If the manufacturer of your speakers recommends a
specific fuse value for their protection, you should obtain
3AG type fuses of that value and substitute them for the
ones supplied. The speaker fuses are included in the
amplifier design solely for speaker protection. They do not
protect, or affect, the amplifier’s operation. A blown
speaker fuse does not indicate malfunction-simply a
higher than normal output signal. If the amplifier produces
a low level distorted signal, a blown speaker fuse is the
most likely cause.
We do not recommend the use of a slo-blo type fuse for
speaker protection. If your speakers can handle the higher
power levels safely, and you wish to install fuses of higher
ratings, we suggest for the sake of the speakers-that
you increase fuse size in l/2 ampere increments. THIS
A M P L I F I E R I S A B L E T O D E L I V E R S U B S T A N -
TIALLY MORE POWER THAN MOST SPEAKERS
CAN SAFELY TOLERATE.
Therefore the Hafler Com-
p a n y cannot assume any responsibility for damage to the
load (loudspeaker) because the choice o f f u s e six is in
your hands
.
The 2 amp fuse, with an 8 ohm load, permits continuous
power up to 32 watts, with peaks well above that. 2-l/2
amps passes 50 watts; 3 amps, 72 watts; 5 amps, 200 watts.
We have included a pair of 5 amp fuses as alternates, for
temporary use as spares; for testing the amplifier at very
high power levels; and for operation into very low load im-
pedances, 5 amps will pass 80 watts into a 4 ohm load. We
suggest that you obtain additional type 3AG fuses of the
appropriate size for your speakers.
Loudspeaker Power Ratings
There are no U.S. standards for rating the power han-
dling capabilities of loudspeakers. As a result the manufac-
turers’ usual “ m u s i c p o w e r ” r a t i n g s , o r s u g g e s t e d
amplifier limits, are of only minimal help in determining
safe operating levels with amplifiers which can deliver
enormous amounts of power. On the other hand, some
speakers, particularly tweeters, can also be damaged
through the use of too small an amplifier if it is driven into
sustained clipping. However, tweeters are also more vul-
nerable to high level signals which are not always notice-
ably loud to the ear (as from electronic music sources), so
you may wish to separately protect the tweeters with fuses
of lower rating. You must take into consideration the type
of music, and the levels you like, to provide long term
trouble-free operation of your speaker choice, when you
have a very powerful amplifier like the DH-500.
If more than one pair of speakers will be connected to
the amplifier, proper protection would dictate that each
speaker be separately fused, since when they are operated
in parallel, the combination of lower impedance and the
higher power handling capacity would suggest a much
larger fuse at the amplifier than would provide individual
protection.
If full power tests are to be performed, particularly at
low load impedances, fuse ratings above 10 amps may be
required for the duration of the test.
CONNECTIONS
AC
The standard unit is intended for 120 volt, 60 Hz opera-
tion. If it is controlled by an AC switch on a preamplifier,
the DH-500 power switch may be left on.
If your line voltage is other than that provided in the
United States, be sure you have the alternate multi-voltage
power transformer, and be sure it is wired for your mains
voltage before you plug in the amplifier. The connections
for other line voltages will be found in the back of this
manual.
Input
Standard phono jacks are installed to accept conven-
tional shielded cables, such as those supplied with your
preamplifier. Be sure that the outer shield connection is
secure, to avoid hum. The length of these cables (so as to
permit remote location of the amplifier, if desired) is lim-
ited only by the output impedance of the preamplifier. If it
is 1000 ohms or less, as with Hafler preamplifiers, cable
lengths of up to 100 feet are permissible without loss of per-
formance. Special low capacitance cables enable even
greater distance between preamp and power amplifier. It is
desirable to keep the left and right input cables close to-
gether throughout their length to minimize the likelihood
of hum pickup. They should cross AC cords at right
angles never running parallel to AC lines at close range.
For professional applications the phono inputjacks may
be easily replaced with l/4” phone jacks. Be sure, in that
case, that they are grounded to the adjacent ground lug.
Output
The loudspeakers connect to the red and black 4-way
binding posts. These are of a particularly heavy duty con-
struction to handle the high currents which may be in-
volved. They are spaced for double banana plugs, or you
can connect stranded wire through the vertical hole in the
shaft which is visible when the cap is unscrewed. To be cer-
tain that no strands of wire are unsecured, a tinned end, or
a spade lug soldered to the wire is recommended.
Be sure to maintain correct speaker phasing by watching
the lead identification in the hookup wire, and the identifi-
cation of the speaker terminals. It is important that the
“sense” of
one
speaker’s connections match the others. If
the speakers are out of phase, you will find that the sonic
image has a “hole in the middle”, and there will be a loss of
bass. This will be most evident on monophonic material.
Correct it by reversing the leads to one speaker.
Select speaker wire of sufficient size to preserve the high
damping factor of the DH-500, Standard 18 gauge lamp
cord is the minimum suggested, and is suitable for short
distances. The next larger wire is #16. There are also a
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