
5 Use as a power amplifier
The ST60 MK IV is essentially a power amplifier
with a
“Passive” pre-amp on the front which
consists of an input selector and a volume control.
When the volume control is turned to
“maximum”
it is effectively out of circuit. The unit is then a
“power amplifier”
On the rear it is already fitted with a 3 position
sensitivity switch.
the same as the integrated version without the
passive preamplifier section, source switching and
volume control.
Topics such as speaker connection, standby,
triode/Ultralinear,
bias
adjustment,
valve
replacement are covered in other parts of this
manual. (See relevant section).
The ST60 MK IV will require some kind of
preamplifier which should have a low output
impedance and an output voltage at least high
enough to drive it to full power without distortion
(950mv or 0.95v). The preamplifier may be valve,
transistor, passive or passive transformer.
As the preamplifier plays an important role in
instructing the power amplifier what to do, the best
results will not be obtained unless correctly
matched. Attention should also be paid to the
RCA to RCA interconnect cable in particular. This
cable should be of good quality but most
importantly fully screened to shield hum. Some
cables designed to work with low impedance
transistor amplifiers may pick up noise or hum if
the are not fully screened.
Earth Loops
Occasio
nally having too many “earths” in a
system can cause a low level hum which ceases
when the preamplifier is disconnected from the
power amplifier
(standby may be used as a “mute”
when making adjustments). A solution may be
found to this problem by using an interconnect
where the shielding is CONNECTED TO ONE
END ONLY.
Switch on Thump
Some transistor
preamplifiers cause a “thump” to
be amplified through the speakers, with the
possibility of damage in some cases. The solution
is to switch on the preamplifier first, with the ST60
MK IV
in “standby” until the preamplifier is working
normally.
Optimum results will be achieved with medium to
low efficiency speakers by having the sensitivity
switch (on the rear) in the LOW (L) position. This
will give the lowest distortion, lowest noise and
highest damping factor (important for modern low
efficiency speakers).
However the ST60 MK IV retains the ability to be
driven by a lower voltage (300mv or 0.3v). This
may be useful when the amplifier is driven
passively or even directly from a low output
source device. In which case move the switch on
the rear to HIGH (H).
6 Getting the best out of your ST60 MK IV
•
Do check the “Bias” with built in meter regularly
•
Do not switch between Ultralinear and Triode without first putt
ing in to “standby”
•
Do not switch off and on without a short rest of 60 seconds
•
Do not leave the amplifier switched on all the time. This wastes valve life
•
Do not adjust the output valve grid bias without reference to the instructions
•
Do not swop the output valves round as they are set up individually
•
Do not operate the amplifier without loudspeakers connected
•
Do not use valves other than listed as there could be danger of shock or overheating
•
Do not operate the amplifier without the valves
•
Do check that the speakers are in phase.
•
Do use efficient, well-designed speakers.
What is safe maximum volume?
The Stereo 60 MK IV will run happily all day long
at undistorted volumes. The valves are hardly
stressed any more at full power than at zero
volume. Running into distortion will however
stress the valves and the rest of the amplifier.
The meter on the front indicates the percentage of
VOLTAGE (not power). When not in use the
meter on the front is measuring the output
voltage. For the ST60 MK IV this is 25v so a
reading of 50% may be calculated:
25x50%=12.5v 12.5
2
/8=19.5 Watts
In the case of 4 ohm speakers divide by 4 instead
of 8. This figure is approximate as the impedance
of any speaker will vary.
Triode
Switch.
This
switch
causes
the
KT88s/KT120/KT150s to operate as
“triode” (e.g.
300B) valves. Generally triodes have a more
linear power curve, and will tolerate more difficult
speaker loads. The power is reduced by
approximately half. Many people prefer this