Headset Noise

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

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.

Well, if it makes you feel any better, the co-pilot's jack in our
Pathfinder has a bit of whine in it, too. None of the other jacks do.

It's not bothersome, and I always figured it was the proximity to the
electrical bus.
--
Jay Honeck
Iowa City, IA
Pathfinder N56993
www.AlexisParkInn.com
"Your Aviation Destination"

On 8 Apr 2006 20:18:37 -0700, "Jay Honeck" wrote:

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.

Well, if it makes you feel any better, the co-pilot's jack in our
Pathfinder has a bit of whine in it, too. None of the other jacks do.

It's not bothersome, and I always figured it was the proximity to the
electrical bus.

Well, I'm sure it would be contrary to the regs to solder a 0.1
microfarad capacitor across the earphone terminals on the right-seat
headset jack. It's even a bad idea to try it with some alligator
clips first to see if it works.

Don

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 .com,
"Jay Honeck" wrote:
Well, if it makes you feel any better, the co-pilot's jack in our
Pathfinder has a bit of whine in it, too. None of the other jacks do.

It's not bothersome, and I always figured it was the proximity to the
electrical bus.

On my installation, the whine is only slightly noticeable with the
headset volume at its lowest, but then I have to turn up the
radio/intercom volume to compensate, which distorts the audio. When I
turn the headset volume up to high, the whine drives me crazy after
about 15-20 minutes or so.

Of course, I would never experiment with a capacitor that may be in
violation of the regs, even if it would enhance comfort and safety.
That's just something that I don't do during the week.


JKG

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