Headset Noise

Maybe I missed it earlier in the thread. But, why have you all ruled out the
possibility of a problem with the alternator?

Of course, there is always some ripple on the DC voltage and a grounding
problem will allow it to be more noticeable. But, it could be that the
grounding is just fine and the ripple got worse. Either a blown diode or a
bad phase.

Changes frequency with RPM? Alternator not regulator. Gets louder with load?
Alternator phase not diode.

I had a whine show up recently. It's was only bothersome with ANR headsets.
If you concentrated, you could also hear it with regular headsets. I checked
grounds and switches. Nothing made the least bit of difference.

Finally, used an oscilloscope to look at the alternator output. Nice
consistent ripple. No spikes. But, it was minimally .5 volt p-p growing to
1.5v p-p with load. Typical alternator output ripple should be more like
20mv to 50mv p-p.

They're rebuilding my alternator now. I'll let you know what happens in a
couple of days.

-------------------------------
Travis
N3094P
Lake Amphib
PWK

wrote in message
...
Tauno Voipio wrote:
: It still has the smell of a ground loop if the intercom
: power supply filtering is OK.

: Maybe a ground loop between the intercom and the radios
: (audio panel)?

Now you're talking. Like I said, I don't know the particulars on the
SPA-400,
but many intercom models are simple single-ended... a ground is a ground
is a ground.
The intercom *does* connect to the audio panel, which connects to the nav,
com, dme,
adf, radio, anklebone, etc, etc.

I suspect if you *truly* were to disconnect absolutely everything that
could
re-ground the intercom (and thus render most of the avionics stack
useless), float the
case and all jacks, you won't hear any whine.

As far as the pilot/copilot side being worse, remember that only the bus
(+14V) goes to the battery on a dedicated line (I don't recall what plane
you're
talking about... if it's composite, I'm wrong). The alternator current
meanders
through the aircraft structure somehow and eventually makes it to the
battery. The
structure could very well favor more of this current on the copilots side
and generate
a few more mV of alternator-induced IR drop there.... or rather where
"there" is the
distance between where the intercom and some other avionics is grounded.

-Cory


--

************************************************** ***********************
* Cory Papenfuss *
* Electrical Engineering candidate Ph.D. graduate student *
* Virginia Polytechnic Institute and State University *
************************************************** ***********************

Travis Marlatte wrote:
. . .

I had a whine show up recently. It's was only bothersome with ANR headsets.
If you concentrated, you could also hear it with regular headsets. I checked
grounds and switches. Nothing made the least bit of difference.

Finally, used an oscilloscope to look at the alternator output. Nice
consistent ripple. No spikes. But, it was minimally .5 volt p-p growing to
1.5v p-p with load. Typical alternator output ripple should be more like
20mv to 50mv p-p.

Travis, if you connected the alternator Field directly to a 2 to 5V DC
supply, connected a heavy load (like a couple of landing lights across
the alternator output), then you spun the alternator in a drill press
or lathe, while viewing the voltage across the load with a scope, you
would see about 4V p-p of ripple. Is that a reason to re-build the
alternator?

No!! It is actually the aircraft battery which "filters" the output of
the alternator, and removes the ripple. If you see ripple, it is
because there is some resistance in the path between the alternator
output and the battery posts, or a bum battery. It could be either in
the positive path or in the ground return. I predict that you will
still have your ripple when you get your freshly o/h alternator back...

Several hundred mV of ripple measured at the B terminal of an
alternator is perfectly normal...

MikeM

Jonathan,

read this
http://tinyurl.com/l4rkk

mikem wrote:
Jonathan,

read this
http://tinyurl.com/l4rkk

Mike M is right. The ripple on the alternator output will be higher
rather than lower.

Did you measure that you indeed had intercom isolation with the ground
disconnected?

In article ,
Tauno Voipio wrote:
It still has the smell of a ground loop if the intercom
power supply filtering is OK.

Maybe a ground loop between the intercom and the radios
(audio panel)?


My wife is summoning, but some quick additional info:

1--When the alt. side of the master is switched off, the noise
disappears completely.

2--The noise is equally noticeable in the right FRONT and right REAR.
It is not noticeable in the left FRONT and left REAR.

3--Gets slightly worse as electrical load is added.

4--Sidetone from the radios is not as loud from the copilot position.

5--There is occasional popping when the mic is keyed from the copilot
position (but never from the pilot position.) The popping seems most
frequent when the engine is advanced to move onto the runway for takeoff
(during which time the mic is often keyed to announce takeoff.) This
electrical popping noise is never heard from the pilot position, and may
or may not reappear for the copilot when operating the mic in flight.


