Or you can put a 70-volt transformer across the amp's output for each pair of
headphones, setting up a mini-70v system. Now you have very low Z and reduced
power available to each pair, and probably lots left over to drive a speaker
which is maybe also on a 70v transformer but on a higher power tap.

Arny Krueger wrote:

How about trying a little engineering?

Based on my studies for testing headphone amplifiers, I think it's safe to
say that the worst case impedance of a pair of headphones is usually about
16 ohms. There are exceptions, but they are relatively rare. The impedance
of a pair of headphones can range up into the 300-600 ohm range.

Based on examination of a range of audio gear, I can say that a very common
driving impedance for headphone jacks driven by dedicated amplifiers is
about 16 ohms.

Therefore, the value of the shunt resistor of our resistive voltage divider
should probably be in the 16 -32 ohm range. 16 ohms if the series resistor
is large, and more like 32 ohms if the series resistor is small.

The power sensitivity of headphones varies greatly, but the voltage
sensitivity is surprisingly similar. Typically 1 volt will drive headphones
to a comfortable level, but some place around 5 volts ears and headphones
can be damaged. I might suggest 2 volts as a reasonable target level.

The operational voltage at the output of a power amp will vary with the
efficiency of the speakers that are typically used with it, and the
preferences of the listeners. A "small" 50 watt amp puts out about 20 volts
at clipping, while a "large" 800 watt amp puts out 80 volts at clipping. The
size of the series resistor should probably range from 16 ohms for a small
amp and efficient speakers, to 300 ohms for a large amp and inefficient
speakers.

Thus, a headphone attenuator should probably be composed of an approximate
16-32 ohm resistor in parallel with the headphones, and a series resistor
ranging from 32 to 300 ohms running to the amplifier output. Higher valued
series resistors are indicated for use with high powered amps. YMMV.

Michael Putrino wrote:
I've driven many AKG 600-ohm headphones with a 50 watt stereo
transistor power amp with no resistor necessary. Just added a parallel
break-out box into which the headphones plugged.

The resistor is there in case something bad happens, like a headphone is
shorted or left plugged in halfway so the two amp outputs are shorted together.
You don't need it, but it is a good idea to have it there for safety.

I would not do this with low-ohm headphones, or with a tube amp with
an output transformer.

You can make it work with a tube amp if you are careful about loading, or
you use an output transformer designed to deal with sloppy loading (like
the Bogen Challengers use... most of the crappy tube amps with 70V output
taps will deal with varying loads well enough because they have to when driving
70V systems).
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."

Thanks for the replies! This project will involve a 60-watt Adcom driving
AKG 240s and Fostex T20s. I could just upgrade my current headphone amp ( a
PAIA kit), but I hate to spend the dough when I have spare amps lying around
(and the time to do this project). I think the idea of a 70v system is good,
but probably too expensive. Using resistors (that I probably have in a parts
drawer) sounds like the way to go! I just need to remember how to wire a
headphone jack -- one of those things that I've learned and forgotten four
or five times...

I appreciate the expertise!

"Fader" wrote in message
.. .
Thanks for the replies! This project will involve a 60-watt Adcom driving
AKG 240s and Fostex T20s. I could just upgrade my current headphone amp
( a
PAIA kit), but I hate to spend the dough when I have spare amps lying
around
(and the time to do this project). I think the idea of a 70v system is
good,
but probably too expensive. Using resistors (that I probably have in a
parts
drawer) sounds like the way to go! I just need to remember how to wire a
headphone jack -- one of those things that I've learned and forgotten four
or five times...

Tip is left. Ring is right. You can figure out what sleeve is all on your
own....

Kendall

In article writes:

I just need to remember how to wire a
headphone jack -- one of those things that I've learned and forgotten four
or five times...

This isn't something that you should learn, it's something you should
learn to figure out. Then you'll know how to do it whenever you need
to wire one.

Plug a set of headphones into a jack and study it. It'll come to you.
The only thing you can do wrong is get left and right switched, but
that's easy to correct. As long as you wire all the jacks the same
way, you only need to switch the wires that go to the amplifier.




