Connecting Headphones in Parallel With a Speaker

[mg]

I would like to connect some headphones in parallel with a single
speaker. The amplifier powering the speaker is a 110-W, Sony STR-DE997
A/V, surround-sound receiver. The speaker has an 8-ohm impedance. The
headphones are Sennheiser model HD 580 (open air) with a nominal
impedance of 300 ohms and a "load rating" of 200 mW.

I'm sort of guessing/figuring that a 3K-ohm resistor in series with
the headphones ought to work. The 3K ohm resistor in parallel with the
8-ohm speaker would yield a net resistance of about 7.9 ohms to the
amplifier. And if power divides the same way current does, that should
provide a maximum of about 300 mW to the headphone. There would only
be a signal to one side of the headphone, of course. I would also put
a headphone volume control in the cord.

As anyone ever done anything like this before? Does anyone know of any
reason it wouldn't work?

---------------------------------

The reason I want to do this, incidentally, is to try to clear up a
problem that I have with hearing dialog in TV movies, etc. The
headphone would be connected to the center speaker of my surround
sound system. The center speaker typically carries the speech part of
the audio, while the other speakers carry mostly the sound effects.
The fact that I would only get sound in one ear probably wouldn't
matter at all since one of my ears is really, really bad and the other
one is only sort of bad.

 

[Jan Panteltje]

I would like to connect some headphones in parallel with a single
speaker. The amplifier powering the speaker is a 110-W, Sony STR-DE997
A/V, surround-sound receiver. The speaker has an 8-ohm impedance. The
headphones are Sennheiser model HD 580 (open air) with a nominal
impedance of 300 ohms and a "load rating" of 200 mW.

I'm sort of guessing/figuring that a 3K-ohm resistor in series with
the headphones ought to work. The 3K ohm resistor in parallel with the
8-ohm speaker would yield a net resistance of about 7.9 ohms to the
amplifier. And if power divides the same way current does, that should
provide a maximum of about 300 mW to the headphone. There would only
be a signal to one side of the headphone, of course. I would also put
a headphone volume control in the cord.

As anyone ever done anything like this before? Does anyone know of any
reason it wouldn't work?

It is better to use a circuit like this:

150 Ohm
-- R1 -----------
from |
amp R2 10 Ohm headphones
|
-----------------

We ant to drive the headphomes from a low impedance, that damps any resonances,
so I will just use 10 Ohms for R2.
Lets see, 110 W (if you amp can do it) in 8 Ohm, U^2 / 8 = 110, so U^ = 880,
so Ueff = sqrt(880) = 30V.
1.5V is enough for big sound pressure on the Senheiser, so you divide by 20.
19 x 10 = 190, so with R2 10 Ohm, make R1 200 Ohm.

The max power in R1 is 30 x 30 / 150 = 900 / 200 = 4.5W
The 'music' power is about 1.10 of that, so .5W
Use a 5 W resistor if you are going to listen to 100% sine waves.
Else use a 1W resistor..... (for R1).

The extra 4.5W will not be noticed much by your amp, especially
as it is purely resistive.

 

[Arny Krueger]

I would like to connect some headphones in parallel with
a single speaker. The amplifier powering the speaker is a
110-W, Sony STR-DE997 A/V, surround-sound receiver.

Interestingly enough, this receiver has a headphone jack.

The speaker has an 8-ohm impedance. The headphones are
Sennheiser model HD 580 (open air) with a nominal
impedance of 300 ohms and a "load rating" of 200 mW.

I own a pair - fine product.

I'm sort of guessing/figuring that a 3K-ohm resistor in
series with the headphones ought to work.

The nominal impedance of the HD 580 s are about 300 ohms, but their
impedance curve wanders around a bit, maybe within a 2:1 range over the
audio band.

The 3K ohm
resistor in parallel with the 8-ohm speaker would yield a
net resistance of about 7.9 ohms to the amplifier. And if
power divides the same way current does, that should
provide a maximum of about 300 mW to the headphone. There
would only be a signal to one side of the headphone, of
course. I would also put a headphone volume control in
the cord.

As anyone ever done anything like this before?

All the time. Since the early days of headphones, series resistors have
been used to reduce power amp output to a range that is suitable for
headphones. The traditional series resistor has been about 100 ohms for
phones in the 8-16 ohm range.

Does anyone know of any reason it wouldn't work?

Depends what you call "work". ;-)

The usual problem with a series resistance is that it often provides a high
source impedance to the headphone. 3K is very high.

