Question on line converter

Alright, I think this idea is bad, but I need a second opinion.  Someone wants to install an amp to their stock HU.  Another person suggests splicing an RCA directly into the speaker line in the back of the stock HU and is saying it will be better than say a scosche line converter. 

If i understand right and correct me if im wrong, the converter changes the speaker power into just a signal so the amp can process it and do what it does.  Now, what would the outcome be if one were to run an already amped signal (from the HU) to the amp?

Any additional informaiton would be wonderful.

Now i had the same question i asked a few days ago, so how would one do what you said here. I seen a friend do this, by splicing the rca's and connecting the wires to the rear speakers in the back deck of his car and everything was just fine like running the rca's do the back of a stereo. I want to do this but i do not know how to, or what wires to connect to each other. My rear speaker has two wires a brown and a white, and the rca's have three wires a red a white and just a plain bare wire. Help. Cole.

OK, I'm not sure I completely understand here but I'll try to clarify. Your RCA’s output about a 3 – 4 volt signal depending on your HU. The amp is designed to take a low current, 3 – 4 volt signal and boost its current and voltage so you get enough juice to move a heavy speaker cone. Aside from thermal problems, inputting an already amplified signal will drive your amp into saturation and clip the output signal at 12 volts. Your amp can only output a voltage as big as you give it (12 volt battery). Picture an input sine wave on its first half cycle. It starts at 0, increases to 4 volts, and decreases back to 0 before it goes negative. When the input signal reaches 4 volts, ideally your output signal will be at the same voltage as the collector, which is biased by the 12 volt batt. Now if you input an already amplified signal, your sine wave starts at 0, goes up to 4 volts again, but this time only half the audio signal is done. The input continues to climb to whatever, 5, 6, 7, 8 volts, but the amp doesn’t care because its already outputting its max voltage of 12 volts, and will continue to do so until the input comes back down to 4 v or less.

I don’t know if I painted a good picture here but I tried my best without a graph. I probably made some assumptions on this too, so anyone who knows what the hell I’m talking about, feel free to correct me. But basically, when all is said and done. NO, you cant throw RCA connectors onto your speaker wires and put them directly to your RCA amp inputs. This is also the reason for input gain control. So that most HU’s pre-amp outputs can be matched to a particular amp without clipping the signal. Sorry if that’s more than you were looking for.

No, a transformer steps up or steps down an AC voltage using the principles of electro-magnetic induction.  An amplifier uses transistor biases to control the direction of electron flow, but it is impossible to create a greater potential difference than you start with.  Meaning you cannot increase your signal voltage any greater than your collecter bias voltage, which in 12 V assuming the battery is just 12 V.  and acctually its 12 minus the .7  volt drop across the semiconductor junction in the transistors. 

Also, devises that turn DC power into sinusoidal AC are called inverters.  AC into DC are rectifiers.