Adding a modern amp to vintage (70s) head unit.

Carmine

...enjoys spaghetti.
Hello. Haven't been on AK in many years, having been mostly into vintage TVs, I migrated to VK. At any rate, I'm hoping an audiophile might be able to help me.

I've got a 1979 Dodge Magnum with the OE AM/FM/CB headunit that works fine. However the stock speakers are weak/shot (and it probably never had the volume to compete with removed T-top noise)! I want to keep the OE radio and simply power some better speakers, which means "amplifier".

Problem is, the OE radio uses a single "-" feed that goes to all the speakers through the sheetmetal. I have all new wiring to the new speakers. I want to feed the amp through the high-level inputs for auto-turn on.

Plenty of gadgets and info for keeping the stock wiring to speakers, but I can find nothing for supplying an amp with a vintage headunit.

The diagram shows how I wired it, but all I get is weak sound and a "protect" light that comes on/off.*(There is an error in my drawing, which shows the - pinouts on the outside of each harness. In reality they are on the inside, + is on the outside. The actual wiring does not have this error.)

An ohmmeter shows continuity between each of the two center harness grounds as well as the amp's ground terminal.

The amp is a Dual 4 channel 4x100
https://www.amazon.com/Dual-Electronics-DA604D-Performance-Amplifier/dp/B00L1OOVE0/ref=mp_s_a_1_84?ie=UTF8&qid=1535467002&sr=8-84&pi=AC_SX236_SY340_QL65&keywords=amplifier,+4-channel,+"high-level"+input%2C+automotive.

I've attached a page from their manual, as well as the OE Mopar wiring diagram (mine is model xs rather than et)

Any ideas how I should do this differently? Thanks
 

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So you don't have the speakers connected to chassis ground anywhere correct? + and - on each speaker goes only to the amp?

Might need to consult with the manual for the amp to see if its compatible with common ground input. Possible you need to tie the L- and R- inputs to ground in order to get this to work right.

Absolute worst case, are you up for some internal hacking? It shouldn't be all that hard to provide an unamplified line-level signal if you open up the stock radio and find the connections between the preamp and power amp section.
 
So you don't have the speakers connected to chassis ground anywhere correct? + and - on each speaker goes only to the amp?

Might need to consult with the manual for the amp to see if its compatible with common ground input. Possible you need to tie the L- and R- inputs to ground in order to get this to work right.

Absolute worst case, are you up for some internal hacking? It shouldn't be all that hard to provide an unamplified line-level signal if you open up the stock radio and find the connections between the preamp and power amp section.

Anything after the amp is the same as a modern car. New wires, new speakers.

High-to-low level converters are available, I don't even need to hack anything. But using those would:

1) add even more wires and complexity.
2) add about $40 in cost, perhaps needlessly.
3) most importantly, it defeats the purpose of choosing an amp with high-level, auto sensing (auto power on/off) inputs and would require me to add some kind of cheesy switch.

The amp appears to have been designed with this type of application in mind. The instructions are lacking and tech support knows essentially; nothing. Figuring out the correct wiring would be a boon to people seeking to improve their stock radio sound without one of those $500 modern-radio-in-old-box conversions.
 
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....
smile.gif


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.

BTW, here's a pic of the car.
 

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