high impedance, low impedance?

[JFLEISC]

I could use some wiring help here. I'm not that good at the terminology but I
know that and always ask advice, so things usually turn out right.
Question; I'm wiring in a used DME and the ident (audio) output has a choice of
'high impedance out' or 'low impedance out'. What's the difference? Is one for
direct to headset and the other for going through the amp in the nav/comm
receiver or something like that? Thanks in advance.

Jim

[Jim Weir]

That's sort of like asking the difference between "high power" and "low power"
for engines. The answers will be totally different between the warbird driver
and the ultralight pilot.

However, in general, "high impedance" in an avionics device means some value in
excess of 1k ohm...more like 10k ohms. Low impedance is less than 1k.

HOWEVER, some manufacturers mean "high impedance" to be phones level (150 ohms)
and "low impedance" to be speaker audio (4-8 ohms or so).

The problem is that you need to do some more digging about the DME in question.

Now *I'VE* got a stupid question of you. DME is a dinosaur. Why in heaven's
name are you loading your airframe with that amount of ironmongery just to give
you about 1% of what a good GPS at 1% of the weight will give you?

Jim


(JFLEISC)
shared these priceless pearls of wisdom:

->I could use some wiring help here. I'm not that good at the terminology but I
->know that and always ask advice, so things usually turn out right.
->Question; I'm wiring in a used DME and the ident (audio) output has a choice
of
->'high impedance out' or 'low impedance out'. What's the difference? Is one for
->direct to headset and the other for going through the amp in the nav/comm
->receiver or something like that? Thanks in advance.
->
->Jim


Jim Weir (A&P/IA, CFI, & other good alphabet soup)
VP Eng RST Pres. Cyberchapter EAA Tech. Counselor
http://www.rst-engr.com

[Ross Oliver]

Jim Weir > wrote:
>Now *I'VE* got a stupid question of you. DME is a dinosaur. Why in heaven's
>name are you loading your airframe with that amount of ironmongery just to give
>you about 1% of what a good GPS at 1% of the weight will give you?


Perhaps because he wants to be legal for IFR without spending $400/yr
on database updates?


Ross Oliver
Cheap ******* Aviator

[JFLEISC]

>Jim Weir > wrote:
>>Now *I'VE* got a stupid question of you. DME is a dinosaur. Why in
>heaven's
>>name are you loading your airframe with that amount of ironmongery just to
>give
>>you about 1% of what a good GPS at 1% of the weight will give you?
>
>
>Perhaps because he wants to be legal for IFR without spending $400/yr
>on database updates?
>
>
>Ross Oliver
>Cheap ******* Aviator

That is precisely it, I got several pieces of eqiupment for next to nothing and
am trying to make my wife's C-172 into an instrument trainer.

Jim

PS I'll bet you're not as cheap as me.

[WJ]

"JFLEISC" > wrote in message
...
> I could use some wiring help here. I'm not that good at the terminology
but I
> know that and always ask advice, so things usually turn out right.
> Question; I'm wiring in a used DME and the ident (audio) output has a
choice of
> 'high impedance out' or 'low impedance out'. What's the difference? Is one
for
> direct to headset and the other for going through the amp in the nav/comm
> receiver or something like that? Thanks in advance.
>
> Jim

Jim, in audio circles (where I kind of know what I'm doing; I usually only
lurk here in r.a.h), the output impedence of a device needs to be set
according to the input impedence of the device it's feeding. In simple
terms, the output impedence acts like a resistor in-line with the output.
Input impedence acts like a resistor tying the input to ground.

Output impedence: Input impedence:

device--resistor--output input-------device
|
|
resistor
|
|
ground

When you connect the two together, the two resistors (impedences) form a
voltage divider that decides how much of the signal is going to make it to
the next stage, and how much gets shunted to ground. In very general terms,
low output impedence coupled to high input impedence is desirable because
that means less of the signal is lost. For that reason, if I were hooking
up a sound system and was in doubt, I'd try the low impedence output setting
first. I'd say that you need to find out what impedence your next stage is
expecting to see.

Cheers,
Walt

[Jay]

Ya you got it right basically. Low impedance will drive a speaker
directly and high impedance is meant to drive the input of an
amplifier. High impedance lines are sensitive to picking up noise and
are often of the shielded variety.

(JFLEISC) wrote in message >...
> I could use some wiring help here. I'm not that good at the terminology but I
> know that and always ask advice, so things usually turn out right.
> Question; I'm wiring in a used DME and the ident (audio) output has a choice of
> 'high impedance out' or 'low impedance out'. What's the difference? Is one for
> direct to headset and the other for going through the amp in the nav/comm
> receiver or something like that? Thanks in advance.
>
> Jim