Recently there have been one or two posts wondering why the headphone
jack of amplifier X produced audible white noise at a constant volume
all the time. People were told that the amp was defective, but I don't
think this is correct.

I believe I know the answer. Some amplifiers of recent vintage (such as
my AMC) have been skimping on parts, and a new design implementation
that is apparently on the rise is to drive the headphone jack with the
speaker power, instead of a separate mini power amp. The result is that
the amplifier's *effective* signal-noise ratio (assuming you don't want
to listen at 130 dB) is ruined when you strap on a pair of headphones,
because it is literally like strapping the speakers to your ears. We
all know that if you put your ear next to the tweeter while the amp is
on, you hear a hissing noise of perhaps 30 dB volume.

I bought a headphone amplifier to eliminate this problem. I couldn't
stand the noise using my amplifier's headphone jack.

-Sean

Sean Fulop wrote:
I believe I know the answer. Some amplifiers of recent vintage (such as
my AMC) have been skimping on parts, and a new design implementation
that is apparently on the rise is to drive the headphone jack with the
speaker power, instead of a separate mini power amp.

IMO, AMC's products may not meet the "competently designed" criteria.

As an objectivist, I've not heard any differencs among any CD players
EXCEPT for an AMC CD8b (or was it a CD6b? I can't remember). It had
audible distortion which I noticed most often with jazz saxophone.
I returend the first one, and the replacement had the same audible
distortion, so I returned it as well. Bad batch? Who knows.

--
Jason Kau
IS FOR EMAIL
IS FOR SPAM
http://www.cnd.gatech.edu/~jkau

Sean Fulop wrote:
Recently there have been one or two posts wondering why the headphone
jack of amplifier X produced audible white noise at a constant volume
all the time. People were told that the amp was defective, but I don't
think this is correct.

I believe I know the answer. Some amplifiers of recent vintage (such as
my AMC) have been skimping on parts, and a new design implementation
that is apparently on the rise is to drive the headphone jack with the
speaker power, instead of a separate mini power amp. The result is that
the amplifier's *effective* signal-noise ratio (assuming you don't want
to listen at 130 dB) is ruined when you strap on a pair of headphones,
because it is literally like strapping the speakers to your ears.

You have to explain how the effective signal-to-noise ratio of the power
amp is ruined.

Since you don't apply tens of volts to the headphone, there is an
attenuator between the amp's output and the headphone jacks if the main
power amp is used to drive the headphones. That attenuator will scale
down the size of the signal *and* the noise.

We
all know that if you put your ear next to the tweeter while the amp is
on, you hear a hissing noise of perhaps 30 dB volume.

Now imagine that hiss being attenuated by a lot before reaching the
headphone jacks.

I bought a headphone amplifier to eliminate this problem. I couldn't
stand the noise using my amplifier's headphone jack.

There is a bad design somewhere, if your headphone jack has excessive
white noise. Not necessarily because the main amp is used to drive the
headphones.


-Sean

"Sean Fulop" wrote in message
...
Recently there have been one or two posts wondering why the
headphone
jack of amplifier X produced audible white noise at a constant
volume
all the time. People were told that the amp was defective, but I
don't
think this is correct.

I believe I know the answer. Some amplifiers of recent vintage
(such as
my AMC) have been skimping on parts, and a new design implementation
that is apparently on the rise is to drive the headphone jack with
the
speaker power, instead of a separate mini power amp. The result is
that
the amplifier's *effective* signal-noise ratio (assuming you don't
want
to listen at 130 dB) is ruined when you strap on a pair of
headphones,
because it is literally like strapping the speakers to your ears.
We
all know that if you put your ear next to the tweeter while the amp
is
on, you hear a hissing noise of perhaps 30 dB volume.

I bought a headphone amplifier to eliminate this problem. I
couldn't
stand the noise using my amplifier's headphone jack.

You're probably right about the cause, but the solution you
propose--buying a separate headphone amp--is certainly an expensive
way to solve the problem. The standard solution is to divide down
the output of the amplifier until the loudness through the headphones
is the same as the loudness over the speakers when the volume control
is set at the same spot. The problem is that headphones vary wildly
in their voltage sensitivity; we have 600 ohms phones, 16 ohm ones,
and everything in between. So the choice of resistors in the divider
will be a compromise.

Since there are 2 resistors involved, it's possible to choose them so
that the ratio is correct for 2 different headphones. With any kind
of luck, this choice of values will work well over the entire range of
headphone impedances. The only problem is the variation in headphone
impedance with frequency. Fortunately, most headphones have
reasonably well behaved impedance curves. (At least mine do :-)

Norm Strong

IMO, AMC's products may not meet the "competently designed" criteria.


This is, of course, quite possible, since I have never heard their CD
players. As a person willing to follow my own perceptions, no matter
their cause (this makes me a "personalist," meaning that if I hear a
difference between two components under sighted conditions, then I say
just exactly that, "I hear a difference").

