Headset Noise

What you describe is classical ground-loop troubles. Your first attempt to
fix the problem should have done it... insulating the jacks from the panel (mics in
particular). If that truly didn't fix it, you may need to look at other ground
locations. You did say you replaced/insulated *all* jacks, right?

You can try relocating the ground on the Sigtronics to another location (like
the same location as where the audio panel connects would be good). With a large
number of older avionics with a single-ended design, the noise could be coming from
other places.

As far as the *location* being the copilot, you could check to make sure that
when the plug is inserted into the jack that it doesn't touch anything when the prongs
spread. IIRC, the SPA-400 has pilot/copilot isolation, so the signal will have
different paths depending on which plug is connected... separate internal circuits.

Basically, troubleshooting ground loops sucks.

-Cory

Jonathan Goodish wrote:
: 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

--

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

In article ,
wrote:
As far as the *location* being the copilot, you could check to make sure
that
when the plug is inserted into the jack that it doesn't touch anything when
the prongs
spread. IIRC, the SPA-400 has pilot/copilot isolation, so the signal will
have
different paths depending on which plug is connected... separate internal
circuits.

I'm not convinced that it's a ground loop because the intercom system
has a single-point ground. I have the system grounded where at least
SOME other avionics are grounded, but not sure that all avionics are
grounded at that point.

Nothing is touching any metal or anything else in the panel--I've
already been down that road.

I'm wondering whether there's just enough noise around the bus area (and
the b+ in particular) to create a problem. The lighter receptacle is
located above the copilot jacks--not sure whether this is relevant to
the problem or not.

Also, I've noticed a crackle interference in BOTH headsets when the
landing light is turned on, but I suspect that the landing light leads
are brushing against the muffler shroud. I need to check this out, but
it's not related to the high-pitched whine as far as I can tell.




JKG

I'm not convinced that it's a ground loop because the intercom system
has a single-point ground. I have the system grounded where at least
SOME other avionics are grounded, but not sure that all avionics are
grounded at that point

Lift that single ground ground and verify with a continuity test that
with power off, the intercom system is now floating. Make sure there
there isn't some other sneak path to ground that you don't know about.

The frequency of the alternator whine isn't so high that it would be
capacitively coupling into all but high impedance circuits.

If it is noise is present on the B+ line, it should be present at both
headset locations, and a simple power supply filter should take care of
it.

I'd be suspicious of the ground/common connection between the
alternator and the battery. It should be done only with heavy wires,
and not involve the airframe to carry the alternator current.

Another ground loop vs B+ capacitive coupling check - does the whine
become worse as the alternator load is increased, say by turning on
pitot heat and the landing lights? If so, it is more likely a ground
problem.

In article .com,
"nrp" wrote:

I'm not convinced that it's a ground loop because the intercom system
has a single-point ground. I have the system grounded where at least
SOME other avionics are grounded, but not sure that all avionics are
grounded at that point

Lift that single ground ground and verify with a continuity test that
with power off, the intercom system is now floating. Make sure there
there isn't some other sneak path to ground that you don't know about.

I am quite confident that there is no sneak path to ground from the
intercom system, because I had everything disconnected when I rewired
(including the wiring harness). All jacks are isolated with fiber
shoulder washers. If there is a sneak path, I'm not sure where it would
be unless the SPA-400 is grounding through the mounting screws or on/off
switch barrel, but in that case I would expect the whine to be
system-wide.



Another ground loop vs B+ capacitive coupling check - does the whine
become worse as the alternator load is increased, say by turning on
pitot heat and the landing lights? If so, it is more likely a ground
problem.

Well, the whine goes away completely when the alternator side of the
master switch is turned off. I would say that it gets slightly worse as
more load is placed on the bus (landing light and pitot heat, for
example.) It is virtually non-existent in the pilot side, and is very
noticeable on the copilot side.

Before I dug into the intercom wiring last year, I did clean up the
ground at the battery box and strobe power supply (strobe noise problem
mostly cured with new power supply.) However, I haven't cleaned
airframe to engine ground or any grounds under the panel (other than
intercom system.)



Thanks,
JKG

Jonathan Goodish wrote:
In article .com,
"nrp" wrote:


I'm not convinced that it's a ground loop because the intercom system

has a single-point ground. I have the system grounded where at least
SOME other avionics are grounded, but not sure that all avionics are
grounded at that point

Lift that single ground ground and verify with a continuity test that
with power off, the intercom system is now floating. Make sure there
there isn't some other sneak path to ground that you don't know about.


I am quite confident that there is no sneak path to ground from the
intercom system, because I had everything disconnected when I rewired
(including the wiring harness). All jacks are isolated with fiber
shoulder washers. If there is a sneak path, I'm not sure where it would
be unless the SPA-400 is grounding through the mounting screws or on/off
switch barrel, but in that case I would expect the whine to be
system-wide.




Another ground loop vs B+ capacitive coupling check - does the whine
become worse as the alternator load is increased, say by turning on
pitot heat and the landing lights? If so, it is more likely a ground
problem.


Well, the whine goes away completely when the alternator side of the
master switch is turned off. I would say that it gets slightly worse as
more load is placed on the bus (landing light and pitot heat, for
example.) It is virtually non-existent in the pilot side, and is very
noticeable on the copilot side.

Before I dug into the intercom wiring last year, I did clean up the
ground at the battery box and strobe power supply (strobe noise problem
mostly cured with new power supply.) However, I haven't cleaned
airframe to engine ground or any grounds under the panel (other than
intercom system.)

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)?

--

Tauno Voipio
tauno voipio (at) iki fi

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 *
************************************************** ***********************

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 *
************************************************** ***********************

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