OPERATION
Press the power switch to the right to turn the amplifier on.
Operation will be signalled by the illuminated lamp in the
switch. If it does not light, the most likely cause is a blown AC
line fuse. That is the single fuse close to the switch inside the
chassis. However, since that fuse is a 15 ampere slo-blo (for 120
VAC lines) and a common branch circuit is also rated at 15
amperes, check the main fuse or breaker panel first.
Standard practice is to turn power amplifiers on last, and
switch them off first, in the electronics chain. This minimizes
annoying noise bursts, or the likelihood of damage to the
speakers from turn-on transients generated by source
equipment. The P505 provides built-in protection from such
transients by incorporating a delay of 3 seconds at turn-on,
before the relay connects the speakers. Some source equipment
may not be silent within 3 seconds, so if you wish to extend the
P505’s delay time, contact the Hafler Technical Service
Department.
The relay serves another purpose - to protect the speakers in
the event an unsafe DC voltage appears at the output. Then the
speakers will be disconnected until the voltage drops to a safe
level. Very high levels of very low frequency energy can
sometimes imitate DC at the output, and might activate the relay
inappropriately. If this is the case, the level of DC protection
provided by the relay can be changed. Contact the factory for
assistance.
If the pilot lamp should ever blink (2-3 times a second) this
signifies that a protective thermal breaker has shut down the
amplifier because of excessive heat sink temperature. The fan
will continue to operate at its highest speed, and as soon as the
heat sink temperature has declined, the amplifier will
automatically return to normal operation. If such a shut down
recurs, you should check for inadequate ventilation, or an
excessive input signal, an extremely low load impedance, or an
input with dangerous signal content (such as oscillation).
Failing any such indication, the fan or the amplifier may require
service. Because of the effective fan cooling, any normal signal
will not cause the amplifier to overheat.
Testing the amplifier to its limits with laboratory signals may
exceed the capacity of a 15 ampere circuit, and may require
higher fuse ratings under severe load conditions. If a back panel
speaker fuse blows, some distorted sound will be heard. Always
check those fuses first. When performing tests at high power
levels, line voltage losses need to be compensated for, or taken
into consideration. Likewise, an inadequately sized line may
suffer voltage drops which adversely effect other electronics
when high power demands occur.
Air filters
The air intake filters should be kept clean to allow maximum
cooling of the amplifier. The protection they provide against dirt
maintains efficient amplifier operation and extends reliability,
so they should not be discarded. They are easily slipped out for
washing in a mild detergent solution, rinsed, and squeezed dry.
They are made of a flame retardant material having a specific
porosity, so exact replacement is best. They may be ordered
from the Hafler Company as part
#AF002.
6
Back Panel Controls
An input level control adjusts the sensitivity of each channel.
They are usually left at full clockwise rotation, for maximum
sensitivity and best signal to noise ratio. Then 2 volts will drive
each channel to full output (4 ohm load). These controls have a
‘B’ taper, with a reduction of 6 dB at 50% rotation.
The ‘Stereo-Mono’ switch selects either normal two channel
operation, or a combined utilization of both channels, which
sends a single input signal through the entire amplifier, and
provides a monophonic ‘bridged’ output across the channels, at
very high power. In the Mono mode, only the Left input and the
Left level control are operative, and the load is connected to the
two Red outputs only. Never make any connections to the black
output terminals in the mono mode. Note that the amplifier’s
specified power output capability in the mono mode is at a
higher load impedance than for stereo operation.
The ‘Float-Chassis’ switch determines the ground (earth)
path of the signal ground. It is normally set in the ‘Chassis’
position, where it is tied directly to the chassis ground. In some
instances, such as where several components are mounted in a
rack, ‘ground loops’ may introduce hum or increased noise.
Moving this switch to the ‘Float’ position may improve this
condition. In this position the signal grounds are referred to the
chassis through a 0.01 mfd capacitor.
Switch settings may be safely changed while the power is on.
Speaker Fuses
The two fuse holders on the back panel protect the speaker
load from excessive drive signals. They are in the feedback loop
from the output stage. When one of these ‘blows’, a distorted
signal will be heard at greatly reduced level. It is to your
advantage to choose a fuse size appropriate to your speakers,
since the P505 can deliver more power than most speakers can
safely tolerate. The David Hafler Company cannot be
responsible for damage to the loudspeakers or the amplifier
resulting from improper fusing.
If the manufacturer of your speakers recommends a fuse
rating, you should install that value in the back panel holders.
When purchasing replacements, do not use the ‘slo-blo’ type.
Type AGC, or 3AG
series are suggested. The P505 is shipped
with 10 ampere fuses installed. This is the maximum value, and
will provide
no
speaker protection, allowing full power testing.
A pair of spare 5 amp fuses is supplied for some protection at
lower power.
There is a distinct difference between fuse ratings for
continuous power and for intermittent ‘peak power’ needs. A 2
ampere fuse with an 8 ohm load permits continuous power up to
32 watts, but will sustain intermittent musical peaks to the
maximum power output. A 3 ampere fuse permits 72 watts
continuously into an 8 ohm load, or a steady 36 watts into a 4
ohm load. A 5 ampere fuse allows 200 watts into 8 ohms, or 100
watts into 4 ohms. Thus even a 5 ampere fuse provides very
little speaker protection.
When the amplifier is operated in Mono, both speaker fuses
are in the circuit. If only one is blown, it is advisable to replace
the other as well, since it has also been stressed to the breaking
point.
#AF002.
Type AGC, or 3AG