What I can say on that score is simply that their mid-level integrated
amplifier, a bargain at $300, has an excellent phono stage and much much
less congestion than the NAD 304 which it replaced. Since it is
established that most solid state power amps sound almost the same
(though the AMC seems to be more solid, with tighter louder-sounding
bass), we can assume that the improvement in congestion is a result of a
better preamplifier, which has long been fingered as a culprit in the
introduction of congestion. In sum, I am more than satisfied with my
AMC, and would happily use it to drive "audiophile" speakers costing
$1000/pair or more.

-Sean

You have to explain how the effective signal-to-noise ratio of the
power amp is ruined.

Since you don't apply tens of volts to the headphone, there is an
attenuator between the amp's output and the headphone jacks if the
main power amp is used to drive the headphones. That attenuator will
scale down the size of the signal *and* the noise.

Right, but some amps such as my AMC don't pad down the voltage nearly
enough for the benefit of the headphones (assuming they do so at all).
The result is that with my Sennheiser 580s, the amplifier's normal hiss
can easily be heard at about a 30 dB constant volume, the music over the
phones is quite loud with the volume barely cracked open (sounds really
good, too), and the volume at anything over 50% will blow the diaphragms
in the phones (though your ears might go first).

The effective S/N ratio under particular listening conditions can be
defined as the difference between the actual noise volume and the
loudest SPL the speakers at hand are going to produce under these
conditions. For most dynamic phones, the maximum SPL is about 105 dB.
In the example under discussion, then, the effective S/N ratio is *at
most* 75 dB, and that's only if you like going deaf. Under normal
listening conditions, where you limit the loudest sounds to be at about
85 dB, the effective S/N ratio is down to 55 dB, about the same as
playing a record. Not a satisfying experience for most, having their
CDs sound as noisy as records.

This, my friends, is what we call a *design flaw*, not a *flaw*, of the
sort that could be corrected with a new unit or a repair shop. And this
particular design flaw seems to be more prevalent, because there have
been recent posts about amplifiers (a Rotel, if I recall, was referred
to in one such post) that have this precise problem.

-Sean

I dealt with this by buying a Behringer HA4700 headphone amplifier.
Great performer. Actually four headphone amplifiers in a single unit.
A steal at its street price of about $100 online. ALPS detented
controls. Not esoteric enough for audiophile dealers. But as good as
the recording industry can produce. And their headphones aren't bad
either.

Sean Fulop wrote:
You have to explain how the effective signal-to-noise ratio of the
power amp is ruined.

Since you don't apply tens of volts to the headphone, there is an
attenuator between the amp's output and the headphone jacks if the
main power amp is used to drive the headphones. That attenuator will
scale down the size of the signal *and* the noise.

Right, but some amps such as my AMC don't pad down the voltage nearly
enough for the benefit of the headphones (assuming they do so at all).

There has to be a pad, since the power amp can output tens of volts. The
pad can be a simple resistor in series.

The result is that with my Sennheiser 580s, the amplifier's normal hiss
can easily be heard at about a 30 dB constant volume, the music over the
phones is quite loud with the volume barely cracked open (sounds really
good, too), and the volume at anything over 50% will blow the diaphragms
in the phones (though your ears might go first).

The effective S/N ratio under particular listening conditions can be
defined as the difference between the actual noise volume and the
loudest SPL the speakers at hand are going to produce under these
conditions. For most dynamic phones, the maximum SPL is about 105 dB.
In the example under discussion, then, the effective S/N ratio is *at
most* 75 dB, and that's only if you like going deaf. Under normal
listening conditions, where you limit the loudest sounds to be at about
85 dB, the effective S/N ratio is down to 55 dB, about the same as
playing a record. Not a satisfying experience for most, having their
CDs sound as noisy as records.

This, my friends, is what we call a *design flaw*, not a *flaw*, of the
sort that could be corrected with a new unit or a repair shop. And this
particular design flaw seems to be more prevalent, because there have
been recent posts about amplifiers (a Rotel, if I recall, was referred
to in one such post) that have this precise problem.

If you think that the output of the power amp is not attenuated enough,
and your headphone is too sensitive, and that your power amp has too
much background noise, here is something cheap that can fix it. Radio
Shack sells an inline headphone volume control. Yes, you guess it, it is
simply a variable resistor. Radio Shack also sells a headphone amp that
runs on batteries for about $20, if you want to have a low impedance
drive for your headphones. It is the same quality as most hi-fi
headphone amps in preamps or receivers.

You can also buy a slightly more expensive headphone distribution amp
used by audio pros. These have attenuators followed by op amps. I bought
one for about $50, and it works very well.

Resistive networks are less costly than a dedicated headphone amp.
But they destroy any amplifier damping to the headphone. And the
result is the same as resistive networks in the speaker line. Since I
finally got a good headphone amplifier, I wouldn't have anything else.

In article qEqgc.18252$ru4.17521@attbi_s52,
Stu-R wrote:

Resistive networks are less costly than a dedicated headphone amp.
But they destroy any amplifier damping to the headphone. And the
result is the same as resistive networks in the speaker line. Since I
finally got a good headphone amplifier, I wouldn't have anything else.

Why put a twenty ohm resistor in series with a 4 ohm resistor and the
headphones across the 4 ohm resistor. 25 to 1 reduction in power (plus
reduction due to higher impedance load than the amp supports so maybe
150 to one. Phones see 4 ohm resistor.