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Old 09-06-2018, 08:10 PM
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Installing 4 channel amp behind 1979 radio.

I have a 1979 Dodge Magnum and would like to retain the OEM AM/FM/CB radio which functions well, but the old speakers cannot keep up with road noise when the T-tops are removed. I'm not an audio snob, I'm only interested in more (clear) volume. Keeping the stock radio is important. I've already replaced the speakers and run new wiring.#

They are heavy 6x9 Pioneer coaxial speakers for the rear and some Rockford Fosgate #3.5" for the front. (They were removed from something I parted out long ago.)

I just purchased a Dual 400w 4-channel amp. It has high-level, signal sensing inputs. ‎Each of the four channels has +/- inputs, which use two supplied harnesses, presumably front/rear). A wire in the middle of each of these, between the +/- wires, is labeled "common ground".#

The radio has a + wire for each of the 4-channels, but just one for negative, which I believe must have gone to body sheetmetal eventually, since the factory speakers only work when attached to the body.#


How would you wire the input of this amp? #
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Old 09-06-2018, 09:03 PM
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Odds are there is already a common negative on the amp input (you can check with a DMM) if there is one ground/negative input connection may suffice.
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Old 09-06-2018, 10:03 PM
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I'm no help, but, great car! Amazing how some of these have really come out of the woodwork in the last few years.

I seem to recall reading that at some point Mopar made a change in the wiring and it might have involved going from using body ground for the speakers to using separate wires. I think other than that you could mount a much newer Chrysler radio without much hassle. What you have is a lot more interesting, though.
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Old 09-07-2018, 08:17 AM
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Thanks for the DVVM idea.

The move away from common-ground radios came sometime in the early 80s. Installing a later model Chrysler radio, up until just after 2000 would be a piece of cake and wouldn't require me to cut the dash... There were some really nice radios offered which would fit.

However, that's not the goal of this upgrade. The point is keeping the car as much a "period piece" as possible, and maintain the novelty of the factory CB. Just as I could pop a flatscreen into an old cabinet, I prefer to keep it original.

In my eyes, adding the amp (and the aux port through the antenna connection) is more like adding a reversible mod to a vintage tv to accept an A/V signal injection.
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Old 09-07-2018, 12:16 PM
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An ohmmeter shows continuity between both of the center grounds on the wiring harnesses AND to the amp's ground terminal.

I'm adding a couple of pics showing both the radio wiring (use the XS model code for front speakers) and the diagram that came with the amp. When I powered this on without any rear channel inputs or outputs, on a "protect" indicator LED came on the amp (no sound). Would this be due to two open channels?
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Old 09-09-2018, 08:04 PM
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An update... Today I removed everything from the car and decided it would be much easier to get this working by mocking it up on a bench. The good news is, I haven't harmed the amp. Using an early 90s Mopar radio, (with individual +/- outputs) I hooked up through the high-level inputs and everything worked as it should.

I've learned this 70s system is called a "floating ground", and that modern amps and radios use a "common ground". There exists a $15 item to turn common-ground outputs to floating-ground... The purpose being to save re-wiring your speakers (which isn't that hard in a car, but I guess people use them for old RVs and Buses, so that could be a PIA.)

https://www.scosche.com/floating-ground-adapter...

Essentially what I need is this, but in reverse... Adding the new - feeds rather than removing them. I'm searching for a schematic online and haven't come up with much more than an Amazon review that complains the FGA is just a bunch of capacitors. I'll continue looking into this. I might have to build some sort of isolator myself.

Of course those of you with more knowledge than I are more than welcome to submit a schematic of your own....

There must be people besides myself who appreciate interfacing with old-tech (mechanical pushbuttons, early vacuum florescent displays, integrated CBs, etc.) but would like to have a bit more power, the ability to transmit via Bluetooth, and isolation/protection of the original head unit.
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Old 09-09-2018, 11:38 PM
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Have you considered a 1:1 audio isolation transformer? They made (make?) them for speakers in building PA applications. They often have multiple impedance taps available on one winding to deal with cases where there may be high impedance lines to reduce power loss in long wire runs between the amp and speakers.
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