If the headphone's impedance curve varies signficantly, the combination of
the series resistor and the impedance of the headphone makes up a
fixed-adjusted equalizer. It changes the frequency response of the transfer
characteristic from the amp output to the listener's ear. It might make the
headphones sound boomy, tinny, dull, excessively bright or just plain weird.

It will work.

There are two common approaches to this problem. One is to use a headphone
amplifier that provides a lower source impedance, and can bypass the power
amp completely. The other is to change the design of the headphone
attenuator for the power amp into a different sort of network that provides
a lower source impedance.

Just a rough cut at a different sort of network. Roughly speaking, a 3 K ohm
resistor in series with a 300 ohm resistor gives about 21 dB attenuation.
You can also get about that much attenuation using a 300 ohm resistor in
series with a 30 ohm resistor. Neither network will appreciably load down a
good power amp. The second network will provide a 100+ times lower source
impedance, one that is in the same range as is commonly provided by
headphone amplifiers.

 

[neon]

what do you need headphones if the speakers are working? the 3k across the speakers is for what exactly? instead of guessing figure the power transfer RATIO 8OHM :XXXX get the idea

 

[Richard Crowley]

"mg" < wrote ...

I would like to connect some headphones in parallel with a single
speaker. The amplifier powering the speaker is a 110-W, Sony STR-DE997
A/V, surround-sound receiver. The speaker has an 8-ohm impedance. The
headphones are Sennheiser model HD 580 (open air) with a nominal
impedance of 300 ohms and a "load rating" of 200 mW.

I'm sort of guessing/figuring that a 3K-ohm resistor in series with
the headphones ought to work. The 3K ohm resistor in parallel with the
8-ohm speaker would yield a net resistance of about 7.9 ohms to the
amplifier. And if power divides the same way current does, that should
provide a maximum of about 300 mW to the headphone. There would only
be a signal to one side of the headphone, of course. I would also put
a headphone volume control in the cord.

As anyone ever done anything like this before?

Similar things have been done perhaps since before you were born.

Does anyone know of any reason it wouldn't work?

3K is very high for this application. You don't really need
any series resistance to prevent overpowering the headphone
since it is such high impedance already (compared to the
speakers). However resistance on the order of 100 ohms
(or higher, up to 1K for modern higher-impedance headphones)
is frequently used as a form of attenuation. Most power amps
are rather noisy, but you don't hear the noise because you
don't listen to speakers with you ear against the grille. But
when you directly connect headphones to the power amp
output, you will likely hear this low-level noise. Attenuating
the audio with a series resistor is a cheap way of dealing
with this issue.

 

[PeterD]

I would like to connect some headphones in parallel with a single
speaker. The amplifier powering the speaker is a 110-W, Sony STR-DE997
A/V, surround-sound receiver. The speaker has an 8-ohm impedance. The
headphones are Sennheiser model HD 580 (open air) with a nominal
impedance of 300 ohms and a "load rating" of 200 mW.

I'm sort of guessing/figuring that a 3K-ohm resistor in series with
the headphones ought to work. The 3K ohm resistor in parallel with the
8-ohm speaker would yield a net resistance of about 7.9 ohms to the
amplifier. And if power divides the same way current does, that should
provide a maximum of about 300 mW to the headphone. There would only
be a signal to one side of the headphone, of course. I would also put
a headphone volume control in the cord.

As anyone ever done anything like this before? Does anyone know of any
reason it wouldn't work?

---------------------------------

The reason I want to do this, incidentally, is to try to clear up a
problem that I have with hearing dialog in TV movies, etc. The
headphone would be connected to the center speaker of my surround
sound system. The center speaker typically carries the speech part of
the audio, while the other speakers carry mostly the sound effects.
The fact that I would only get sound in one ear probably wouldn't
matter at all since one of my ears is really, really bad and the other
one is only sort of bad.

If you are hooking up to more than one speaker, realize that the
negative speaker terminals are usually not common, and cannot be
connected together.

You can get a 'speaker pad' to do the level adjusting, they are
available in a number of different impedences.

 

[GregS]

"mg" < wrote ...

Similar things have been done perhaps since before you were born.


3K is very high for this application. You don't really need
any series resistance to prevent overpowering the headphone
since it is such high impedance already (compared to the
speakers). However resistance on the order of 100 ohms
(or higher, up to 1K for modern higher-impedance headphones)
is frequently used as a form of attenuation. Most power amps
are rather noisy, but you don't hear the noise because you
don't listen to speakers with you ear against the grille. But
when you directly connect headphones to the power amp
output, you will likely hear this low-level noise. Attenuating
the audio with a series resistor is a cheap way of dealing
with this issue.