Thanks
JKG

Have you checked to see that the mike wires are shielded. Ideally the mike
wires should be run in one shielded cable and the speaker wires run in
another shielded cable. the shields should be grounded only at the intercom.
The pilot and co-pilot wiring run in different areas so an unshielded pilot
headset wiring may be OK were as the co-pilot may require better shielding.

"Jonathan Goodish" wrote in message
...
I have a Sigtronics SPA-400 intercom in my Cherokee. Everything works
fine, except that the right seat position has an annoying amount of
electrical noise in the headset. It is a high-pitch whine (alternator
noise), and the pitch seems to follow RPM. This is not noticable in the
pilot's headset, even with the headset volume at full. It is much less
noticable in the right seat headset with the headset volume turned all
the way down, but obviously this makes things harder to hear for the
right seat occupant (often another pilot).

In an attempt to rectify the problem, I replaced the intercom wiring
with shielded Tefzel cable, replaced all of the jacks, and isolated all
of the jacks from the panel with fiber shoulder washers and brought
everything back to a single point ground. That helped, but only slightly.

I'm trying to figure out what's causing it or, at least, how to stop it
on that particular jack. Obviously, the intercom wiring runs adjacent to
the electrical buses on that side of the panel, so I'm not sure what
role the buses may play, especially since I'm using shielded cable.

Does anyone have any ideas?


Thanks,
JKG

In article . com,
"mikem" wrote:

Jonathan,

read this
http://tinyurl.com/l4rkk


Mike,

That's very helpful, but unless I'm missing something, I'm not sure that
it explains why I'm only having the whine problem on the right side of
the cabin (in both front and rear jacks). All jacks are isolated from
the airframe, and I'm running a common ground back to the intercom
ground. The braided shield is floating at the jack ends and grounded at
the intercom ground point. The whine on one side of the cabin would
make more sense to me if the jacks were grounded locally to the
airframe, but not sure how noise would enter (or exit) the system on one
side of the cabin with the intercom system on a single-point ground.

Outside of the stuff I've posted to the newsgroup, one other oddity:
when I activate the marker beacon output on the audio panel, the whine
is more noticeable.

Obviously, it sounds like there's a ground problem somewhere, but I
don't know where to start. I'm baffled as to why I'm only hearing the
problem on one side of the intercom system and not the other, with none
of the jacks or PTT switches grounded directly to the airframe. The
noise must be alternator-induced, because it is only present when the
alternator field is activated. If something was looping through the
audio panel or somewhere else, why would it only affect the jacks on one
side of the aircraft?



JKG

Jonathan Goodish wrote:

That's very helpful, but unless I'm missing something, I'm not sure that
it explains why I'm only having the whine problem on the right side of
the cabin (in both front and rear jacks). All jacks are isolated from
the airframe, and I'm running a common ground back to the intercom
ground. The braided shield is floating at the jack ends and grounded at
the intercom ground point. The whine on one side of the cabin would
make more sense to me if the jacks were grounded locally to the
airframe, but not sure how noise would enter (or exit) the system on one
side of the cabin with the intercom system on a single-point ground.

I concur. I dont think it is a ground loop issue. I'm thinking a wiring
error, or short which occurs only when headphone plugs are pushed in,
or an internal fault inside the SPA400. Too bad you dont live close, I
would lend you mine to see if it made a difference.

Outside of the stuff I've posted to the newsgroup, one other oddity:
when I activate the marker beacon output on the audio panel, the whine
is more noticeable.

Obviously, it sounds like there's a ground problem somewhere, but I
don't know where to start. I'm baffled as to why I'm only hearing the
problem on one side of the intercom system and not the other, with none
of the jacks or PTT switches grounded directly to the airframe. The
noise must be alternator-induced, because it is only present when the
alternator field is activated. If something was looping through the
audio panel or somewhere else, why would it only affect the jacks on one
side of the aircraft?

The SPA400 drives the pilot headphone jack differently than the other
three jacks. Is the problem only on the copilot's jack, or also on the
back-seat jacks?

I'm puzzled by the "crackle". The alternator whine caused by a ground
loop is usually musical, pitch varies with engine speed. Crackles are
usually vibration induced intermittent contacts such as an improperly
seated audio panel, or nav-com. These usually behave with the engine
off, but can be induced by bashing the panel with your hand. Crackle
could also mean loose electrical connections to the bus, unshielded
P-leads, noisy electric-powered gyro instruments, bad fuel senders,
vibrating point mechanical alternator regulator (hopefully yours is
solid-state) or a vibration-sensitive Over Voltage Protection Relay. If
you have a good DVM, connect it to main bus, and read the voltage prior
to engine start, during runup and flight to see if there are any
fluctuations which correlate with the crackle you are hearing...

