Page 5
4 Power Amplifier Version
The ST60 MK III PA M is essentially 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 MKIII PA M will require some kind of
preamplifier which should have a low output
impedance and an output voltage at least high
enough to drive the ST60 MKIII PA M 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
Occasionally 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
MKIII PA M
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 III PA M 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).
5 How to get the best out of your amplifier
•
Do check the “Bias” with built in meter regularly
•
Do not switch between Ultralinear and Triode without first
putting in to “standby”
•
Do not switch off and on without a short rest of 60 seconds
(to reset the ‘soft start’)
•
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 check that the speakers are in phase.
•
Do use efficient, well-designed speakers.
What is safe maximum volume?
The Stereo 60 MKIII 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 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
sound, but depending upon your set up, you may
hear no difference. The gain on the volume
control remains the same, so no difference will be
noticed in volume. But the maximum will be half.
As
switching
will
stress
the
output
transformers
it
is
essential
that
triode/Ultralinear switching be done when the
amplifier is switched into standby mode.
DO NOT LEAVE SWITCHED ON 24/7
Whilst the amplifier will sound at its best when it is
properly warmed up, there is no advantage
leaving it switched on when it is not in use. It is
using electricity and valves have a finite life of
very roughly 5000 hrs
WHICH IS ONLY SEVEN
MONTHS CONTINUOUS USE!
(Or 4½yrs at 3
hrs a day).
Conversely the valves and other
components are stressed more at switch on;
therefore do not switch on and off unnecessarily.
We would always advise that any item of home
electronics is switched off when not in use