Putting headphones on the power amp output was always kind of cheap way of
doing it. I don't think the resistance is high, but it may cause the headphones
to be rather quiet at normal settings. 3K is the simple resistance,
because it will limit its temperature to a .3 watt @ 100 watts. I would normally
use two resistors, series, parallel, to get more versitility in wattage
and ohms. The 3K is not really going to get that .3 watts unless 100 watts
is maintained. So with a average of 20 % wattage is only .05 watts
dissipation in the resistor if the 3K is used as a simple series component.


greg

 

[Richard Crowley]

"GregS" wrote ...

Putting headphones on the power amp output was always
kind of cheap way of doing it.

It was pretty much the ONLY way of doing it. What alternatives
were there? Even today, separate headphone amps are as
rare as hens' teeth. Remember, we are talking about an end-
user reproduction system, not a production studio.

I don't think the resistance is high, but it may cause the
headphones to be rather quiet at normal settings.

Which is practically the definition of "too high".

3K is the simple resistance,
because it will limit its temperature to a .3 watt @ 100 watts.

The headphones are self-limiting by virtue of their relatively
high impedance. As others have observed, throwing 3K in
series will emphasize frequency-related impedance variation
and make the headphones sound worse.

 

[GregS]

"GregS" wrote ...

It was pretty much the ONLY way of doing it. What alternatives
were there? Even today, separate headphone amps are as
rare as hens' teeth. Remember, we are talking about an end-
user reproduction system, not a production studio.


Which is practically the definition of "too high".


The headphones are self-limiting by virtue of their relatively
high impedance. As others have observed, throwing 3K in
series will emphasize frequency-related impedance variation
and make the headphones sound worse.

I guess since I bought my Hitachi preamp in the late 70's, I just
plugged any headphones into the headphone output, driven by an internal
headphone driver chip. I guess I got used to that having been done RIGHT.
I agree that most times, its done crudely. I used to think it would screw
up all the damping factor, well it does, but it does not seem to matter
with phones.

greg

 

[GregS]

I guess since I bought my Hitachi preamp in the late 70's, I just
plugged any headphones into the headphone output, driven by an internal
headphone driver chip. I guess I got used to that having been done RIGHT.
I agree that most times, its done crudely. I used to think it would screw
up all the damping factor, well it does, but it does not seem to matter
with phones.

The other main thing, its almost always problematic with a headphone
plugged in, not to have level mismatches when switching from phone off to phone
on, or speaker output on or off. Having an additional volume control
on the headphone sure helps.

greg

 

[Richard Crowley]

"GregS" wrote ...

I agree that most times, its done crudely. I used to think it would screw
up all the damping factor, well it does,

Headphones are generally high enough impedance that "damping
factor" is NOT a factor at all.

but it does not seem to matter with phones.

Headphone diaphragms move only a microscopic amount
compared to speakers. They don't need "damping factor".

What you are calling "crude" is perceived as simple, functional,
and even elegant from other perspectives.

 

[GregS]

"GregS" wrote ...

Headphones are generally high enough impedance that "damping
factor" is NOT a factor at all.


Headphone diaphragms move only a microscopic amount
compared to speakers. They don't need "damping factor".

What you are calling "crude" is perceived as simple, functional,
and even elegant from other perspectives.

I would argue damping factor is independant of levels, as
long as the levels are in the linear range.

greg

 

[Arny Krueger]

"GregS" wrote ...

Headphones are generally high enough impedance that
"damping factor" is NOT a factor at all.


Headphone diaphragms move only a microscopic amount
compared to speakers. They don't need "damping factor".

Since damping factor is closely linked to source impedance, and source
impedance is closely linked to the desirable sitaution where the headphones
are driven by a source with reasonably flat frequency response...

 

[Richard Crowley]

"Arny Krueger" wrote ...

"Richard Crowley" wrote

Since damping factor is closely linked to source impedance, and source
impedance is closely linked to the desirable sitaution where the
headphones are driven by a source with reasonably flat frequency
response...

I think we have lost track of what "damping factor" is.

The most compliant headphone element is extremely stiff compared
(on a proportional basis) to even an average modern LF speaker driver.

I agree with Arny that throwing a (relatively) large series resistance
in the line will just exacerbate the frequency-response roughness
(because of varying impedance). But that is a different phenomenon
than "damping factor".

I agree that it is desirable to feed headphones (or most anything
else, for that matter) from a low-impedance, flat (freq-resp) source.
But "damping factor" needs a low impedance load as well, else you
can't "control" the position of the transducer.

 

[Rich Grise]

The fact that I would only get sound in one ear probably wouldn't
matter at all since one of my ears is really, really bad and the other
one is only sort of bad.