Jonathan Goodish wrote:
: That's very helpful, but unless I'm missing something, I'm not sure that
: it explains why I'm only having the whine problem on the right side of
: the cabin (in both front and rear jacks). All jacks are isolated from
: the airframe, and I'm running a common ground back to the intercom
: ground. The braided shield is floating at the jack ends and grounded at
: the intercom ground point. The whine on one side of the cabin would
: make more sense to me if the jacks were grounded locally to the
: airframe, but not sure how noise would enter (or exit) the system on one
: side of the cabin with the intercom system on a single-point ground.

The jacks may be a red herring. If they truly are isolated from the structure
and grounded only to the intercom, you're right... they *cannot* be causing a ground
loop problem. The shield could still be allowing some noise coupling, but I doubt it.

As far as the right side/left side stuff, remember that there's a *BIG*
current flowing out of the alternator (well, *into* the alternator's ground). That
current is just meandering through the structure somehow. It could certainly be more
preferential to one side or the other when you're talking a few miliohms total through
the structure.

: Outside of the stuff I've posted to the newsgroup, one other oddity:
: when I activate the marker beacon output on the audio panel, the whine
: is more noticeable.

Does the *audio panel* run to the same single-point ground as the intercom?
If they're spatially separated (especially if one is on port the other's on
starboard), it could do what you are describing. Of course *any* chunk of avionics
grounded elsewhere could be inducing the ground loop as well.

: Obviously, it sounds like there's a ground problem somewhere, but I
: don't know where to start. I'm baffled as to why I'm only hearing the
: problem on one side of the intercom system and not the other, with none
: of the jacks or PTT switches grounded directly to the airframe.

Do you have the schematics available for the intercom? I managed to find a
set for both of the intercoms I've had in my plane. They generally have different
circuitry for the pilot vs. copilot. That might allow for different coupling of the
noise.

The : noise must be alternator-induced, because it is only present when the
: alternator field is activated. If something was looping through the
: audio panel or somewhere else, why would it only affect the jacks on one
: side of the aircraft?

Again, different inputs. It *does* seem a bit odd, but I haven a hard time
believing that it's the shield. Again, you're *SURE* that the jacks are floating,
right? ... and they don't accidentally contact any chunk of airframe when a jack is
plugged in? For the longest time we didn't have side panels in our plane and I just
had the rear seat intercom jacks zip-tied to the side. Once in awhile they'd move and
rattle against the airframe and make a serious crackle racket in the intercom system.
Just a thought.

-Cory

--

************************************************** ***********************
* Cory Papenfuss *
* Electrical Engineering candidate Ph.D. graduate student *
* Virginia Polytechnic Institute and State University *
************************************************** ***********************

In article .com,
"mikem" wrote:
I concur. I dont think it is a ground loop issue. I'm thinking a wiring
error, or short which occurs only when headphone plugs are pushed in,
or an internal fault inside the SPA400. Too bad you dont live close, I
would lend you mine to see if it made a difference.

Well, I rewired the intercom with shielded cable and was pretty darn
careful to follow the wiring schematic, and the system does function, so
I'm fairly confident that it is wired correctly. However, I do
understand that just about anything is possible.

Believe it or not, I actually sent the SPA-400 back to Sigtronics a few
months ago for them to bench test, and it came back fine. That's not to
say that they didn't miss something, as I'm not sure what testing
procedure is used.



The SPA400 drives the pilot headphone jack differently than the other
three jacks. Is the problem only on the copilot's jack, or also on the
back-seat jacks?

The problem exists ONLY in the right side of the cabin (copilot and
right rear passenger.) The pilot side is fine (both front and rear.)
You can hear a very, very faint whine on the pilot's side if there is
radio silence, but it's almost unnoticeable even with the headset volume
set at max. Despite the whine on the copilot side of the cabin, voice
quality is still very good throughout the system.



I'm puzzled by the "crackle". The alternator whine caused by a ground
loop is usually musical, pitch varies with engine speed. Crackles are
usually vibration induced intermittent contacts such as an improperly
seated audio panel, or nav-com. These usually behave with the engine
off, but can be induced by bashing the panel with your hand. Crackle
could also mean loose electrical connections to the bus, unshielded
P-leads, noisy electric-powered gyro instruments, bad fuel senders,

I think I have two incidents of "crackle." One is only present when the
landing light is turned on, and I suspect that one of the landing light
leads in the cowl is occasionally contacting the muffler shroud.

The other one, however, sounds like a "chop chop chop" which increases
in speed with engine RPM and is often (but not always) present when the
throttle is advanced when positioning onto the runway. It is only heard
when the copilot PTT is keyed. It is never heard when the pilot's PTT
is keyed.

As for the regulator, I'm not sure when solid-state regulators became
standard equipment on airplanes, but as far as I know, mine hasn't been
changed since the airplane was manufactured in 1977.



JKG