--
I'm really Mike Rivers - )
However, until the spam goes away or Hell freezes over,
lots of IP addresses are blocked from this system. If
you e-mail me and it bounces, use your secret decoder ring
and reach me he double-m-eleven-double-zero at yahoo

In article .net writes:

Tip is left. Ring is right.

Unless it's the other way around. This is true for most if not all
modern headphones, as they come from the factory, if they're built
according to instructions. It's sort of an industry standard, but
there's no law against wiring it the other way. If the headphone cable
has been repaired or replaced, there's nothing saying it was done
correctly other than trust in the one doing the repair.

Like anything else that you're building (so you have control over it),
it never hurts to check before you put it into service and run into
something like Mondoslug did in a studio, where the headphones were
reversed with respect to the monitor speakers.



--
I'm really Mike Rivers - )
However, until the spam goes away or Hell freezes over,
lots of IP addresses are blocked from this system. If
you e-mail me and it bounces, use your secret decoder ring
and reach me he double-m-eleven-double-zero at yahoo

I believe some phone jacks (including some that I have) actually have *four*
solder points to allow a momentary break of the signal to a speaker when the
phones are inserted. Maybe this is why I've had a mental block that
headphone jacks were difficult...

Anyway, thanks, and I'll post the results of my soldering soon!

Tip is left. Ring is right.

Ever notice that Spock's wife was T'pring?

I often wondered if she worked in the Vulcan telecom industry.

Also, what about running the headphone signal on xlr? (And I mean the
amplified speaker output). Didn't a Furman headphone amp do that and use
passive boxes that attached to mic stands? I thought it did -- and it
worked! It would definitely be easier to wire breakout boxes and
interconnect them with xlr cable.

"Fader" wrote in message ...
Does anyone know how to use a regular stereo amp (D-150, for instance) to
drive headphones rather than speakers? I heard of this being done a long
time ago, but can't remember.

Thanks!

Fader,
I have a .jpg schematic of a headphone distribution circuit that I
built to use with a Samson Servo 120 power amp. It works, sounds fine
with my AKG K240s. If you want the schematic, email me privately.

DaveT

The resistor is there in case something bad happens, like a headphone is
shorted or left plugged in halfway so the two amp outputs are shorted together.
You don't need it, but it is a good idea to have it there for safety.

--scott

I used this for 10 years without resistors...plugging and unplugging
headphones many many times...and yes, somtimes left half-way inserted
by accident. Most transistor power amps today (and for many years in
the past as well) have short-circuit protection, and most of the time,
headphones do not short when going bad...that I'm aware of; usually
they have a break somewhere in their cable.

He can use the resistors if he likes, but I still think they are not
necessary. By the way, the amp was an old 50 watt per channel Kustom
amp we had doing nothing else, so it came in handy for this purpose.

Mike Putrino

In article writes:

I believe some phone jacks (including some that I have) actually have *four*
solder points to allow a momentary break of the signal to a speaker when the
phones are inserted. Maybe this is why I've had a mental block that
headphone jacks were difficult...

Be sure you know what those terminals do.

There are jacks with switches that are opened or closed when a plug is
inserted. The most common application around the studio is normalled
patchbay jacks. The top row jacks are connected to the bottom row
jacks through a switch. With no plug inserted, it's as if a jack was
patched to the one directly below it (or somehwere else if that's how
your patchbay is wired). Inserting a plug breaks that connection and
you can patch it wherever you want.

You could wire the speaker through the switch contacts on the jack so
that when you plug in headphones, the switch breaks the connection to
the speaker. You'll need a jack with two switches, one for each
channel, assuming it's a stereo amplifier.




--
I'm really Mike Rivers - )
However, until the spam goes away or Hell freezes over,
lots of IP addresses are blocked from this system. If
you e-mail me and it bounces, use your secret decoder ring
and reach me he double-m-eleven-double-zero at yahoo