You don't have to get sound in only one ear - just parallel the left and
right phones. The impedance will be half, but take that into account for
your attenuator.

Cheers!
Rich

 

[GregS]

"Arny Krueger" wrote ...

I think we have lost track of what "damping factor" is.

The most compliant headphone element is extremely stiff compared
(on a proportional basis) to even an average modern LF speaker driver.

I agree with Arny that throwing a (relatively) large series resistance
in the line will just exacerbate the frequency-response roughness
(because of varying impedance). But that is a different phenomenon
than "damping factor".

I have seen plenty of speaker curves, but I'm lost as far as headphone Z curves.

greg

 

[[email protected]]

"GregS" wrote ...


Headphones are generally high enough impedance
that "damping factor" is NOT a factor at all.

Well, no, on several fronts.

First, if you actually MEASURE the impedance of most
headphones, you will be hard pressed to find frequency-
dependent variation everyone here seems to assume exist.
Most headphones, regarldess of their rated impedance,
exhibit almost purely resistive impedance.

With no or at most insignificant frequency-dependent
impedance to worry about, driving them from a source
with a reletively high source resistance does not pose
a problem.

Secondly, the reason headphones do not exhibit these
sorts of variations is becuas ethey are so heavily damped
mechanically and acoustically.

Headphone diaphragms move only a microscopic amount
compared to speakers. They don't need "damping factor".

It has abosolutely nothing to do with how far they move.
It has everything to do with how much energu is stored
in mechanical reatiances like suspension compliance
and diaphragm mass, and how much is dissipated
in losses like electrical resistance and mechnical
friction. The ratio between these two, which in essence
determines the frequency-dependent electrical impedance,
remains pretty much the same regardless of how far
the diaphragms move.

In the case of many headphones, the loss portion of
the electrical, mechanical and acoustical portions of
FAR overwhelms the reactive portions, thus a nearly
resistive impedance.

What you are calling "crude" is perceived as simple,
functional, and even elegant from other perspectives.

And technically quite justifiably correct, once you actually
know what the headphone impedance actually looks like.

Oh, and lastly, "damping factor" is a crock of shit,
just more so with headphones.

 

[[email protected]]

I would argue damping factor is independant of levels, as
long as the levels are in the linear range.

I would instead argue that damping factor is a useless
concept when it comes to speakers in general and to
headphones especially, given their common electrical
properties.

 

[[email protected]]

I have seen plenty of speaker curves, but I'm lost as
far as headphone Z curves.

I have measured quite a few, ranging from ear buds to
open-type phones to large closed-type professional
varieties from a number of manufacturers spanning
several decades and almost without exception, they
all present a nearly flat, resistive, frequency-independent
impedance that varies but a few percent over the 10
octaves commonly considered "audio."

 

[mg]

Interestingly enough, this receiver has a headphone jack.


I own a pair - fine product.


The nominal impedance of the HD 580 s are about 300 ohms, but their
impedance curve wanders around a bit, maybe within a 2:1 range over the
audio band.


All the time. Since the early days of headphones, series resistors have
been used to reduce power amp output to a range that is suitable for
headphones. The traditional series resistor has been about 100 ohms for
phones in the 8-16 ohm range.


Depends what you call "work". ;-)

The usual problem with a series resistance is that it often provides a high
source impedance to the headphone. 3K is very high.

If the headphone's impedance curve varies signficantly, the combination of
the series resistor and the impedance of the headphone makes up a
fixed-adjusted equalizer. It changes the frequency response of the transfer
characteristic from the amp output to the listener's ear. It might make the
headphones sound boomy, tinny, dull, excessively bright or just plain weird.

It will work.

There are two common approaches to this problem. One is to use a headphone
amplifier that provides a lower source impedance, and can bypass the power
amp completely.

Your comments prompted me to do a search on Google and I found
something called the "ART HeadTap Headphone Tap" (#180628). It has 4
connections as follows:

1. Amplifier Parallel In.
2. Amplifier Parallel Out.
3. Headphone Out.
4. Headphone Out.

In addition, it has a stereo/mono switch and a know for adjusting the
sound level. It looks like it actually allows for the connection of 2
headphones, but, of course, I will only connect one. There's a PDF
schematic that shows two resistors in series going to the headphones.
I guessing that might be similar to what you mention below.

http://tinyurl.com/26pjt4
http://img3.musiciansfriend.com/dbase/pdf/spec/180628.pdf
http://www.musiciansfriend.com/product/ART-HeadTap-Headphone-Amp?sku=180628&src=3SOSWXXA

Does that appear to be something that would allow me to tap into a
speaker cable and insert some headphones?