gmcjetpilot
Joined: 04 Nov 2006
Posts: 170
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 Posted: Tue Jan 29, 2008 11:00 pm Post subject: Bridged Amps, miss match impedance (was: Dumb questions) |
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| Quote: | From: "Robert L. Nuckolls, III" <nuckolls.bob(at)cox.net (nuckolls.bob(at)cox.net)>
Subject: Re: Dumb questions
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>What Rick is referring to is the differential balanced PAIR
>of wires that goes to each speaker. This is a technique
> that allows TWO amplifiers to drive both wires of a
> speaker out
To add to Bob's comments, "bridged amps", each speaker
wire, Pos and Neg are powered by amps, so don't ground
the speaker's Neg to gnd or airframe gnd. It might be best to
use the pre-amp output of the radio to drive your intercom.
When you connect a car's "bridge amp" to an intercom, with
on single common gnd you have an ISSUE? Most just use the
Pos from L & R & don't connect the Neg speaker wires at
all. It works, but some amps detect an imbalance & may not
work, shutting down, protecting itself. Use a 10uF capacitor
in line with the Pos speaker output, tricking it. For the
ground connection on the intercom? Leave it disconnected
or ground to the airframe (I'm talking about the ground on
the intercom's AUX audio input, not the receiver/CD player.)
I wouldn't use the hi-pwr speaker outputs at all. Most quality
radios today have preamp outputs. Using the speaker output
to drive your little mill-watt intercom is over driving to the
extreme. I guess you can hook-up some speakers to the
high pwr speaker drivers, and "rock-out" at the air show?
Impedance
The other issue is impedance. The impedance from car
receiver speaker output's is typ 8 ohm. Even little portable
players have 16-31 ohm head-phone outputs typically. The
INPUT to the intercom is typically 600-1000 ohms. This
miss-match will reduce the volume or gain. It might work,
it might not. The best way to handle this is a matching
transformer. A cars pre-amp output is typically of 2v to 5v,
so the mismatch may not be a big issue, since it has pwr
to overcome the inefficiency. Battery powered iPods on
the other hand have low pwr output and are affected by
miss-match more, so volume levels can be unacceptably low.
You can buy a solution from these guys. ($35?)
http://www.cirruslog.com/products/imp_adp/garmin_296.htm
(don't use this with a bridged amp, for low power no bridge)
You can make your own. Radio Shack to the rescue.
[url=http://www.radioshack.com/product/index.jsp?productId 103254]http://www.radioshack.com/product/index.jsp?productId 103254[/url]
You need two for stereo.
Model: EI-19 Catalog #: 273-1380
Wire diagram:
http://www.cirruslog.com/products/imp_adp/garmin11.gif
EI-19 may not be the perfect transformers; if you have a
better suggestion or source let us all know.
*** Stereo to Mono Quandary:
If you want to go from stereo to mono (with a portable
low power device) don't tie the L & R together to make
mono directly. You may damage your iPod. Regardless,
it's just poor form. A resistor/capacitor pair is sometimes
used on each L & R channel, isolating them, before
connecting them together to make mono. However with
the miss-match mentioned above, the efficiency is already
pretty low; combined w/ the resistor/capacitor and small
amp, of a portable device, you might not have much volume.
Some will use a resistor/capacitor in line with each
speaker output (L&R) before tying them together.
Another way, use the matching/isolation transformer
above (EI-19) and wire like this.
http://img114.imageshack.us/img114/856/matchingtransformerfz7.jpg
(The radio shack audio transformer has a center tap
on the 1000 ohm side. Again not for high powered bridge
amps, just little low drive MP3, pocket AM/FM radios.)
The third way to handle low volume of portable devices
is this isolation / pre-amp. Here is a home made design
you build from your own parts (not a kit).
http://www.minidisc.org/headbanger.html
Enjoy George ATP/CFI - RV7
Be a better friend, newshound, and [quote][b]
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