John Richards....
Your assumption about the value of the resistors is a little low but OK but
you do not need the big 40 Watt power rating in the headphone series
resistors.
Generally, all you would require is one 220 ohm @ 1 or 2 watt series
resistor in each channel. In addition, the value of the resistor can be
juggled quite a bit too...... I just pulled schematics from a half dozen
Stereo Receivers here at my shop.... Yamahas, Kenwood, Denon, etc......
values from 68 to 360 ohms were found, most were usually @ 1 Watt except for
one older Sony 75 watt per channel unit which used a 2 watt resistor
rating. The 220 ohm @ 1 Watt resistor value was found in a 100 watt per
channel Yamaha.
The head phones at 300 ohms in series with a 220 ohm resistor will provide
an amplifier load of about 520 ohms per channel..... this won't allow the
amp output stages to work very hard and the relatively high resistance
holds down the current to low levels.....unlike driving full power into 8
ohm speaker loads where higher current will be required and heat will be
produced in the power output stages.
The max instantaneous power rating just in the headphones series resistors
is less than 1 watt each.... and that's at your 120 Watt output..... which
you would never be anywhere near that high level, especially with headphones
on .
Watch your wiring and be careful that you do not accidentally short the
speaker terminals..... solid state amps will try to deliver the maximum
amount of current into a load..... and a low ohm or dead short will allow
destructive currents in the power output stages.
--
Best Regards,
Daniel Sofie
Electronics Supply & Repair
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"John Richards" wrote in message
news
I would like to drive Sennheiser HD 580 headphone from the speaker
terminals
of a 120 watt amp. The headphones have a nominal impedance of 300 ohms,
power handling of .2 watts and sensitivity is 97dB (at 1 mW?). My
intention
is to wire resistors in series across the outputs of the amp to limit the
voltage to the headphones thereby assuring that the headphones will not be
over driven and the volume control will operate in a reasonable range.
The output voltage of the amp should be about 31 volts (120 W into 8
ohms).
If I wire 4 10 ohm, 10 watt resistors in series for a load of 40 ohms
across
the speaker terminals, the total output power of the amp would be about 24
watts into 40 ohms and should be low enough for the 4 10 watt resistors
(40
watts) to dissipate the power and would provide about 6 volts across each
resistor. Connecting the headphones across one of the resistors would
provide a maximum of about 6 volts to drive the headphones. This
configuration would result in a maximum power of about 120 mW (6 volts
into
300 ohms) which should be plenty of power without the likelihood of
destroying the headphones. The reason I chose 4 10 ohm resistors is that
this value and power rating is easy to find and cheap (Radio Shack). I
could have used 4 2 ohm resistors to provide an 8 ohm load for the amp but
the power rating of each resistor would have to be at least 30 watts
(preferably 40 or 50) which would probably be expensive and require heat
sinks. I'm assuming that the amp will perform as well into 40 ohms (or
any
load as long as the power output is adequate - how about 1 watt into 1
kohm?) as it will into 8 ohms (?).
I know this project is trivial to most of the people on this group but I
would just like some confirmation that my plan is reasonable.
Thanks
John